In many conditions surface hardening instead of throughout hardening only is enough to serve the reasons. Gears are illustrations. Surface hardening is attained via case carburizing, induction or nitriding heating. Steels comprising 0.1 to 0.25 percent C are best well suitable for case carburizing. Good combination of tough core as lesser hardness and high surface hardness is attaining by case carburizing of nickel steel. Case hardness of 60 RC along with a core hardness of 33 to 38 RC offers best results in case of gears. The case hardness is because of residual compressive stress introduced on the surface via penetration of C and N2.
Surface hardening is categorized into two types as:
(a) Along with no addition of any of element from outside but simply transforming outer layer to martensite. This could be attained by heating the surface via gas flame or causing magnetic induction hence complete austenite transformation arises on surface. On retained austenite and quenching martensite form on surface whilst on the inner side pearlite ferrite is the major phase.
(b) The second technique is named as case hardening whether C and/or N2 are introduced in the surface layer. In carburizing the part is within material or atmosphere rich in C and on heating such C is absorbed and released in steel. Only just case carburizing is extra effectively performed with heating steel part in the natural or atmosphere gas, coke oven gas, propane or butane or the volatized form of liquid hydrocarbons similar to benzene and terpenes. Volatilized form of alcohol and glycols or ketones is also utilized. In such cases the thickness of hardened layer is proportional to root of the treatment time in hour.
Liquid carburizing comprises in dipping the part in fused mixture of carbonates, cyanides and chlorides. Baths maintained at 840oC to 900oC generate a case depth of 0.075 to 0.75 millimeter 0.5 to 3.0 millimeter case depth is attainable if bath is kept at 900 to 950oC. Plain low alloy steel and carbon steel can be carburized in liquid bath.
Nitriding of steel surface is the absorption of N2 in the surface. Nascent N2 for such causes is acquired from ammonia. Molten cyanide or sodium cyanide bath maintained at 560oC is quite effective within nitriding mostly if thin case is required. Plain C steel are not good for nitriding since iron nitride consequently formed is extremely brittle. Steels alloyed along with Al and Chromium and Nickel, Copper, Si and Mn are better nitrided than plain Carbon steel.
Carbonitriding, nitrocarburizing or gas cyaniding is a procedure similar to gas carburizing whether ammonia is also added to carburizing atmosphere. This procedure produces better hardened case than carburizing.