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Heat treatment process
- 1. Heat treatment process Seminarand Technical writing (CR798) Autumn 2021 Course Instructor- Prof. Debasish Sarkar Presented by AMRENDRA KUMAR PhD Scholar 520CR1001 Department of Ceramic Engineering, National Institute of Technology, Rourkela, Odisha
- 2. OUTLINE OF PRESENTATION •Definition • Classification • Purpose • Principles • Annealing • Normalising • Hardening • Tempering • Hardenability
- 3. Heat treatment definition Heattreatment is a process by which any material gets heated at particular temperature held at that temperature for certain time and cooled in a desired atmosphere to get desired property. It is used to get • Better machinability • To relieve stresses • To get desired microstructure • To get improved ductility
- 4. Classification of heattreatment • Annealing • Normalising • Tempering • Hardening • Case hardening • Surface hardening
- 5. Purpose of heattreatment • To relieve internal stresses caused due to cold working • To harden and strengthen metal • To improve machinability • To increase or decrease grain size • To improve ductility and toughness • To improve electrical and magnetic properties • To homogenize structure • To improve wear and tear resistance
- 6. Principles of heattreatment The principles involve behind heating process that any alloy will experience change in structure if it is heated at certain temperature and again changes when it is cooled to room temperature. Cooling rate is an important factor in heating process. In heating of steel, fast heating will develop martensitic or hard structure whereas slow cooling develop pearlitic or soft structure. Thus due to heat treatment process in material we get variety of useful properties which are required in further processing.
- 7. Stages of heattreatment process • First metal or alloy is heated to a desired temperature. • Then it will hold at that temperature for a sufficient time to undergo necessary changes. • Then it will be cooled in a certain atmosphere to room temperature either it will be fast cooling or slow cooling.
- 9. Annealing A material whichis in unstable stage or metastable stable will be heated to that temperature which remove instability of material and then cooled preferably slow cooling to room temperature. So that room temperature stable structure will be obtained. The purpose of annealing is to get stable structure To reduce hardness To improve machinability To get desired microstructure To improve mechanical properties
- 10. Types of annealing •Full annealing • Process annealing • Spheroidise annealing • Diffusion annealing
- 11. Full annealing Full annealinginvolves heating the steel to austenitic zone temperature, holding it at sufficient time period mainly dependent on the thickness of material so that it fully converts into austenite and cool slowly through transformation range in furnace atmosphere. It is done to • refine grain • Remove internal strain • Improve machinability • Induce softness
- 12. Process annealing It ismainly carried out to remove the effects of cold work and induce softness in the material so that the plastic deformation can takes place in wire and sheet industries. The material is heated below lower critical temperature, hold for some time at that temperature and then cool slowly to room temperature. This process is mainly carried out in mild steel industries and low carbon steel industries.
- 13. Spheroidise annealing This processis mainly performed for high carbon steel which are difficult to machine. The heating takes place above lower critical temperature usually 730-770℃ and then cool slowly to 600 ℃. The rate of cooling is 25-30℃ mainly in furnace. holding lower critical temperature line cooling temperature heating
- 14. Diffusion annealing This processis performed to homogenise the austenitic grain when heating takes place above upper critical temperature. It mainly follows by full annealing for fine grained casting structure.
- 15. Normalising Normalising is mainlya final stage heat treatment process which is performed when material suffers from high internal stresses. The material is mainly heated above 40-50℃ above upper critical temperature, hold for very short duration and then quenched in air. So it is called air quenching. The microstructure obtained after normalising consists of ferrite and pearlite for hypoeutectoid steel and pearlite and cementite for hypereutectoid steel.it increases yield point, ultimate tensile strength and impact strength.
- 16. Hardening It is aprocess of heat treatment by which the hardness of steel will be increased by quenching. The parts which undergo heavy duty service are hardened by this method. The process involved heating of steel having high carbon percentage will be heated 30-60 ℃ above upper critical temperature, held at that temperature for 30 min to 1 hour and then rapidly quenched in suitable medium either its water, brine solution.
- 17. Factors on whichhardness is dependent are: • Nature and properties of medium in which material is quenched. • Quenching temperature • Rate of cooling • Composition of steel • Size of sample • Impurities present
- 18. Tempering After hardening processthere is a need of increasing ductility. That can be done by tempering process. The process involves heating of steel near lower critical temperature, held at that temperature for 3 to 5 minute and then cooling in air or water either slowly or rapidly depending on the need. It is done to relieve from internal stress, reduce hardness and increase percentage elongation.
- 19. Case hardening The componentslike gear, bearing surfaces and cam shafts must be tough, shock resistant and wearing high load capability. The steel used having low carbon percentage does not appreciably respond to heat treatment process. So there is a need of increasing carbon percentage at surfaces. So this problem will be resolved by case hardening. The process involves increasing carbon content at the surface by diffusion of carbon monoxide in the contact of surface having 870-950 ℃.
- 20. Types of casehardening • Pack carburizing • Liquid carburizing • Gas carburizing • Nitriding • Cyaniding • Carbonitriding
- 21. • Pack carburizing:In this process material is packed into cast iron or steel boxes along with carburizing material. Carburizing material may be wood, charcoal together with barium carbonate. The heating is done between 900- 950 ℃ then held for 5 hours depending upon the depth of case desired. • Liquid carburizing: In this process the work piece is immersed in liquid carburizing material at 870-950 ℃ for 5 min to 1 hour depending upon the depth of case desired. The liquid carburizing medium consists of sodium cyanide with some percentage of sodium carbonate and barium chloride. • Gas carburizing: in this process the components gets heated in an atmosphere which will deposit carbon on the surface of workpiece at 900 ℃. The gaseous atmosphere consists of 20% CO, 40% hydrogen and 40% nitrogen.
- 22. • Cyaniding: inthis process carbon and nitrogen are introduced on the surface of workpiece by heating it to a certain temperature and holding it in contact with molten cyanide to form a thin layer. It is used in screws, nuts and small gears. The heating is done from 800- 870 ℃ and holding it from 30 min to 3 hour depending on the case thickness. • Carbonitriding: It is same as cyaniding but the main difference is in cyaniding the components come in contact of liquid molten salt bath but in carbonitriding it comes in contact of gaseous atmosphere. Both carbon and nitrogen will simultaneously deposited on the surface of component. The gaseous atmosphere is in 3:1 proportion of natural gas and ammonia.
- 23. Surface hardening The surfacelayer of metals are hardened up to certain depth by this process. The core remains unaffected by this process because heating and cooling takes place in a rapid way. Types of surface hardening: • Induction hardening • Flame hardening
- 24. Induction hardening: In thisprocess the surface to be hardened is surrounded by inductor. A high frequency current will be flowed through it. The work is kept in such a way that it will not touch the inductor. The heating effect is produced by eddy current and hysteresis loss. The temperature suitable for this process will be above 768℃.
- 25. Flame hardening This processis also based on rapid heating and rapid cooling. An oxyacetylene flame is used to heat the workpiece above critical temperature. Quenching is done by spray of water. The heating torch may be stationary or movable. After this process the surface becomes too hard and core remains soft.
- 26. REFERENCES • Material scienceand metallurgy by O.P. KHANNA • Material Science by G.K. Narula • Material science by Callister
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