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Heat treatment of steel prepared by chaudhari M.M


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Heat Treatment is defined as an operation or combination of operation, involving heating and cooling of a metal or alloy in its solid state with the object of changing the characteristics of the material.
The amount of carbon present in plain carbon steel has a pronounced effect on the properties of steel and on the selection of suitable heat treatments to attain certain desired properties. Steels can be heat treated to produce a great variety of microstructures and properties. Generally, heat treatment uses phase transformation during heating and cooling to change a microstructure in a solid state.

you have easy understand the concepts of heat treatment processes
5.Surface hardening:
a.Case hardening(carburizing)
d.Flame hardening

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Heat treatment of steel prepared by chaudhari M.M

  1. 1. Heat Treatment Of Steel Prepared by- Chaudhari M.M SVCP(Sinhgad), PUNE Mechanical Department Prof-Chaudhari M.M
  2. 2. Heat Treatment • Heat Treatment is defined as an operation or combination of operation, involving heating and cooling of a metal or alloy in its solid state with the object of changing the characteristics of the material. • The amount of carbon present in plain carbon steel has a pronounced effect on the properties of steel and on the selection of suitable heat treatments to attain certain desired properties. • Steels can be heat treated to produce a great variety of microstructures and properties. Generally, heat treatment uses phase transformation during heating and cooling to change a microstructure in a solid state. Prof-Chaudhari M.M
  3. 3. Why we apply heat treatment process?:- • Cause relief of internal stresses developed during cold working, welding, casting, forging etc. • Harden and strengthen metals. • Improve machinability • Change grain size • Soften metals for further (cold) working as in wire drawing or cold rolling. • Improve ductility and toughness • Increase, heat, wear and corrosion resistance of materials. • Improve electrical and magnetic properties. • Homogenize the structure to remove coring. • Spheroidise tiny particles, such as those of Fe₃C in steel, by diffusion. Prof-Chaudhari M.M
  4. 4. Stages of Heat Treatment:- • Stage l-Heating the metal slowly to ensure a uniform temperature. • Stage 2-Soaking (holding) the metal at a given temperature for a given period of time. • Internal structural changes take place. • soaking period depends on the chemical analysis of the metal and the mass of the part • Stage 3-Cooling the metal to room temperature. • To cool the metal, you can place it in direct contact with a COOLING MEDIUM composed of a gas, liquid, solid, or combination of these. Prof-Chaudhari M.M
  5. 5. Transformation of heating under equilibrium condition 1.Transformation of pearlite to austenite  It is take place at lower critical temperature of 7230C.  At this temperature pearlite changes to austenite because of nucleation process 2.Transformation of austenite to pearlite  Transformation of austenite begins with the nucleation of certain platelet at austenite grain boundary  The diffusion of carbon atoms austenite surroundings the cementite take place & the cementite further. Austenite carbon contains ferrite More carbon rejection & cementite platelet Below temp 7230CPearlite Formed Carbon Diffusion Repeat process Temp 5500C Prof-Chaudhari M.M
  6. 6. TTT Diagram  During the heat treatment austenite transform into various form.  Time & Temperature at which austenite Transformation take place has significant impact on new matter which is ultimately decide properties of steel  This study is done at constant Temperature also known as ‘Isothermal transformation’.  TTT diagram also known as ‘IT’ as well as “C” or “S curve”  Some terms:- 1. Austenite 2. Ferrite 3. Pearlite 4. Bainite 5. Martensite 6. Cementite Prof-Chaudhari M.M
  7. 7. Constituents of steel & Iron 1. Ferrite Iron which contain little or no carbon is called ferrite. The maximum solubility carbon in iron is 0.02% at 7230C. Its very soft and ductile that’s called alpha(ἁ) iron. 2. Cementite This is definite carbide of iron (Fe3C) which is extremely hard, being harder than ordinary harden steel or glass. Its presence in iron maximum percent of carbon is 6.67% or 8%. Its intermediate compound which contain 93.3% Fe. Prof-Chaudhari M.M
  8. 8. Cont.… 3. Pearlite Its mixture of ferrite and cementite, Pearlite is eutectoid of steel. 4. Martensite This is brittle mass of fibrous or needle like structure & the chief constituent of hardened steel. Its produced by the rapid quenching of high carbon steel from a slightly high temp than the max temp of critical interval. Ferrite (87.5%) Cementite (12.5%) Pearlite Prof-Chaudhari M.M
  9. 9. Cont.… 5. Austenite- The solid solution of gamma() iron is called austenite. Its formed when carbon steel with more than 1.1% carbon is quenched rapidly from about 11450C. 6. Bainite Its formed from austenite in temp range below 5300C. Its like pearlite mixture of ferrite & iron carbide but in different form. Its harder, tougher & stronger than ferrite –pearlite. Prof-Chaudhari M.M
  10. 10. Cont.… Prof-Chaudhari M.M
  11. 11. Heat treatment processes:- • Annealing • Normalizing • Hardening • Tempering • Surface hardening: • Case hardening(carburizing) a) Nitriding b) Cyaniding c) Flame hardening Heattreatmentprocesses:- Annealing Normalizing Hardening Tempering Surface hardening Case hardening(carburizing) Prof-Chaudhari M.M
  12. 12. Prof-Chaudhari M.M
  13. 13. 1. Annealing- It’s the process of heating the steel slightly above the critical temperature of steel i.e. 7230C & allowing it to cool down very slowly. Purposes of annealing:- • To soften the metals • To improve machinability • To refine grain size due to phase recrystallization. • To increase ductility of metal • To prepare steel for subsequent treatment. • To modify electrical and magnetic properties. • To receive internal stresses. • To remove gases. • To produce a definite microstructure. Prof-Chaudhari M.M
  14. 14. Classification of Annealing Annealing Full annealing Bright Annealing Box Annealing Isothermal Annealing Spheroidise Annealing Sub-critical Annealing Prof-Chaudhari M.M
  15. 15. a. Full Annealing/Conventional Annealing  Steel is heated 300C-500C above upper critical temperature at sufficient time to allow the material to fully form austenite.  After the above Austenization it slowly cooled in furnace & percentage of carbon is 0.3% - 0.6%.  The steel is heated above A3 (for hypo-eutectoid steels) & A1 (for hyper-eutectoid steels) → (hold) → then the steel is furnace cooled to obtain Coarse Pearlite.  Coarse Pearlite has low (↓) Hardness but high (↑) Ductility.  For hyper-eutectoid steels the heating is not done above Acm to avoid a continuous network of proeutectoid cementite along prior Austenite grain boundaries (presence of cementite along grain boundaries provides easy path for crack propagation). Advantages- 1.Refine grain 2.Remove strains 3.Improve formability. A1 A3 Acm  T Wt%C 0.8% 723C 910C Full Annealing FullAnnealing Prof-Chaudhari M.M
  16. 16. b. Bright Annealing • Its critical production process. If proper annealing is not done tube surface can crack, stained and may lead to easy corrosion. • In this process the stainless tube is heated to this high temp in presence of inert gases like nitrogen. • Tube surface must be cleared & free of foreign matter. Prof-Chaudhari M.M
  17. 17. c. Box Annealing • Annealing a metal or alloy in a sealed container under conditions that minimize oxidation. In box annealing a ferrous alloy, the charge is usually heated slowly to a temperature below the transformation range, but sometimes above or within it, and is then cooled slowly; this process is also called close annealing or pot annealing. Prof-Chaudhari M.M
  18. 18. d. Isothermal Annealing • Its process is hypoeutectoid steel is heated above the upper critical temp due to structure rapidly convert into austenite form. • Then steel cooled lower critical temp is 6000C- 7000C Purpose 1. Improved machinability 2. Better surface finish & homogeneous structure. Prof-Chaudhari M.M
  19. 19. e. Spheroidise Annealing  This is a very specific heat treatment given to high carbon steel requiring extensive machining prior to final hardening & tempering. The main purpose of the treatment is to increase the ductility of the sample.  Like stress relief annealing the treatment is done just below 7230C. Prof-Chaudhari M.M
  20. 20. f. Sub-critical Annealing • Sub-critical annealing is also known as ‘Process annealing’. • Process mainly suited for low carbon steel. • Components are heated above the recrystallization temp. • The aim of process annealing is to restored ductility of the cold worked material. Deformed crystal Undeformed crystal Recrystallization annealing • Application:- 1. Casting 2. Forging 3. Press work 4. Rolled stock. Prof-Chaudhari M.M
  21. 21. 2. Normalizing  The sample is heat above A3 | Acm to complete Austenization. The sample is then air cooled to obtain Fine pearlite. Fine pearlite has a reasonably good hardness and ductility.  In hypo-eutectoid steels normalizing is done 50C above the annealing temperature. Purpose 1. To reduce segregation in casting or forgings 2. To harden the steel slightly 3. Modification of steel grain structure easily. 4. Refine grain structure prior to hardening A1 A3 Acm  T Wt% C 0.8 % 723C 910C Normalization Normalization Prof-Chaudhari M.M
  22. 22. 3.Hardening  The sample is heated above A3 | Acm to cause Austenization. The sample is then quenched at a cooling rate higher than the critical cooling rate.  The quenching process produces residual strains (thermal, phase transformation).  The transformation to Martensite is usually not complete and the sample will have some retained Austenite.  The Martensite produced is hard and brittle and tempering operation usually follows hardening. This gives a good combination of strength and toughness. A1 A3 Acm  T Wt% C 0.8 % 723C 910C Full Annealing Hardening Hardening Prof-Chaudhari M.M
  23. 23. Cont.…  Purpose:- 1.Reduce brittleness 2. Relieves internal stresses 3.Improve wear resistance. • Quenched media 1.Water 2. Brine solution 3. Oil 4.Air Blast 5. Molten Salt Prof-Chaudhari M.M
  24. 24. 4. Tempering • It consist of heating quenched, hardened steel in martensitic condition to some predetermined temperature between room temp & the critical temp of the steel for a certain length of the time followed by air cooling. • Heating a hardened steel below the lower critical temperature. • Cooling the steel either rapidly or slowly susceptible to temper brittleness. Prof-Chaudhari M.M
  25. 25. 4. Tempering • Purpose:- 1. Increase toughness 2. Decrease hardness 3. Stabilize structure 4. Improve ductility 5. Increase percentage of elongation • Types:- 1. Low temp 2. Medium temp 3. High temp • Stages:- 1. Stage I (500 C-2000 C) 2. Stage II (2000 C-3000 C) 3. Stage III (4000 C-7500 C) Prof-Chaudhari M.M
  26. 26. Austenite Pearlite Pearlite + Bainite Bainite Martensite 100 200 300 400 600 500 800 723 0.1 1 10 102 103 104 105 Eutectoid temperature Ms Mf t (s) → T→  + Fe3C a. Martempering & Austempering  These processes have been developed to avoid residual stresses generated during quenching.  In both these processes Austenized steel is quenched above Ms (say to a temperature T1) for homogenization of temperature across the sample.  In Martempering the steel is then quenched and the entire sample transforms simultaneously to martensite. This is followed by tempering.  In Austempering instead of quenching the sample, it is held at T1 for it to transform to Bainite. Martempering Austempering T1 Prof-Chaudhari M.M
  27. 27. Surface Heat treatments • Case-hardening or surface hardening is the process of hardening the surface of a metal object while allowing the metal deeper underneath to remain soft, thus forming a thin layer of harder metal (called the "case") at the surface. • Types a. Flame hardening b. Induction Hardening c. Case Hardening (Carburizing) d. Nitriding e. Cyniding Prof-Chaudhari M.M Example
  28. 28. a. Flame hardening • Gas flames raise the temperature of the outer surface above the upper critical temperature. • The core will heat by conduction. • Water jet quench the components. Advantages:- 1. Very cheap method 2. Very suitable for hardening 3. Used to very big parts. Disadvantages:- 1. Irregular shape part difficult to hardening. 2. Its poor surface property Prof-Chaudhari M.M Example
  29. 29. b. Induction hardening • Induction hardening is a form of heat treatment in which a metal part is heated by induction heating and then quenched. And its suitable for circular parts only. • It involves processes are:- 1. Heat by alternative magnetic field. 2. Immediate quench by water. • Advantages:- 1. Fast & clean operation 2. Low cost operation 3. Tough & hard case obtained 4. Apply both internal & external surface • Application:- 1. Automobile shaft & crank Shaft 2. Automobile parts. 3.Piston rod & spur gear. Prof-Chaudhari M.M
  30. 30. c. Carburizing • Carburizing, also referred to as Case Hardening, is a heat treatment process that produces a surface which is resistant to wear, while maintaining toughness and strength of the core. • This treatment is applied to low carbon steel parts after machining, as well as high alloy steel bearings, gears, and other components. • Types:- 1.Pack carburizing- placed metal into charcoal & 10% barium carbonate. Case is 0.25-6 mm. 2. Gas carburizing- gas uses is CO, N2, H2. case is 0.25-1 mm Advantages:- 1. Quick type of process 2. Distortion is very small 3. Rapid heat transfer 4. Any shape can be treated. Prof-Chaudhari M.M Disadvantages:- 1. Carburizing salt very poisonous 2. Cleaning of complex part difficult.
  31. 31. d. Nitriding • Nitriding is a heat treating process that diffuses nitrogen into the surface of a metal to create a case-hardened surface. • These processes are most commonly used on low-carbon, low-alloy steels. • case thickness is 0.001 & 0.002 inch, temp range is 6000C. • Nitriding components precipitate out 1.Gas Nitriding:- heat in Ammonia 2. Liquid Nitriding:- Dip in molten cyanide bath. Properties:- 1. Hardness is high 2. Good corrosion resistance 3. Less coefficient of friction. Applications:- 1.Valve seats, guides, gears, gauges, ball races 2.Aircraft engine, aero engine cylinder. Prof-Chaudhari M.M
  32. 32. e. Cyaniding • Both carbon and nitrogen absorbs in this process. • Produce thin shell is 0.25-0.75 mm. • Low carbon steel is heated between 8000C to 8900C in molten sodium cyanide. • Main drawback is cyanide is very poisonous. • For more hardenability after cyaniding process material directly quenched into water or oil. Applications:- 1.Nuts and Bolts, gears, screws. Prof-Chaudhari M.M
  33. 33. Prof-Chaudhari M.M Thank You..!!