It is a surface hardening process, often applied on low carbon steels which don’t respond to heat treatment. The process consist in packing the steel piece in charcoal powder and covered from outside. The charcoal powder cuts of air. While heated in a furnace the carbon from charcoal penetrates the surface and upon quenching the carburized surface retains stiffness or hardness. The process is also termed as carburizing and steels containing 0.1 to 0.25% C are easily carburized. The mechanism of hardening is twofold. Firstly the carbon of iron is extremely hard and because of quenching this is retained in the surface. Secondly because of increase in C in the surface layer, the residual compressive stress is produced. Surface hardening is advantageous in case of gear teeth because the inner bulk material still keeps softer and tougher and hence combined advantage of harder surface and tougher core mereases the life.
This may be pointed out that surface is the weakest region of the material in any type of form. Fatigue cracks initiate late in stiff or hard surface and the wear resistance in better also.
Besides carburizing by packing there are other methods of surface hardening of steels. In gas carburizing the metal is heated in gas atmosphere in controlled manner to ignor oxidation and allows absorption of the gas in the surface. Gasses utilized for the purpose are coke oven gas, natural gas, propane or butane. In the process it named as cyaniding the steel part is heated covered along with the mixture of potassium ferrocyanide and potassium bichromate. In several cases the mixture is replaced by powdered potassium cyanide. An extremely hard case is produced by cyaniding. A thin hardened layer is produced by immersion of part in the heated cyanide solution. Natriding is now another method of case hardening. This consists in heating the part in the presence of not associated ammonia in the range of 570°C to 61 0°C.
The surface hardness of 60 Rockwell C associated along with core hardness of 33 to 38 RC is produced by carburizing. Nitriding can produce surface hardness upto 70 RC and core stiffness or hardness of 27 to 47 RC. Natrided parts could be tempered. Each steel can be nitrided however those containing aluminum develop extremely high hardness. For gears Nitralloy 135 and 135 modified is a highly preferred material. This contains Carbon-0.3510.41, Mn-0.55, Si-0.3, Chromium-1.2011.60, AlIuminium-1.00, Mo-0.2010.35. This material enhances surface hardness of 65-70 and core hardness of 30-36 RC. AISI 4340 as Carbon-0.40, Mn-0.70, Si-0.30, Chromium-0.80, Mo-0.25, Nickel-1.00 and AISI 4140 as C-0.40, Mn-0.90, Si-0.30, Chromium-0.95, Mo-0.20 are two other steels that are nitrided and utilized to make gears.
The high frequency current while passed through body of a part results in surface heating. Once heated such way, the part is quenched in water. This type of treatment resulting into hardened surface is named as induction hardening for which steels containing 0.4 to 0.5% C is excellent.