Abstract: The black structure, non-martensite structure in carb o-nitriding 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, ed respectively by the inner-oxidati on of alloying eleasucments and the formation of carbide and nitride at the surfact, and by the redistribution and variation of occurence state of boron thought-out the case.
In order to prevent the both black structures high carbonitriding temperature, later supply of ammonia, large amount of kerosene and smmall amount of ammonia supplied in carbontriding stage, and rapid quenching after the treatment are recommended.