Surface Hardening
In many situations surface hardening instead of through hardening only is sufficient to serve the purpose. Gears are examples. Surface hardening is achieved through case carburising, nitriding or induction heating. Steels containing 0.1 to 0.25% C are best suited for case carburising. Good combination of tough core (lesser hardness) and high surface hardness is achieved by case carburising of nickel steel. Case hardness of 60 RC with a core hardness of 33 to 38 RC gives best results in case of gears. The case hardness is due to residual compressive stress introduced on the surface by penetration of C and N2.
Surface hardening is classified into two types:
1. Without addition of any element from outside but only transforming outer layer to martensite. This could be achieved by heating the surface by gas flame or causing magnetic induction so that complete austenite transformation occurs on surface. On quenching martensite and retained austenite form on surface while on the inner side peralite-ferrite is the main phase.
2. The second method is called case hardening in which C and/or N2 are introduced in the surface layer. In carburising the part is surrounded by material or atmosphere rich in C and on heating this C is released and absorbed in steel. Recently case carburising is more effectively performed by heating steel part in the atmosphere or natural gas, coke oven gas, butane or propane or the valatised form of liquid hydrocarbons like terpenes and benzene. Volatilised form of alcohol and glycols or ketones are also used. In these cases the thickness of hardened layer is proportional to root of the time of treatment in hour.
Liquid carburising consists in dipping the part in fused mixture of chlorides, carbonates and cyanides. Baths maintained at 840oC to 900oC produce a case depth of 0.075 to 0.75 mm. 0.5 to 3.0 mm case depth is attainable if bath is maintained at 900 to 950oC. Plain carbon steel and low alloy steel can be carburised in liquid bath.
Nitriding of steel surface is the absorption of N2 in the surface. Nascent N2 for this purpose is obtained from ammonia. Molten cyanide (sodium cyanide) bath maintained at 560oC is quite effective in nitriding particularly if thin case is desired. Plain C steel are not good for nitriding because iron nitride so formed is very brittle. Steels alloyed with Al and Cr and Ni, Cu, Si and Mn are better nitrided than plain C steel.
nitrocarburising , Carbonitiriding, or gas cyaninding is a process similar to gas carburising in which ammonia is also added to carburising atmosphere. This process produces better hardened case than carburising.