<P> Carbonitriding is similar to cyaniding except a gaseous atmosphere of ammonia and hydrocarbons is used instead of sodium cyanide . If the part is to be quenched, it is heated to 775--885 ° C (1,427--1,625 ° F); if not, then the part is heated to 649--788 ° C (1,200--1,450 ° F). </P> <P> Ferritic nitrocarburizing diffuses mostly nitrogen and some carbon into the case of a workpiece below the critical temperature, approximately 650 ° C (1,202 ° F). Under the critical temperature the workpiece's microstructure does not convert to an austenitic phase, but stays in the ferritic phase, which is why it is called ferritic nitrocarburization . </P> <P> Parts that are subject to high pressures and sharp impacts are still commonly case - hardened . Examples include firing pins and rifle bolt faces, or engine camshafts . In these cases, the surfaces requiring the hardness may be hardened selectively, leaving the bulk of the part in its original tough state . </P> <P> Firearms were a common item case - hardened in the past, as they required precision machining best done on low carbon alloys, yet needed the hardness and wear resistance of a higher carbon alloy . Many modern replicas of older firearms, particularly single action revolvers, are still made with case - hardened frames, or with case coloring, which simulates the mottled pattern left by traditional charcoal and bone case - hardening . </P>

Which form of pure carbon is so hard that it can be used in cutting tools