MICROSTRUCTURAL & MICRO HARDNESS ANALYSIS OF DEFECTIVE AND DEFECT FREE MULTI PASS WELDED COUPONS X00105837 JOSEPH STYNES - Copy

MICROSTRUCTURAL & MICRO HARDNESS ANALYSIS OF DEFECTIVE AND DEFECT FREE MULTI PASS WELDED COUPONS X00105837 JOSEPH STYNES - Copy

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  Institute of TechnologyTallaght Department of Mechanical Engineering ProjectTitle: Microstructural & Micro Hardness Analysis of Defective and Defect Free Multi-pass Welded Coupons Supervisor - Brian O’Brien Student name - Joseph Stynes Student number - X00105837 Date of Submission: 24/04/16 Declaration I hereby declare that this is my original work produced without the help of any third party. Signed: Date: 24/04/16 Lecturer Feedback
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  i Abstract Multi-pass welds aredifferent in nature to single pass welds, not simply because of the number of weld passes but because of the effect that repeated thermal cycling has on the final microstructure. Because the initial weld is not exposed to the cooling ambient air for any length of time, as would be the case with a single pass weld, its final mechanical properties are quite different. The initial heat energy required, to raise the material to a point where the material becomes molten within the weld fusion zone, decreases with successive weld passes. Once the initial weld pass has been completed then the material temperature has greatly increased. The re-heating of the previous weld metal deposit by subsequent weld passes acts to normalise the previous weld microstructure. This ensures that any residual stresses within the material are reduced. The nature of multi-pass welds guarantee a level of preheat into the material and, upon completion of the welding operation, slows the cooling rate down and greatly reduces the likelihood of the formation of cracks within the welds and the parent material. When the final weld is completed, whether it is a multi-pass fillet weld or a multi-pass butt weld, its microstructure differs significantly from the previous welds. This is because the final weld is exposed to the cool ambient air, unlike the previous welds. This ensures that the final welds have a different cooling rate than the underlying welds and therefore a higher level of hardness. By sectioning these multi-pass weld specimens and hot mounting them using a phenolic resin they are then subject to successive planar grinding and polishing. They are then finally etched with a 2% nitric acid solution to highlight the grain structure of the material. This is done to allow for the microscopic examination of the weld microstructure. Photographic images are taken to further examine the microstructure from the parent metal through to the heat affected zone and the weldment. This is a crucial step to help clearly identify any flaws that maybe present. Vickers Micro Hardness tests are then carried out to determine the hardness values of the different zones of interest. Acknowledgements I wish to thank my supervisor Brian O’Brien for his ongoing support and encouragement throughout this project. I would also like to thank Elaine McGeough for patiently training me on the use of the metallographic equipment in the materials lab. I am also very thankful for the knowledge and help given to me by Brian O’Donnchadha on the use of the hardness testing and microscopy equipment in the metrology lab. Thanks also to Alan Somers for allowing me access to the manufacturing lab anytime that I needed it. A special thanks to Chris Keogh for his help with a critical part of this project.
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  ii Contents Institute of TechnologyTallaght.............................................................................................1 Chapter 1 Project Introduction..............................................................................................1 1.1 Project proposal & Project description........................................................................1 1.2 Why was the project being undertaken? .....................................................................1 1.3 Aims and Objectives ...................................................................................................1 1.4 Objectives for the Literature review............................................................................2 1.5 Methodology of approach to undertaking of project...................................................2 1.6 The preparation and welding of the selected samples .................................................3 1.7 Material needed for weld coupons ..............................................................................3 1.8 The preparation of the welded samples to be mounted and polished ready for Microscopic inspection..........................................................................................................4 1.9 The Testing for hardness variation across the welded samples using Vickers Micro Hardness testing.....................................................................................................................4 Chapter 2 Literature review..................................................................................................5 2.1 Introduction.................................................................................................................5 2.2 Microstructural analysis of steels................................................................................5 2.3 Ferrite ..........................................................................................................................6 2.4 Cementite ....................................................................................................................7 2.5 Pearlite.........................................................................................................................7 2.6 Austenite......................................................................................................................8 2.7 Martensite....................................................................................................................9 2.8 Previous research and analysis of Carbon steel microstructure ................................10 2.9 Welding processes.....................................................................................................10 2.10 Welding procedure.................................................................................................11 2.11 Welding technique .................................................................................................12 2.12 Welding consumable material selection ................................................................12 2.13 Material selection...................................................................................................13 2.14 Heat treatment procedure.......................................................................................13 2.15 Welding of samples ...............................................................................................13 2.16 Mounting of welded samples.................................................................................14 2.17 Grinding and polishing of mounted samples .........................................................14 2.18 Microscopic analysis of weld samples ..................................................................15 2.19 Hardness testing and analysis of weld samples .....................................................15
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  iii 2.20 A reviewof research papers into the field of weld microstructures and their defects and the causes of such defects .................................................................................16 Chapter 3 Preparation of test samples ................................................................................24 3.1 Design of welded coupons ........................................................................................24 3.2 Preparation of MMA welded carbon steel pipe (API 5L).........................................24 3.3 Preparation of TIG welded carbon steel pipe (API 5L) ............................................25 3.4 Preparation of MMA welded mild steel plate ...........................................................25 3.5 Preparation of TIG welded mild steel plate ..............................................................26 3.6 Preparation of MMA and TIG welded medium carbon steel flat (EN 8) .................26 3.7 Preparation of MMA and TIG welded high carbon tool steel flat (O1tool steel) .....27 3.8 Welding procedure for test samples..........................................................................28 3.9 Sectioning of weld samples.......................................................................................31 3.10 Compression mounting of samples........................................................................31 3.11 Planar grinding and polishing of samples..............................................................32 3.12 Table of variables for planar grinding and polishing.............................................33 Chapter 4 Methods of Evaluation of Project Data..............................................................35 4.1 Macroscopic analysis ................................................................................................35 4.2 Microscopic analysis.................................................................................................35 4.3 Micro Hardness testing..............................................................................................35 4.4 Representing the micro hardness results graphically using Microsoft Excel............36 4.5 Sample Table of Results for Vickers Micro Hardness Tests ....................................36 4.6 Determining the Yield Strength of the Weld specimens...........................................36 4.7 How was the Arc Energy and Heat Input of the welding processes determined? ....37 4.8 Determining the variations in hardness in multi-pass welds from root of weld to cap of weld..................................................................................................................................37 Chapter 5 Analysis of results..............................................................................................38 5.1 Analysing and interpreting images of weld specimen microstructure ......................38 5.2 Comparing the HAZ of the four steels......................................................................42 5.3 Analysing & interpreting macroscopic images of defective and defect free weld specimens.............................................................................................................................43 5.4 Sample graphs of Vickers hardness plots..................................................................47 5.5 The formation and propagation of cracks in welded specimens ...............................48 Chapter 6 Discussion..........................................................................................................49 6.1 Differences in Arc Energy and Heat Input of MMA and TIG..................................49
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  iv 6.2 The effectof multiple weld passes on the mechanical properties .............................50 6.3 Yield strength of weld specimens .............................................................................51 6.4 Hardenability.............................................................................................................52 6.5 Strengthening by Grain Size Reduction....................................................................54 6.6 Phase Transformations in Weld Specimens..............................................................55 6.7 Heat Treatment of Carbon Steels ..............................................................................56 6.8 Conclusions ...............................................................................................................58 Chapter 7 APPENDICES ...................................................................................................61 7.1 Graph of Difference in Vickers Micro Hardness From Root To Cap .......................65 7.2 Graph of Differences in Yield Strength of All Weld Specimens ..............................65 7.3 Table of Vickers Hardness values and Yield Strengths ............................................66 Bibliography.............................................................................................................................67
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  1 Chapter 1 ProjectIntroduction 1.1 Project proposal & Project description To investigate the microstructure and micro hardness of defective and defect free welded coupons using low carbon mild steel, API 5L carbon pipe, medium carbon steel (EN8), and O1 tool steel utilising two different welding processes, MMA and TIG. The photographic images taken were compared so as to illustrate the differences in microstructure. Micro hardness tests were conducted using Vickers micro hardness testing, the results from testing the different materials were compared graphically on an Excel chart. The Vickers hardness values were used to determine the yield strength of the materials. 1.2 Why was the project being undertaken? This project was undertaken to investigate the effect of the heat input of the welding processes MMA and TIG on the microstructure of low carbon mild steel, API 5L carbon pipe, and medium carbon steel (EN8) as well as the alloy O1 tool steel. Sound defect free samples, welded to ASME IX standard, and defective samples created using poor technique were made. They were subjected to microscopic examination so as to graphically illustrate and compare the varying effects on the microstructure from heat input during the welding operation on the different grades of carbon steel and tool steel. There was a need to conduct various hardness tests on the prepared samples so as to investigate whether or not there is a change in hardness across the zones of the sample, from the base metal to the Heat Affected Zone (HAZ) and through to the weldment on all four steel grades. There was a need to examine the effect of imparting differing levels of heat into the material. 1.3 Aims and Objectives The main aims of this project are as follows:  To investigate and analyse the effects of welding on the microstructure of mild steel, API 5L carbon pipe, medium carbon steel (EN8), and O1 tool steel. The microstructure from parent metal to Heat Affected Zone (HAZ) and weldment are to be identified.  To analyse, interpret and compare the different structures of the mild steel, API 5L carbon pipe, medium carbon steel (EN8), and 01 tool steel welded coupons, and discuss their differences.  To investigate and compare the effects of heat input caused by utilizing different welding processes, namely MMA & TIG.  To identify and explain the causes of any weld defects present. Different weld defects will occur using different welding processes, this was achieved by intentionally utilizing poor operator technique.  To conduct Rockwell hardness and Vickers Micro Hardness tests on mild steel, API 5L carbon pipe, medium carbon steel (EN8), and O1 tool steel welded coupons to ascertain if the hardness changed across the zones from parent metal to HAZ to weldment.
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  2 1.4 Objectives forthe Literature review The main aims of the Literature review The main aims of the literature review was to review relevant technical literature relating to the project that was undertaken and that has been published and peer reviewed by experts in the field of welding technology. Questions askedand areas that were examined  What was the effect of welding on the micro structure of mild steel and API 5L, medium carbon steel and O1 tool steel?  How was the microstructure of medium carbon steel and O1 tool steel affected by improper pre-weld and post-weld heat treatment?  Did the varying levels of heat input from different welding processes have an effect on the microstructure of the different steels being welded?  What were the type and nature of the weld defects present in the steels being welded using both MMA and TIG?  What difference in hardness values were recorded between mild steel, API 5L,medium carbon steel (EN 8), and O1 tool steel across the different areas of the welded coupons from the parent metal into the heat affected zone (HAZ) and the weldment itself ?, did they compare to published results ? Sources used for the literature review Sources used include those listed below but also contain many others  The Welding Institute Cambridge (TWI) (1).  The American Welding Society (AWS) (2).  The James F Lincoln Foundation (3)  The American Society of Mechanical Engineers (ASME) (4)  The American Society of Materials (ASM) (5)  The Newnes Engineering Materials Handbook (6). 1.5 Methodology of approach to undertaking of project Plan of practical work to be undertaken  Phase one – This phase included the cutting, machining, and welding of a selection of steels.  Phase two – This phase encompassed both the sectioning and mounting of the welded samples as well as the planar grinding and polishing of the samples.  Phase three – The final phase was the microscopic examination and photographic imaging of the material microstructures as well as the Vickers micro hardness testing of welded samples.
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  3 1.6 The preparationand welding of the selectedsamples The aim of Phase one of this project was to weld a selection of joints using four different materials, mild Steel, API 5L carbon pipe, medium carbon steel (EN8), and O1 tool steel that had been prepared to specific criteria. These criteria were as follows:  Joint type and Joint configuration  Joint bevel angle  Joint root gap dimension  Joint root face dimension  Pre-heat and post-heat temperature (where applicable) The project used a selection of welded coupons that were welded to a defect free standard (ASME IX 1G, 3G, 6G). Other samples were welded knowingly using an improper technique to induce defects into the weldments for the purpose of examination. These samples were welded using two processes, MMA (Manual Metal Arc), and TIG (Tungsten Inert Gas). Once those materials had been prepared in the correct manner the welding operation was carried out according to industry standard WPS (Welding Procedure Specification), these standards and procedures were clearly explained and referenced further in the report. However to intentionally introduce defects into a selection of samples a number of steps were taken. These steps included inadequate gas shielding so as to introduce porosity into the weldment, inadequate amperage and improper welding technique so as to introduce lack of fusion between the weldment and the base metal. The parameters are clearly laid out in Table 1 below. 1.7 Material needed for weld coupons Material Mild steel plate Mild steel plate API 5L seamless pipe API 5L seamless pipe Medium carbon steel (EN8) flat Medium carbon steel (EN8) flat 01 Tool steel flat 01 Tool steel flat Joint type Butt Butt Butt Butt Butt Butt Butt Butt Inclusive bevel angle 70o 70o 70o 70o 35o 35o 35o 35o Number of coupons 2 2 2 2 2 2 2 2 Material thickness 10 mm 10 mm 9mm Sch. 80 9mm Sch. 80 5 mm 5 mm 6 mm 6 mm Welding processes MMA TIG MMA TIG MMA TIG MMA TIG Welding consumable electrode Oerlikon E7016/ Lincoln E7018-1 Oerlikon ER 70s-2 Oerlikon E7016/ Lincoln E7018-1 Oerlikon ER 70s-2 ESAB OK 68.81 E312 Stainless SIF ER 312 Stainless ESAB OK 68.81 E312 Stainless SIF ER 312 Stainless Heat treatment None None None None None None Yes (Specified later) Yes (Specified later) Table 1 Materials & Processes and Welding consumables
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  4 1.8 The preparationof the welded samples to be mounted and polished ready for Microscopic inspection The aim of Phase two of the project was to microscopically analyse and interpret the effects of welding on the micro structure of the test samples then to compare the results. This, however, was only possible once the welded coupons had been prepared in the proper fashion by cutting the specimens to the required dimensions and then preparing them for mounting and polishing using the metallographic grinding & polishing machine, then the microscopic examination commenced. When conducting the microscopic examination, photographic images were taken of the weld specimen microstructure. 1.9 The Testing for hardness variation across the welded samples using Vickers Micro Hardness testing The aim of Phase three of the project was to conduct hardness tests using the Buehler Vickers Micro Hardness testing machine. Once the hardness tests had been completed then photographic images were taken of the hardness test indentations. How were the test data and results presented? The photographic images recorded were used to graphically illustrate different zones within the microstructure and to show the point at which these zones changed from parent metal to HAZ to weldment and how the grain size changed across those zones. The Vickers hardness test results were displayed on Excel graphs. The different zones that the values were obtained from were clearly labelled on graphs to explain how hardness changed across zones. Analysis of the primary results and test data The photographic imaging and the Vickers micro hardness test data were analysed to identify and determine:-  How the microstructure changed across weld zones  How hardness changed  The effects that material composition as well as welding technique had on weld strength  The location and cause of weld defects Analysis of secondary results and test data  Yield strength of weld specimens – Comparing the yield strength of the welded specimens to those of raw materials that have not been affected by the heat energy of welding, and determining if this is a problem for design.  Arc Energy and Heat Input – Comparing the arc energy and heat input from of the two welding processes MMA & TIG and determining if there are differences. What effect would those differences in heat input have on the resulting microstructure?
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  5 Chapter 2 Literaturereview 2.1 Introduction The aim of the literature review The aim of this chapter was to familiarise the reader with Microstructural & Micro Hardness analysis of welded coupons of various grades of carbon steel as well as alloyed tool steel. The literature review included referenced research into the changes in the micro structure and micro hardness of the materials due to the heat input caused by the welding operation and the addition of filler materials into the weldment. The review detailed published work to date on the effects of various welding processes on the microstructure and micro hardness of various steels as seen in Table 2 below. Material Mild steel API 5L Grade B EN 8 Medium carbon steel 01 Tool steel Carbon content 0.10% - 0.15% 0.25% - 0.30% 0.40% - 0.45% 0.90% - 0.95% Table 2 Material Carbon Content (7) Typical applications for these materials are as listed below:-  Mild steel – Probably the most broadly used steel used in general fabrication. Used in the fabrication of a vast array of things such as roof trusses, fire escapes, gates, railings etc. etc. Excellent weldability.  API 5L Grade B – API 5L Grade B seamless carbon steel pipe is used in the petrochemical industry for the transport of oil and gas. It is also used in the power generation industry for the transport of steam. Good weldability.  EN8 Medium carbon steel – This material is used in the fabrication of shafts, pins, rolls, spindles, gears etc. etc. Good weldability provided precaution is taken to follow the heat treatment process required.  01 Tool steel – 01 Tool steel is used in the fabrication of cutting tools such as high quality combat knives, taps & dies, reamers, intricate forging dies, paper cutting machine knives. Poor weldability, even if proper heat treatment procedure is followed. This gives a clearer understanding of the different materials that were used in this investigation and their common uses & applications within industry. 2.2 Microstructural analysis of steels Microstructural analysis is the inspection and interpretation of grain structure, including anomalies and flaws within metallic structures at the microscopic level. The arrangement of the grain structure can be viewed microscopically to understand the importance of stringent materials manufacturing processes and welding procedures as well as engineering design in general. Flaws within a material structure or the welds joining such materials must be avoided. With varying carbon content present in the different samples, it was shown that the samples have a varied grain structure when viewed microscopically. In the past, microstructures of various carbon steels have been examined and although different due to their varying carbon content and the heat treatment processes they undergo they are still similar in some ways.
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  6 They mainly containfive different regions:-  Ferrite  Cementite  Pearlite  Austenite  Martensite. The largest constituent part of low carbon steel would be ferrite. It is crucial to this literature review and to the project as a whole to understand the nature of these different regions. The need to identify and interpret these regions within the metallic structure and how they interact to produce defective structures or sound defect free structures is crucial. The nature of these structures and how they are formed are discussed and graphically illustrated in the course of this chapter. 2.3 Ferrite Ferrite has a BCC (Body Centred Cubic) crystal structure and is, to the greatest part, pure iron with a negligible carbon content of approximately 0.005%. (8) This however is not counting the other small alloying elements such as manganese, silicon, and phosphorous, sulphur, and copper. In Figure 3 of the image below, a ferrite grain structure of AISI 1010 steel can be seen. The ferrite grains are the large white patches and the pearlite (containing layers of ferrite and cementite) are the small black spots interspersed throughout the material. AISI 1010 is the American designation for the European equivalent of low carbon mild steel with a carbon content of maximum 0.1 % - 0.13 % carbon. (9) Figure 1 Ferrite with Pearlite at the grain boundaries (10)
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  7 2.4 Cementite Cementite isformed when the amount of carbon present in the steel has exceeded the amount of carbon that the ferrite has the ability to dissolve within its microstructure below 727o C, the excess carbon turns into a very hard brittle substance called iron carbide or Fe3C. (11) Iron Carbide is the component which gives steel its strength, however this is not always favourable if this exceeds the amount of Fe3C required for desired strength. This is to be avoided in low carbon steel because it can turn the material hard and brittle. It also drastically reduces its toughness and ductility, this in turn severely limits the ability of the material to be cold worked. (8) Cementite when combined with ferrite forms layers or lamellae called pearlite. In Figure 4 in the image below, a ferritic grain structure of AISI 1018 steel can be seen with large dark regions which are pearlite containing ferrite and cementite. AISI 1018 is an American designation of the European equivalent of a type of mild steel with a maximum carbon content of 0.18 % carbon. (12) Figure 2 Cementite & Ferrite (pearlite) at the grain boundaries of Ferrite (13) 2.5 Pearlite Pearlite is not a phase and it contains layers or lamellae of varying levels of cementite and ferrite. Microscopically it appears as white layers of ferrite and dark layers of cementite. The thickness of these ferrite layers and cementite layers depend on the material composition or the manufacturing processing that the material may have undergone. (14) It is a very desirable material property because it combines the strength and hardness of Cementite with the property that is crucial for a vast range of steels which comes from ferrite, that property is ductility. The fact that pearlite has a lamellar or layered structure enables the material to inhibit the propagation of cracks, but this does not eradicate their growth. It merely makes the growth of such cracks much more difficult. However pearlite comes in varying forms such as coarse pearlite or fine pearlite. Coarse pearlite produces thicker lamellae and is formed when the material is cooled slowly, this affects the mechanical properties of the material by reducing its strength. Fine pearlite on the other hand produces thinner lamellae and is more desirable because the strength of the material is significantly enhanced. Fine pearlite is produced when the material is cooled rapidly. In Figure 3 below an image clearly shows that there is a distinction between the lamellae of ferrite and cementite. (8)
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  8 Figure 3 Lamellarspacing of pearlite containing ferrite and cementite (15) 2.6 Austenite Austenite has an FCC (Face Centred Cubic) crystal structure. Austenite is the structure of iron or Gamma (γ) iron when brought above the critical temperature of 900o C and below 1500o C, depending on the percentage of carbon within the material. This structure is incredibly important because it is this region, when cooled in a controlled fashion, from which other crystal structures are attained. This region obviously does not exist at room temperature in carbon steels. At temperatures above 1400o C the arrangement of the crystal structure changes from FCC (Face Centred Cubic) to BCC (Body Centred Cubic). This phase is referred to as delta (δ) ferrite. Low carbon mild steel melts at approximately 1540o C. (16) Images for austenite at temperature ranges between 900o C and 1500o C are not readily available however images for austenitic steel at room temperature are. In Figure 4 an image of the grain structure of fine grained austenitic steel at room temperature is shown. Figure 4 Austenitic grain structure of fine grained steel at room temperature (17)
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  9 2.7 Martensite Martensite isformed when steel, with adequate carbon content, is cooled extremely fast from the austenite region which internally locks in the carbon in solid solution forming an extremely hard and brittle structure. Martensite does not appear on the Iron Carbon diagram because it is not an equilibrium phase. Equilibrium phases form with the aid of very slow cooling rates that enable the diffusion of carbon , not rapid cooling. The arrangement of the crystal structure, after the material has been rapidly cooled to form Martensite, is BCT (Body Centred Tetragonal). This means that the BCC (Body Centred Cubic) structure has been stretched along its vertical axis. This process of rapid cooling or quenching is widely used in industry to attain desirable mechanical properties such as higher strength and hardness values for a specific material. The extent to which a given material can be strengthened and hardened is dependent on the carbon content of the material. However to make the material useable whereby it won’t suffer any catastrophic failure, the material will need to undergo a heat treatment process called tempering. This involves multiple stages of heat treatment adhering to a specific procedure for a given material. This process of heat treatment allows for the retaining of the desirable properties of Martensite and therefore relieving the material of its brittle structure which then increases the material’s strength and toughness. (18) In Figure 5 below it can clearly be seen what Martensite looks like. In Figure 5 a typical martensitic microstructure of a medium carbon steel (EN8 / AISI 1040) is shown. The white component in the grain structure is ferrite and the dark component is cementite. AISI 1040 steel is the American designation for the European equivalent of medium carbon steel EN8 with a carbon content of approximately 0.40 %. Figure 5 Martensite in AISI 1040 Medium Carbon Steel (19)
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  10 2.8 Previous researchand analysis of Carbon steel microstructure In the course of researching for this literature review it became apparent that there was a substantial amount of research material and case studies available relating to the effects on the HAZ and the microstructure in general. However, sourcing research material relating specifically to the theme of this project proved rather difficult to come by. That said, working with what material was available, it was important to break down what the most important points of discussion and further investigation should be. The factors that are crucial for discussion are as follows:-  Welding processes  Welding procedure  Welding technique  Welding consumables  Material selection  Heat treatment procedures (if required)  Welding of samples  Mounting of welded samples  Grinding and polishing of mounted samples  Microscopic analysis of weld samples  Hardness testing and analysis of weld samples 2.9 Welding processes The welding processes that were of interest and their respective effects on the materials relating to this literature review are firstly TIG (Tungsten Inert Gas) or, as it is sometimes referred to, GTAW or (Gas Tungsten Arc Welding). The second process that was of interest was MMA (Manual Metal Arc) or, as it is sometimes referred to, SMAW (Shielded Metal Arc Welding). These processes have different operational characteristics as well as different environments in which they are commonly used. TIG Welding TIG welding was developed and perfected because of the need to be able to weld magnesium and aluminium for the aircraft industry cleanly without the risk of defects occurring. The modern TIG welding process and the first modern TIG torch was called Heliarc and was patented in 1942 by Russell Meredith who worked for Northrop aircraft (now Northrop Grumman). (20) TIG welding allows for a more precise control of the molten weld pool and the arc is not erratic as the MMA process can sometimes be. TIG is a much more flexible process when the need to control the heat input into a material is crucial. Heat input into stainless steel and more exotic alloys needs to be tightly controlled and functions of modern TIG welding equipment such as pulsing of the arc, so as to prevent burn through and scorching of the material, help to limit the heat input into the material.
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  11 MMA Welding MMA isrecognised as the first ever electric arc welding process. There was early experimentation with an electric arc created by a high voltage DC circuit used to melt two pieces of Iron together in the 1860’s. In 1865 there was a patent issued to an English inventor by the name of Wilde for the first ever electric welding process. However in 1885 a British patent was granted to two inventors, Nikolas De Bernados and Stanislav Olszewski, for their invention. Their process consisted of the work being connected to the negative pole of a DC circuit and the electrode connected to the positive pole of the fixed position electrode holder using a carbon electrode. Two years later in 1887 Nikolas De Bernados was granted a patent for his adapted invention in Russia which consisted of the work being connected to the negative pole of a DC circuit and the electrode being connected to the positive pole of a non- fixed position electrode holder. This was a vast improvement on the previous invention because of the fact that the electrode holder could be manipulated at will. This was the birth of modern electric arc welding. Two years later in 1889 another Russian inventor made a giant leap forward with the technology of the time. Carbon electrodes were replaced by bare metal electrodes, the precursor to modern flux coated manual metal arc welding electrodes. MMA is without doubt the most dependable, the most versatile, the most used welding process across industry anywhere in the world. It is even used underwater. (21) 2.10 Welding procedure Welding procedure refers to the specifics of set up prior to the welding operation commencing. Welding procedures have been developed over the years by welding engineers in conjunction with materials, mechanical, and design engineers. There are a number of organisations that design welding procedures and certify welding personnel to specific standards. They also train welding inspectors to witness welding approval tests and to carry out the necessary inspections to certify that the welded joints are fit for service. Organisations such as TWI, Lloyds, AWS, and ASME design and certify to appropriate standards. (22) (23) (24) (25). The specifics referred to are as follows:  Material selection.  Material preparation.  Material fit up.  Joint design, Joint preparation, and Joint position.  Welding consumable or filler rod selection.  Utilising an appropriate welding process to carry out the welding operation.  Specifying appropriate welding parameters such as amperage, inter pass temperatures, voltage, travel speed, wire feed speed.  Specifying exact tensile and compressive loads that the finished welded join must withstand.  Specifying an appropriate inspection method relevant to the appropriate welding code to certify that the finished welded joint has withstood the tests deemed necessary to certify the welded joint fit for service.
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  12 2.11 Welding technique Weldingtechnique refers to the approach taken to the exact way in which a welding operation will be carried out. There are a multitude of different variables that would dictate what welding technique is appropriate for a given situation. These variables would include material thickness, joint position, welding process, filler material selection and so on. Different scenarios might dictate a specific deliberate hand movement of the welding rod or torch. A certain scenario might call for stringer beads (26) to limit heat input into the material whereas another scenario might call for a weave pass. (27) Each and every scenario, regarding welding operations, is different and calls for a different technique to be used. Knowing and understanding this only comes with experience. 2.12 Welding consumable material selection Welding consumables vary hugely for the welding of different materials, using different processes, in different environments. Welding using the manual metal arc process requires the using of flux coated wire rods, which are inserted into the electrode holder and once struck off the material to be welded an electric arc is initiated and welding commences. A weld is created and the slag is chipped off. The slag acts to shield the developing molten weld pool from oxygen and other atmospheric contaminants that would render the weld unsatisfactory. (28) Welding using the tungsten inert gas process requires a torch held non consumable electrode in the form of a ground tungsten electrode which is used to initiate and maintain a high frequency electric arc between the tungsten electrode and the base metal. Additional filler rod material (if required) would be added to the weld pool by way of a long filler rod that is dipped in and out of the weld pool as the tungsten is moved along the developing weld pool without touching the base metal which would lead to unwanted contamination of the molten weld pool. (29) When selecting a filler material appropriate to the welding operation being carried out then certain considerations must be taken into account. Attention to detail regarding the following must be taken.  Selecting a filler material with a chemical composition that closely matches the base metal if materials joining is the focus of the welding operation.  Do the mechanical properties of the welding consumables match or exceed those of the base metal and those required for an in service welded joint.  Has the proper storage procedure of the welding electrodes been stringently adhered to, to prevent degradation of the welding electrodes.  If surfacing was the primary focus of the welding operation, had the correct electrode been selected bearing in mind what the surfacing layer was intended for. Was the layer to be used as a buffer layer if indifferent materials are to be joined together or was the purpose of the layer a hard facing layer to prevent corrosion, abrasion, or erosion.
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  13 2.13 Material selection Materialselection refers to the selection of appropriate materials required for a specific purpose. Materials, in the case of metals, would be selected as an appropriate material required for the manufacture of sheet metal products or structural beams based on their mechanical properties and chemical composition. If, for instance, a metal was required to fabricate marine hardware then the material chosen would need to contain alloying elements that would prevent or inhibit corrosion such as chromium and molybdenum. (30) If, for instance, a material is required to enter service as a tool die in a drop forging machine or a hydraulic punch for punching holes in other steels then this material has to possess some very desirable qualities to withstand daily service in a very punishing environment. These forging or punching dies would likely be made from material such as AISI O1 tool steel or AISI D2 tool steel. (31) Therefore, as one would imagine, there are a number of variables that would dictate what materials would be selected for the intended purpose. 2.14 Heat treatment procedure Heat treatment is a process whereby varying levels of heat over a set time cycle is administered to metals to impart specific mechanical properties into those metals. Heat treatment processes such as annealing (32) may be used so as to be able to work that material i.e. drilling, machining, welding. Welding of carbon steels with a carbon content above 0.4% would be very difficult if not impossible to weld if that material was in its hardened state. Another example would be if a similar material was in its annealed state and the properties that were sought was increased hardness then, depending on the materials chemical composition and carbon content, according to specific heat treatment procedures relating to the specific material then it would be possible to significantly increase a materials hardness by using a specific heat treatment procedure. This procedure, depending on composition, would typically involve heating an annealed steel up to the austenite region and holding that material at that temperature for a specific period of time and then quenching the material in an appropriate medium such as oil or water. Then it would be required that the material be tempered to regain some of the materials more desirable properties such as toughness and ductility but, by virtue of this, the material will lose some of its hardness. It would also rid the material of undesirable properties such brittleness. (33) 2.15 Welding of samples The welding of samples for testing and evaluation should be carried out in as much of a controlled environment as possible. It is imperative that when such welding operations are to be undertaken it should be clear what procedures and protocol need to be adhered to so as to minimise any deviation from what is required. If the object is to microscopically examine defect free welded samples then it would be essential to limit the chance of introducing unwanted defects into the welded samples. The welding operation including material fit up and joint preparation should be undertaken by trained experienced personnel according to a recognised and appropriate welding procedure specification (WPS). (34)
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  14 Having undertaken towork to a relevant welding procedure specification, the materials would then need to be secured or fixed in a welding fixture so as to help limit any distortion in the test pieces, however sometimes a small percentage of distortion is unavoidable depending on the material being welded and how it is fixed. The joint type that would be required for this work would be a single V open root butt weld. (35) The welds would be completely fused at the root before being cleaned and prepared for the next runs so as to fill up the plate and pipe chamfer with sound defect free weld metal. (36) When the entire butt weld is completed then the finished weld metal should be slightly proud of the surface of the base material. This would maintain the integrity of the original material thickness adjoining the weld area. 2.16 Mounting of welded samples The preparations required to mount a welded sample in a thermoset plastic would be done by sectioning the sample using an appropriate cutting method. The sample would be sectioned using a thin cutting disc which is enclosed in a sealed environment and flooded with coolant to prevent the sample being scorched by overheating. This method would be used as opposed to a band saw because the roughness of the finished cut of a band saw is not appropriate for the first stage of grinding samples to prepare them for polishing. These samples are cut to the maximum width of 25 mm according to the Buehler handbook for mounting samples. (37) These samples would be sectioned transversely leaving the end of the weld visible and ready for mounting. Once the samples had been sectioned and cleaned with alcohol removing all traces of cutting fluid they would be ready for mounting. The samples are placed inside the hollow cylindrical chamber of a Buehler compressive mounting machine with a hydraulic ram at its base. The sample would then be set face down with the face to be examined facing downward. A measured amount of granulated phenolic thermoset resin (38) is poured into the chamber and the chamber sealed shut with a twist lock clamp. The chamber would be preheated and then the resin brought up to liquefying temperature and then pressurised by the hydraulic ram so that the material formed a strong bond once cooled. The material then cools down and once the cycle is finished, the sample would be extracted from the chamber and the welded sample would be formed into a solid plastic mounting fixture similar in design to a miniature hockey puck. (39) 2.17 Grinding and polishing of mounted samples Mounted samples would be placed into a segmented cylindrical fixture with six slots in the fixture which house the samples ready for grinding and polishing. The samples are then clamped face down by a series of adjustable pneumatic actuators which exert the desired amount of pressure onto the samples pressing them against the abrasive pads on the turntable. (40) Depending on the material to be ground, a specific amount of pressure would be selected and then a specific grade of abrasive used depending on which stage of grinding the samples are undergoing. The variables for planar grinding & polishing and the pressures required as well as the time required are laid out in Table 3 on page 34 further on in this report. The first stage in grinding would be to use a relatively coarse water lubricated abrasive pad to remove any surface deformation due to sectioning of the sample prior to mounting.
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  15 Continuing on fromthis would be progressively finer stages of abrasion using finer abrasive pads to guarantee a clean flat surface with which to commence the initial stages of polishing. The first stage of polishing removes any remnants of surface deformation to provide a smooth mirror like finish. Multiple stages of polishing using a progressively finer water based solution containing diamonds in suspension from 30 microns down to 1 micron would be used. To highlight the area of interest ready for microscopic examination it would be appropriate to use an etching solution that would increase the definition of the microstructure ready for inspection. This step would be crucial to reveal the detail of the microstructure and grain boundaries of the welded sample. (41) 2.18 Microscopic analysis of weld samples Sometimes it is possible to see the grain structure of materials and any possible defects they may have at the macroscopic level, that is to say the structural components can be viewed with the naked eye. However to understand and interpret the microstructure of polished and etched samples it would be necessary to view them microscopically so as to identify the different regions within the sample and to identify defects, if any, present within the grain structure of the material. (42) When materials are to be viewed microscopically, depending on what level of detail one wishes to observe, microscopy equipment with varying power of magnification is necessary for the investigation. The most common and most accessible form of microscopy is optical microscopy. For this kind of microscopic examination to be viable the samples to be examined need to be highly polished and have a mirror like finish. Depending on what regions of the grain structure and grain boundaries that are of interest, would dictate what kind of etching solution would be required. Chemical etching is necessary to reveal a metal’s microstructure, it is by selective use of particular etching solutions that chemically degrade a particular area of the grain structure preferentially to another. The areas that are likely be attacked by the etching solutions would be points of high energy such as areas where defects are present as well as the grain boundaries. (43) 2.19 Hardness testing and analysis of weld samples There are multiple hardness test techniques that are used to determine the hardness of materials such as Rockwell, Brinell, Vickers. There is also another technique that is of particular use when it is necessary to examine the micro hardness of small samples. This micro hardness testing technique is called Vickers Micro Hardness. This technique utilises a diamond indenter which is shaped like an inverted pyramid, this indenter is forced into the material’s surface using specific applied loads from between 1.0 grams and 1000 grams. (44) The indentation that is left on the surface of the material can be accurately measured, this measurement can then be converted into a hardness number using formulae that are specific to the Rockwell, Brinell (HB), Vickers Micro Hardness (HV) testing techniques. (45)
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  16 To accurately testthe hardness variations across a welded sample it would be necessary to make indentations across the sample from the base metal across into the heat affected zone and into the weldment. These indentations are made both horizontally and vertically right across the welded sample to obtain a truly accurate picture of the variation in hardness within the microstructure. Those figures would be used to create a graph via an Excel spreadsheet so as to graphically illustrate how those hardness values vary across the sample due to the varying levels of heat input into the material. The carbon content within the material would also be one of the main variables that determine the variation in hardness values. (46) 2.20 A review of research papers into the field of weld microstructures and their defects and the causes of such defects Influence of welding method on microstructural creation of welded joints. Čičo P., Kalincová D., Kotus M.,. Special Issue - S50-S56, Zvolen - Slovakia : Czech Agriculture Journals, 2011, Vol. 57. (47) The research carried out, as illustrated in this paper, relates to the analysis and interpretation of the effects that various arc welding processes have on the microstructure and metallurgical quality of welded steel joints. The samples were welded with the MMA welding process as well as the MIG/MAG welding process. Analysis of the welded steel joints at the macro and microstructural level confirmed that grain structure and grain size is influenced by the particular welding parameters. The material used for the welding operation was a low carbon steel. For the MMA welding process the welding consumable used were listed under the Slovak designation of “EB-121” – These rods are the Slovak equivalent of what is known in western Europe and America as 7018 Low Hydrogen electrodes. (48) (49) For the MIG/MAG Welding process the consumables that were used were listed under the Slovak designation of “ESAB OK Autorod 12.58” – The western European and American equivalent of this MIG/MAG wire is known as ER70S-3. (50) The exponential heat change caused by the rapid input of heat energy into the material being welded is indicative of what happens during most arc welding procedures. The resultant microstructure is dependent on what kind of welding process is used and the heat input necessary to create a given microstructure is directly proportional to the operational arc voltage and welding amperage. The material thickness is related to the rate of heat dissipation from the welded joint throughout the material. The specific temperature for the material where austenite is formed and its rate of transformation is dependent not just on the chemical composition and the mechanical properties of the material but also its dimensions. According to the results obtained during the course of this investigation it was discovered that there is more heat imparted into the base metal and, by virtue of that fact, the HAZ by the MMA welding process than the MIG/MAG welding process. The slower deposition rate of weld metal using the MMA process as opposed to the much quicker deposition rate of MIG/MAG is directly related to the higher rates of heat input by the MMA process into the material.
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  17 Therefore it canbe concluded, according to this research, using the specific consumables & materials and parameters that the HAZ is 20 % to 30% narrower using the MIG/MAG process. This is a point of interest that needs to be investigated in relation to the differences in heat input between MMA and TIG which relates directly to this project. It can be safely asserted that the welding processes, along with the welding parameters, strongly influence the physical and mechanical properties of the finished welded joints. This is a factor that must be taken into account when deciding on what kind of process would be the most appropriate for the welding operation that needs to be carried out. Defects - Hydrogen cracks in steels - Identification. Bill Lucas, Gene Mathers, David Abson. Cambridge : Connect (Orignal) - Amended version by The Welding Institute, 2000 (51) Hydrogen induced cracking (HIC) comes under a few different names such as cold cracking as well as delayed cracking. Regardless of what the defect is called, the end result of the defect is the same. This kind of critical defect occurs in ferritic steels. These defects are caused by the presence of diffusible hydrogen within the steel or the welding electrodes, a microstructure prone to HIC, and sufficient tensile loads. From past research is has become understood that in carbon manganese steels, hydrogen induced cracking originates most frequently in the HAZ at the toes of the weld but can also travel from the HAZ and into the weldment itself, as seen in Figure 6 below. Figure 6 Hydrogen Induced Crack originating at the toes of the root weld and extending through the weldment (52) This is due the formation of a brittle micro structure in the HAZ. These types of cracks can be intergranular (along grain boundaries) or they can be transgranular (across grain boundaries) or a combination of both. Intergranular cracks occur more predominantly in the HAZ of the harder materials which, if they were carbon steels, would have a high carbon content. It requires much greater energy for cracks to propagate across grain boundaries of harder materials. For a crack to propagate it would seek the path of least resistance and for harder materials that would be along the grain boundaries. Transgranular cracking would occur in the relatively softer and more ductile materials where it requires less energy for the crack to propagate across grain boundaries and into neighbouring grains. Transgranular cracking occurs more frequently in carbon manganese steels. In Carbon Manganese steels hydrogen induced cracks would occur more regularly in the base metal. If care was taken to select the correct electrodes which should be stored at the correct temperatures in a controlled environment then the likelihood of HIC occurring would be greatly reduced. (53) Great care should be taken to ensure that the weld deposit would have a lower carbon content than the base metal.
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  18 It would becrucial to be aware of what the hardenability of the parent metal is. It is important therefore to calculate the carbon equivalent or CEIIW of the material. (54) The higher the value of the CE the more susceptible to HIC the base metal is. Steels with a CE value of <0.4 are not prone to HIC, provided hydrogen controlled welding electrodes are used. (55) Carbon equivalent formula (CEIIW) --- 𝑪𝑬 𝑰𝑰𝑾 = 𝑪 + 𝑴𝒏 𝟔 + 𝑪𝒓+𝑴𝒐+𝑽 𝟓 + 𝑵𝒊+𝑪𝒖 𝟏𝟓 This relates to the project being undertaken because the materials being welded range in carbon content from 0.15% carbon up to 0.95% carbon. Residual stresses in welded structures. Leggatt, R.H. 144 - 151, Cambridge: International Journal of Pressure Vessels and Piping, 2008, Vol. 85. (56) There are critical factors to consider when determining the residual stresses in welded structures or welds in general. These factors are usually broken down into separate categories.  Pre fabrication residual stresses Residual stresses existing in individual components caused by particular manufacturing processes used. These stresses exist prior to any welding operation being carried out.  Material properties The pre-existing properties of the parent material, whether they be thermal properties or mechanical properties or chemical composition, all play a role in what kinds of residual stresses already exist within a material. What is also of paramount importance is the nature of the microstructure and whether there are any pre-existing flaws in the microstructure.  Welded joint geometry Welding joint geometry contributes greatly to residual stresses. Whether the geometry is relating to the uneven leg length of a weld (57) or the joint configuration (58) such as a butt weld or a fillet weld, these factors have significant bearing on the resultant residual stresses of a finished fabrication.  Welded joint restraints External restraints on welded structures whether they be alignment fixtures such as clamps or fabrication jigs, contribute to the restriction of free movement of the components or parts being welded. This imparts residual stresses into the base material and the weldment.  Welding procedures Welding procedure and technique also have considerable significance when attempting to ascertain what is considered best practice to avoid or limit the resulting residual stresses in a finished welded fabrication. This encompasses such factors as the welding amperage, the arc voltage, and the inter pass temperatures when talking about multi pass welds.
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  19  Post weldor post fabrication residual stresses These residual stresses on finished welds or finished fabrications exist partly because of the restricting and aligning of component parts during the welding and fabrication processes. These stresses are sometimes avoidable by using pre-setting distortion control techniques (59) , however this is not always possible. Residual stresses may also arise due to the nature of the service conditions and the environment in which the fabrication is located in.  Magnitude of residual tensile stresses There is an assumption regarding the magnitude of tensile residual stresses in a weld after welding ceases, that the tensile residual stress is equal in magnitude to the yield strength of the weldment or the base metal. This does not apply to all materials or weld types. Generally tensile residual stresses occur at the interface between the weldment and the parent metal, i.e. the heat affected zone (HAZ). Due to the fact that during cooling there is contraction of both zones of material, i.e. the weldment and the parent metal, therefore this causes tensile residual stress. These stresses are magnified if there is a significant difference between the mechanical properties of the weld filler material and the parent metal. Therefore it is crucial to closely match filler material with the parent metal prior to any welding operation. (60)  Material properties The tensile residual stresses will approach a magnitude that is sufficient for the weld or the parent material to yield if certain circumstances are met.  Circumstance 1- If the weld or welded structure is constrained and the free contraction of the weldment is prevented, this will cause significant tensile residual stresses.  Circumstance 2- If the strain, due to thermal contraction from post welding temperature down to room temperature, exceeds that of the yield strain of the parent material.  Mathematical theory to calculate tensile residual stresses If the tensile residual stresses approach a magnitude where the material yields this will be due to the thermal strain exceeding yield strain of the material. This is expressed mathematically as (61) :- 𝜶(𝑻𝒔 − 𝑻𝒐) ≥ 𝝈𝒚 𝑬
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  20 Where: α isthe coefficient of thermal expansion for the specific material Ts is the softening temperature (a point where yield is 10% of normal yield) To is the ambient or room temperature E is the Young’s modulus for the specific material σy is the yield strength of the material at ambient temperature  Restraint of welded joints If a weld or welded structure is in anyway restricted in its free movement during thermal expansion or contraction then it is said to be “restrained”. As can be imagined, in a structure with multiple welds in multiple directions, welded joints may experience restraint in uniaxial direction, thus developing tensile residual stresses within the fabrication. If consideration is given to the need for pre-setting the fabrication components where possible then the restraint experienced by the structure can be considerably less.  Uniaxial stress directions in butt welded plates As shown in Figure 7 below the stress experienced in butt welded plates is in a multiaxial direction but importantly they are not of equal magnitude. This is due to the fact that there would be different restraints on the weldment and the parent metal in different directions. Longitudinal shrinkage stress is present in the weld due to the parent metal resisting the rate at which the weld is shrinking. Transverse shrinkage, or shrinkage across the weld, on the finished cap welds are being resisted by the previous filler welds in the open butt chamfer. The upper weld surface experiences tensile residual stresses, this however changes from tensile residual stress to compressive residual stress as it nears the root of the butt weld. The residual stress changes direction from compressive residual stress in the mid depth of the butt welded plate back to tensile residual stress as it nears the bottom surface of the butt welded plates. The average transverse residual stresses are, as one would imagine, tensile. (62) However the average transverse residual stresses change as one approaches the ends of the plate were the average transverse residual stresses become compressive. This is in contrast to the mid weld section of the plates where the average residual stresses are tensile. The through depth residual stresses are entirely compressive in nature. Figure 7 Multi Axial Stress Direction In Double V Butt Welded Plates (63) σx = Longitudinal Direction σy = Transverse Direction σz = Through Thickness
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  21 Metallographic Basics DonaldC. Zipperian, Ph.D. http://www.metallographic.com/Technical/Basics.pdf. http://www.metallographic.com. [Online] Pace Technologies. [Cited: 22nd November 2015.] (64) Metallography is the in depth investigation and analysis of the microstructure of materials. This is a field of study that is crucial in ascertaining whether a material has been manufactured in the correct manner or if it has been subject to stringent post manufacturing processes so that its microstructure and its mechanical properties have not been detrimentally affected. For the sake of determining if a material can withstand the in service demands that will be placed upon it, it is crucial to have proven methods of analysis that will aid in the identification of inherent flaws or defects that are present within a material’s microstructure. There are a number of steps that are crucial in metallographic preparation. (65) They include:-  Initial preparation – By documenting the original initial condition of the specimen it would be a reference for future investigations. It is important to take this first initial step so as to record how the specimens appear before they are exposed to the environment for a prolonged period which may hasten corrosion.  Cutting and sectioning – Initial cutting of specimens, especially large specimens, maybe carried out using a conventional band saw. But when the specimens are to be sectioned it is absolutely crucial that these specimens are sectioned within the area of interest using an appropriate cutting method. The method that is common in the metallographic preparation industry is using a very fine surface abrasive cutting disc aided by coolant so as to prevent the scorching of the sample. This step is really important because this provides the initial surface that is necessary for proper mounting and for the first stages of planar grinding which provides controlled abrasion for scratch removal from the sample surface.  Compression mounting of samples – Compression mounting of samples is carried out by fixing or “mounting” the sample within a confined shape which aids in the ease of handling for analysis and storage. This shape is usually a circular shaped disc approximately 30 mm to 50 mm in diameter and approximately 25 mm thick that has the appearance of a hockey puck. The material used for hot compressive mounting is usually a thermoset polymer which comes in granular form. The granular thermoset is poured into a cylindrical chamber in which the prepared and sectioned sample is housed. The housing of this cylindrical chamber is locked shut and the cycle is commenced. This cycle consists of a preheating period of two minutes followed by a heating period of four minutes whereby the thermoset polymer is heated to 400oC. Depending on the exact mounting material being used and the specimens being prepared, the sample is subjected to 300 bar of pressure. Once these phases of the cycle are complete then the final phase commences and this is the cooling phase which last approximately two minutes. Once this phase has finished then the cycle is complete, the sample is then released from the compressive mounting machine and ready for the first stages of planar grinding.
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  22  Planar grinding– This is the initial stage in the preparation of mounted samples. This step is crucial for a number of reasons. It prepares the surface for stock removal of material to attain a plain smooth surface that is devoid of deep scratches. Once the deep scratches have been removed then the successively finer stages of abrasion can commence. The abrasive pads are usually 240 grit, 320 grit, 400 grit, 600 grit, and finally 800 grit. (66) The higher number of grit particles per unit area means that this particular pad is a finer finish pad. These stages of abrasion must be adhered to and there are no shortcuts to attaining the surface required for initial polishing. There are various parameters that must also be adhered to such as speed of rotating head on metallographic grinding machine, whether the sample is to be ground in a complimentary or contradictory direction to that of the rotating head. The duration of grinding cycle and the downward pressure per sample(s) are also important parameters whereby proper guidelines laid out in industry standard equipment manuals must be adhered to.  Initial or “rough” polishing – This, as its name implies, is the first or “rough” polishing stage which prepares the surface of the specimen for finer polishing by removing the really fine scratches that were left on the specimen surface by the last final abrasion stage. Once the surface has been prepared in this manner then it will make it easier for the specimen to be polished more finely. The polishing stages, in contrast to the abrasion stages, require only a few minutes to complete. At this stage of preparation the grain structure of the sample would not change but the surface would appear smoother and more polished. Rough polishing is initiated by using a 9 micron down to 3 micron diamond suspension solution as a lubricant and polishing solution. (67) The polishing cloth that is used in conjunction with the diamond abrasive solution is made from nylon.  Final Polishing – The reason for the final polishing stage is that it prepares the specimen surface for etching. This is just a final polishing stage using a 1 micron alumina slurry in conjunction with a napped micro cloth that has the texture and appearance of velvet. (68) This stage can only commence once all surface damage has been removed and the previous successive rough polishing stages have properly carried out.  Etching – The reason for etching is because of the need to provide contrast which highlights the features of interest in the specimen ready for microscopic inspection. Etching allows for the visual inspection of a specimen and gives texture to the features such as grain structure, grain boundaries, and zone boundaries. The etchant is used to selectively corrode specific areas within the microstructure. (69) Chemical etchants are usually of an acidic or basic nature and are used in relatively mild solution. This gives definition and highlights the areas of interest that need to be inspected. Once an etchant has been applied to a specimen it is allowed to take effect for a number of minutes or a number of seconds in most cases. It is then immersed in deionized water which ceases the chemical reaction of the etchant and then the surface is cleaned using ethanol. The specimen is then dried under a warm air dryer.
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  23  Microscopic analysis– Microscopic examination is carried out using microscopes of varying magnification and resolution. Microscopic analysis of a specimen’s microstructure provides the materials engineer with specific information relating to grain structure and any changes that it may have undergone due to any heat treatment process, welding process, or forming process. It is also crucial in identifying any inherent flaws or defects in the microstructure of a material such as defects caused during the manufacturing process whether this is a casting or a forging or a welded fabrication. There is a real need to be able to examine a material’s grain structure and to determine whether it has a fine grain structure or a coarse grain structure because this information can help the materials engineer to determine the material’s mechanical properties such as ductility and hardness.  Hardness testing – Hardness testing is incredibly important and the information attained from standardised testing procedures allows the material engineer to relate this information to other material properties such as tensile strength, ductility, and wear resistance. Micro Hardness testing is carried out using the Vickers procedure. This method is carried out using indenters that are subject to loads between 1 gram and 1000 grams of force. This procedure is used to measure the hardness of different zones within a material such as the parent material, the heat affected zone or the HAZ and the weldment. Vickers Micro Hardness Formula (70) :- HV = 𝟐𝑷 𝑺𝒊𝒏( 𝜽 𝟐 ) 𝑳 𝟐 = 𝟏.𝟖𝟓𝟒 𝑷 𝑳 𝟐 Where P = Applied load in kg L = Average length of diagonals in mm ϴ = Angle between opposite facets of diamond indenter (136o)
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  24 Chapter 3 Preparationof test samples 3.1 Designof welded coupons The coupons that were welded were prepared in accordance with the ASME IX (American Society of Mechanical Engineers section 9) welding procedure specification. (71) The samples that were used were prepared using four different materials. Mild steel, EN 8, API 5L, and O1 Tool Steel were used for this investigation. These samples were welded using two different electric arc welding processes, MMA and TIG. Half of the samples were welded to a defect free standard, and the other half were purposefully welded with defects. For comparison there was a control sample of each material to show the difference between the microstructure of the material which had not been subjected to any kind of microstructural change due to heat input from welding or any heat treatment process. 3.2 Preparation of MMA welded carbon steel pipe (API 5L) The pipe that was used was 4.5” schedule 80 seamless carbon steel pipe (carbon content approx. 0.25%). (72) The pipe bevel was machined in the lathe with a 65o to 70o inclusive angle (.i.e. 32.5o to 35o for each chamfered pipe) with a 2.5mm root gap to accommodate the MMA electrode diameter of 2.5 mm. The root face or landing edge was approximately 2.5 mm in width. The pipe was set up in preparation for welding by aligning the two most appropriate points on the pipe where there was no misalignment of the pipe inside wall. This was not easy to do because all pipe is slightly elliptical. Once the pipe had been aligned it was secured in place with clamps. Once secured, the pipe was welded with “bullet” tacks to permanently fix the root spacing. Using a clock face as a reference, the pipe was tack welded at 12 o’clock, it was then rotated until this bullet tack was at 1 o’clock. Two further bullet tacks were made at 3 o’clock, and 9 o’clock respectively. Bullet tacks were used so the pipe would be fixed in position without any root contamination. This is shown in Figure 9. Figure 8 4.5" Schedule 80 seamless carbon steel pipe secured with bullet tacks Figure 9 Illustration of pipe tack welded using bullets Parent metal Round steel rod Tack weld Root gap remains virgin and free of contamination
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  25 3.3 Preparation ofTIG welded carbon steel pipe (API 5L) As with the preparation of the MMA welded carbon steel pipe, the joint fit up and preparation for the TIG welded pipe was the same. Although the filler wire was of a slightly smaller diameter than the MMA electrodes, the filler wire was 2.4 mm. The procedure for tacking the pipes however was quite different. Given the fact that there was no possibility of getting contamination, such as slag inclusions in the root, because there was no flux on the TIG filler wire and the atmospheric shielding for the welding pool came not from flux coated electrodes but from inert gas shielding, there was therefore no need to use “Bullet” tacks and the pipe was root tacked as normal. The pipe was tacked at 1 o’clock, 3 o’clock, and 9 o’clock respectively. The tacks however had to be substantial if one were to avoid root cracks. The reason why the tacks had to be substantial was that the ends of the tacks were feathered at each end to ensure sound fusion and continuity of weld when the welds were started and finished at the ends of each individual tack. This is shown in Figure 10. Figure 10 4.5" Schedule 80 seamless carbon steel pipe secured with root tacks 3.4 Preparation of MMA welded mild steel plate Preparation and joint fit up of the plates were the same as the pipe regarding inclusive angle of the butt joint, the root gap, and the landing edge. The difference was where the bevel on the pipe was machined in the lathe, the bevel on the plates were flame cut and ground. To ensure that there was sound fusion at the start of the root in the butt weld it was advisable to use “run off” plates. By using run off plates it ensured that the welding amperage was at the maximum output selected once fusion of the root commenced. These were the plates that were welded to each end of the parent material that was to be welded. They secured the parent material plates in place and maintained the proper root gap but their primary function was to act as a starting point at which the welding arc was initiated. This is shown in Figure 11.
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  26 Figure 11 10mm Thick mild steel plate MMA butt joint 3.5 Preparation of TIG welded mild steel plate The joint preparation and joint fit up of the TIG butt welded mild steel plate was the same as the MMA butt welded mild steel plate, the only difference was the welding process. This is shown in Figure 12. Figure 12 10 mm Thick mild steel plate TIG butt joint 3.6 Preparation of MMA and TIG welded medium carbon steel flat (EN 8) The EN 8 weld coupons were much smaller in thickness than the pipe and plate samples. The EN 8 samples were 5 mm in thickness. The butt joint was only 40 mm in width before sectioning. The samples were bevelled to no more than 35o inclusive angle, because there was no need to have such a lean angle due the material being half the thickness of the mild steel plates. Due to the fact that the material was only 5 mm thick there was no need for a wide root gap, a maximum of 2mm root gap was sufficient for full penetration using both MMA and TIG welding processes. The EN 8 butt joints for both MMA and TIG were prepared in exactly the same fashion and were identical other than the fact that they were welded using a different process. Figure 13 shows the sample prepared for both processes.
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  27 Figure 13 EN8 6 mm butt joint for MMA & TIG 3.7 Preparation of MMA and TIG welded high carbon tool steel flat (O1tool steel) The tool steel specimens were prepared from a piece of ground flat stock of O1 tool steel which was 70 mm in width by 6 mm in thickness. The material was supplied in the annealed state. To try and limit, as much as possible, the chance of immediate brittle fracture upon welding due to the shape of the butt joint (a single V butt), it was decided that the joint configuration should be simulated. It was preferable to weld the tool steel in this fashion to avoid the possibility of stress concentrations in the joint. What was meant by simulated is that it would not be two separate pieces of tool steel joined together but a single piece of tool steel with a U shaped trough milled through its length with a bull nose cutter. This radial shaped simulated U shaped butt would limit the chances of immediate failure, although failure upon cooling was expected anyway because this was a joining operation and not a surfacing operation where the chances of cracking and ultimate failure could be avoided. The tool steel sample prior to machining was marked by single red lines where the U shaped trough was milled. Once the sample was milled it was cut into three individual samples. This is shown in Figure 14. Figure 14 Bull Nose milling of U shaped trough (73) Bull nose cutters U shaped milled trough
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  28 Figure 15 O1Tool steel prior to milling of U shaped trough 3.8 Welding procedure for test samples Welding procedure for MMA welding of carbon steel pipe (API 5L) and mild steel plate The pipe and plate were preheated to a low preheat temperature just to eradicate any hydrogen present in the material prior to welding. The welding consumables used for the root were of a hydrogen controlled specification with <15ml of diffusible hydrogen per 100 grams of weld metal deposit. They were of AWS specification E7016 (Oerlikon Spezial) which are a low hydrogen potassium coated manual metal arc electrode. The rods were re-dried at 300oC – 350oc as per manufacturer’s specifications (74) for 2 hours prior to being stored in a rod heating quiver during the welding operation. The rods used for the hot pass, the fill, and the cap runs were also of an extremely low hydrogen controlled specification with <5ml of diffusible hydrogen per 100 grams. The rods used were of AWS specification E7018-1 (Lincoln Electric Conarc 49c) which are a low hydrogen iron powder coated manual metal arc electrode. These rods were also re-dried at 300o C – 350o C as per manufacturer’s specifications (75) for 2 hours prior to being stored in a rod heating quiver during the welding operation. The finished welded pipe and plate are shown in Figures 16 and 17 below. Figure 16 4.5 Inch Schedule 80 MMA Welded API 5L carbon steel pipe Welding capping runs using E7018-1 consumable 4&1/2” Schedule 80 seamless carbon steel pipe - Grade API 5L Pipe fixed in pipe clamp at HLO 45O
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  29 Figure 17 10mm MMA butt welded mild steel plate Figure 18 BOC Welding Preheat Temperature Chart (76) Welding procedure for TIG welding of carbon steel pipe (API 5L) and mild steel plate As with the previous welding procedure for pipe and plate the specimens were preheated to a low pre heat temperature as shown Figure 18. This was done to eradicate any hydrogen present prior to commencing the welding operation. The welding electrodes, or filler wire as it is referred to when speaking of TIG welding consumables, were of AWS specification ER 70s-2 (BÖhler super steel). The root, fill, and cap runs for both pipe and plate were carried out using the same filler wire, the gas shielding for the welding operation was argon. Welding procedure for TIG & MMA welding of medium carbon steel (EN 8) The welding procedure for welding medium carbon steel (EN 8) involved a preheating procedure that varies with choice of welding consumable. However if welding sections <18 mm in thickness a rigorous heat treatment process can be avoided. (77) However in this case it was advisable to preheat the material to eradicate any hydrogen present to prevent hydrogen infused cracking (HIC). For this procedure a nickel based welding consumable known as 312 stainless (ESAB OK 68.81) was used. (78) 300 mm long x 100 mm wide x 10 thick - mild steel plate Weaved capping run using E7018-1 welding electrodes
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  30 The reason forthis choice was because the nickel based electrode had a thermal expansion coefficient that was fairly close to that of the base material which greatly reduced the chances of cracking if the joint was in any way restrained. (79) The welding procedure for MMA and TIG welding of EN 8 was the same as regards the choice of welding consumables. 312 stainless electrodes (80) and 312 TIG filler wire (81) was used in both processes, however in the case of TIG welding, a gas lens was used on the TIG torch to provide adequate gas shielding and prevent oxidisation of the weldment. (82) This is shown in Figure 19. Figure 19 Gas diffusion pattern of gas lens Vs standard nozzle (83) Figure 20 MMA welded EN 8 medium carbon steel Figure 21 TIG welded EN 8 medium carbon steel
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  31 Welding procedure forMMA and TIG welding of O1 tool steel Joining O1 tool steel is not advisable because of the possibility of HIC and brittle fracture due to the very high carbon content of 0.95 %. O1 tool steel is repaired when there maybe surface cracks or wear. It is usually “buttered” with a layer of softer material such as a nickel based 312 stainless filler material before it is “hard faced” with a material that closely matches the tool steel. In this case a weld was made in a trough that was milled in the material to a depth of 4 mm, this simulated a U shaped butt joint. Due to the small size of the samples they only needed to be preheated to 150oC prior to welding. (84) They were then stress relieved at 650oC for a short time and then allowed to cool naturally in the furnace overnight. (85) This procedure was only done for one MMA sample and one TIG sample. The second set of samples were pre-heated to 150oC but after welding they were allowed to cool in the air. The third set of samples were only slightly warmed up to eradicate any hydrogen present and the material were only hand warm prior to welding. After welding, the third set of samples were allowed to cool in the air. In all cases, all welding of O1 tool steel was carried out using 312 stainless MMA electrodes or 312 stainless filler wire. 3.9 Sectioning of weld samples Prior to sectioning the samples to the size required for mounting, they were cut using a band saw. The samples were then further cut down to size using a thin high speed abrasive disc which was flooded with coolant to prevent the preliminary surface from getting scorched. This gives the sample its required preliminary surface needed for mounting. This operation was carried out using the Buehler Abrasimet 2. (86) This machine is shown in Figure 22. Figure 22 Buehler Abrasimet 2 abrasive disc cutter 3.10 Compression mounting of samples The compression mounting of the specimens was done by placing the samples face down inside an enclosed cylindrical chamber which houses a hydraulic ram. On top of the specimen was poured a measured amount of granulated phenolic polymer. Once the samples had been placed correctly inside the chamber and the granulated polymer had been poured into the chamber, the chamber was then locked and closed with the aid of a twist lock clamp. The cycle was then initiated. The cycle included the pre heating for 1 minute of the polymer, it was then that the material was fully heated to 300oC and pressurised to 30 bar via the hydraulic ram for 3 minutes. Once this time had elapsed the sample underwent a cooling down period of 2 minutes within the enclosed chamber which allowed the polymer to solidify and cool down, housing the sample securely. The cycle then terminated. This was carried out using a Buehler Simplemet 3000. (87) This is shown in Figure 23.
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  32 Figure 23 ABuehler Simplimet 3000 compression mounting sampling machine 3.11 Planar grinding and polishing of samples Planar grinding of samples The first stage of the abrasion was carried out using 240 grit silicon carbide abrasive paper, which was primarily used for stock removal so as to provide a flat surface devoid of any major scratches or uneven surfaces. The number of cycles of planar grinding varied with every sample, because some had very good preliminary surfaces which were very flat where some others were slightly uneven. After using 240 grit until a completely flat even surface was obtained with all major scratches removed, the next stage of planar grinding commenced. The next grade of abrasive paper used was 320 grit, which further removed any slightly less obvious scratches and was used for 2 minutes per cycle. Once 320 grit had been used for a period of time the next stage of planar grinding commenced. At this stage the abrasive used was 600 grit SiC paper which is a much finer grade of abrasive. This was used to finely grind the sample surface and ensure that all the real fine scratches were running in the same direction, this meant that all other major scratches had been removed. The final stage of planar grinding was done using an 800 grit SiC paper. This extremely fine abrasive paper was used to very finely grind the surface of the sample in preparation for polishing. At this stage the surface scratches had been entirely removed. Polishing of samples The first stage of polishing was carried out using a nylon pad in conjunction with a 9μm diamond water based solution. The solution contained fine diamond in suspension, this polishing lubricant was sprayed onto the turntable at 15 second intervals. This stage of polishing consisted of one 4 minute cycle which refined the surface to a really clean finish. The second stage of polishing was carried out using a much finer nylon cloth pad in conjunction with a finer water based solution with a diamond particle size of 5μm. This stage gave a blemish free finish to the sample surface. This stage of polishing consisted of one four minute cycle and the polishing lubricant was also sprayed onto the turntable in 15 second intervals. The final stage of polishing was carried out using a fine velvet like pad called a microcloth in conjunction with an extremely fine alumina slurry which contained 1μm alumina particles in suspension. This stage of polishing gave a mirror like finish to the sample and was devoid of any surface defects. The variables of all the grinding and polishing operation are clearly outlined in Table 3 on page 34.
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  33 Etching of samples Etchingof the samples was carried out to aid in the preferential corrosion of the microstructure on the sample surface. This was done by using a 2% nitric acid solution called “Nital”. The samples were cleaned with ethanol and then dried under an air dryer prior to the application of the acid solution. Once the etchant was applied to the surface with a cotton bud it was allowed to settle for five seconds. Once this time had elapsed there was a very clear definition of the surface grain structure. The samples were then immersed in clean fresh water to cease the chemical reaction of the acid attacking the surface grain structure. The samples were then taken out of the water and dried under an air dryer. The samples were then ready for macroscopic and microscopic examination. 3.12 Table of variables for planar grinding and polishing Table 3 on page 34 clearly lays out the variables for planar grinding and polishing. This includes the abrasives used as well as the coolant/lubricant for each stage of surface preparation. The pressure required to secure the samples to the platen, the platen speed, the head speed, and the cycle time are all clearly laid out in the table. All planar grinding and polishing was carried out using the Buehler Phoenix 4000 automatic sample grinding & polishing machine. (88) This machine is shown in the photo in Figure 24 below. Figure 24 Buehler Phoenix 4000 sample grinding and polishing machine
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  34 Table 3 PlanarGrinding and Polishing Variables METHODS OF ABRASION TYPE OF ABRASIVE PAPER/CLOTH SIZE/TYPE OF ABRASIVE DIRECTION PLATEN SPEED (RPM) HEAD SPEED (RPM) PRESSURE PER SAMPLE (Lbs) CYCLE TIME (MINUTES) PLANAR GRINDING SiC PAPER P 240 GRIT CONTRA 200 120 5 UNTIL PLANE PLANAR GRINDING SiC PAPER P 300 GRIT CONTRA 200 120 5 2 MINUTES PLANAR GRINDING SiC PAPER P 600 GRIT CONTRA 200 120 5 2 MINUTES PLANAR GRINDING SiC PAPER P 800 GRIT CONTRA 200 120 5 2 MINUTES STAGE 1 POLISHING ULTRAPAD 9 μm WATER BASED DIAMON SOLUTION COMPLIMENTARY 300 120 5 4 MINUTES STAGE 2 POLISHING NYLON or TEXMET 3 μm WATER BASED DIAMON SOLUTION COMPLIMENTARY 300 120 5 4 MINUTES FINAL POLISHING MICROCLOTH 1 μm WATER BASED DIAMON SOLUTION CONTRA 80 60 2.5 2 MINUTES
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  35 Chapter 4 Methodsof Evaluation of Project Data 4.1 Macroscopic analysis The first step in analysing the polished & etched samples was to view and evaluate the microstructure under a low powered microscope. This allowed the evaluation of the weld structure, the weld definition and the number of weld passes as well as their orientation. Macroscopic examination also helped in locating and identifying any possible obvious surface defects such as porosity, slag inclusions, lack of fusion etc. this facilitated closer inspection of the samples. It is an accepted procedure to photograph each section of the specimen at a macro level. The sample was photographed from the parent metal to the parent metal/HAZ boundary, the HAZ, and the weldment itself. It was also advisable to perform this procedure right across the sample because of the possibility of variation in the sample. In this investigation, macroscopic examination was performed using a “Meiji” 7x-45x binocular zoom microscope. 4.2 Microscopic analysis Microscopic examination was carried out for many reasons, not least was to view and assess the grain structure of the material from the photographic images taken. When it comes to microscopic examination of metals and welded components it is standard practice to closely examine the grain structure of the material to look for any imperfections or unusual patterns of grain growth. For example, in some metals, a refined grain structure gives materials desirable toughness properties. However with a refined grain structure for the same material, this would reduce the ductility of the material. 4.3 Micro Hardness testing Micro hardness testing was a procedure that was used in this project to evaluate the variations in hardness across the specimens from the parent material through to the HAZ and the weldment. Vickers Micro Hardness testing is an industry standard method for evaluating the variations in hardness due to the carbon content of the selected materials which affects their hardenability. When trying to ascertain the hardness variations of small samples using multiple small indentations travelling from top to bottom and transversely across the sample, this was the only viable method when trying to obtain more accurate refined figures. Figure 25 Vickers Micro Hardness Testing Procedure (89)
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  36 4.4 Representing themicro hardness results graphically using Microsoft Excel Once a series of figures had been obtained from the micro hardness tests and the distances of the indentations in the sample relative to a reference point had been recorded, then those values where plotted on a graph using Microsoft Excel. Once the graph displayed what was to be expected from the figures then the rest of the hardness tests commenced. When all those figures had been accurately compiled then they were also plotted on an Excel graph. This is an example of a typical hardness profile for a material with hardness above 50 HRC. (90) Graph 1 Example of Micro Hardness Profile of 0.4 % Carbon Steel (91) 4.5 Sample Table of Results for Vickers Micro Hardness Tests The table below is a sample of the average hardness results obtained while conducting the Vickers Micro Hardness tests. The complete test data is compiled within the appendices. Table 4 Average Vickers Micro Hardness Values for the Test Samples 4.6 Determining the Yield Strength of the Weld specimens The Vickers Hardness results obtained during the course of this investigation were used to ascertain the yield strength of those weld specimens at different regions throughout the materials. The yield strength of the weld specimens were calculated for different zones right across the materials from the parent metal to the HAZ and the weldment itself. MATERIAL JOINT TYPE WELDING PROCESS HV IN PARENT METAL HV IN HEAT AFFECTED ZONE HV IN WELD MILDS STEEL BUTT MMA 135.133 165 154.6 MILDS STEEL BUTT TIG 146.1 179.55 182.7 API 5L CARBON PIPE BUTT MMA 176.25 189.1 162.35 API 5L CARBON PIPE BUTT TIG 156.675 193.5 175 EN8 MEDIUM CARBON STEEL BUTT MMA 202.45 145.75 227.433 EN8 MEDIUM CARBON STEEL BUTT TIG 176.85 169.85 481.75 01 TOOL STEEL (ANNEALED) BUTT MMA 184.5 399 230.5 01 TOOL STEEL (ANNEALED) BUTT TIG 184.5 312 207.5 TABLE OF AVERAGE VICKERS MICROHARDNESS VALUES ACROSS TEST SAMPLES
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  37 To calculate theyield strength of the weld specimens, the Vickers Micro Hardness values were multiplied by 3. This is only applicable to carbon steel. This gave a valid value for the yield strength. Yield strength is directly related to hardness, and as one would expect, therefore as the hardness increases so too does the yield strength. An example of how this was displayed graphically is shown in Graph 2 below. This is illustrated in greater detail in the analysis chapter further on in this report. A typical yield strength value for mild steel that has not been affected by heat energy from welding would be approximately 248 MPa (92). Typical yield strength for API 5L would be 414 MPa (93). Typical yield strength for EN 8 would be 465 MPa (94). Typical yield strength for O1 tool steel would be 1500 MPa (95). Graph 2 Yield Strength of MMA Welded Mild Steel 4.7 How was the Arc Energy and Heat Input of the welding processes determined? The Arc Voltage of the welding processes was calculated using a multi-meter with its probes connected to the Earth and the Work leads of the welding equipment. This was done for the MMA and TIG welding processes. Using a piece of material that was marked to a predetermined length for the start and finish of the weld (50 mm), a test weld was conducted and the time taken to complete the weld was recorded with a stop watch. This was necessary to determine the travel speed. Using the AE (Arc Energy) formula, values for both the MMA and TIG welding processes were determined. Each welding process had an inherent efficiency rating and this was factored into the calculation to determine the Heat Input. The AE formula and the exact values obtained for both the MMA and TIG welding processes are shown in the analysis section of the report. 4.8 Determining the variations in hardness in multi-pass welds from root of weld to cap of weld It was observed that the hardness of multi-pass welds change from the root weld into the hot pass, and throughout the filler welds and into the final cap weld. It was observed that the hardness increased nearer to the surface of the cap weld and the resulting microstructure was quite different from the underlying microstructure. This is explained in the analysis section.
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  38 Chapter 5 Analysisof results 5.1 Analysing and interpreting images of weld specimen microstructure In this section the resulting microstructure of the different materials and the processes used to weld them were compared for similarity. Also the specimen microstructure was compared from the welded samples, the heat treated samples, and the unprocessed control samples. Mild steel microstructure comparison Figure 26 Mild Steel Control Sample Microstructure 20 x 10 Magnification Figure 27 Mild Steel Parent Metal MMA 20 x 10 Magnification As can be clearly seen from the images above there was little or no difference between the grain structure of the mild steel control sample in Figure 26 and the parent metal of the MMA welded mild steel specimen in Figure 27. The proportions and distribution of both ferrite and pearlite are very similar in both specimens. There was a very small amount of diffusion of carbon from the ferrite into the cementite. There was a minute difference in average hardness from 134.18 HV in the control sample to an average of 135.13 HV in the parent metal sample. Ferrite is the white/grey grain structure in the image opposite Pearlite is the black substance situated between the ferrite grains
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  39 API 5L microstructurecomparison Figure 28 API 5L Control Sample 20 x 10 Magnification Figure 29 API 5L Parent Metal 20 x 10 Magnification The API 5L control sample in Figure 28 and the parent metal of the weld sample in Figure 29 appear slightly different. This is due to the fact that the parent metal, although a significant distance away from the weld fusion zone, has had its temperature elevated to a point where it minutely changed the material microstructure. The elevated temperature has allowed for the diffusion of carbon atoms out of the ferrite and into the cementite. This diffusion of carbon atoms into cementite raised the materials hardness in the parent metal section of the sample to 176.6 HV from the control sample average of 174 HV. The closer to the HAZ the higher the hardness became. This, however, depended on a number of factors. Primarily it depended on the carbon content of the material and its hardenability, but it also depended on the size of the sample. Was there sufficient surrounding parent material for the resulting accumulation of heat to dissipate into so that the material can retain its hardness or would the material become in effect annealed due to the specimen becoming saturated with heat? A darker ferritic grain structure due in part to the higher carbon content and the higher manganese content which gives the ferritic grain structure a dark blue/brown tint White/grey grain structure of ferrite more prominent than in previous image. Black pearlite grain structure more prominent than in previous image.
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  40 EN8 Medium carbonsteel microstructure comparison Figure 30 EN8 Medium Carbon Steel Control Sample 20 x 10 Magnification Figure 31 EN8 Medium Carbon Steel Parent Metal 20 x 10 Magnification There was a clear visual difference between the EN8 control sample in Figure 30 and the EN8 parent metal sample in Figure 31. However, unlike the previous API 5L sample, the hardness of the EN8 control sample exceeded that of the EN8 parent metal. The reason for this was clear, unlike the previous mild steel plate samples and the API 5L pipe samples which were of sufficient dimensions that would allow for the dissipation of accumulated heat. The EN 8 samples were very small (40 mm wide in total) and this, in effect, was the cause of the material becoming annealed. The average hardness value of the EN 8 control sample was 234.6 HV but surprisingly the EN 8 parent metal sample hardness value was 212.2 HV. Instead of seeing a gradual increase in hardness across the parent metal and a sharp rise in hardness when into the HAZ, the opposite occurred and the hardness once into the HAZ dropped significantly. This happened in spite of the fact that the material contained 0.4 % carbon that would allow the material in ideal circumstances to attain a maximum hardness of approximately 58 HRC. A darker Martensitic microstructure mixed with white Ferrite Average Hardness = 234.6 HV Average Hardness = 212.2 HV Greater proportion of white Ferrite grains visible
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  41 O1 Tool steelmicrostructure comparison Figure 32 O1 Tool Steel Control Sample 20 x 10 Magnification Figure 33 O1 Tool Steel Parent Metal 20 x 10 Magnification (Annealed) The O1 tool steel control sample in Figure 32 showed a stark difference when compared to the O1 tool steel parent metal sample in Figure 33. One reason for this difference was that the parent metal belonged to a sample that was annealed post welding. Even after post weld heat treatment to remove the brittleness from the material the parent metal MMA welded O1 tool steel sample retained a typical hardness profile. The average hardness value of the O1 tool steel control sample was 218.933 HV, whereas the parent metal sample hardness value (after annealing) was 150 HV. But the hardness value steadily increased across the parent metal sample and a sharp rise was observed in the HAZ and then a drop in hardness across the weldment as would be typical. The annealed welded sample in Figure 33 displayed a discoloured microstructure after undergoing heat treatment. Average Hardness = 218.933 HV Average Hardness = 150 HV Annealed O1 Tool Steel control sample displays a dark Martensitic like structure interspersed with small Ferrite clusters. Discoloured welded O1 Tool Steel which was annealed post weld. The image shows discoloured Ferrite.
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  42 5.2 Comparing theHAZ of the four steels Figure 34 MMA Welded MildSteel HAZ 20 x 10 Figure 35 MMA Welded API 5L HAZ 20 x 10 Figure 36 MMA Welded EN8 HAZ 20 x 10 Figure 37 MMA Welded O1 Tool Steel HAZ 20 x 10 The HAZ images from Figure’s 34, 35, 36, and 37 display typical HAZ features for the selected materials mild steel, API 5L, EN8, and O1 tool steel. The mild steel showed a typically uniform distribution of ferrite and cementite. The API 5L shows a higher proportion of cementite to ferrite. The EN8 hardness value in the HAZ was NOT what was expected. It was expected that there would be a sharp rise in the HAZ value from the parent metal value. This is due to the fact that the EN8 weld specimen was far too small to begin with. If the weld specimen was of a sufficient size then the accumulated heat would have had adjacent parent material where it could dissipate too. The resulting drop in hardness within the HAZ of the EN8 sample was because the HAZ in effect became annealed. The MMA welded O1 tool steel sample showed a typical HAZ for this particular material. Average HAZ Hardness = 165 HV Average HAZ Hardness = 189.1 HV Post Weld Annealed Average HAZ Hardness = 159.97 HV Average HAZ Hardness = 399 HV
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  43 5.3 Analysing &interpreting macroscopic images of defective and defect free weld specimens Mild steel weld specimens Figure 38 MMA Welded MildSteel (Defect Free) Figure 39 MMA Welded MildSteel (Defective) Figure 40 TIG Welded Mild Steel (Defect Free) Figure 41 TIG Welded Mild Steel (Defective) In Figure 38, the weaved capping run of the MMA welded mild steel specimen had a distinctly different grain structure than the rest of the sample. Due to a lower heat input from the MMA this led to a more localised normalising of the grain structure which gave a more coarse appearance. Also the fact that the cap cooled at a quicker rate because it was exposed to the atmosphere unlike the previous welds, gave a more coarse appearance to the cap. The MMA welded mild steel specimen in Figure 39 contained multiple flaws due to inadequate welding current. These flaws included slag inclusions, undercut, and no root penetration. The TIG welded mild steel specimen in Figure 40 was defect free and displayed a more refined grain structure due to a more generalized annealing because of the much higher heat input. The TIG welded mild steel specimen in Figure 41 had defects that included porosity and lack of sidewall fusion. These flaws were due to inadequate gas shielding and inadequate welding current respectively. Sound inter pass fusion, sidewall fusion, and root fusion. Weld Root Defective weld with multiple flaws. Sound inter pass fusion, sidewall fusion, and root fusion. Defective weld with multiple flaws. Corrosion HAZ Multi pass layering of welds Dendrites in weld cap Undercut on the cap Slag Inclusion Inter pass lack of fusion No root penetration Weld Root No sidewall fusion Porosity Weld Cap
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  44 API 5L Weldspecimens Figure 42 MMA Welded API 5L (Defect Free) Figure 43 MMA Welded API 5L (Defective) Figure 44 TIG Welded API 5L (Defect Free) Figure 45 TIG Welded API 5L (Defective) In Figure 42, the capping runs of the MMA welded API 5L specimen have a grain structure that was also quite different than the previous welds. The dendrites are prominent in the surface layer because upon cooling the capping runs were exposed to the atmosphere. In Figure 43 the MMA welded API 5L had very obvious defects such as undercut, voids, and slag inclusions. The undercut was caused by improper rod angle and the other defects were caused by low welding current. The TIG welded API 5L specimen in Figure 44 shows a defect free weld specimen. The TIG welded API 5L sample in Figure 45 shows a defective weld with multiple weld defects such as voids, lack of sidewall fusion, and lack of penetration. All these defects were caused by the welding current being set too low and using improper welding technique. Dendrites in cap Sound inter pass fusion, sidewall fusion, and root fusion. Defective weld with multiple flaws. Defective weld with multiple flaws. Sound inter pass fusion, sidewall fusion, and root fusion. Root Weld Undercut and devoid of weldment Slag inclusion Root Weld HAZ No sidewall fusion and lack of penetration Void and no sidewall fusion HAZ
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  45 EN8 Weld specimens Figure46 MMA Welded EN8 (Defect Free) Figure 47 MMA Welded EN8 (Defective) Figure 48 TIG Welded EN8 (Defect Free) Figure 49 TIG Welded EN8 (Defective) As can be seen from Figure 46, the MMA welded EN 8 specimen shows a perfectly defect free sample. The weldment displays a mirror like finish after etching with a nitric acid solution because 312 stainless was resistant to granular attack from the nitric acid. In Figure 47 the MMA welded EN 8 specimen displayed serious weld defects. Those defects included slag inclusions and no sidewall fusion, both were caused by the welding current being set too low. In Figure 48 the TIG welded EN 8 specimen was welded to a satisfactory standard, however the cap weld was slightly excessive. In Figure 49 the TIG welded EN 8 specimen shows serious weld defects such as insufficient sidewall fusion and a complete lack of root penetration. HAZ Welded using 312 SS consumable. Sound inter pass fusion, sidewall fusion, and root fusion. Defective weld with multiple flaws. Slag inclusions and no sidewall fusion Sound inter pass fusion, sidewall fusion, and root fusion. Defective weld with multiple flaws. Lack of sidewall fusion and lack of root penetration. Welded using 312 SS filler. wire.
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  46 O1 Tool steelspecimens Figure 50 MMA Welded O1 Tool Steel (Defect Free) Figure 51 MMA Welded O1 Tool Steel (Defective) Figure 52 TIG Welded O1 Tool Steel (Defect Free) Figure 53 TIG Welded O1 Tool Steel (Defective) The MMA welded O1 tool steel specimen in Figure 50 was a defect free specimen. There was a distinct visible difference in grain structure between the parent material and the HAZ. The MMA welded O1 tool steel specimen in Figure 51 displayed serious flaws. What was most evident was the major mid weld crack. There was also an insufficient depth of weld filler material in the joint chamfer. The TIG welded O1 tool steel specimen in Figure 52 was a visibly defect free weld specimen. The TIG welded O1 tool steel specimen in Figure 53 was a defective weld specimen which displayed an identical major crack mid weldment just like the MMA welded O1 tool steel specimen in Figure 51. Sound inter pass fusion, sidewall fusion, and root fusion. Defective weld with multiple flaws. Sound inter pass fusion, sidewall fusion, and root fusion. Defective weld with multiple flaws. Welded using 312 SS consumable. HAZ Major crack in weldment Insufficient depth of filler material HAZ Welded using 312 SS consumable. Major crack in weldment Crack propagating into weldment from the parent metal Annealed Normalised Welded at room Temperature Welded at room Temperature
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  47 5.4 Sample graphsof Vickers hardness plots Graph 3 Vickers Micro Hardness plot of Mild Steel MMA Graph 4 Vickers Micro Hardness plot of EN8 MMA Graph 5 Vickers Micro Hardness plot of API 5L MMA Graph 6 Vickers Hardness plot of 01 Tool SteelMMA The data plotted in Graphs 3,4,5, and 6 represent the Micro Hardness data plotted of all four project materials, Mild Steel, API 5L, EN 8, and O1 Tool Steel. The data shows the fluctuation in hardness across the three zones of interest from Parent Metal to Heat Affected Zone (HAZ) and the Weld itself.
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  48 5.5 The formationand propagation of cracks in welded specimens Figure 54 MMA Welded O1 Tool Steel Specimen with Hydrogen Infused Crack In some of the tool steel specimens, which were heat treated pre-weld and post weld, severe cracks appeared in the parent metal and also the weldment itself. This was due to this materials’ poor weldability and the possibility of diffusible Hydrogen being contained within the cooling weldment. Also the conflicting thermal expansion rates of the parent material and the welding filler material was a source of concern when trying to minimise the potential for the initiation of cracks and their possible propagation to catastrophic failure. The result of not following rigorous heat treatment procedures regarding the correct method for welding O1 Tool Steel is clearly illustrated in Figures 51 and 53 on page 46. Using the carbon equivalent formula (CEIIW) --- 𝑪𝑬 𝑰𝑰𝑾 = 𝑪 + 𝑴𝒏 𝟔 + 𝑪𝒓+𝑴𝒐+𝑽 𝟓 + 𝑵𝒊+𝑪𝒖 𝟏𝟓 The weldability of the material will be determined based on the chemical composition of 01 tool steel. (96) 𝑪𝑬 𝑰𝑰𝑾 = 𝟎. 𝟗𝟓 + 𝟏. 𝟐𝟓 𝟔 + 𝑪𝒓 + 𝑴𝒐 + 𝑽 𝟓 + 𝑵𝒊 + 𝑪𝒖 𝟏𝟓 The carbon equivalent of the O1 tool steel is ≈ 1.158. This far exceeds the accepted limit for good weldability of 0.4. Coefficients of thermal expansion for O1 tool steel and 312 stainless steel filler material Figure 55 COTE For O1 Tool Steel (97) Figure 56 COTE For 312 StainlessSteel (98) Weld Nugget Coarse grain structure at the Initiation point of Hydrogen infused crack at the toes of the weld Refined grain structure at the point where the crack rapidly propagates MMA Welded 01 Tool Steel Specimen
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  49 Chapter 6 Discussion 6.1Differences in Arc Energy and Heat Input of MMA and TIG Differences in arc energy and heat input between arc welding processes affect the resulting material properties due to the amount of heat energy imparted to the material. In this project there were two arc welding processes used, MMA and TIG. Arc Energy and Heat Input are both ways of measuring the amount of heat energy imparted to the material, however heat input takes account of the process efficiency in the calculation. The value is expressed in Kilojoules per unit length of welded material, in this case it is kJ/mm. The test piece was welded with a single pass weld over a measured distance of 50 mm for both processes. The Arc Energy and Heat Input equations are expressed mathematically as follows: Arc Energy (AE) = ( 𝟔𝟎) 𝒙( 𝑽) 𝒙(𝑰) ( 𝟏𝟎𝟎𝟎) 𝒙(𝒗) Heat Input (HI) = ηAE Where V = Arc Voltage (Volts) I = Welding Current (Amps) v = Travel Speed (mm/Minute) η = Process Efficiency Table 5 Table of Results for Arc Energy and Heat Input The results for Heat Input left no doubt about the significant variation in Heat Input between MMA and TIG. TIG imparts a larger quantity of heat energy per unit length into the material. As shown in Figures 57 and 58 on page 50, there is a significant difference in the microstructure between the TIG weld specimen and the MMA welded specimen. Strangely the TIG welded specimen shows an enlarged and coarse grain structure within the HAZ but a higher HV value. The TIG welded specimen has a large Ferrite grain structure containing fine pearlite at the grain boundaries. The MMA welded specimen shows a smaller proportion of Ferrite surrounded by a coarse Pearlite structure, it had a lower HV value. This is reflected in the Micro Hardness values for both these specimens. The peak Vickers Hardness in the HAZ for the TIG welded specimen was 159.6 HV, whereas the MMA welded specimen had a Vickers Hardness of 149.5 HV. The dip in the HAZ Vickers Hardness values for both specimens was not what was expected. This was due, in part, to the inadequate size of the samples which were far too small. The specimens effectively became annealed in the HAZ because of the build-up of heat. With a high heat input value this leads to a much slower cooling rate which promotes grain growth. Also excessive Ferrite grain growth in the HAZ of the EN 8 TIG welded sample is due partly to the higher chromium content of the TIG filler wire as opposed to the MMA welding rods even though both were 312 stainless. TIG filler chromium content was 32 % (81) and MMA rod content was 29% (80). PROCESS ARCV(Volts) WELDINGI(Amps) EFFICIENCY(η) WELDTESTL(mm) TIME(Seconds) TRAVELSPEED(mm/min) AE(kJ/mm) HI(kJ/mm) %DIFFERENCE MMA 19 75 0.8 50 17 176.471 0.4845 0.3876 TIG 75 120 0.6 50 22 136.364 3.96 2.376 TABLEOFRESULTSFORARCENERGYANDHEATINPUT 513.00%
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  50 Images of EN8 Medium Carbon Steel welded with MMA and TIG Figure 57 TIG Welded EN8 HAZ (20x10 Magnification) Figure 58 MMA Welded EN8 HAZ (20x10 Magnification) 6.2 The effect of multiple weld passes on the mechanical properties The resulting microstructure and mechanical properties that are created due to the effect of multi-pass welds on a material differ significantly from single pass welds. Because the initial weld is not exposed to the cooling ambient air for any length of time, as would be the case with a single pass weld, its final mechanical properties are quite different. The initial heat energy required, to raise the material to a point where the material becomes molten within the weld fusion zone, decreases with successive weld passes. Once the initial weld pass has been completed then the material temperature has greatly increased. The successive weld passes can be carried out with a higher travel speed while using the same welding current, this means that a lower heat input is imparted to the material at the same time ensuring sound inter-pass fusion. By increasingly lowering the heat input into the material this aids in limiting grain growth. The re-heating of the previous weld metal deposit by subsequent weld passes acts to normalise the previous weld microstructure. This ensures that any residual stresses within the material are reduced. The nature of multi-pass welds guarantee a level of preheat into the material and, upon completion of the welding operation, slows the cooling rate down and greatly reduces the likelihood of the formation of cracks within the welds and the parent material. When the final weld is completed, whether it is a multi-pass fillet weld or a multi- pass butt weld, its microstructure differs significantly from the previous welds. This is because the final weld is exposed to the cool ambient air, unlike the previous welds. This ensures that the final welds have a different cooling rate than the underlying welds. Graphically the microstructure appears quite different to the underlying microstructure of the previous welds. What can be clearly seen in multi-pass welds are the lines where the welds have been layered on top of each other. Also on the final welds, just below the surface, the dendrites can clearly be seen. In the image shown in Figure 59, the Vickers Hardness is higher in the surface cap weld than in the root and subsequent welds. This is displayed graphically in Graph 7. All Vickers Micro Hardness values were taken in the centre of the weld from root to cap. Less Prominent Ferrite structure than would be seen in mild steel Martensitic Like structure containing small amounts of Ferrite
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  51 Figure 59 MMAMild Steel Butt Weld Graph 7 Graph Displaying Difference in Vickers Micro Hardness from Root to Cap 6.3 Yield strength of weld specimens The yield strength of a material is closely related to the hardness of the material. With the increase in hardness comes an increase in yield strength, depending on carbon content. If a plain carbon steel (0.15% - 0.85%), once quenched, is tempered at a temperature not exceeding 250o C then it will attain maximum hardness. At a tempering temperature not exceeding 350o C then it will attain maximum yield strength. (99) The yield strength of a plain carbon steel is calculated simply by multiplying the Vickers Hardness value by 3.17 (100) (only applicable to carbon steel). The yield strength of the weld specimens used in this project vary due to the difference in carbon content. The carbon content of the specimens for this project ranges from 0.15% C - 0.95% C. The variation in yield strength of the different materials across zones from parent metal into the HAZ and into the weldment, are illustrated in Graph 8. As can be seen from the graph, the higher carbon containing materials, gave a much higher yield strength due to carbon content. The variation in yield strength of welded carbon steels and un-welded carbon steels is an important factor to consider when designing welded components. It would need to be determined whether or not if the brittle nature of very hard HAZ’s would mean that higher yield strength in those areas was actually a serious design fault. The results of such faults are clearly illustrated in Figures 51 and 53 on page 46. Dendrites Weld Inter-pass layers
• 57.
  52 Graph of YieldStrength of Weld Specimens Graph 8 Graph of Weld Specimen Yield Strengths 6.4 Hardenability The chemical composition of a material and the alloying elements within that material determine its ability to from Martensite, this is called “Hardenability”. For different steels there is a particular relationship between the mechanical properties and the cooling rate of that material. Hardenability is not hardness. Hardness is the ability of a material to resist indentation whereas hardenability is a measure of how much that hardness decreases as the distance away from the quenched surface increases. As the distance away from the quenched surface increases, the cooling rate is slower and therefore the ability of the material to form Martensite diminishes. This will give a lower Rockwell Hardness value than the quenched part. Figure 60 Time Temperature Cooling Curve for SAE 1040 (EN 8)
• 58.
  53 Images of SAE1040 (EN 8) After Undergoing Different Heat Treatments Figure 61 AnnealedEN 8 (0.4 % C) @ 20x10 Mag. Figure 62 NormalisedEN 8 (0.4 % C) @ 20x10 Mag. Figure 63 QuenchedEN 8 (0.4 % C) @ 20x10 Mag. Figure 64 Tempered EN 8 (0.4 % C) @ 20x10 Mag. Hardenability of Heat Treated Samples Using the TTT diagram in Figure 60, it can be seen that SAE 1040 when quenched will attain up to 57 HRC if cooled within 10 seconds, cooling at a rate of 634o C per second. The HRC value for the quenched sample was 47 HRC, this was cooled in under 10 seconds so therefore the material must have had a carbon content less than 0.4 %. The HRB value for the annealed sample is76.85 HRB, which according to the TTT diagram will cool at a rate of approximately 1o F/minute. It will take approximately 27.78 hours to cool completely. The normalized sample gave a hardness value of 83.85 HRB. The tempered sample gave a Rockwell hardness value of 37 HRC, which would mean that it cooled in approx. 25 seconds. MATERIAL HEAT TREATMENT VICKERS HARDNESS (HV) ROCKWELL SCALE ROCKWELL HARDNESS (HR) SAE 1040 (EN 8) ANNEALED 143.4 HRB 76.85 SAE 1040 (EN 8) NORMALISED 164.4 HRB 83.85 SAE 1040 (EN 8) QUENCHED 477.867 HRC 47 SAE 1040 (EN 8) TEMPERED 360.133 HRC 37 TABLE OF VICKERS TO ROCKWELL CONVERSIONS Enlarged coarse grain structure due to prolonged cooling time Less coarse grain structure due to faster cooling in ambient air Needle like shape of Martensite Fine Grain Structure of Tempered Martensite
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  54 6.5 Strengthening byGrain Size Reduction When a material is held at an elevated temperature, above the recrystallization temperature, for a period of time beyond the time required for full recrystallization then grain growth occurs. Diffusion of atoms occur across the grain boundaries more easily because of the added energy within the system due to the higher temperature experienced by the material. Larger grains, having less overall grain boundary surface area per unit volume than smaller grains, diffuse atoms at a lesser rate than the smaller grains. This allows for the grains to grow larger. Grain size has a significant effect on the strength and toughness properties of a material. There is a direct correlation between the strength and hardness of carbon steel and the grain size within the microstructure. Boundaries between grains inhibit the movement of dislocations and any slip that may possibly occur. Grains are positioned at different orientations relative to each other and because slip planes are not trans-granular, meaning slip planes cannot travel across grains, the smaller the grain size then the smaller distance that the atoms can move along that slip plane. The smaller grains then greatly improve the yield strength of the material. The size of the finished grain structure is dependent upon carbon content and the rate at which the material is cooled from the liquid phase. Figure 65 AnnealedSAE 1040 (EN 8) @ 20X10 Mag. Figure 66 QuenchedSAE1040 (EN 8) @ 20x10 Mag. Figure 67 O1 Tool SteelParent Metal @ 20x10 Mag. Figure 68 O1 Tool SteelHAZ @ 20x10 Mag. 143.40 HV 477.867 HV 188.5 HV 543 HV Larger Grain Structure Refined Needle Like Grain Structure Larger Grain Structure Refined Smaller Grain Structure 430.2 MPa 1433.601 MPa 565.5 MPa 1629 MPa
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  55 6.6 Phase Transformationsin Weld Specimens Figure 69 Iron Carbon Phase Diagram (101) Figure 70 MMA Welded MildSteel Microstructure (10x Mag.) Solidified Weld Solid – Liquid Transition Zone RecrystallizedZone PartiallyTransformedZone Unaffected Parent Metal TemperedZone Grain Growth Zone
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  56 Mechanical Properties ofWeld Specimens during and after Phase Transformation The mechanical properties of a welded joint are related to its microstructure and the number of welding cycles that it has undergone. The number of welding cycles i.e. the number of weld passes performed to fill out a butt weld or a fillet weld, have a direct impact on the final mechanical properties of the weldment and the surrounding HAZ and parent material. However there are many other factors that determine what kind of microstructure and mechanical properties will be attained on completion of the welding operation. The welding parameters such as arc voltage, welding current, travel speed, and welding process all influence the final microstructure and mechanical properties of the finished welded joint. Upon initiation of the welding arc and once the weld pool is moving, the semi solid cooling weldment left in its wake forms a cast type microstructure which starts at the edges of the weld pool. Grains start to form and grow from the weld pool edges towards the centre of the weld. The size of the grains formed in the weld depend on the rate of cooling of the weld metal and its surrounding parent material. The HAZ, the zone between the fusion zone and the parent material forms a different kind of microstructure. The width of the HAZ is determined by the temperature during the welding operation. Once the temperature exceeds 550o C (102), which it will during welding, the microstructure of the HAZ will change. As shown in Figure 69, only tempering of the HAZ happens below the Eutectoid temperature of 723o C. Above the Eutectoid point there is partial phase transformation to austenite. Above the recrystallization temperature of 912o C within the fully austenite region, there is no significant grain growth until all carbides present have been dissolved. At the other side of the HAZ where it interfaces with the weld fusion zone the carbides have been fully dissolved, this allows the grain structure to grow and have a more coarse appearance and therefore is softer relative to the HAZ. Moving outwards from the weldment and the HAZ towards the zone of the parent metal a point is reached where the grain structure of the material is no longer affected. 6.7 Heat Treatment of Carbon Steels Heat treatment of carbon steels is a way of changing the microstructure to attain the desirable mechanical properties required. Depending on what application the material will be used for and what service conditions it will experience will dictate what heat treatment, if any, will be necessary for the material. When carbon steels are heat treated they undergo phase transformations which alter the grain structure. If a material needs to machined or welded then it would be advisable to “Anneal” the material first. If a material needs increased toughness after being work hardened by rolling or folding then it should be “Normalized”. If a material needs increased hardness then the material should be quenched in an appropriate medium. If a material has been quenched and if it is to be used in service then it must be tempered to relieve the stress and embrittlement and by virtue increasing its strength. The various heat treatments carried out on the specimens relating to this project are as follows:  Annealing  Normalizing  Quenching  Tempering
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  57 Images of HeatTreated Samples SAE 1040 (EN 8) Figure 71 AnnealedSAE 1040 (50 x 10 Mag.) Figure 72 NormalizedSAE1040 (50 x 10 Mag.) Figure 73 QuenchedSAE1040 (50 x 10 Mag.) Figure 74 Tempered SAE 1040 (50 x 10 Mag.) As is clearly shown in Figures 71, 72, 73, and 74, the different heat treatment processes have a significant effect on the microstructure of the medium carbon steel. When a material is annealed it allows the material to cool at a very slow rate which promotes grain growth which is clearly shown in Figure 71. In Figure 72 there is a slightly smaller grain structure which is brought about because the material is cooled at a much quicker rate in the ambient air, thus increasing toughness and strength by normalizing. In Figure 73 there is significantly visible reduction in grain size because the material has been quenched and thus locking the carbon in solution because of the rapid rate of cooling. This forms Martensite and the structure is extremely hard, but brittle. In Figure 74 the grain structure is slightly less “needle like” because it has been tempered. This heat treatment procedure will relive the material of internal stress at the expense of a proportion of its hardness but in return its strength has been greatly increased. Very Large Grain Structure Slightly Smaller Grain Structure Sharp Needle Like Grain Structure More Refined Tempered Grain Structure
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  58 6.8 Conclusions Upon reflectionthere are areas of concerns and they should be addressed so that if similar work was to be carried out in the future, these areas of concern could be avoided. Weld Specimen Materials The materials used in this project were those near to hand and the best that was available. After attempts at sourcing medium carbon steel (EN 8) in the size that was needed had failed, a compromise was made to include the use of this material in the project. EN 8 of a dimension that was far from ideal was used, and because of its inadequate size the Vickers Hardness values that were recorded were the reverse of what was expected. This is purely because the material was far too small and the material properties reflected this fact. If carrying out a similar project in the future it would be advantageous to use materials of equal dimensions (width. length, thickness) for all four steel used, so as to help limit any factors that have the possibility to produce unexpected results. Welding Consumables The welding consumables used were not all new consumables but were prepared, as much as was possible, according to the ASME IX code of practice. The consumables, (low hydrogen) need to be stored and preheated in accordance with the above code prior to any welding being carried out. But due to the cost of these consumables, older consumables (although sealed) were used to reduce cost. This may have introduced diffusible Hydrogen into the molten weld pool which can cause Hydrogen infused cracking. Also there is a possibility that this may have had an adverse effect on the x-ray quality of those welds. But since there is no possibility of submitting the welds to an x-ray, there is no way of being sure either way. Planar Grinding and Polishing Procedure After extensive planar grinding and polishing of more than two dozen samples, doubts have been raised as to the validity of the procedure that was followed. The procedure concerned all the variables of this operation. For some of the samples, the mild steel, the API 5L, and the medium carbon EN 8 there were no significant concerns but the O1 tool steel samples did not yield the planar surface that was expected when the advised Buehler procedure was followed. It took an unusually long time for stock removal to remove all scratches and when it was suspected that the surface of the O1 tool steel was ready to move to a finer grade of abrasive, when commenced it had the adverse effect and introduced new scratches to the specimens.
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  59 Yield Strength ofwelded carbon steel Vs yield strength un-welded carbon steel The yield strength of the lower carbon steels (mild steel, API 5L) after welding have a HAZ hardness that does not represent a problem that would lead to brittle fracture. There is no significant fluctuation in hardness from the parent metal to the HAZ and the weldment. The exceptionally high carbon O1 tool steel, however, shows significant variations between hardness in the parent metal into the HAZ and the weld. There is a significant rise in hardness in the HAZ of O1 tool steel and this has led to brittle fracture in some O1 specimens which displayed high yield strength figures as shown in Figures 51 & 53 on page 46. All O1 tool steel specimens, except the annealed specimens exceeded the typical un-welded yield strength of O1 tool steel and ultimately failed by cracking. The typical yield strength for EN8 was exceeded, and in the case of the TIG welded specimen, it was exceeded by more than 3 times the yield strength in the HAZ for the un-welded EN 8. However none of the EN 8 specimens failed. None of the mild steel specimens or the API 5L specimens failed and they were all within acceptable design limits. The complete table of Vickers hardness values and yield strength values are in the Table 6 on page 79. Multi Pass Weld Test Pieces Vs Single Weld Test Pieces When the microscopic examination of the weld specimens was commenced it was noticed that there was a layered type of microstructure which sometimes made it difficult to clearly define zone interfaces. When the Vickers Micro Hardness testing commenced it was presenting a very confusing looking microstructure that made it difficult to clearly locate regions of interest for hardness testing. It became very difficult to accurately pinpoint a suitable location for the diamond indenter. Upon reflection it would be advisable to use single pass butt welds on thinner plates or single pass fillet welds, using a wider variety of materials and processes. This would allow for a broader comparison of not just materials but of welding processes also. Arc Energy and Heat Input of MMA Vs TIG It was concluded that the TIG welding process imparted a far greater amount of heat energy into the material during the welding process than the MMA process does. This high energy input meant that there was a much slower cooling rate for the samples welded with the TIG welding process. This resulted in excessive grain growth in some samples, namely Figure 57 on page 50. The need for using the Carbon Equivalent Formula The carbon equivalent formula is an extremely important source of critical information which was used to determine the weldability of the project materials. It was demonstrated that without proper adherence to recommended heat treatment procedures, any carbon steels with a carbon equivalent above 0.4 would produce undesirable mechanical properties when welded.
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  60 The anomaly ofvaried hardness values in the EN 8 weld specimens The anomaly of obtaining an unexpected decrease in hardness in the HAZ of both the TIG welded and the MMA welded specimens, coupled with unexpected corresponding microstructure for the same zones of both specimens was difficult to explain. It would have been expected that as the hardness increased, the corresponding grain structure for the TIG welded EN 8 specimen would have been more refined rather than the coarse grain structure that was obtained. The finer grain structure that was obtained from the MMA welded EN 8 specimen was what was expected but the corresponding hardness value was lower than that for the coarser grain structure of the TIG welded EN 8 specimen. The only conclusion that can be drawn from the relatively coarse Ferrite microstructure of the TIG welded EN 8, as opposed to the finer microstructure of the MMA welded EN 8, is the fact that the TIG filler wire (312 stainless) contains 32% Chromium which at higher temperatures promotes the growth of Ferrite. The ESAB MMA welding rods however, although also 312 stainless, but only containing 29% Chromium could be a factor for the disparity in hardness value and microstructure grain size. EN 8 Microstructure for MMA & TIG with corresponding graphs Figure 75 HAZ of EN 8 (20 x 10 Magnification) Graph 8 Hardness Profile across EN 8 Weld Specimen Figure 76 HAZ of EN 8 (20 x 10 Magnification) Graph 9 Hardness Profile across EN 8 Weld Specimens HAZ Hardness = 159.6 HV TIG Coarse Grain Structure MMA Finer Grain Structure HAZ Hardness = 149.5 HV
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  61 Chapter 7 APPENDICES Graph10 Vickers Micro Hardness For Mild Steel (MMA) Graph 11 Vickers Micro Hardness For Mild Steel (TIG) Graph 12 Vickers Micro Hardness For EN 8 (MMA)
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  62 Graph 13 VickersMicro Hardness For EN 8 (TIG) Graph 14 Vickers Micro Hardness For API 5L (MMA) Graph 15 Vickers Micro Hardness For API 5L (TIG)
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  63 Graph 16 VickersMicro Hardness For O1 Tool Steel ANNEALED (MMA) Graph 17 Vickers Micro Hardness For O1 Tool Steel ANNEALED (TIG) Graph 18 Vickers Micro Hardness For O1 Tool Steel NORMALISED (MMA)
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  64 Graph 19 VickersMicro Hardness For O1 Tool Steel NORMALISED (TIG) Graph 20 Vickers Micro Hardness For O1 Tool Steel ROOM TEMPERATURE (MMA) Graph 21 Vickers Micro Hardness For O1 Tool Steel ROOM TEMPERATURE (TIG)
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  65 7.1 Graph ofDifference in Vickers Micro Hardness From Root To Cap Graph 22 Difference in Vickers Micro Hardness From Root To Cap 7.2 Graph of Differences in Yield Strength of All Weld Specimens Graph 23 Graph of Yield Strengths of All Weld Specimens
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  66 7.3 Table ofVickers Hardness values and Yield Strengths Table 6 Table of Vickers Hardness Values and Corresponding Yield Strengths MATERIAL CARBONCONTENT(%) WELDPROCESS AVERAGEHV(PM) AVERAGEHV(HAZ) AVERAGEHV(WELD) AVERAGEYIELDMPa(PM) AVERAGEYIELDMPa(HAZ) AVERAGEYIELDMPa(WELD) MILDSTEEL 0.15 MMA 135.133 165 154.6 428.37161 523.05 490.082 MILDSTEEL 0.15 TIG 146.1 179.55 182.7 463.137 569.1735 579.159 API5L 0.25 MMA 177.867 193.1 162.35 563.83839 612.127 514.6495 API5L 0.25 TIG 152.4 179.8 192.8 483.108 569.966 611.176 EN8 0.4 MMA 207.133 159.967 227.433 656.61161 507.09539 720.96261 EN8 0.4 TIG 176.85 169.85 481.75 560.6145 538.4245 1527.1475 O1TOOLSTEEL 0.9MMANORMALISED MMA 188.5 543 236.5 597.545 1721.31 749.705 O1TOOLSTEEL 0.9TIGNORMALISED TIG 465.5 602 218 1475.635 1908.34 691.06 O1TOOLSTEEL 0.9MMAANNEALED MMA 184.5 399 230.5 584.865 1264.83 730.685 O1TOOLSTEEL 0.9TIGANNEALED TIG 312 423 207.5 989.04 1340.91 657.775 O1TOOLSTEEL 0.9MMAROOMTEMP. MMA 220 594 226.5 697.4 1882.98 718.005 O1TOOLSTEEL 0.9TIGROOMTEMP. TIG 513 538 227.5 1626.21 1705.46 721.175 TABLEOFCONVERSIONSFROMVICKERSHARDNESSTOYIELDSTRENGTH
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