Microstructural Change near Retained Austenite Interface during the Formation Process of White Etching Area under Rolling-slip Contact

The microstructural change near interface between retained austenite and martensite during the formation process of White Etching Area (WEA) under rolling contact is investigated to characterize formation mechanism of WEA using the disk on roller type rolling contact fatigue equipment. These examinations were carried out under rolling/slip contact. When the slip ratio between a roller and a disk is 14%, acicular structure and WEA have been formed below the contact area. The sample has been investigated using Optical Microscope (OM), Scanning Ion Microscope (SIM), Transmission Electron Microscope (TEM) and X-ray diffraction equipment.It is found that acicular structures have been formed before an appearance of WEA. It is considered that acicular structures are formed by shear deformation of martensites. These are at angles of 30° or 160° with rolling contact surface. These angles have been suggested a direction of shear stress that acts on material under this experimental condition. In this microstructural change process, volume fraction of retained austenite has been decreased after the formation of acicular structures. The microstructural change of retained austenite occurs under different contact stress 4.2 GPa and 3.3 GPa, though hardness of both specimens is not change. Volume fraction of retained austenite under WEA formation process decreases with rolling contact time.In this microstructural change process, many voids have been formed near the interface between retained austenite and martensite. This is suggested that the interface acts as voids formation points in the formation process of WEA under rolling/slip contact.