Abstract: The black structure, non-mar tensite structure in Carboni trided case, is easy to occure in boron steel, when the heat treatment process is out of control, which reflects the decline of hardenability of case material. There are two kinds of black structure, surface black structure and transition region black structure,caused respectively by the inner-oxidation of alloying elements and the formation of carbide and nitride at the surface, and by the redistribution and variation of occurrence state of boron throughout the case.
In order to prevent the both black structures, high carbonitriding temperature, later supply of ammonia, large amount of kerosene and small amount of ammonia supplied in carbonitriding stage, and rapid quenching after the treatment are recommended.