Volume 7, Issue 2 (February 2010)
Microstructure and Fatigue Strength of the Bearing Steel 52100 after Shortened Bainitic Treatment
Quenching to obtain martensite is the mostly applied process for standard rolling element bearings. Isothermal treatment in the lower bainitic range is used as an alternative method to generate favorable compressive residual stress on the surface of components, e.g., in spherical roller bearings. The duration of the bainitic treatment, however, is much longer than that of a martensitic treatment because more or less a complete transformation of austenite to bainite is usually requested. This causes higher energy consumption and a longer production period. Therefore it is desirable to perform bainitic treatment with a shortened process duration. In the present work possible processes for shortening the bainitic treatment of the bearing steel 52100 were primarily investigated by dilatometric experiments. Some selected processes were carried out in an industrial salt bath. The microstructures of bainite were observed by optical microscope, transmission electron microscope, and field emission scanning electron microscope. These were compared to martensitic microstructures. The cyclic fatigue strength of the steel after shortened bainitic treatments was examined using a rotating-bar fatigue test. The results show that the fatigue resistance while maintaining the requested minimum hardness of 58 HRC was even enhanced significantly through the shortened treatments particularly by means of a two-step bainitic treatment. The process duration was only about 25 % of the conventional time. The influence of the bainitic microstructure on the fatigue strength of the steel is discussed.