THE EFFECT OF SUB-ZERO TREATMENT ON THE CASE HARDNESS AND THE STRENGTH OF SOME CARBURIZED STEELS
The hardness distribution of carburized case, tensile strength and compressive strength were measured, after sub-zero treatment was applied to some Ni-Cr, Ni-Cr-Mo and Cr-Mo carburized steels. The results were as follows.
1) The carburized case of Cr-Mo (containing high % of Mo) and Ni-Cr-Mo steels were not so hard as we expected owing to the appearance of a large amount of retained austenite. But sub-zero treatment improved such a poor condition of carburized case, that is, the hardness became very high.
2) Generally speaking, the tensile strength of carburized specimens decreased after they were sub-zero treated, because the internal stress increased. But these specimens recovered their strength after tempering, because the internal stress was eliminated.
3) In general, the compressive strength of carburized specimens increased after they were sub-zero treated. This phenomenon can be explained by the same reason. But in Cr-Mo steels (containing high % of Mo), it was reverse. This phenomenon can be explained by the brittle carburized case with complex carbides in Cr-Mo steels.