The oldest known method of producing a hard surface on steel is case hardening or carburizing. The steel used for this purpose is usually a low - carbon steel of about 0.15 per cent carbon, which does not respond appreciable to heat treatment. In course of the process, the outer layer is converted into a high carbon steel with a carbon content ranging from 0.9 to 1.2 % carbon. If it receives proper heat treatment , it will have an extremely hard surface on the outside and a soft ductile core.
This process, in brief is merely heating iron or steel to red heat , in contact with some carbonaceous materials such as wood , bone or leather charcoal, with compounds such as carbonates of barium, calcium or sodium which are termed "energizers". These energizers are added with the materials to increase the concentration of the carbon monoxide and thus improve the rate of carburizing. Iron, at temperatures close to and above its critical temperature, has an affinity for carbon. The carbon thus enters the metal to form a solid solution with iron and converts the outer surface into high carbon steel.
On completion of the carburizing treatment the composite steel, having a high carbon case and low carbon core, is heated in different ways depending on the results desired. Since there is some grain growth in the steel during the prolonged carburizing treatment, the work should be heated to the critical temperature of the core and then cooled, thus refining the core structure. The steel should then be reheated to a point above the transformation range of the case and quenched to produce a hard , fine structure. The lower heat-treating temperature of the case results from the fact that steels having high amount of carbon are normally austenized for hardening just above the lower critical temperature.
Alloy steels are usually quenched in oil and carbon steels in water . This treatment will produce a hard fine - grained case, while the core of the structure will retain the properties of low carbon steel.
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