The Effect of Isothermal Heat Treatment on the Rolling Contact Fatigue of Carburized Low Carbon Microalloyed Steel

In case of the low carbon chromium steel that have widely been used for the carburized gears or rolling bearings, the austenite grain coarsening may be occasionally occurred during carburizing. To restrain this phenomenon, most of hot forged parts have been given to an isothermal heat treatment or normalizing immediately after hot forging and/or prior to carburizing. Therefore, their production cost includes unexpected additional expenses caused by such a non-value added process. To confirm the possibility of an energy saving, as well as attaining a superior fatigue life, in the production of automotive parts to be carburized, the austenite grain coarsening with the manufacturing process of the microalloyed steel, containing Nb and B as a grain refining elements, was investigated. The heat treatment characteristics and rolling contact fatigue behavior of the carburized specimens with the isothermal heat treatment were also investigated. In spite of omitting the isothermal heat treatment after hot forging, the abnormally coarse austenite grains were not found out in the carburized specimens. However, the rolling contact fatigue life of the carburized specimens, in which the isothermal heat treatment was omitted, was shorter than that of isothermally heat-treated specimens.