THE EFFECT OF INITIAL MICROSTRUCTURE ON THE HARDENING OF 100CR6 BEARING STEEL

Bearing steels are used for parts requiring high hardness and wear resistance. Conventional hardening of the bearing steel is carried out at 840-870 degrees C after reheating of the initial structure of ferrite and carbides. At this temperature range, the hardening occurs without dissolving all carbides in order to prevent coarsening of austenite grains and formation of cracks in martensite plates following the quenching step. In this study, it is aimed to obtain a fine-grained initial microstructure by full austenitization before the hardening. For this purpose, the full austenitization followed by quenching to obtain the initial microstructure of martensite. Finer and well-distributed carbides reprecipitate at the hardening step. The effects of initial microstructure on phase transformations have been investigated by hardening of different initial microstructures namely martensite, martensite and carbide or ferrite and carbide, followed by the hardening treatment at 840 degrees C using a precision dilatometer. Accordingly, different heat treatment cycles also helped to understand the effect of different austenitizing temperature before the hardening step. Dilatometer tests were carried out to determine to critical transformation temperatures. Microstructural characterization and image analysis performed to determine the carbide size and distribution depending on the initial structure. It was found that phase transformations and final carbide size and distribution are sensitive to the initial microstructure.