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Metallurgy

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Metallurgy by Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh.

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Metallurgy

  1. 1. Mohammud Hanif Dewan, Maritime Lecturer & Trainer, Bangladesh METALLURGY
  2. 2. DUCTILITY • A metal is ductile when it may be drawn out in tension without rupture. • Wire drawing depends upon ductility for its successful operation. • A ductile metal must be both strong and plastic • With many materials ductility increase rapidly with heat. • Is the property of a material which enables it to be drawn easily into wire form • The percentage elongation and contraction of area, as determined from a tensile test are a good practical measures of ductility • Ability to undergo permanent change in shape without rupture or loss of strength if any force applied. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 2
  3. 3. MALLEABILITY • The ability to be hammered or rolled out without cracking. • Very few metals have good cold malleability, but most are malleable when heated to a suitable temperature • The material that can be shaped by beating or rolling is said to be malleable. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 3
  4. 4. ELASTICITY • The elasticity of a metal is its power of returning to its original shape after deformation by force. • The ability to return to the original shape or size after having been deformed or loaded. • All strain in the stressed material disappears upon removal of the stress. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 4
  5. 5. PLASTICITY • The property of flowing to a new shape under pressure/stress and retaining on the new shape after removal of pressure/stress. • This is a rather similar property to malleability, and involves permanent deformation without rupture. • It is opposite to elasticity • The ability to deform permanently when load is applied. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 5
  6. 6. Modulus of Elasticity E defined as the ratio of tensile stress to strain and determined in a tensile test. Modulus of Rigidity G defined as the ration of shear stress and strain and determined in a torsion test. Bulk Modulus K defined as the ration of pressure and volumetric strain and found with specialised equipment for liquids. Poisson’s ratio ν defined as the ratio of two mutually perpendicular strains and governs how the dimensions of a material change such as reduction in diameter when a bar is stretched. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 6
  7. 7. TOUGHNESS • Resistance to fracture by blows. • The materials usually have high tenacity combined with good or fair ductility. • Toughness decreases with heating. • A combination of strength and the ability to absorb energy or deform plastically. • A condition between brittleness and softness. • A materials ability to sustain variable load conditions without failure.. • Materials could be strong and yet brittle but a material is tough has strength 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 7
  8. 8. HARDNESS • The hardness of a metal is a measure of its ability to withstand scratching, wear and abrasion, indentation by harder bodies, etc. • The machine ability and inability to cut are also hardness property which is important for workshop process. • Hardness also decreased by heating • A material’s resistance to erosion or wear will indicate the hardness of the material • A material’s ability to resist plastic deformation usually by indentation 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 8
  9. 9. HARDNESS MATERIALS LIST: Hard materials are diamonds and glass. Soft materials are copper and lead. Hardness is measured by comparing it to the hardness of natural minerals and the list is called the Moh scale. The list runs from 1 to 10 with 1 being the softest ands 10 the hardest. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 9 10 Diamond 9 Corundum 8 Topaz 7 Quartz 6 Feldspar 5 Apatite 4 Fluorite 3 Calcite 2 Gypsum 1 Talc
  10. 10. BRITTLENESS • Opposite of toughness. • A brittle material breaks easily under a sharp blow, although it may resist a steady load quite well. • Brittle materials are neither ductile or malleable, but they often have considerable hardness. • As a lack of ductility • Strong materials may also be brittle 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 10
  11. 11. STIFFNESS/RIGIDTY - This is the property of resisting deformation within the elastic range and for ductile materials is measured by the Modulus of Elasticity. A high E value means that there is a small deformation for a given stress. - The property of a solid body to resist deformation, which is sometimes referred to as rigidity. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 11
  12. 12. Strength • The greater the load which can be carried the stronger the material and strength of the material will be higher. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 12
  13. 13. Tensile strength • This is the main single criterion with reference to metals. • This is the ability of a material to withstand tensile loads without rupture when the material is in tension • It is a measure of the material’s ability to withstand the loads upon it in service. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 13
  14. 14. If the material is ductile, we look for the point at which it starts to stretch like a piece of plasticine. This point is called the yield point and when it stretches in this manner, we call it PLASTIC DEFORMATION. If the material is not ductile, it will snap without becoming plastic. In this case, we look for the stress at which it snaps and this is called the ULTIMATE TENSILE STRENGTH. Most materials behave like a spring up to the yield point and this is called ELASTIC DEFORMATION and it will spring back to the same length when the load is removed. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 14
  15. 15. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 15 The tensile test is carried out with a standard sized specimen and the force required to stretch it, is plottedagainsttheextension. Typical graphs are shown below.
  16. 16. Ultimate tensile strength (UTS) (Tensile strength or Ultimate Strength) - It is the maximum stress that a material can withstand while being stretched or pulled before failing or breaking. Tensile strength is not the same as compressive strength and the values can be quite different. - UTS is usually found by performing a tensile test and recording the engineering stress versus strain. The highest point of the stress-strain curve (see point 1 on the engineering stress/strain diagrams below) is the UTS. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 16
  17. 17. Stress vs. Strain curve typical of aluminum. 1 Ultimate Strength 2 Yield Strength 3 Proportional Limit Stress 4 Rupture 5 Offset Strain (usually 0.002) 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 17
  18. 18. Compressive Strength • This is the ability of a material to withstand Compressive (squeezing) loads without being crushed when the material is in compression. Shear Strength • This is the ability of a material to withstand offset or traverse loads without rupture occurring. Fatigue Strength • This is the property of a material to withstand continuously varying and alternating loads. Yeild Strength The stress a material can withstand without permanent deformation. Torsional Strength This governs the stress at which a material fails when it is twisted and a test similar to the tensile test is carried out, only twisting the specimen instead of stretching it. This is a form of shearing. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 18
  19. 19. HEAT TREATMENT
  20. 20. • Heat treatment is a general term referring to a cycle of heating and cooling which alters the internal structure of a metal and thereby changes its properties • Metal and alloys are heat treated for a number of purposes however the primarily to:- 1. Increase their hardness and strength 2. To improved ductility 3. To soften them for subsequent operations (cutting etc) 4. Stress relieving 5. Eliminate the effects of cold work 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 20
  21. 21. HEAT TREATMENT OF STEEL The mechanical properties of materials can be changed by heat treatment. Let’s first examine how this applies to carbon steels. CARBON STEELS In order to understand how carbon steels are heat treated we need to re-examine the structure. Steels with carbon fall between the extremes of pure iron and cast iron and are classified as follows. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 21
  22. 22. All metals form crystals when they cool down and change from liquid into a solid. In carbon steels, the material that forms the crystals is complex. Iron will chemically combine with carbon to form IRON CARBIDE (Fe3C). This is also called CEMENTITE. It is white, very hard and brittle. The more cementite the steel contains, the harder and more brittle it becomes. When it forms in steel, it forms a structure of 13% cementite and 87% iron (ferrite) as shown. This structure is called PEARLITE. Mild steel contains crystals of iron (ferrite) and pearlite as shown. As the % carbon is increased, more pearlite is formed and at 0.9% carbon, the entire structure is pearlite.5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 22 NAME Dead mild CARBON % 0.1 – 0.15 TYPICAL APPLICATION pressed steel body panels Mild steel Medium carbon steel High carbon steels Cast iron 0.15 – 0.3 0.5 – 0.7 0.7 – 1.4 2.3 – 2.4 steel rods and bars forgings springs, drills, chisels engine blocks
  23. 23. 1538 1130 2.0 oC 695 910 0.4 0.8 1.2 AUSTENITE AUSTENITE + FERRITE FERRITE + PEARLITE HYPO-EUTECTOID STEELS PEARLITE Mixture of Ferrite & Cementite EUTECTOID STEELS AUSTENITE AUSTENITE + CEMENTITE AUSTENITE + CEMENTITE HYPER-EUTECTOID STEELS IRON – CARBON EQUILIBRIUM DIAGRAM 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 23
  24. 24. IRON IRON-CARBON DIAGRAM Ferrite Austenite Steel Cast iron Pearlite Pearlite and Cementine Pearlite and Carbide Eutectic eutectoid
  25. 25. 1538 1130 2.0 oC 695 910 0.4 0.8 1.2 AUSTENITE FERRITE + CEMENTITE AUSTENITE + CEMENTITEAUSTENITE + FERRITE FERRITE + PEARLITE CEMENTITE + PEARLITE AUSTENITE + LIQUID IRON – CARBON EQUILIBRIUM DIAGRAM 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 25
  26. 26. AUSTENITE • A solid solution of Carbon in face-centred cubic iron (Allotropic), containing a maximum 0f 1.7 % carbon at 1130oC • It is soft, ductile and non-magnetic and also exist in the plain carbon steel above the upper critical range. • It may however occur at room requirement, however, occur at room temperatures in certain alloy steels 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 26
  27. 27. FERRITE • Ferrite is nearly pure iron.A solid solution of Carbon in body-centred cubic  iron, containing a maximum of 0.04 % Carbon at 695oC. • At room temperature, small amounts of manganese, silicon and other elements may be dissolved in iron as well as up to 0.007 % Carbon. • Found only in Hypoeutectoid steel • It is softest constitute of steel and very ductile and readily cold-worked 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 27
  28. 28. CEMENTITE • A hard brittle compound of iron and Carbon with the formula Fe3C • The hardest constituent of steel • This may exist in the free state usually as a grain boundary film, or as a constituent of the eutectoid pearlite 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 28
  29. 29. PEARLITE • This is the eutectoid structure consisting of alternate lamination of ferrite and cementite. • It contains 0.83% Carbon and is formed by the breakdown of the austenite solid solution at 695oC • The properties of pearlite are harder and stronger than ferrite, but softer and more ductile than cementite 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 29
  30. 30. If the carbon is increased further, more cementite is formed and the structure becomes pearlite and cementite as shown. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 30
  31. 31. HEAT TREATMENT of CARBON STEELS Steels containing carbon can have their properties (hardness, strength, toughness etc) changed by heat treatment. Basically if it is heated up to red hot and then cooled very rapidly the steel becomes harder. Dead mild steel is not much affected by this but a medium or high carbon steel is. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 31
  32. 32. Principle of heat treatment of steel • Metals are never heated to the melting point in heat treatment. • Therefore, all the reactions within the metal during the heating and cooling cycle, take place while the metal is in the solid state • During ordinary heat treating operations, steel is seldom heated above 983oC. • In using the iron-iron carbide diagram, we need only to concern ourselves with that part which is always solid steel. • The area where the Carbon content is 2% or less and the temperature is below 1130oC 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 32
  33. 33. COOLING RATE • Cooling rate is the most important part of heat treatment. • Different cooling rates are now considered as they have a significant effect on the properties of the metal. SLOW COOLING • Austenite is transformed to course pearlite. • Slightly more rapid cooling may produce fine pearlite in which the layers of ferrite and cementite are thinner. INTERMEDIATE COOLING • Austenite transforms to a material called Bainite instead of the usual pearlite. • When etched, Bainite gives a dark appearance and shows a circular or needle like form. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 33
  34. 34. FAST COOLING • By quenching in water, the transformation of austenite is suppressed until about 318oC at which point a new constituent called Martensite(quite brittle) begins to form instead of the Bainite or pearlite of slower cooling rate. • As the temperature drops lower, the transformation become complete. • This temperature vary with the alloy content of the steel 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 34
  35. 35. TIME TEMPERATURE TRANSFORMATION • In order to obtain steels with the desired properties, we must have some control over the transformation process, and this is indicated in the TTT diagram • TTT diagram are used to predict the metallurgical structure of a steel sample which is quenched in the austenite region and held to constant elevated temperature below 729oC. • This is known as Isothermal transformation 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 35
  36. 36. Time (sec) oC 0 760 725 650 590 540 430 316 260 190 90 TIME TEMPERATURE TRANSFORMATION DIAGRAM Ferrite form Pearlite starts Pearlite forms Pearlite is complete Coarse Pearlite Fine Pearlite Bainite forming Upper Bainite Lower Bainite 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 36
  37. 37. TIME TEMPERATURE TRANSFORMATION • However since heat treatment usually involves continuous cooling, TTT diagrams are not directly applicable but can be modified to be useful in at least a qualitative way for continuous cooling condition 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 37
  38. 38. THE AFFECT OF PROCESSING and MANIPULATION ON METALS When a metal solidifies grains or crystals are formed. The grains may be small, large or long depending on how quickly the material cooled and what happened to it subsequently. Heat treatment and other processes carried out on the material will affect the grain size and orientation and so dramatically affect the mechanical properties. In general slow cooling allows large crystals to form but rapid cooling promotes small crystals. The grain size affects many mechanical properties such as hardness, strength and ductility. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 38
  39. 39. MANIPULATIVE PROCESSES These are processes which shape the solid material by plastic deformation. If the process is carried out at temperatures above the crystallisation temperatures, then re-crystallisation occurs and the process is called HOT WORKING. Otherwise the process is called COLD WORKING. The mechanical properties and surface finish resulting are very different for the two methods. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 39
  40. 40. HOT ROLLING This is used to produce sheets, bars and sections. If the rollers are cylindrical, sheet metal is produced. The hot slab is forced between rollers and gradually reduced in thickness until a sheet of metal is obtained. The rollers may be made to produce rectangular bars, and various shaped beams such as I sections, U sections, angle sections and T sections. Steel wire is also produced this way. The steel starts as a round billet and passes along a line of rollers. At each stage the reduction speeds up the wire into the next roller. The wire comes of the last roller at very high speeds and is deflected into a circular drum so that it coils up. This product is then used for further drawing into rods or thin wire to be used for things like springs, screws, fencing and so on. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 40
  41. 41. COLD ROLLING The process is similar to hot rolling but the metal is cold. The result is that the crystals are elongated in the direction of rolling and the surface is clean and smooth. The surface is harder and the product is stronger but less ductile. Cold working is more difficult that hot working. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 41
  42. 42. FORGING In this process the metal is forced into shape by squeezing it between two halves of a die. The dies may be shaped so that the metal is simply stamped into the shape required (for example producing coins). The dies may be a hammer and anvil and the operator must manipulate the position of the billet to produce the rough shape for finishing (for example large gun barrels). 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 42
  43. 43. COLD WORKING Cold working a metal by rolling, coining, cold forging or drawing leaves the surface clean and bright and accurate dimensions can be produced. If the metal is cold worked, the material within the crystal becomes stressed (internal stresses) and the crystals are deformed. For example cold drawing produces long crystals. In order to get rid of these stresses and produce “normal” size crystals, the metal can be heated up to a temperature where it will re-crystallise. That is, new crystals will form and large ones will reduce in size. If the metal is maintained at a substantially higher temperature for a long period of time, the crystals will consume each other and fewer but larger crystals are obtained. This is called “grain growth”. Cold working of metals change the properties quite dramatically. For example, cold rolling or drawing of carbon steels makes the stronger and harder. This is a process called “work hardening”. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 43
  44. 44. HOT WORKING Most metals (but not all) can be shaped more easily when hot. Hot rolling, forging, extrusion and drawing is easier when done hot than doing it cold. The process produces oxide skin and scale on the material and producing an accurate dimension is not possible. Hot working, especially rolling, allows the metal to re- crystallise as it is it is produced. This means that expensive heat treatment after may not be needed. The material produced is tougher and more ductile. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 44
  45. 45. LIQUID CASTING AND MOULDING When the metal cools it contracts and the final product is smaller than the mould. This must be taken into account in the design. The mould produces rapid cooling at the surface and slower cooling in the core. This produces different grain structure and the casting may be very hard on the outside. Rapid cooling produces fine crystal grains. There are many different ways of casting. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 45
  46. 46. SAND CASTING A heavy component such as an engine block would be cast in a split mould with sand in it. The shape of the component is made in the sand with a wooden blank. Risers allow the gasses produced to escape and provide a head of metal to take up the shrinkage. Without this, the casting would contain holes and defects. Sand casting is an expensive method and not ideally suited for large quantity production. Typical metals used are cast iron. Cast steel and aluminium alloy. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 46
  47. 47. DIE CASTING Die castings uses a metal mould. The molten metal may be fed in by gravity as with sand casting or forced in under pressure. If the shape is complex, the pressure injection is the best to ensure all the cavities are filled. Often several moulds are connected to one feed point. The moulds are expensive to produce but this is offset by the higher rate of production achieved. The rapid cooling produces a good surface finish with a pleasing appearance. Good size tolerance is obtained. The best metals are ones with a high degree of fluidity such as zinc. Copper, aluminium and magnesium with their alloys are also common. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 47
  48. 48. CENTRIFUGAL CASTING This is similar to die casting. Several moulds are connected to one feed point and the whole assembly is rotated so that the liquid metal is forced into the moulds. This method is especially useful for shapes such as rims or tubes. Gear blanks are often produced this way. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 48
  49. 49. MACHINING Machining processes involve the removal of material from a bar, casting, plate or billet to form the finished shape. This involves turning, milling, drilling, grinding and so on. Machining processes are not covered in depth here. The advantage of machining is that is produces high dimensional tolerance and surface finish which cannot be obtained by other methods. It involves material wastage and high cost of tooling and setting. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 49
  50. 50. Heat treatment Methods • Annealing • Normalizing • Hardening • Tempering 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 50
  51. 51. Annealing • heat treatment that alters the microstructure of a material causing changes in properties such as strength, hardness, and ductility • It the process of heating solid metal to high temperatures and cooling it slowly so that its particles arrange into a defined lattice
  52. 52. Stages in annealing Heating to the desired temperature , Holding or soaking at that temperature, Cooling or quenching ,usually to room temperature . • In practice annealing concept is most widely used in heat treatment of iron and steals
  53. 53. Purpose of annealing • It is used to achieve one or more of the following purpose . 1. To relive or remove stresses 2. To include softness 3. To alter ductility , toughness, electrical, magnetic. 4. To Refine grain size 5. To remove gases 6. To produce a definite microstructure .
  54. 54. Application Annealing process is employed in following application • Casting • Forging • Rolled stock • Press work ….
  55. 55. 3 Types of Annealing: I. Process Annealing II. Full annealing III. Spheroidising 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 55
  56. 56. i. Process Annealing  Carried out on cold-worked low carbon steel sheet or wire in order to relieve internal stress and to soften the metals. • The steel is heated to 550 to 650oC below the critical point. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 56 Increase in ductility reduce in TS & hardness
  57. 57. ii. Full Annealing  It carried out on hot-worked and cast steels in order to obtain grain refinement with high ductility.  It also produces a softer steel with better machinability • For steels – heating above critical point (30 - 50oC) then - holding at this temperature for a time (thickness) - followed by slow cooling usually in furnace. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 57
  58. 58. iii. Spheroidizing Annealing  To remove coarse pearlite and making machining process easy .  It forms spherodite structure of maximum soft and ductility easy to machining and deforming. • The process is limited to steels in excess of 0.5% carbon. This steel can be softened by annealing at 650 – 750oC just below the lower critical point, when the cementite of the pearlite balls up or spheroidizes. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 58
  59. 59. Defects uncontrolled temperature i. Overheating  Heated above the actual temperature or to long maintained at annealing temperature: austenite grain growth will occur and make the metal weak and brittle ii. Burning  If heated above the upper critical point to temperature, Brittles films of oxide are formed which make the steel unsuitable. For further use and must be remelted. iii. Under annealing  The original pearlite will have change to several small austenite grains. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 59
  60. 60. NORMALIZING  For hypoeutectoid steels - heating above critical point (30 - 50oC) - holding at this temperature for a time (thickness) & - followed by cooling in still air. • Produces maximum grain refinement and consequently the steel slightly harder and stronger than a fully annealed steel. • However the properties will vary with section thickness 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 60
  61. 61. HARDENING  Hardening is process in which Medium and High carbon steels (0.4 – 1.2%) is heated to a temperature above the critical point (until red hot), held at this temperature and quenched (rapidly cooled) in water, oil or molten salt baths. • Hardening producing a very hard and brittle metal. At 723 Deg C, the ferrite changes into Austenite structure. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 61
  62. 62. TEMPERING Tempering is a process of heat treating, which is used to increase the toughness of iron-based alloys. To remove some of the brittleness from hardened steels, tempering is used. The metal is heated to the range of 220- 300 deg C and cool in the air. • Tempering is usually performed after hardening, to reduce some of the excess hardness, and is done by heating the metal to some temperature below the critical temperature for a certain period of time, then allowed to cool in still air. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 62
  63. 63. QUENCHING • To harden by quenching, a metal (usually steel or cast iron) must be heated into the austenitic crystal phase and then quickly cooled. • Quenching Media:  Brine (water and salt solution)  Water  Oil  Air  Turn off furnace 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 63
  64. 64. CASE HARDENING • Low carbon steels cannot be hardened by heating due to the small amounts of carbon present. So, Case hardening seeks to give a hard outer skin over a softer core on the metal. • The addition of carbon to the outer skin is known as carburising. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 64
  65. 65. • When temperature region 200 – 450oC the martensite decomposes into ferrite and the precipitation of the fine particles of carbide occurs known. as troostite • At higher temperatures 450 – 650oC the carbide particles coalesce thus producing fewer and larges particles which provide fewer obstacles to dislocations resulting further increasing toughness while decrease in strength and hardness and known as sorbite. • Sorbite is ideal for components subject to dynamic stresses such as crankshaft and connecting rod 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 65
  66. 66. ……………………………………………………………………… ……………………………………………………………………… ……………………………………………………………………… …………………………………… ……………………………………………………………………… ……………………………………………………………………… ……………………………………………………………………… …………………………………… ……………………………………………………………………… ……………………………………………………………………… ……………………………………………………………………… …………………………………… ……………………………………………………………………… ……………………………………………………………………… ……………………………………………………………………… …………………………………… ……………………………………………………………………… ……………………………………………………………………… ……………………………………………………………………… …………………………………… ……………………………………………………………………… ……………………………………………………………………… ……………………………………………………………………… …………………………………… ……………………………………………………………………… ……………………………………………………………………… ……………………………………………………………………… …………………………………… ……………………………………………………………………… ……………………………………………………………………… ………………………………………………………… SORBITEMARTENSITE TROOSTITE 200 400 600 oC Hardness 200 800 600 400 1000 EFFECT OF TEMPERING 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 66
  67. 67. ALLOYS 5/28/2015 Mohd. Hanif Dewan, 67 Nickel - One of the most widely used alloying elements in steel. In amounts 0.50% to 5.00% its use in alloy steels increases the toughness and tensile strength without detrimental effect on the ductility. Chromium - Gives resistance to wear and abrasion. Chromium has an important effect on corrosion resistance and is present in stainless steels in amounts of 12% to 20%.
  68. 68. ALLOYS 5/28/2015 Mohd. Hanif Dewan, 68 •Molybdenum - Increases hardenability, toughness to quenched/tempered steels. It also improves the strength of steels at high temperatures (red- hardness). •Vanadium - Steels containing vanadium have a much finer grain structure than steels of similar composition without vanadium.
  69. 69. CREEP 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 69 • Creep is strain increase with time under constant load. • Creep is temperature dependent – the higher the temperature the greater the effect
  70. 70. FRETTING A type of wear that occurs between tight-fitting surfaces subjected to cyclic relative motion of extremely small amplitude. Usually, fretting is accompanied by corrosion, especially of the very fine wear debris. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 70 FRETTING CORROSION The accelerated deterioration at the interface between contacting surfaces as the result of corrosion and slight oscillatory movement between the two surfaces.
  71. 71. IMPURITIES 5/28/2015 Mohd. Hanif Dewan, 71 Sulphur – The presence of free sulphur in a steel product is detrimental to its properties, especially toughness. Phosphorous – Its presence in steel is usually regarded as an undesirable impurity due to its embrittling effect, for this reason its content in most steels is limited to a maximum of 0.050%.
  72. 72. Welding Metallurgy 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 72
  73. 73. Heat Affected Zone Welding Concerns 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 73
  74. 74. Heat Affected Zone Welding Concerns  Changes in Structure Resulting in Changes in Properties  Cold Cracking Due to Hydrogen Two major concerns occur in the heat affected zone which effect weldability these are, a.) changes in structure as a result of the thermal cycle experienced by the passage of the weld and the resulting changes in mechanical properties coincident with these structural changes, and b.) the occurrence of cold or delayed cracking due to the absorption of hydrogen during welding. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 74
  75. 75. First let’s review the thermal cycles experienced in the heat affected zone as a result of the passage of the weld. The figure illustrated here shows the temperature vs time curve at various distances from the weld metal. Note that almost every thermal cycle imaginable occurs over this short distance of the heat affected zone. Thus a variety of structural and property variations are expected. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 75
  76. 76. Look At Two Types of Alloy Systems 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 76
  77. 77. There are two types of alloy systems which we will consider, those which do not have an allotropic phase change during heating like copper, and those which have an allotropic phase change on heating like steel. We will first consider those materials which do not have an allotropic phase change. The top schematic illustrates this type of material. We will however consider that this material has been cold worked (not the elongated cold worked grains present in the base material (region A). The weld metal is represented by region C, and the heat affected zone is region B. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 77
  78. 78. Note that the heat of welding has effected the structure of this material even though there are no allotropic transformations. Recall that cold worked structures undergo recover, recrystalization and grain growth when heated to ever increasing temperatures. So it is in this material. As we traverse from the cold worked elongated grains in the unaffected base metal, we come to a region where the cold worked grains undergo recovery and then shortly there after they recrystalize into fine equaled new grains. Traversing still closer to the weld region we note grain growth where the more favorably oriented grains consume neighboring grains and grain growth occurs. The grains within the weld epitaxially nucleate from the grains in the heat affected zone at the fusion boundary, and grain growth continues into the solidifying weld metal making very large grains.5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 78
  79. 79. Introductory Welding Metallurgy, AWS, 1979 Cold Worked Alloy Without Allotropic Transformation 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 79
  80. 80. One of the factors that occur when cold worked grains recrystalize and grain grow occurs we have already discussed, and that is the material softens. Thus the heat affected zone and weld metal will not hold the same strength level as the cold worked base metal. Another consequence of increased grain size is perhaps equally important and that is that the larger grains are more brittle. A “Charpy” impact test is used to determine how much impact energy a structure will absorb over various temperature ranges. Note that the larger grain size material will become brittle and not absorb much of an impact load even at temperatures around room temperature and above. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 80
  81. 81. Welding Precipitation Hardened Alloys Without Allotropic Phase Changes Welded In: • Full Hard Condition • Solution Annealed Condition 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 81
  82. 82. A second way of strengthening materials without allotropic phase changes is by precipitation strengthening. (The first we just discussed was cold working). Recall that in precipitation strengthening, the base metal is solutionized, rapidly cooled and then aged at some moderately elevated temperature to promote precipitate formation. There are two ways that precipitation hardened material can be welded. One is to weld on the full hard, that is the already aged base metal. The second is to weld on material which has been solution annealed and rapidly cooled, but not yet given the ageing heat treatment. In either case, when welding, the heat affected zone will see some additional time at temperature (varied temperature over the distance of the HAZ) as illustrated above, and this will effect the aged or overaged condition of the precipitates. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 82
  83. 83. Annealed upon Cooling 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 83
  84. 84. When welding on the already aged (full hard) material, the unaffected base metal will have aged precipitates that are just the right size for strengthening. The heat affected zone, on the other hand, will experience some additional heating. In the region farthest from the weld the heat will be sufficient to overage the precipitates with the resulting loss in strength. In regions closer to the weld, the heat will be so excessive that the temperature will exceed the two phase region and the single phase solutionizing region on the phase diagram will be entered. Again, a loss in strength will occur, but this region at least might be able to be re- aged to recover some strength. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 84
  85. 85. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 85
  86. 86. Let us now turn our attention to the materials which do have an allotropic phase change during heating. A typical material like steel is ferrite at low temperatures and transforms to austenite when heated. Each time the material goes through one of these phase changes, new finer equaled grains grow starting from the grain boundaries of the previous grains present. So in the case of cold worked steels in the base metal, the elongated cold worked grains will undergo recovery, recrystalization and grain growth just as discussed above. But now the recrystallized grains at higher temperature will undergo the allotropic phase change, reducing the grain size again which then is followed by grain growth at still higher temperature (nearer the weld). This variation in grain structure is schematically shown in the lower figure above. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 86
  87. 87. Introductory Welding Metallurgy, AWS, 1979 Steel Alloys With Allotropic Transformation 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 87
  88. 88. This illustration shows the various regions in the heat effected zone and what microstructure would be predicted as related to the iron-carbon phase diagram. Note that at the far extent of the element in the base metal, ferrite and commentate arte expected. Closer to the weld some dual phase ferrite austenite will occur at temperature of welding. Closer yet we would expect single phase austenite, and then maybe some austenite of delta ferrite and liquid mixtures until at the maximum temperature the liquid phase would be present as the welding arc traverses. These are the structures at temperature, but we now must consider what happens during cooling. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 88
  89. 89. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 89
  90. 90. We have already seen that the cooling rate from welding can vary depending upon a number of weld variables. The two most important are preheat and heat input. The cooling rate is fastest when no preheat and low heat input are used to make the weld. On the other hand, the cooling rate is slowest when high preheat and high heat input are employed. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 90
  91. 91. Introductory Welding Metallurgy, AWS, 1979 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 91
  92. 92. As we have learned before, the cooing rate from austenite can effect the room temperature structure as defined by the continuous cooling transformation diagram. Rapid cooling results in non-equilibrium hard brittle martensite. Slow cooling results in some higher temperature transformation products such as bainite, ferrite and pearlite which tend to be softer. Examining two welding procedures here, one with no preheat (number 1) and the other with preheat (number 2) we find some differences in structure. The no preheat weld has a narrower HAZ and rapid cooling and the austenite transforms to martensite on cooling giving a hard martensite peak near the fusion line. The weld with preheat has a wider HAZ, a slower cooling rate producing ferrite pearlite and bainite and the fusion line peak is softer. There is also more outer HAZ region grain growth and overaging so that the softening in the HAZ is greater. Thus, once again, welding procedures have to be carefully tailored for the material being welded. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 92
  93. 93. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 93
  94. 94. How does the hydrogen get into the heat effected zone where the cold cracking is often observed? Liquid metal can absorb more hydrogen than solid austenite, and austenite more than ferrite. When welds are made on wet material or with wet electrodes, the hydrogen is absorbed into the liquid. As the liquid solidifies, if forces some of the hydrogen which it is trying to get rid of into the surrounding hot austenite. If there is still too much to be absorbed even in a supersaturated solid, some hydrogen porosity may form in the weld metal, a sure sign that poor procedures were followed. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 94
  95. 95. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 95
  96. 96. During cooling, the cooler material tries to push hydrogen out while at the same time the solidifying weld metal tries to push hydrogen out. Note that the large grained region of the HAZ which just may have the hardest most susceptible martensitic microstructure thus acquired hydrogen from both directions and a supersaturated condition exists there. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 96
  97. 97. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 97
  98. 98. The hydrogen then slowly diffuses to any location where is can relieve the stress of being stuck in the lattice in the supersaturated condition. The hydrogen atoms are often carried by dislocation and the preferred site for collection is often inclusions. At this point, they can either weaken the surrounding structure or the hydrogen atoms can recombine and form molecular hydrogen gas and exert an internal pressure. As this pressure grows, the crack slowly expands until a critical size is reached and catastrophic failure occurs. This takes time at low temperature , thus the common name of cold cracking or delayed cracking applies. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 98
  99. 99. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 99
  100. 100. The time after welding has an effect. As time proceeds, the hydrogen diffuses away from the high concentration in the most critical portion of the heat affected zone. If hydrogen diffuses away before the critical crack length is reach, the weld has occurrence of some micro cracks but catastrophic failure does not occur. On the other hand, if hydrogen diffusion is slower than that failure may occur. Elevated temperature post weld treatment will allow fast hydrogen diffusion and may help in the reduction of cold cracking. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 100
  101. 101. Dickinson5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 101
  102. 102. The above diagram summarizes the discussions about delayed cracking. The red regions are crack sensitive regions while the blue represents the safe region. Materials with high hardenabilty will promote the formation of martensite, and materials with high carbon content will produce a harder martensite. Increases in heat input and preheat and stress reliving practices increases the safety against hydrogen delayed cracking. And the decrease in hydrogen in the welding process likewise increases the safety region. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 102
  103. 103. Why Preheat? • Preheat reduces the temperature differential between the weld region and the base metal – Reduces the cooling rate, which reduces the chance of forming martensite in steels – Reduces distortion and shrinkage stress – Reduces the danger of weld cracking – Allows hydrogen to escape 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 1030.1.1.5.1.T9.95.12
  104. 104. Using Preheat to Avoid Hydrogen Cracking • If the base material is preheated, heat flows more slowly out of the weld region – Slower cooling rates avoid martensite formation • Preheat allows hydrogen to diffuse from the metal 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 104 Cooling rate T - Tbase)2 Steel Cooling rate T - Tbase)3 T base T base
  105. 105. Why Post-Weld Heat Treat? • The fast cooling rates associated with welding often produce martensite • During postweld heat treatment, martensite is tempered (transforms to ferrite and carbides) – Reduces hardness – Reduces strength – Increases ductility – Increases toughness • Residual stress is also reduced by the postweld heat treatment 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 105 Carbon and Low-Alloy Steels 0.1.1.5.1.T10.95.12
  106. 106. Postweld Heat Treatment and Hydrogen Cracking • Postweld heat treatment (~ 1200°F) tempers any martensite that may have formed – Increase in ductility and toughness – Reduction in strength and hardness • Residual stress is decreased by postweld heat treatment • Rule of thumb: hold at temperature for 1 hour per inch of plate thickness; minimum hold of 30 minutes 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 106 Steel
  107. 107. Base Metal Welding Concerns 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 107
  108. 108. Lamellar Tearing • Occurs in thick plate subjected to high transverse welding stress • Related to elongated non-metallic inclusions, sulfides and silicates, lying parallel to plate surface and producing regions of reduced ductility • Prevention by – Low sulfur steel – Specify minimum ductility levels in transverse direction – Avoid designs with heavy through-thickness direction stress 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 108 Cracking in Welds 0.1.1.5.2.T14.95.12
  109. 109. Improve Cleanliness Improve through thickness properties Buttering 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 109
  110. 110. This illustrates how the rolled out inclusions (mainly MnS) can de-bond from the base metal matrix and under the action of short transverse (through thickness) stresses they can actually link to form a stepped like fracture. Improving cleanliness of the steel during steel processing, and improving through thickness properties by steel making processed line calcium or rare earth treatment which produces inclusions which to not roll out a long stringer during plate processing can help. Also laying a weld bead on top of the plate which has lower strength and improved ductility before welding the attachment can help by letting the weld bead take the shrinkage stresses rather than transmitting them into the base plate. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 110
  111. 111. Multipass Welds • Heat from subsequent passes affects the structure and properties of previous passes – Tempering – Reheating to form austenite – Transformation from austenite upon cooling • Complex Microstructure. • In a multi-pass weld, the heating and cooling cycles of one pass are superimposed upon those of previous passes. Portions of previous passes are heated high enough to form austenite again, and upon cooling this austenite once again can transform to ferrite and pearlite or to martensite. Some portions of previous weld passes will not transform to austenite but will be tempered by the heat from subsequent passes. All in all, this leads to a rather complicated structure in multi-pass welds. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh.
  112. 112. Multipass Welds • Exhibit a range of microstructures • Variation of mechanical properties across joint • Postweld heat treatment tempers the structure – Reduces property variations across the joint 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 112 Steel
  113. 113. Reheat Cracking • Mo-V and Mo-B steels susceptible • Due to high temperature embrittlement of the heat-affected zone and the presence of residual stress • Coarse-grained region near fusion line most susceptible • Prevention by – Low heat input welding – Intermediate stress relief of partially completed welds – Design to avoid high restraint – Restrict vanadium additions to 0.1% in steels – Dress the weld toe region to remove possible areas of stress concentration5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 113 Cracking in Welds 0.1.1.5.2.T15.95.12
  114. 114. Steels containing molybdenum or vanadium resist creep at elevated-temperature. These steels, along with thick sections of high-strength, low-alloy steels, are subject to reheat cracking in combination with residual stress and low creep-ductility in the HAZ. During postweld heat treatment, cracks form along the grain boundaries in the HAZ, particularly in the coarse- grained region near the fusion line. Defects at the weld toe can promote reheat cracking; therefore, grinding or peening the weld toe can help prevent this cracking. The cracked area must be heat treated to restore ductility prior to repair. Then it can be cut out beyond the ends of the cracks and rewelded. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 114
  115. 115. Knife-Line Attack in the HAZ • Cr23C6 precipitate in HAZ – Band where peak temperature is 800- 1600°F • Can occur even in stabilized grades – Peak temperature dissolves titanium carbides – Cooling rate doesn’t allow them to form again 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 115 Weld HAZ Knife-line attack Stainless Steel
  116. 116. A discrete band in the heat affected zone of the austenitic stainless steel welds experiences peak temperatures in the 800°-1600°F temperature range associated with sensitization. Chromium carbide precipitation in this region can lower the chromium content near the grain boundaries to less than 12%, thereby causing sensitization. Stabilized grades can also suffer from knife-line attack. Elevated temperatures in the heat-affected zone can dissolve titanium and niobium carbides. The fast cooling rates in the welded joint do not allow these carbides to reform. This leaves excess free carbon, which can then form chromium carbides. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 116
  117. 117. WELDING FAULTS 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 117 Root Faults For deep vee multi run welds the first run or root weld is critical to the quality of the welds laying on top. Typical faults may be caused by too high or low a current of too large a rod.
  118. 118. Fusions Faults The three main causes of this is too low current for rod, too high a travel rate or when too small a rod is used on a cold surface. Bead Edge Defects normally in the form of under cutting or edge craters. The main cause for this is incorrect current setting. Too high will lead to undercutting, too low to edge craters. Similar efects may occur at the correct current due to incorrect arc length. Edge faults are particularly common in vertical welding or 'weave' welding. The general cause for the latter being a failure to pause at the extremes of the weave. Edge defects are stress raisers and lead to premature weld failure. Porosity May have many causes the most common being moisture in the rod coating or in the weld joint. Poor rod material selection is also a factor 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 118
  119. 119. Heat Cracks this is a destructive fault caused generally due to incompatiablity of the Weld material and weld Rod. Indeed in some cases the material may be deemed unweldable. Heat cracks occur during or just after the cooling off period and are caused by impurites in the base metal segrateing to form layers in the middle of the weld. The layers prevent fusion of the crystals. The two main substances causing this are Carbon and Sulphur. A switch to 'basic' electrodes may help. Anouther cause is temsion acroos the weld which , even without segregation in the weld, cause a crack. This occurs during a narrow critical temerpature range as the bead coagulates. During this period the deformation property is small, if the shrinkage of the base material is greater than the allowed stretch of the weld then a crack will result. One method of preventing this is to clamp the piece inducing a compressive force on the weld during the cooling period 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 119
  120. 120. Shrinkage Cracks Thes form due to similar effect of allowed weld deformation being less than base metal shrnkage although it is not associated with the critical temerpature rang above and therefore cannot be elleviated by compression. The use of 'basic' electrodes can help Hydrogen cracks This is generally associated what either hardened material or material hardened during the welding process. The hydrogen source can be moisture, oil, grease etc. Ensuring that the rod is dry is essential and preheating the weld joint to 50'C will help. The cracking occurs adjacent to the weld pool and allied to the tension created during the welding porcess will generate a through weld crack. Slag Inclusion This common fault is caused by insufficient cleaning of the weld between runs. If necessary as well as using a chipping hammer and brush grind back each weld run with an angle grinder. Once the slag is in the weld it is near impossible to removed it by welding only5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 120
  121. 121. Metallurgical Testing 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 121
  122. 122. Non-Destructive Testing - This is carried out on components rather than on test pieces, they are designed to indicate flaws occurring due or after manufacture. They give no indication of the mechanical properties of the material. - Surface flaws may be detected by visual means aided by dye penetrant or magnetic crack detection. - Internal flaws may be detected by X-ray or ultrasonic testing. - In addition to this there are special equipment able to exam machine finish. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 122
  123. 123. Liquid Penetrant Methods - The surface is first cleaned using an volatile cleaner and degreaser. - A fluorescent dye is then applied and a certain time allowed for it to enter any flaws under capillary action. Using the cleaning spray, the surface is then wiped clean. - - An ultra violet light is shone on the surface, any flaws showing up as the dye fluoresce. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 123
  124. 124. Dye penetrant method - The surface is cleaned and the low viscosity penetrant sprayed on. - After a set time the surface is again cleaned. - A developer is then used which coats the surface in a fine white chalky dust, then the dye seeps out and stains the developer typically a red colour. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 124
  125. 125. Magnetic crack detection - A component is place between two poles of a magnet. - The lines of magnetism concentrate around flaws. - Magnetic particles are then applied, in a light oil or dry sprayed, onto the surface where they indicate the lines of magnetism and any anomalies/ abnormalities like the below figures. Limitation of Magnetic test: This method of testing has a few limitations. - Firstly it cannot be used on materials which cannot be magnetised such as austenitic steel and non-ferrous metals. - Secondly it would not detect a crack which ran parallel to the lines of magnetism. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 125
  126. 126. 5/28/2015 126 Tensile Test
  127. 127. - When a material is tested under a tensile load, it changes shape by elongating. - Initially the extension is in proportion to the increasing tensile load. - If a graph is plotted showing extension for various loads, then a straight line is obtained at first. - If the loading is continued the graph, deviates as shown. - Within the limit of the straight line, if the load is removed the material will return to its original length which is elastic limit of the specimen. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 127 Tensile Testing Method
  128. 128. 128
  129. 129. - When the test piece reaches the Yield point (Yu), there is a failure of the crystalline structure of the metal, not along the grain boundaries as it has been the case, but through the grains themselves. This is known as “slip”. - A partial recovery is made at the lower yield point (YL), then the extension starts to increase. - If the load is removed at any stage along the “Load-Extension” curve after Yield Point (YL), the material will have a corresponding permanent deformation. This termed “permanent set”. - Maximum loading occurs at the “ultimate Load” (S). and after Yield Point (YL) to Ultimate Load (S) is the plastic limit. - Ultimate Load (S) to Breaking point (B) this stage local wasting or extension will start which termed “necking”. Normally this starts at about the centre of the specimen and will rapidly be followed by failure up to breaking point (B). 129 Tensile Testing Method
  130. 130. Proof Test 5/28/2015 130
  131. 131. Proof Stress: For a material which does not have a marked yield point such as Aluminium, there is a substitute stress specified. This is termed “the proof stress”. - Proof stress is determined from a load/extension or stress/strain graph. 131
  132. 132. Proof Testing Method • Hard steels and non-ferrous metals (Aluminium) do not have defined yield limit, therefore a stress, corresponding to a definite deformation, (0.1% or 0.2%) is commonly used instead of yield limit. This stress is called proof stress or offset yield limit (offset yield strength): • σ0.2%= F0.2% / S0 • The method of obtaining the proof stress is shown in the picture. • As the load increase, the specimen continues to undergo plastic deformation and at a certain stress value its cross-section decreases due to necking . 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 132
  133. 133. • At point S in the Stress-Strain Diagram the stress reaches the maximum value, which is called ultimate tensile strength (tensile strength): σt= FS / S0 • Continuation of the deformation results in breaking the specimen - the point B in the diagram (from Ultimate load S to breaking point B) • The actual Stress-Strain curve is obtained by taking into account the true specimen cross-section instead of the original value. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 133
  134. 134. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 134
  135. 135. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 135
  136. 136. Creep Testing - Creep tests are carried out under controlled temperature over an extended period of time in the order of 10,000hrs. - The test piece is similar to the type used for tensile tests and creep is usually thought of as being responsible for extensions of metal only. In fact creep can cause compression or other forms of deformation 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 136
  137. 137. - Temperature of the test is at recrystallization point around 400oC. For other metals the recrystallization temperature is different (200oC for copper and room temperature for tin and lead). - At the start of the test the initial load must be applied without shock. - This load, normally well below the strength limit of the material, will extend the test piece slowly. - The load is kept steady through the test and the temperature is maintained accurately. - Extension is plotted and is seen to proceed in three distinct stages. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 137
  138. 138. HARDNESS TESTING The basis of the Brinell hardness testing is the resistance to deformation of a surface by a loaded steel ball. Oil is pumped into the chamber between the pistons until there is sufficient pressure to raise the Weight so that it is floating. The ball is now forced into the specimen material at the same force. The loading for steel and metals of similar hardness is 3,000Kg. The load is allowed to act for 15 sec to ensure that plastic flow occurs. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 138
  139. 139. - The surface diameter of the indentation is measured with the aid of a microscope which is traversed over the test piece on a graduated slide with a vernier. - Cross wires in the microscope, enable the operator to accurately align the instrument. - Both the loading and ball diameter (10mm) are known, by measuring the indentation diameter the hardness can be calculated. For softer materials the loading is reduced, Copper being 1000Kg and Aluminium 500Kg. The diameter of the indentation must be less than half the ball diameter. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 139
  140. 140. - The thickness of the specimen must be not less than 10x the depth of the impression. The edge of the impression will tend to sink with the ball if the surface has been work hardened; otherwise the local deformation will tend to cause piling up of the metal around the indent If the hardness test is used on very hard materials, the steel ball will flatten. This method is not reliable for reading over 600. It is used in preference to other methods where the material has large crystals, e.g. Cast iron. Mild Steel 130, Cast Iron 200, white cast iron 400, nitrided surface 750. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 140
  141. 141. Under low temperature conditions , impact or shock loading on a material can cause cracking in a material which is normally ductile at room temperature. To find out the Critical stressing in a material, Griffith equation, sc = Kic / ж Pc where, sc = the critical stress in a material Kic = the fracture toughness of a material Pc = the micro-crack length within the materials 141 BRITTLE FRACTURE TESTING
  142. 142. BRITTLE FRACTURE TESTING - The presence of these micro cracks (porous materials or defects) can act to cause transcrystalline type failures with a bright crystalline appearance. - Testing is carried out via the Charpy notched piece test at various temperatures between -200o to +200oC - To reduce the effects of brittle fracture the carbon content in carbon steels is kept as low as practical. - Grains within the materials are kept as small as possible by heat treatment and normalizing. - Alloying elements may also be added. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 142
  143. 143. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 143
  144. 144. 144 Transition Temperatures • As temperature decreases a ductile material can become brittle - ductile-to-brittle transition – The transition temperature is the temp at which a material changes from ductile-to-brittle behavior • Alloying usually increases the ductile-to-brittle transition temperature. FCC metals remain ductile down to very low temperatures. For ceramics, this type of transition occurs at much higher temperatures than for metals.
  145. 145. Factors which affect the transition temperature are 1. Elements: - Carbon, silicon, phosphorus and sulphur raise the temperature. - Nickel and manganese lower the temperature. 2. Grain size: - the smaller the grain size the lower the transition temperature, hence grain refinement is beneficial. 3. Work hardening: - this appears to increase transition temperature. 4. Notches: - possibly occurring during assembly e.g. weld defects or machine marks. - Notches can increase tendency to brittle fracture. 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 145
  146. 146. 18/80 stainless steel 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 146 It is this property of stainless steel that makes it so suitable for use in LPG carriers. Hardness Testing
  147. 147. ANY QUESTION? THANK YOU! 5/28/2015 Mohd. Hanif Dewan, Chief Engineer and Maritime Lecturer & Trainer, Bangladesh. 147

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