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Heat treatment part 2

1. The document discusses various heat treatment processes for steels including annealing, normalizing, and hardening. 2. Annealing involves heating and slow cooling to soften steel by refining grain structure. Types include stress relief, spheroidizing, and full annealing. 3. Normalizing refines grain size by heating above the critical temperature and slow cooling in air. 4. Hardening increases hardness and wear resistance by heating and quenching in water or oil to form martensite.

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Prepared by Prof. Naman M. Dave Assistant Professor, Mechanical Engg. Dept. Gandhinagar Institute of Technology MATERIAL SCIENCE & METALLURGY 2131904 Chapter 8 Heat Treatment Processes
Please do not blindly follow the presentation files only, refer it just as reference material. More concentration should on class room work and text book-reference books.
Steels Can be heat treated to alter properties by either ... B) Heating and rapid cooling (Quenching) 1. Hardening 1. Full hardening / Through Hardening 2. Surface Hardening / Case Hardening / Selective Hardening (i) Nitriding (ii) Induction Hardening (iii) Carburising – (a) Liquid Carburising (b) Gas Carburising (c) Pack Carburising (iv) Cyaniding (v) Electron Beam Hardening (vi) Flame Hardening (vii) Laser Beam Hardening 2. Tempering 1. Low Temperature Tempering 2. Medium Temperature Tempering 3. High Temperature Tempering 3. Aus-tempering 4. Mar-tempering A) Heating and slow cooling 1. Annealing 1. Stress-relief annealing 2. Process annealing 3. Spheroidize annealing 4. Full annealing 5. Bright and Black Annealing 6. Iso-thermal annealing 2. Normalising Prof. Naman M. Dave
Prof. Naman M. Dave
Objectives / Purposes  Softening the steel  Refining grain structure  Improve mechanical, physical, electrical and magnetic properties  Relieving Internal Stresses  Improve machinability Heat Treatment Processes for Steels Annealing Prof. Naman M. Dave
Heating Temperature > Normally heating temperature for annealing is as under • Hypo-eutectoid Steels Upper Critical Temp. : (723-910OC) + 30-50OC • Eutectoid Steel Critical Temp. (723OC) + 30-50OC • Hyper-eutectoid Steels : Lower Critical Temp. (723OC) + 30-50OC However, some special annealing cycles (like spheroidise annealing and process annealing) are carried out below lower critical temperature also. Annealing Prof. Naman M. Dave
Holding Time 1 minute per mm of maximum thickness cross section OR 1/2 hr. Per 1 inch of maximum thickness cross section Cooling Rate : Very slow e.g. 25-30OC per hour. Refer CR1 shown in the T.T.T. Diagram here. > Cooling Medium : Cooling in furnace or heap of ashes or in an insulated chamber Final Micro-Structure Hypo-eutectoid Steel : Coarse Pearlite + Ferrite Eutectoid Steel : Coarse Pearlite Hyper-eutectoid Steel : Coarse Pearlite+Cementite Annealing Prof. Naman M. Dave
Types of Annealing There are different types of Annealing Processes. Some of the most widely used processes are… 1.Stress Relief Annealing 2.Process Annealing 3.Spheroidize Annealing 4.Full Annealing 5.Bright Annealing and Black Annealing 6.Isothermal Annealing Prof. Naman M. Dave
Stress Relief Annealing • Stress relief annealing process consists of three steps. • The first step is heating the cold worked steel to a temperature between 5000C and 5500C i.e. below its recrystallization temperature. • The second step involves holding the steel component at this temperature for 1-2 hours. • The final step is to cool the steel component to room temperature in air. • The stress relief annealing partly relieves the internal stress in cold worked steels without loss of strength and hardness i.e. without change in the microstructure. • It reduces the risk of distortion while machining, and increases corrosion resistance. Since only low carbon steels can be cold worked, the process is applicable to hypo eutectoid steels containing less than 0.4% carbon. This annealing process is also used on components to relieve internal stresses developed from rapid cooling and phase changes. Annealing Prof. Naman M. Dave
Spheroidise Annealing • Spheroidise annealing is one of the variant of the annealing process that produces typical microstructure consisting of the globules (spheroid) of cementite or carbides in the matrix of ferrite. The following methods are used for spheroidise annealing. • Holding at just below A1:Holding the steel component at just below the lower critical temperature (A1) transforms the pearlite to globular cementite particles. But this process is very slow and requires more time for obtaining spheroidised structure.
Spheroidise Annealing • Thermal cycling around A1: In this method, the thermal cycling in the narrow temperature range around A1 transforms cementite lamellae from pearlite to spheroidal. Figure depicts a typical heat treatment cycle to produce spheroidised structure. During heating above A1, cementite or carbides try to dissolve and during cooling they try to re-form. This repeated action spheroidises the carbide particles. Spheroidised structures are softer than the fully annealed structures and have excellent machinability. This heat treatment is utilized to high carbon and air hardened alloy steels to soften them and to increase machinability, and to reduce the decarburization while hardening of thin sections such as safety razor blades and needles.
Recrystallization or Process Annealing • Recrystallization annealing process consists of heating a steel component below A1 temperature i.e. at temperature between 6250C and 6750C (recrystallization temperature range of steel), holding at this temperature and subsequent cooling. • This type of annealing is applied either before cold working or as an intermediate operation to remove strain hardening between multi- step cold working operations. In certain case, recrystallization annealing may also be applied as final heat treatment. • The cold worked ferrite recrystallizes and cementite tries to spheroids during this annealing process. Recrystallization annealing relieves the internal stresses in the cold worked steels and weldments, and improves the ductility and softness of the steel. Refinement in grain size is also possible by the control of degree of cold work prior to annealing or by control of annealing temperature and time. Annealing Prof. Naman M. Dave
Full Annealing • Full annealing process consists of three steps. First step is heating the steel component to above A3 (upper critical temperature for ferrite) temperature for hypoeutectoid steels and above A1 (lower critical temperature) temperature for hypereutectoid steels by 30-500C (Figures 1 and 2). In Figure 2, the terms α, γ and Fe3C refer to ferrite, austenite and cementite phases. • The second step is holding the steel component at this temperature for a definite holding (soaking) period of at least 20 minutes per cm of the thick section to assure equalization of temperature throughout the cross-section of the component and complete austenization. Final step is to cool the hot steel component to room temperature slowly in the furnace, which is also called as furnace cooling. The full annealing is used to relieve the internal stresses induced due to cold working, welding, etc, to reduce hardness and increase ductility, to refine the grain structure, to make the material homogenous in respect of chemical composition, to increase uniformity of phase distribution, and to increase machinability. Annealing
Isothermal Annealing • Isothermal annealing consists of four steps. The first step is heating the steel components similar as in the case of full annealing. The second step is slightly fast cooling from usual austenitizing temperature to a constant temperature just below A1. Third step is to hold at this reduced temperature for sufficient soaking period for the completion of transformation and the final step involves cooling the steel component to room temperature in air. Fig. depicts heat treatment cycles of full annealing and isothermal annealing. The terms α, γ, P, PS and PF. • Reduced annealing time, especially for alloy steels which need very slow cooling to obtain the required reduction in hardness by the full annealing. • More homogeneity in structure is obtained as the transformation occurs at the same time throughout the cross section. • Improved machinability and surface finish is obtained after machining as compared to that of the full annealed components. • Isothermal annealing is primarily used for medium carbon, high carbon and some of the alloy steels to improve their machinability. Annealing
1. Stress Relief Annealing • Heating up to a temperature of 500-550 OC • Iso-thermal holding • Cooling in still air up to room temperature 2. Process Annealing • Heating up to a temperature of 600-650 OC • Iso-thermal holding • Cooling in still air up to room temperature 3. Spheroidize Annealing • This Annealing process is used to convert the carbides in a steel into globular form • Heating & cooling alternately in the temperature range of 650- 700 OC • Iso-thermal holding • Very Slow Cooling (furnace cooling) up to room temperature Annealing
4. Full Annealing • Heating up to UCT (723-910 OC) + 30-50 OC for hypo-eutectoid steels (less than 0.8% C) & • LCT (723 OC) + 30-50 OC for eutectoid and hyper-eutectoid steels (more than 0.8% C) • Iso-thermal holding • Very Slow Cooling (furnace cooling or insulated chamber cooling) up to room temperature 5. Bright Annealing and Black Annealing • Annealing process carried out in protective atmosphere prevents dis-colouration of the steel. This process is called as Bright Annealing. • When components to be annealed are surrounded by reducing agents like charcoal and annealed in a box then oxidation of these components is reduced. This process is Box or Black annealing. Prof. Naman M. Dave Annealing
6. Isothermal Annealing • Medium or High Carbon Steels have their T-T-T curves positioned substantially away from temperature axis. This demands pro-longed annealing cycles. • The steels are fast cooled up to the subcritical i.e. below A1 and then held isothermally there until the transformation is completed. Subsequently these steels are cooled to room temperatures. This saves cycle time substantially. Prof. Naman M. Dave Annealing
Temperature(OC) Time 550 500 (1) Stress Relief Annealing Range (2) Process Annealing Range (3) Spheroidize Annealing Range 25 960 LCT 600 UCT-1910 (4) Full Annealing Range 763 723 700 650 UCT-21130 Prof. Naman M. Dave Annealing
Objectives / Purposes  Softening the steel  Eliminate carbide network at grain boundaries  Relieving Internal Stresses  Refining grain structure  Improve mechanical, physical, electrical and magnetic properties Heat Treatment Processes for Steels Normalizing Prof. Naman M. Dave
Heating Temperature > • Hypo-eutectoid Steels: Upper Critical Temp. (723-910OC) + 50-70OC • Eutectoid Steel: Critical Temp. (723OC) + 50-70OC • Hyper-eutectoid Steels: Upper Critical Temp. (723-1130OC) + 50-70OC Normalizing Prof. Naman M. Dave
Holding Time About 15 minutes Cooling Rate : Slow e.g. 50-100OC per hour. Refer CR3 shown in the T.T.T. Diagram here. > Cooling Medium : Cooling in still ambient air Final Micro-Structure: • Hypo-eutectoid Steel: Fine Pearlite + Ferrite • Eutectoid Steel: Fine Pearlite • Hyper-eutectoid Steel: Fine Pearlite + Cementite Prof. Naman M. Dave Normalizing
910 Temperature(OC) Time 763 25 LCT UCT-1 1130 Normalising Range UCT-2 723 1180 Prof. Naman M. Dave Normalizing
Objectives / Purposes 1. To increase the hardness 2. To increase the wear resistance Heating Temperature > • Hypo-eutectoid Steels: Upper Critical Temp. (723-910OC) + 10-30OC • Eutectoid Steel: Critical Temp. (723OC) + 10-30OC • Hyper-eutectoid Steels: Lower Critical Temp. (723OC) + 10-30OC Heat Treatment Processes for Steels Hardening Hardening
Holding Time • 1 minute per mm of maximum thickness cross section OR • 1/2 hr. Per 1 inch of maximum thickness cross section Cooling Rate > • Faster than Critical Cooling Rate. For e.g. If CCR is 250OC per hour then we • have to cool at say 250 + 30 to 50 i.e. 280-300OC. In other words the • temperature drop per hour should be more than 250 OC. Prof. Naman M. Dave Hardening
Cooling Medium : Quenching medium are many. For e.g. 1) 5-10% Caustic Soda (Very Drastic Quench) 2) 5-20% Brine (NaCl) 3) Cold Water 4) Warm Water 5) Mineral oil 6) Animal oil 7) Vegetable oil 8) Air Normally water is recommended for quenching of plain carbon steels and oil is recommended for quenching alloy steels Prof. Naman M. Dave
Final Micro-Structure >  Steels with C% below 0.3% do not harden.  For Steels with C% higher than 0.3%, the final microstructure will be Martensite + Retained Austenite + Carbides.  Relative amount of these phases will depend on composition of Steel, hardening temperature, soaking time and temperature of quenching medium.  For e.g. If the cooling rate becomes slower than critical cooling rate then it can result in formation of pearlite and bainite also. Needle type micro-structure indicating Martensite, white coloured structures are a mixture of carbides and retained austenite Prof. Naman M. Dave
27 910 Temperature(OC) Time 763 25 LCT UCT-1 1130 Hardening Range UCT-2 723 960 Prof. Naman M. Dave Hardening
Hardenability of Steels  Hardness is a measure of resistance to wear.  Hardening is done to improve the hardness of steel i.e. To improve the wear resistance of steel.  Hardenability of steel means the depth and distribution of hardness attained by quenching. It is defined in following ways : 1. It is ease with which the steel can be hardened or ability of steel to get hardened. (Qualitative definition) 2. It is the length of the test bar up to which 50% martensite is achieved OR It is the length of the test bar up to which 50 RC hardness is achieved. (Quantitative definition).  It is noted by a notation like this : J50 = 5. It means 50 RC hardness can be obtained in the test bar upto 5/16 inch length from the quenched end.  For testing the hardenability of a steel, there is a test called Jominy Quench Test. This will be explained in lab.  All steels are not hardenable. For e.g. low C steels are not hardenable. This is due to their low Carbon content. Hardenability Prof. Naman M. Dave
29 Note : Only the alloys highlighted in blue colour can be full hardened Hardenability Prof. Naman M. Dave
Factors affecting hardenability are as under...  Alloying content : Higher alloying means good hardenability  Homogeneity of Austenite : Higher homogeneity means good hardenability  Grain size of Austenite : Higher the size better is the hardenability  Presence of un-dissolved carbides : Lesser carbides better hardenability  Section of steel : Smaller the section better is the hardenability  Quenching medium : Selecting the quenching medium as per the size of the casting will give better results for full hardening. Hardenability
Objectives / Purposes 1. To reduce hardness 2. To eliminate retained austenite 3. To relieve internal stresses induced by quenching 4. To improve toughness and ductility 5. To improve other mechanical properties Heating Temperature > Low temperature tempering : 150-250OC Med. temperature tempering : 350-450OC High temperature tempering : 500-650OC Heat Treatment Processes for Steels Tempering Tempering Prof. Naman M. Dave
Holding Time : • Low temperature tempering cycle : Heating results in formation of low C martensite from Martensite. Hence holding time depends on Martensite content in the micro-structure (after hardening). Holding should be done untill C% in Martensite decreases and reaches the value of 0.3%. Tempering Cooling Rate : -> Slower than hardening See the curve shown on T.T.T. Dig. Here • Medium temperature tempering cycle : Heating results in formation of Bainite from retained austenite. Hence holding time depends on Retained Austenite (RA) content in the micro-structure (after hardening). The holding should be done untill all the RA converts to Bainite. • High temperature tempering cycle : Heating results in formation of ferrite from Martensite. Hence holding time depends on Martensite content in the micro-structure (after hardening). The holding should be done untill all Martensite converts to ferrite by losing carbon.
Tempered Martensite (Black colour) + Carbides (White colour) + Retained Austenite (White colour). Cooling Medium : Cooling in air Final Micro-structure : -> Tempered Martensite (having lower C% than Martensite formed after hardening) + ‘ε’ or Epsilon Carbide (Fe2.4C) (having higher C% than Cementite i.e. Fe3C + Retained Austenite. The quantity of retained austenite after tempering is lesser than before. Tempering <-Tempering Colors and Temperatures
Surface Hardening Concept Surface - Pearlite + Cementite - High Hardness - Highly Wear Resistant Core - Ferrite - Low hardness - Ductile - Capable of withstanding stress Cross section of a case hardened gear teeth  The Carbon content in the steel determines whether it can be directly hardened or not. If the Carbon content is low (for example less than 0.3%) then an alternate means exists to increase the Carbon content of the surface. The part then can be heat-treated by either quenching in liquid or cooling in still air depending on the properties desired.  Note that this method will only allow hardening on the surface, but not in the core, because the high carbon content is only on the surface. This is sometimes very desirable because it allows for a hard surface with good wear properties (as on gear teeth or knife), but has a tough core that will perform well under impact loading.  It is also possible to add additional carbon or chromium or boron to the outer surface of a component low in carbon which will make the surface sufficiently hard.  This process is known as case hardening or surface hardening or selective hardening. Prof. Naman M. Dave
I. Thermal Treatments 1) Induction Hardening 2) Flame Hardening 3) Laser Beam Hardening* 4) Electron Beam Hardening* II. Thermo-Chemical Treatments 1) Nitriding a) Gas Nitriding b) Liquid Nitriding 2) Carburizing* a) Solid / Pack Carburizing* b) Gas Carburizing* c) Liquid Carburizing (Cyaniding) 3) Carbo-nitriding* 4) Chroming* 5) Boronizing* Surface Hardening Concept Prof. Naman M. Dave
36 Thermal Treatment Induction Hardening  In Induction hardening, the steel part is placed inside a electrical coil which has alternating current through it. This induces Eddy Current on the outer surface of the component and heats it up.  Depending on the amperage of current, frequency of current and heating time depth of hardening can be controlled .  Temperature is around 750-800OC.  The heated region is then quenched by water jets to achieve the desired hardness.  Tempering can be done to eliminate brittleness.  Maximum 6 mm of depth can be achieved in about 6 seconds time.  Example of application : Gear teeth hardening Coil Work Piece Work Piece (Gear) Coil Prof. Naman M. Dave
Flame Hardening  A high intensity oxy-acetylene flame is applied to the selective region. The temperature is raised high enough to be in the region of Austenite transformation.  The "right" temperature is determined by the operator based on experience by watching the color of the steel. Flame hardening of a flat cross section work piece Flame hardening of a circular cross section work piece using lathe (MOVING) (STATIONARY) (Stationary) Work piece (Rotating) Thermal Treatment Prof. Naman M. Dave
 The overall heat transfer is limited by the torch and thus the interior never reaches the high temperature.  The heated region is quenched to achieve the desired hardness.  Tempering can be done to eliminate brittleness.  The depth of hardening can be increased by increasing the heating time.  Maximum 6 mm of depth can be achieved. A gear teeth being surface hardened using flame hardening method Thermal Treatment Flame Hardening Prof. Naman M. Dave
Flame Hardening  Large parts, which will not normally fit in a furnace, can be heat-treated using this method.  Example of application : Lathe Bed Hardening, Large Gears’ Teeth hardening, Large Sprockets’ teeth hardening, etc. Thermal Treatment Prof. Naman M. Dave
Thermo-chemical Treatment Nitriding  Introduction of nitrogen into the outer surface of steel parts in order to give an extremely hard, wear resisting surface is called as nitriding or nitrogen hardening.  Nitride compounds precipitate out during one of the following processes a) Gas nitriding - heat in ammonia b) Liquid nitriding - dip in molten cyanide bath  A schematic layout of gas nitriding plant is shown in the next slide. Prof. Naman M. Dave
41 Gas Nitriding  The part to be nitrided is placed in a box made of heat resistant steel. This box also has provision for inlet and outlet of ammonia gas (2 holes drilled on the walls – one for inlet of ammonia gas and another for outlet of ammonia gas)  The box (along with the component) is then placed in the furnace and heated to around 450- 550OC.  Then a steady flow of Ammonia gas is injected in the box.  On coming in contact with the steel, the ammonia vapour gets dissociated and nascent nitrogen is released. This nitrogen combines with other elements in outer surface of steel like C, Si, Mn, Ni, Cr, Mo, etc and forms nitrides which are extremely hard. After that, quenching is done.  Case thicknesses are between 0.5 to 0.8 mm with hardness up to 70 RC. Thermo-chemical Treatment Prof. Naman M. Dave
Solid Carburizing or Pack carburizing  The component is placed in a Cast Iron box along with a carbon rich material (e.g. charcoal). The box is placed in a furnace and heated to a temperature around 900-950 oC and maintained at that temperature for some period (1 hr for 0.1 mm depth).  The carbon is absorbed into the austenite on the surface of the component.  The depth of penetration of carbon into the surface depends on the temperature and the time spent in the furnace.  After enriching the surface with carbon, oil quenching is done. Carburizing  Low-carbon steel is heated in a carbon-rich environment and then quenched.  Depth achieved by carburizing is 0.025 to 4 mm with hardness around 60 RC. Thermo-chemical Treatment
43 Gas carburizing 1. A mild carburizing gas is generated from LPG (Liquified Petroleum Gas). This gas is fed into the furnace containing the heated steel component. 2. Another method of gas carburizing is called liquid feed or drip feed method. In this method, an organic liquid such as a mixture of iso-propyl or methyl alcohol with benzene is dripped directly into the furnace for generating a specific furnace atmosphere. • In any case, the gas mixture breaks to form an atmosphere containing N2, Ha, CH4, CO2, and H2 (due to heat). The component to be carburized is heated externally and made to travel through this atmosphere of the furnace and thus their surface becomes rich in Carbon content. It is then quenched to form a hard external surface whereas the core remains soft and ductile. The depth depends upon time and temperature of exposure to carbon rich atmosphere. HIGH FREQUENCY ELECTRIC CURRENT GAS-CARBURISING UNIT Gas Carburizing is conceptually the same as pack carburizing, except that gas is supplied to a heated component through the furnace. Thermo-chemical Treatment
44 Liquid Carburizing (Cyaniding)  The steel parts are immersed in a molten carbon rich bath maintained at 850-950OC for periods ranging from 15 minutes to 3 hours.  Sometimes the heating / holding can be intermittent i.e. there can be two baths – one at 850-950OC and another at 760OC. This done so as to minimize the distortion of shape of product during heat treatment. After that quenching is done.  This process produces a thin, hard shell that is harder than the one produced by other carburizing methods, and can be completed in 3 hours compared to several hours as in other methods so the parts have less opportunity to become distorted.  Sketch on the left illustrates an electric salt bath furnace used for cyaniding.  It is typically used on small parts such as bolts, nuts, screws and small gears.  The major drawback of cyaniding is that cyanide salts are poisonous. Therefore, safety concerns have led to non-toxic baths that achieve the same result. ELECTRIC SALT BATH FURNACE Thermo-chemical Treatment
Carbo-nitriding  Use both carbon and nitrogen – hardness around 70 RC – maximum depth of hardened surface 0.07 – 0.5 mm Chromizing  Pack or dip in chromium-rich material - adds heat and wear resistance Boronizing  Pack or dip in boron-rich material - Improves abrasion resistance, coefficient of friction Thermo-chemical Treatment Prof. Naman M. Dave
Sr. No. Thermo- chemical Treatment Hardness of Surface Depth of Hardened Layer from Surface 1. Nitriding 70 RC 0.5 – 0.8 mm 2. Carburizing 60 RC 0.025 - 4 mm 3. Carbo- Nitriding 70 RC 0.07 – 0.5 mm 4. Chromizing 70 RC - 5. Boronizing 70 RC - Comparison of different thermo-chemical Treatment Thermo-chemical Treatment Prof. Naman M. Dave
Thank You

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Heat treatment part 2

  • 1.
    Prepared by Prof. NamanM. Dave Assistant Professor, Mechanical Engg. Dept. Gandhinagar Institute of Technology MATERIAL SCIENCE & METALLURGY 2131904 Chapter 8 Heat Treatment Processes
  • 2.
    Please do notblindly follow the presentation files only, refer it just as reference material. More concentration should on class room work and text book-reference books.
  • 3.
    Steels Can beheat treated to alter properties by either ... B) Heating and rapid cooling (Quenching) 1. Hardening 1. Full hardening / Through Hardening 2. Surface Hardening / Case Hardening / Selective Hardening (i) Nitriding (ii) Induction Hardening (iii) Carburising – (a) Liquid Carburising (b) Gas Carburising (c) Pack Carburising (iv) Cyaniding (v) Electron Beam Hardening (vi) Flame Hardening (vii) Laser Beam Hardening 2. Tempering 1. Low Temperature Tempering 2. Medium Temperature Tempering 3. High Temperature Tempering 3. Aus-tempering 4. Mar-tempering A) Heating and slow cooling 1. Annealing 1. Stress-relief annealing 2. Process annealing 3. Spheroidize annealing 4. Full annealing 5. Bright and Black Annealing 6. Iso-thermal annealing 2. Normalising Prof. Naman M. Dave
  • 4.
  • 5.
    Objectives / Purposes Softening the steel  Refining grain structure  Improve mechanical, physical, electrical and magnetic properties  Relieving Internal Stresses  Improve machinability Heat Treatment Processes for Steels Annealing Prof. Naman M. Dave
  • 6.
    Heating Temperature > Normallyheating temperature for annealing is as under • Hypo-eutectoid Steels Upper Critical Temp. : (723-910OC) + 30-50OC • Eutectoid Steel Critical Temp. (723OC) + 30-50OC • Hyper-eutectoid Steels : Lower Critical Temp. (723OC) + 30-50OC However, some special annealing cycles (like spheroidise annealing and process annealing) are carried out below lower critical temperature also. Annealing Prof. Naman M. Dave
  • 7.
    Holding Time 1 minuteper mm of maximum thickness cross section OR 1/2 hr. Per 1 inch of maximum thickness cross section Cooling Rate : Very slow e.g. 25-30OC per hour. Refer CR1 shown in the T.T.T. Diagram here. > Cooling Medium : Cooling in furnace or heap of ashes or in an insulated chamber Final Micro-Structure Hypo-eutectoid Steel : Coarse Pearlite + Ferrite Eutectoid Steel : Coarse Pearlite Hyper-eutectoid Steel : Coarse Pearlite+Cementite Annealing Prof. Naman M. Dave
  • 8.
    Types of Annealing Thereare different types of Annealing Processes. Some of the most widely used processes are… 1.Stress Relief Annealing 2.Process Annealing 3.Spheroidize Annealing 4.Full Annealing 5.Bright Annealing and Black Annealing 6.Isothermal Annealing Prof. Naman M. Dave
  • 9.
    Stress Relief Annealing •Stress relief annealing process consists of three steps. • The first step is heating the cold worked steel to a temperature between 5000C and 5500C i.e. below its recrystallization temperature. • The second step involves holding the steel component at this temperature for 1-2 hours. • The final step is to cool the steel component to room temperature in air. • The stress relief annealing partly relieves the internal stress in cold worked steels without loss of strength and hardness i.e. without change in the microstructure. • It reduces the risk of distortion while machining, and increases corrosion resistance. Since only low carbon steels can be cold worked, the process is applicable to hypo eutectoid steels containing less than 0.4% carbon. This annealing process is also used on components to relieve internal stresses developed from rapid cooling and phase changes. Annealing Prof. Naman M. Dave
  • 10.
    Spheroidise Annealing • Spheroidiseannealing is one of the variant of the annealing process that produces typical microstructure consisting of the globules (spheroid) of cementite or carbides in the matrix of ferrite. The following methods are used for spheroidise annealing. • Holding at just below A1:Holding the steel component at just below the lower critical temperature (A1) transforms the pearlite to globular cementite particles. But this process is very slow and requires more time for obtaining spheroidised structure.
  • 11.
    Spheroidise Annealing • Thermalcycling around A1: In this method, the thermal cycling in the narrow temperature range around A1 transforms cementite lamellae from pearlite to spheroidal. Figure depicts a typical heat treatment cycle to produce spheroidised structure. During heating above A1, cementite or carbides try to dissolve and during cooling they try to re-form. This repeated action spheroidises the carbide particles. Spheroidised structures are softer than the fully annealed structures and have excellent machinability. This heat treatment is utilized to high carbon and air hardened alloy steels to soften them and to increase machinability, and to reduce the decarburization while hardening of thin sections such as safety razor blades and needles.
  • 12.
    Recrystallization or ProcessAnnealing • Recrystallization annealing process consists of heating a steel component below A1 temperature i.e. at temperature between 6250C and 6750C (recrystallization temperature range of steel), holding at this temperature and subsequent cooling. • This type of annealing is applied either before cold working or as an intermediate operation to remove strain hardening between multi- step cold working operations. In certain case, recrystallization annealing may also be applied as final heat treatment. • The cold worked ferrite recrystallizes and cementite tries to spheroids during this annealing process. Recrystallization annealing relieves the internal stresses in the cold worked steels and weldments, and improves the ductility and softness of the steel. Refinement in grain size is also possible by the control of degree of cold work prior to annealing or by control of annealing temperature and time. Annealing Prof. Naman M. Dave
  • 13.
    Full Annealing • Fullannealing process consists of three steps. First step is heating the steel component to above A3 (upper critical temperature for ferrite) temperature for hypoeutectoid steels and above A1 (lower critical temperature) temperature for hypereutectoid steels by 30-500C (Figures 1 and 2). In Figure 2, the terms α, γ and Fe3C refer to ferrite, austenite and cementite phases. • The second step is holding the steel component at this temperature for a definite holding (soaking) period of at least 20 minutes per cm of the thick section to assure equalization of temperature throughout the cross-section of the component and complete austenization. Final step is to cool the hot steel component to room temperature slowly in the furnace, which is also called as furnace cooling. The full annealing is used to relieve the internal stresses induced due to cold working, welding, etc, to reduce hardness and increase ductility, to refine the grain structure, to make the material homogenous in respect of chemical composition, to increase uniformity of phase distribution, and to increase machinability. Annealing
  • 14.
    Isothermal Annealing • Isothermalannealing consists of four steps. The first step is heating the steel components similar as in the case of full annealing. The second step is slightly fast cooling from usual austenitizing temperature to a constant temperature just below A1. Third step is to hold at this reduced temperature for sufficient soaking period for the completion of transformation and the final step involves cooling the steel component to room temperature in air. Fig. depicts heat treatment cycles of full annealing and isothermal annealing. The terms α, γ, P, PS and PF. • Reduced annealing time, especially for alloy steels which need very slow cooling to obtain the required reduction in hardness by the full annealing. • More homogeneity in structure is obtained as the transformation occurs at the same time throughout the cross section. • Improved machinability and surface finish is obtained after machining as compared to that of the full annealed components. • Isothermal annealing is primarily used for medium carbon, high carbon and some of the alloy steels to improve their machinability. Annealing
  • 15.
    1. Stress ReliefAnnealing • Heating up to a temperature of 500-550 OC • Iso-thermal holding • Cooling in still air up to room temperature 2. Process Annealing • Heating up to a temperature of 600-650 OC • Iso-thermal holding • Cooling in still air up to room temperature 3. Spheroidize Annealing • This Annealing process is used to convert the carbides in a steel into globular form • Heating & cooling alternately in the temperature range of 650- 700 OC • Iso-thermal holding • Very Slow Cooling (furnace cooling) up to room temperature Annealing
  • 16.
    4. Full Annealing •Heating up to UCT (723-910 OC) + 30-50 OC for hypo-eutectoid steels (less than 0.8% C) & • LCT (723 OC) + 30-50 OC for eutectoid and hyper-eutectoid steels (more than 0.8% C) • Iso-thermal holding • Very Slow Cooling (furnace cooling or insulated chamber cooling) up to room temperature 5. Bright Annealing and Black Annealing • Annealing process carried out in protective atmosphere prevents dis-colouration of the steel. This process is called as Bright Annealing. • When components to be annealed are surrounded by reducing agents like charcoal and annealed in a box then oxidation of these components is reduced. This process is Box or Black annealing. Prof. Naman M. Dave Annealing
  • 17.
    6. Isothermal Annealing •Medium or High Carbon Steels have their T-T-T curves positioned substantially away from temperature axis. This demands pro-longed annealing cycles. • The steels are fast cooled up to the subcritical i.e. below A1 and then held isothermally there until the transformation is completed. Subsequently these steels are cooled to room temperatures. This saves cycle time substantially. Prof. Naman M. Dave Annealing
  • 18.
    Temperature(OC) Time 550 500 (1) StressRelief Annealing Range (2) Process Annealing Range (3) Spheroidize Annealing Range 25 960 LCT 600 UCT-1910 (4) Full Annealing Range 763 723 700 650 UCT-21130 Prof. Naman M. Dave Annealing
  • 19.
    Objectives / Purposes Softening the steel  Eliminate carbide network at grain boundaries  Relieving Internal Stresses  Refining grain structure  Improve mechanical, physical, electrical and magnetic properties Heat Treatment Processes for Steels Normalizing Prof. Naman M. Dave
  • 20.
    Heating Temperature > •Hypo-eutectoid Steels: Upper Critical Temp. (723-910OC) + 50-70OC • Eutectoid Steel: Critical Temp. (723OC) + 50-70OC • Hyper-eutectoid Steels: Upper Critical Temp. (723-1130OC) + 50-70OC Normalizing Prof. Naman M. Dave
  • 21.
    Holding Time About 15minutes Cooling Rate : Slow e.g. 50-100OC per hour. Refer CR3 shown in the T.T.T. Diagram here. > Cooling Medium : Cooling in still ambient air Final Micro-Structure: • Hypo-eutectoid Steel: Fine Pearlite + Ferrite • Eutectoid Steel: Fine Pearlite • Hyper-eutectoid Steel: Fine Pearlite + Cementite Prof. Naman M. Dave Normalizing
  • 22.
  • 23.
    Objectives / Purposes 1.To increase the hardness 2. To increase the wear resistance Heating Temperature > • Hypo-eutectoid Steels: Upper Critical Temp. (723-910OC) + 10-30OC • Eutectoid Steel: Critical Temp. (723OC) + 10-30OC • Hyper-eutectoid Steels: Lower Critical Temp. (723OC) + 10-30OC Heat Treatment Processes for Steels Hardening Hardening
  • 24.
    Holding Time • 1minute per mm of maximum thickness cross section OR • 1/2 hr. Per 1 inch of maximum thickness cross section Cooling Rate > • Faster than Critical Cooling Rate. For e.g. If CCR is 250OC per hour then we • have to cool at say 250 + 30 to 50 i.e. 280-300OC. In other words the • temperature drop per hour should be more than 250 OC. Prof. Naman M. Dave Hardening
  • 25.
    Cooling Medium : Quenchingmedium are many. For e.g. 1) 5-10% Caustic Soda (Very Drastic Quench) 2) 5-20% Brine (NaCl) 3) Cold Water 4) Warm Water 5) Mineral oil 6) Animal oil 7) Vegetable oil 8) Air Normally water is recommended for quenching of plain carbon steels and oil is recommended for quenching alloy steels Prof. Naman M. Dave
  • 26.
    Final Micro-Structure > Steels with C% below 0.3% do not harden.  For Steels with C% higher than 0.3%, the final microstructure will be Martensite + Retained Austenite + Carbides.  Relative amount of these phases will depend on composition of Steel, hardening temperature, soaking time and temperature of quenching medium.  For e.g. If the cooling rate becomes slower than critical cooling rate then it can result in formation of pearlite and bainite also. Needle type micro-structure indicating Martensite, white coloured structures are a mixture of carbides and retained austenite Prof. Naman M. Dave
  • 27.
  • 28.
    Hardenability of Steels Hardness is a measure of resistance to wear.  Hardening is done to improve the hardness of steel i.e. To improve the wear resistance of steel.  Hardenability of steel means the depth and distribution of hardness attained by quenching. It is defined in following ways : 1. It is ease with which the steel can be hardened or ability of steel to get hardened. (Qualitative definition) 2. It is the length of the test bar up to which 50% martensite is achieved OR It is the length of the test bar up to which 50 RC hardness is achieved. (Quantitative definition).  It is noted by a notation like this : J50 = 5. It means 50 RC hardness can be obtained in the test bar upto 5/16 inch length from the quenched end.  For testing the hardenability of a steel, there is a test called Jominy Quench Test. This will be explained in lab.  All steels are not hardenable. For e.g. low C steels are not hardenable. This is due to their low Carbon content. Hardenability Prof. Naman M. Dave
  • 29.
    29 Note : Only thealloys highlighted in blue colour can be full hardened Hardenability Prof. Naman M. Dave
  • 30.
    Factors affecting hardenabilityare as under...  Alloying content : Higher alloying means good hardenability  Homogeneity of Austenite : Higher homogeneity means good hardenability  Grain size of Austenite : Higher the size better is the hardenability  Presence of un-dissolved carbides : Lesser carbides better hardenability  Section of steel : Smaller the section better is the hardenability  Quenching medium : Selecting the quenching medium as per the size of the casting will give better results for full hardening. Hardenability
  • 31.
    Objectives / Purposes 1.To reduce hardness 2. To eliminate retained austenite 3. To relieve internal stresses induced by quenching 4. To improve toughness and ductility 5. To improve other mechanical properties Heating Temperature > Low temperature tempering : 150-250OC Med. temperature tempering : 350-450OC High temperature tempering : 500-650OC Heat Treatment Processes for Steels Tempering Tempering Prof. Naman M. Dave
  • 32.
    Holding Time : •Low temperature tempering cycle : Heating results in formation of low C martensite from Martensite. Hence holding time depends on Martensite content in the micro-structure (after hardening). Holding should be done untill C% in Martensite decreases and reaches the value of 0.3%. Tempering Cooling Rate : -> Slower than hardening See the curve shown on T.T.T. Dig. Here • Medium temperature tempering cycle : Heating results in formation of Bainite from retained austenite. Hence holding time depends on Retained Austenite (RA) content in the micro-structure (after hardening). The holding should be done untill all the RA converts to Bainite. • High temperature tempering cycle : Heating results in formation of ferrite from Martensite. Hence holding time depends on Martensite content in the micro-structure (after hardening). The holding should be done untill all Martensite converts to ferrite by losing carbon.
  • 33.
    Tempered Martensite (Blackcolour) + Carbides (White colour) + Retained Austenite (White colour). Cooling Medium : Cooling in air Final Micro-structure : -> Tempered Martensite (having lower C% than Martensite formed after hardening) + ‘ε’ or Epsilon Carbide (Fe2.4C) (having higher C% than Cementite i.e. Fe3C + Retained Austenite. The quantity of retained austenite after tempering is lesser than before. Tempering <-Tempering Colors and Temperatures
  • 34.
    Surface Hardening Concept Surface -Pearlite + Cementite - High Hardness - Highly Wear Resistant Core - Ferrite - Low hardness - Ductile - Capable of withstanding stress Cross section of a case hardened gear teeth  The Carbon content in the steel determines whether it can be directly hardened or not. If the Carbon content is low (for example less than 0.3%) then an alternate means exists to increase the Carbon content of the surface. The part then can be heat-treated by either quenching in liquid or cooling in still air depending on the properties desired.  Note that this method will only allow hardening on the surface, but not in the core, because the high carbon content is only on the surface. This is sometimes very desirable because it allows for a hard surface with good wear properties (as on gear teeth or knife), but has a tough core that will perform well under impact loading.  It is also possible to add additional carbon or chromium or boron to the outer surface of a component low in carbon which will make the surface sufficiently hard.  This process is known as case hardening or surface hardening or selective hardening. Prof. Naman M. Dave
  • 35.
    I. Thermal Treatments 1)Induction Hardening 2) Flame Hardening 3) Laser Beam Hardening* 4) Electron Beam Hardening* II. Thermo-Chemical Treatments 1) Nitriding a) Gas Nitriding b) Liquid Nitriding 2) Carburizing* a) Solid / Pack Carburizing* b) Gas Carburizing* c) Liquid Carburizing (Cyaniding) 3) Carbo-nitriding* 4) Chroming* 5) Boronizing* Surface Hardening Concept Prof. Naman M. Dave
  • 36.
    36 Thermal Treatment Induction Hardening In Induction hardening, the steel part is placed inside a electrical coil which has alternating current through it. This induces Eddy Current on the outer surface of the component and heats it up.  Depending on the amperage of current, frequency of current and heating time depth of hardening can be controlled .  Temperature is around 750-800OC.  The heated region is then quenched by water jets to achieve the desired hardness.  Tempering can be done to eliminate brittleness.  Maximum 6 mm of depth can be achieved in about 6 seconds time.  Example of application : Gear teeth hardening Coil Work Piece Work Piece (Gear) Coil Prof. Naman M. Dave
  • 37.
    Flame Hardening  Ahigh intensity oxy-acetylene flame is applied to the selective region. The temperature is raised high enough to be in the region of Austenite transformation.  The "right" temperature is determined by the operator based on experience by watching the color of the steel. Flame hardening of a flat cross section work piece Flame hardening of a circular cross section work piece using lathe (MOVING) (STATIONARY) (Stationary) Work piece (Rotating) Thermal Treatment Prof. Naman M. Dave
  • 38.
     The overallheat transfer is limited by the torch and thus the interior never reaches the high temperature.  The heated region is quenched to achieve the desired hardness.  Tempering can be done to eliminate brittleness.  The depth of hardening can be increased by increasing the heating time.  Maximum 6 mm of depth can be achieved. A gear teeth being surface hardened using flame hardening method Thermal Treatment Flame Hardening Prof. Naman M. Dave
  • 39.
    Flame Hardening  Largeparts, which will not normally fit in a furnace, can be heat-treated using this method.  Example of application : Lathe Bed Hardening, Large Gears’ Teeth hardening, Large Sprockets’ teeth hardening, etc. Thermal Treatment Prof. Naman M. Dave
  • 40.
    Thermo-chemical Treatment Nitriding  Introductionof nitrogen into the outer surface of steel parts in order to give an extremely hard, wear resisting surface is called as nitriding or nitrogen hardening.  Nitride compounds precipitate out during one of the following processes a) Gas nitriding - heat in ammonia b) Liquid nitriding - dip in molten cyanide bath  A schematic layout of gas nitriding plant is shown in the next slide. Prof. Naman M. Dave
  • 41.
    41 Gas Nitriding  Thepart to be nitrided is placed in a box made of heat resistant steel. This box also has provision for inlet and outlet of ammonia gas (2 holes drilled on the walls – one for inlet of ammonia gas and another for outlet of ammonia gas)  The box (along with the component) is then placed in the furnace and heated to around 450- 550OC.  Then a steady flow of Ammonia gas is injected in the box.  On coming in contact with the steel, the ammonia vapour gets dissociated and nascent nitrogen is released. This nitrogen combines with other elements in outer surface of steel like C, Si, Mn, Ni, Cr, Mo, etc and forms nitrides which are extremely hard. After that, quenching is done.  Case thicknesses are between 0.5 to 0.8 mm with hardness up to 70 RC. Thermo-chemical Treatment Prof. Naman M. Dave
  • 42.
    Solid Carburizing orPack carburizing  The component is placed in a Cast Iron box along with a carbon rich material (e.g. charcoal). The box is placed in a furnace and heated to a temperature around 900-950 oC and maintained at that temperature for some period (1 hr for 0.1 mm depth).  The carbon is absorbed into the austenite on the surface of the component.  The depth of penetration of carbon into the surface depends on the temperature and the time spent in the furnace.  After enriching the surface with carbon, oil quenching is done. Carburizing  Low-carbon steel is heated in a carbon-rich environment and then quenched.  Depth achieved by carburizing is 0.025 to 4 mm with hardness around 60 RC. Thermo-chemical Treatment
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    43 Gas carburizing 1. Amild carburizing gas is generated from LPG (Liquified Petroleum Gas). This gas is fed into the furnace containing the heated steel component. 2. Another method of gas carburizing is called liquid feed or drip feed method. In this method, an organic liquid such as a mixture of iso-propyl or methyl alcohol with benzene is dripped directly into the furnace for generating a specific furnace atmosphere. • In any case, the gas mixture breaks to form an atmosphere containing N2, Ha, CH4, CO2, and H2 (due to heat). The component to be carburized is heated externally and made to travel through this atmosphere of the furnace and thus their surface becomes rich in Carbon content. It is then quenched to form a hard external surface whereas the core remains soft and ductile. The depth depends upon time and temperature of exposure to carbon rich atmosphere. HIGH FREQUENCY ELECTRIC CURRENT GAS-CARBURISING UNIT Gas Carburizing is conceptually the same as pack carburizing, except that gas is supplied to a heated component through the furnace. Thermo-chemical Treatment
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    44 Liquid Carburizing (Cyaniding) The steel parts are immersed in a molten carbon rich bath maintained at 850-950OC for periods ranging from 15 minutes to 3 hours.  Sometimes the heating / holding can be intermittent i.e. there can be two baths – one at 850-950OC and another at 760OC. This done so as to minimize the distortion of shape of product during heat treatment. After that quenching is done.  This process produces a thin, hard shell that is harder than the one produced by other carburizing methods, and can be completed in 3 hours compared to several hours as in other methods so the parts have less opportunity to become distorted.  Sketch on the left illustrates an electric salt bath furnace used for cyaniding.  It is typically used on small parts such as bolts, nuts, screws and small gears.  The major drawback of cyaniding is that cyanide salts are poisonous. Therefore, safety concerns have led to non-toxic baths that achieve the same result. ELECTRIC SALT BATH FURNACE Thermo-chemical Treatment
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    Carbo-nitriding  Use bothcarbon and nitrogen – hardness around 70 RC – maximum depth of hardened surface 0.07 – 0.5 mm Chromizing  Pack or dip in chromium-rich material - adds heat and wear resistance Boronizing  Pack or dip in boron-rich material - Improves abrasion resistance, coefficient of friction Thermo-chemical Treatment Prof. Naman M. Dave
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    Sr. No. Thermo- chemical Treatment Hardness of Surface Depth ofHardened Layer from Surface 1. Nitriding 70 RC 0.5 – 0.8 mm 2. Carburizing 60 RC 0.025 - 4 mm 3. Carbo- Nitriding 70 RC 0.07 – 0.5 mm 4. Chromizing 70 RC - 5. Boronizing 70 RC - Comparison of different thermo-chemical Treatment Thermo-chemical Treatment Prof. Naman M. Dave
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