Improving Rolling Contact Fatigue Life of （提高滚动接触疲劳寿命）.pdf

NTN TECHNICAL REVIEW No.71 2004 Technical Papers Improving Rolling Contact Fatigue Life of Bearing Steels Through Grain Refinement Chikara OOKI Kikuo MAEDA Hirokazu NAKASHIMA It is well known that yield strength can be improved by reducing the ferritic grain size (i.e. Hall-Petch’s Law) and that through a reduction in the ferritic grain size an increase in a steel’s fatigue life is observed. However, the specific effect of grain refinement on rolling contact fatigue life has not been thoroughly investigated. The primary obstacle in researching hardened steels (which possess a martensitic structure) has been the difficulty in obtaining small, uniform grain sizes. Recently, some new methods for the grain refinement of hardened steels have been discovered. One such method is called “Ausforming”, which induces a large deformation at temperatures over Ac1. Unfortunately, this remarkable method is as yet unavailable for practical manufacturing. Keeping this in mind, we set out to develop a specialized heat-treatment process. The main objective was to obtain a grain refined martensitic structure within JIS- SUJ2 (SAE52100 equivalent) bearing steel, produced by a standard manufacturing process, while at the same time minimizing productivity loss. Thus far, we have succeeded in creating a prior austenite grain size of approximately 5 m in diameter (half that of the conventional grain size). It is believed that the packet or block size