Heat treatment process

Purpose of Heat treatment processes with basic detailed description and comparative study of different process such as bulk & surface process .

1. GUJARAT Technological University Dr . S & S.S Gandhy G overnment Engineering College , Surat Branch :- Mechanical Engineering Subject :- Material science and metallurgy

2. Heat treatment Process Purpose :- Improve metals properties ( Ex. Mechanical , Electrical etc..) Change in Grain structure Process :- It consists of 1. Heating 2. Soaking ( Holding ) 3. Cooling

3. Types of Heat treatment Process Annealing Normalizing Hardening Tempering Carburizing Nitriding

4. Annealing process Types of annealing process :- Stress relief annealing Recrystallization annealing Spheroidizing annealing Full annealing

5. Types of annealing process Parameters Steps 1.
Stress relief annealing 2.
Recrystallization annealing 3. Spheroidizing annealing 4. Full annealing Purpose Relieve Stresses that have been absorbed by metal from different process Remove strain Harding and Internal Stresses Improve machinibility Remove all structural imperfections Process Step 1 Heating the cold work steel at temp. between 500-550°c Heating the steel components at temp. between 625-675°c Heating the steel components at temp. between 650-700°c Heating the steel components at a slowly raising temp. about 30-50°c above upper critical temperature.

6. Parameters Steps 1.
Stress relief annealing 2.
Recrystallization annealing 3.
Spheroidizing annealing 4.
Full annealing Step 2 2. Holding the steel components at this temp. for
1-2 hours. 2. Holding the steel components at this temp. for sufficient time . 2. Heating and cooling alternate for a 8hour period of time. 2. Holding the steel components at this temp. for at least 20min per cm of the thick section . Step 3 3. To cool the steel components at room temperature in air 3. To cool the steel components at room temperature in air 3. Heating at temp. above lower critical temp. 730-770°c with very slow cooling in furnace 3. To cool the hot steel components to room temp. slowly in the furnace Benefits Reduce risk of distortion while machining
Increase corrosion resistance Allows recovery process by reduction of work hardening effect. Steel have carbide and it shape is sphere . Sphere shape are soft so we can improve machinibility. Steel become soft & ductile A microstructure having small grains and uniform grain structure

7. Parameters Steps 1.
Stress relief annealing 2.
Recrystallization annealing 3.
Spheroidizing annealing 4.
Full annealing No loss in strength & Hardness
No change in microstructure Increase equiaxed ferrite grains
Decease strength & Hardness
Increase ductility Increase Ductility & toughness Decrease Hardness & Strength -- Applicable Low carbon steel Hypo Eutectoid Steel Low carbon steel Hypo Eutectoid Steel Medium & High carbon steel Low & Medium carbon steel

8. Normalizing process Purpose :- Inhance Mechanical properties of a material by refining microstructure Normalizing involves Heating Soaking Cooling

9. The Normalizing Process Heating :- The metal is heated in a furnace for normalizing heat treatment process. The temperature of the furnace is kept between 750-980 °C (1320-1796 °F), depending upon the carbon content in the material. Soaking :- The material is kept at the temperature above austenite temperature for 1-2 hours, until all the ferrite converts into austenite. Cooling :- The ferrite converts into austenite, and then cooled to room temperature in still air or Nitrogen.

10. Applicable Metals :- Iron based alloys (tool steel, carbon steel, stainless steel, and cast iron)
Nickel-based alloys
Copper
Brass
Aluminum

11. Common Applications of Normalizing Ferritic stainless steel stampings in the automotive industry may be normalized following the work hardening that occurs during their forming process.
Nickel-based alloys in the nuclear industry may be normalized following the thermal microstructure alteration that occurs following welding.
Carbon steel may be normalized after it is cold-rolled to reduce the brittleness caused by work hardening.

12. Difference between Annealing & Normalizing Process

13. Hardening Process :- - Material heated to austenitic range and followed by sudden Quenching. - Mainly used for increasing hardness of material. Purpose of Hardening :- - To develop High Hardness in material. - To improve strength. - To improve wear resistance.

14. Step - 2 :- Holding - At heating temperature for some time. Step - 3 :- Quenching or Rapid cooling - Cooling at a faster rate than the critical cooling rate. - Different Quenching mediums are :- 1.Water 2.Brine 3.Oil 4.Salt solution

15. Result :- - Due to rapid cooling Austenite will transform to Martensite. - Internal stresses will be created in the material because carbon gets trapped and they do not get enough time to diffuse out of the structure.

16. TTT – diagram :- M : Martensite A : Austenite P : Pearlite B : Bainite N : Nose of curve M s : Martensite start temperature M f : Martensite finish temperature

17. Result of different cooling mediums :- 1. Furnace cooling - Slowest cooling - Coarse or fine pearlite 2. Air cooling - Moderate cooling - Pearlite or Bainite 3. Oil quench - Faster cooling - Martensite 4. Water quench - Fastest cooling - Martensite

18. Important points about Martensite :- - It forms during Quenching (rapid cooling). - Austenite to martensite transformation is a diffusion less transformation because of very quick arrangement of atoms, there is not enough time for atoms to diffuse. - Body- centered Tetragonal (BCT) structure. - Very high hardness and brittleness.

19. Tempering Process :- - Tempering is always followed after Hardnening process. - Sub critical heat treatment process. - Martensite obtained is very much brittle that a small impact can break it, so tempering process is carried out to improve its toughness. - This process also reduces the internal stresses which are produced during rapid cooling.

20. Purpose of Tempering :- - To relieve internal stresses. - To improve ductility. - To improve toughness. - To reduce brittleness. - To reduce hardness to some extent.

21. Procedure :- Step - 1 :- Reheating hardened steel - Heat below critical temperature (below 700⁰C). Step - 2 :- Holding - Holding at that temperature for period of time. Step - 3 :- Cooling - Slow cooling in air. - Cooled to the room temperature.

22. Depending on temperature , Tempering process can be classified as :- 1. Low temperature Tempering (150⁰ to 250⁰C) 2. Medium temperature Tempering (350⁰ to 500⁰C) 3. High temperature Tempering (500⁰ to 650⁰C) Result :- - At tempering temperature, carbon atoms diffuses out and martensite transforms to fine cementite and softer ferrite structure so internal stresses gets relieved. - Good combination of toughness, strength & hardness in material is obtained.

23. Difference between Hardening and Tempering :- Hardening Tempering 1. Heating occurs a bove critical temperature ( Above 723⁰C). 1. Heating occurs below critical temperature ( Below 723⁰C). 2. Cooling rate is very fast as it is quenched in water or oil. 2. Cooling rate is slow as compared to hard- - ening process as it is cooled in air. 3. Austenite transforms to Martensite . 3. Martensite transforms to fine cementite and softer ferrite. 4. Internal stresses are produced. 4. Internal stresses are relieved. 5. Improves hardness and brittleness. 5. Improves toughness and reduces hardness and brittleness to some extent.

24. Carburizing Process :- Definition :- A thermo-chemical process in which carbon is diffused on the surface of steel is called Carburizing. Purpose of Carburizing process :- - To increase surface hardness so that it will respond well to heat treatment. - To increase strength. - To increase wear resistance.

25. Important points about Carburizing :- - Thermo-chemical process. - Mainly done on low carbon steel (0.02 - 0.3%C). - Carried out on above critical temperature (>750⁰C). - Fully austenite phase is required for carburizing because austenite is having very good carbon solubility. - Carbon content increases upto 0.7-0.9% after carburizing.

26. Types of Carburizing :- 1. Solid or Pack carburizing :- - Low carbon steel is packed with carbonaceous solid mixture of 80% charcoal powder and 20% barium chloride. - Specimen and this mixture is kept in a sealed container. - Then heat upto 790⁰ to 845⁰C for about 6 to 8 hours, then cooled to room temperature.

27. - Reactions :- BaCo 3 = Bao + Co 2 (This Co 2 reacts with charcoal) Co 2 + C = 2Co 2Co + Fe = FeC + Co 2 - Advantages :- 1. It is simple method and economical. 2. No atmosphere furnace is required. - Disadvantages :- 1. Carburizing time is very long. 2. Difficult to control surface carbon and case depth.

28. 2. Liquid carburizing :- - Specimen is kept in molten salt baths or various salt mixtures : Sodium cyanide (20-50%), Sodium carbonate (40%). - The salt is usually a cyanide – chloride – carbonate mixture and is highly toxic. - Then it is heated on about 845⁰ to 950⁰C temperature. - Time required is about 55 min to 1 hour.

29. - Reactions :- BaCl 2 + 2NaCN = Ba(CN) 2 + 2NaCl Ba(CN) 2 + Fe = FeC + BaCN 2 - Advantages :- 1. Rapid rate of penetration. 2. Free from oxidation. - Disadvantages :- 1. Sodium cyanide is highly poisonous; hence care should be taken while storage. 2. Salt sticks to the components.

30. 3. Gas carburizing :- - Specimen is kept in a gaseous atmosphere of methane or propane. - Methane and propane diluted with carrier gas which is a mix- ture of N 2 , Co, Co 2 , H 2 , CH 4 . - Heating is carried out between 900⁰ to 950⁰C temperature for about 3 to 4 hours.

31. - Reactions :- C 3 H 8 = 2CH 4 + C CH 4 + Fe = FeC + 2H 2 CH 4 + Co 2 = 2Co + 2H 2Co + Fe = FeC + Co 2 - Advantages :- 1. It gives more uniform case depth. 2. Total time of carburization much less than the pack carburization. - Disadvantages :- 1. Furnace and gas generators are expensive. 2. Handling of fire hazards and toxic gases are difficult.

32. Nitriding Process :- Definition :- A thermo-chemical process in which nitrogen is diffused on the surface of steel is called Nitriding process. Purpose of Nitriding Process :- - To increase surface hardness. - To improve wear resistance. - To improve corrosion resistance. - To improve fatigue life.

33. Important points about Nitriding :- - Thermo-chemical process. - Most commonly used on high carbon, low alloy steels. It can also used for medium carbon steels, titanium, aluminium and molybdenum. - Carried out on below critical temperature (<723⁰C). - Better modification than Carburizing.

34. Procedure :- - Nitrogen in monoatomic form is diffused on the surface of steel and very hard nitrides of iron are formed. - Resulting nitride case is much harder than carbu - rized case. - Subcritical temperatures are used and hardness is gained without quenching.

35. - Source of nitrogen used in diffusion process is ammonia and the nitriding temperature is 500⁰ to 575⁰C. - Reaction :- 2NH 3 = 2N + 3H 2 - Nitrogen is diffused on steel and hydrogen is exhausted. - Nitrided case is categorized into three zones :- 1. White layer, which is brittle. 2. A hard nitride layer. 3. A diffusion zone of decreasing hardness.

36. Advantages :- - Process requires less time. - Improves hardness, fatigue properties, resistance to wear and corrosion. - No cracks produced during process. Disadvantages :- - High furnace cost required. - Medium used is expensive. - Necessity of using special alloy steels.

37. Difference between Carburizing and Nitriding :- Carburizing Nitriding Carburizing is done in presence of carbonaceous environment. Nitriding is done in presence of nitrogenous environment. 2. Carbon gets diffused on surface of steel. 2. Nitrogen gets diffused on surface of steel. 3. Heating is carried out above critical temperature. 3. Heating is carrired out below critical temperature. 4. After heating, Quenching is required. 4. After heating, Quenching is not required.

38. Reference material Material Science and Engineering ( An introduction) William D. Callister Design For Heat treatment Lecture
IIT Bombay
Nptel Online resources http://www.metalsupermarkets.com http://www.inspection-for-industry.com Figures from Google sources

39. Any Questions