1. Case Hardening Case – hard, wear resistant. Core – soft, tough, shock resistant. 0.1% carbon.. 0.9% carbon. Addition of carbon and nitrogen on the surface of the steel.  Carburizing  Nitriding.  Cyaniding.  Carbonitriding.

2. Local hardness:  Flame hardening.  Induction hardening. Carburizing: Introduce of Carbon, absorption and diffusion. Heated to 870 to 925 o c, gaseous, solid or liquid carbon containing substance. o case depth is about 1.25mm o Hardness after heat treatment is Rc 65.

3. Gears, camshafts, bearings.. a)Pack carburizing, b) Gas carburizing, c) Liquid Carburizing. Pack Carburizing: Wood, charcoal, charred leather, 900 – 995 o c, 5 hours, slowly cooled. Formation of carbon monoxide, Dissociation of CO with the evolution of carbon

4. Enrichment of the steel surface layer with carbon. 2CO CO 2 + C 2CO + 3Fe Fe 3 C + CO 2 Diffusion of carbon, absorbed by the steel surface, deep into the metal. Copper can be covered, coated with a mixture of fireclay.  Gas Carburizing: Mass production, 900 o c, 3 to 4 hrs, 20% CO, 40% hydrogen, 40% Nitrogen.

5. Co 2 is removed by passing over heated coke or charcoal at 1000 o c. CO,N 2, H 2 and propane or butane, 3Fe + 2CO Fe 3 C + CO 2 3Fe + 2CO + H 2 Fe 3 C + H 2 O CH 4 + 3Fe Fe 3 C + 2H 2 Depths can vary 0.25mm, 0.5 – 1.0mm for automobile work, 0.37mm for roller bearings. Liquid Carburizing: 0.10 – 0.25mm, bath containing 20 to 50% solid sodium cyanide, 40% sodium carbonate, sodium chloride or barium chloride.

6. Heated 870 – 950 o C Dipped for 5min to 1hr, 2NaCN + 2O 2 Na 2 CO 3 + 2N + CO Case depth 0.25, liquid carburizing permits cases as depth as 6.25mm.. Nitriding: Case depth is about 0.381mm. Extreme hardness. Case has improved corrosion resistance.

7. Example : valve seats, guides, gears, gauges, bushings. Process: Inside heat resistant metal, filled with ammonia, sealed, placed in furnace, heated to 500 o C. NH 3 3H + N. 500 o c for 40 to 100 hrs. Very high surface hardness, Minimum distortion, Good fatigue resistance, No machining is reqquired

8. Cyaniding: Carbon and nitrogen Case depth is about 0.25mm, hardness is about Rc 65, negligible dimension change. Screws, nuts and bolts, small gears. Low carbon steel, 800 – 870oC Sodium cyanide bath – 30min to 3hrs, quenching in oil or water bath,, 30% NaCN, 40% Na 2 CO 3 and 30%NaCl

9. Hardening (Quenching): Increases hardness by quenching, Tools and machine parts, heat treatable steels, nodular graphite irons and cast iron. 0.3 to 0.6% C formation of martensite, Steel with sufficient carbon ( 0.35 to 0.75%C ) 30 to 50 o c above A 3 line. 15 to 30 min per 25mm of cross section. Is cooled rapidly or quenched in a suitable medium (brine, water, oil etc)  composition of steel  nature and properties of quenching medium,  quenching temperature.

10. Hardening (Quenching):  size of the object  homogeneity of austenite  degree of agitation  degree of cooling  surface condition of metal.  alloying element. Quenching medium: 5 to 10% caustic soda. 5 to 20% brine ( NaCl) Cold water

11. Warm water Mineral oil (obtained during the refining of crude petroleum) Animal oil ( produced by boiling the blubber of seal and whale or by rendering down other animal tissue) Vegetable oil (linseed, cotton seed) Air. Quenching characteristic : Temperature of quenching medium Heat of vaporization Specific heat Thermal conductivity of quenching medium

12. Viscosity Agitation ( the rate of movement of work piece or flow of coolant) Tempering : -Martensite and retained austenite -Martensite is brittle, hard and highly stressed, cracking, distortion of hardened article. -Retained austenite is unstable, changes with time, dimensions may alter, - therefore heated to below lower critical temperature.

13.  heating hardened steel below lower critical temperature.  holding it at that temperature for 3 to 5 min for each mm of thickness or dia.  cooling the steel either rapidly or slowly. Purpose of tempering:  Relieve residual stresses  improve ductility  Improve toughness  Reduce hardness  increase % elongation.

14. Stages of tempering: 1)At low temp ( 80 – 200 o c) a hexagonal close packed carbide (epsilon carbide) begins to form, martensite changes from tetragonal to body centered cubic. 2) at 200 to 300 o c retained austenite to bainite 3) at 300 to 475 o c cementite (Fe 3 C) from epsilon carbide and changes from low carbon martensite to cubic ferrite. 4) at 450 to 705 o c, cementite agglomerates, structure becomes an aggregates of ferrite with cementite in quite fine spheres, that is tempered martensite and tempered bainite. Tempering classification:

15. 1.Low temperature Tempering: 150 – 250 o c, internal stresses are reduced, toughness and ductility get increased, contains martensite, cutting tools, low alloy steels, surface hardened and carburized component. 2. Medium temperature tempering: 350 – 450 o c, troostite structure, hardness and strength of steels decrease, % elongation and ductility increases. Highest elastic limit with sufficient toughness, Coil springs, laminated springs, hammers, chisels etc. 3. High temperature tempering: 500 – 650 o c, eliminates internal stresses completely, develops a sorbite structure, high ductility with high hardness, connecting rod, shafts, gears etc.

16. Temperature: Pyrometer, temperature indicating paints and cryons, tempering colours. When the steel is heated the oxide film on the surface first assumes a pale – yellow colour and gradually thickens, with increase in temperature, until it is dark blue. 220 o c – pale yellow, 230 o c – straw, 240 o c – dark straw, 250 o c – light brown, 270 o c – purple, 280 o c – deeper purple 290 o c – bright blue 300 0 c – dark blue.

17. Interrupted quenching: Interrupted quenching procedure, It is two stage process – 1) first quenched in a medium which cools it rapidly past the nose of the TTT diagram, 2) The piece is further quenched in a second medium which cools it rapidly enough to avoid the bainite transformation. Martempering : a)Heated to above the critical range to make it all austenite. b) quenched into a salt bath maintained at a temp above Ms and is held at this temp long enough until the temp is uniform, with transformation of the austenite. c) subsequent cooling the work piece in air through the martensite range.

18. Martensite with a minimum of stresses, distortion, and cracking. Further tempered to increase ductility. No large section is possible, exceeds the start of transformation of austenite into bainite, alloying element is added to slow down the critical cooling rate. Austempering: It is not a hardening treatment, Bainite is formed in this, It resembles tempered martensite, Limitations is restricted to relatively thin sections. Steels of high harden ability, larger sections can be used, Disadvantage is long time is required of transformation from austenite to bainite.

19. Austempering : a)Heated to above the critical range to make it all austenite. b) quenched into a salt bath or lead bath held in the bainite range. c) the steel piece remains in the bath until the austenite is completely transformed to bainite. d) allowed to cool to room temp, the rate being im-material. Greater ductility and toughness along with high hardness, Less distortion, Less danger of quenching cracks.

20. Ausforming, maraging: Hardenability: