Abstract
Quenching of steels normally produces heighest hardness at the surface and successively lower hardness into the center of the article. In oil quenching of bearing grade steels, however, an inverse hardness distribution (called inverse quench-hardening, hereinafter) is sometimes found. In this investigation, the inverse quench-hardening was examined. The main results are as follows
(1) The inverse quench-hardening is caused by a rapid change of cooling rate during cooling. This phenomenon is particularly liable to occur when the steels are quenched into oil having long vapor blanket cooling stage and a low characteristic temperature.
(2) This phenomenon occurs more often at the upper part than the lower part of the article treated. This is due to that the vapor blanket cooling stage is longer at the upper part than the lower part.
(3) The inverse quench-hardening is attributed to higher consumption of incubation period for perlitic transformation at the surface than at the center of the steel during vapor blanket cooling stage in quenching. In such a case, the hardening at the surface becomes difficult even by rapid cooling in the following boiling stage.
