SURFACE HARDENING HEAVY CROSS SECTION - IMPOSSIBLE TO COOL QUICKLY TO PRODUCE A UNIFORMLY MARTENSITIC STRUCTURE THROUGHOUT A SOFT UNHARDENED CORE DUE TO RELATIVELY SLOW COOLING RATE. CORE WITH FINE PEARLITE, SURFACE MARTENSITIC- CALLED MASS EFFECT OF HEAT TREATMENT 0.1% C – TOUGH & SOFT; 0.8% C – HARD & BRITTLE CERTAIN APPLICATIONS WHERE CORE AND SURFACE SHALL HAVE DIFFERENT PROPERTIES. Eg: Cam, Gears, Shafts to have hard and wear resistant surfaces, but TOUGH, shock resistant cores HARDNESS- TOUGHNESS- MALLEABILITY- [COMPRESSION ](eg: rolling, forging)- DUCTILITY- [TENSION ](eg: wires drawn)

Carbon or Nitrogen to penetrate to some depth for hardness to increase on surface Flame or Induction hardening for localised purposes CASE HARDENING – eg; Wrought Iron to steel by Cementation Carbon diffused into Iron (of Fe 3 C structure)

CASE HARDENING Solid, Liquid or Gaseous medium Release carbon at surface, absorb interstitially to steel- By DIFFUSION DEPTH DERIVED BY SECOND LAW

FICK’S LAWS OF DIFFUSION

FIRST LAW dn/dt = no. of moles of B atoms crossing per unit time D= Diffusion coefficient A= Planar area dc/dx= concentration gradient If J = flux flow / unit area per unit time,

PACK CARBURISING Packing work in 25 Cr, 20 Ni heat resisting steel boxes with 50 mm gap with carburising material. Heated slowly to 850 – C, maintained for 8 hrs according to depth needed. Temperature Carburising Time Depth of case

CHARCOAL WITH BARIUM CARBONATE AS ENERGISER (10 to 15%). Process depends on presence of CO 2C + O 2 2 CO At surface, releases C atoms 2CO CO 2 + C C dissolved interstitially at surface of steel. Ba CO 3 Ba O + CO 2 CO2 + C 2CO

LIQUID BATH Mixture of salts of Sodium Cyanide, Sodium carbonate, Sodium/barium Chloride Melted in pots to C, work immersed for 5 min to 1 hour Then basket quenched- hard and clean surface For shallow- 0.1 to 0.25,mm; for small parts

GAS CARBURISING In batch type or continuous furnaces. Far widely used Clean compact plant Heated to C for 3 to 4 hours Hydrocarbons methane and propane partly burnt in furnace, diluted with carrier gas to get required carbon POTENTIAL ( ie carbon content maintained in equilibrium in the surface film- of 0.8% desirable)

After carburising, HT necessary to strengthen and toughen the core. Hardens case too.

NITRIDING Resembles Carburising-(interstitial penetration during heating with nitriding agent ) Hardness depends on the formation of hard nitrides For alloy steels with Al, Cr, Mo, V (these form strong nitrides)

Plus points: No quenching needed, so cracking /distortion least. High surface hardness of 1150 H obtained Resistance to fatigue failure good Resistance to corrosion good, (on unpolished surface) Hardness retained at C ( in carburising falls near C) Economical for large no. of components Clean process, (cyaniding with water rinsing environmentally not good) Minus points: Initial outlay higher than for case hardening Overheating removes hardness completely.

INDUCTION HARDENING AND FLAME HARDENING USE OF INDUCTION COILS Gas flame derived rom acetylene, propane or natural gas. Manually operated torches used for small areas or localised surfaces.