Hydrogen-Induced Microstructural Change under Mode U Fatigue for a Tempered Bearing Steel
Shinji FUJITA, Yoji MINE, Saburo MATSUOKA and Yukitaka MURAKAMI
Abstract:Slip band-like microstructural change formed under the cyclic shear stress coupled with hydrogen has a crucial importance to clarify the basic mechanism in the presence of hydrogen of Mode U fatigue failure. The authorsf previous research works have indicated that the slip band-like microstructural changes are formed by the hydrogen enhanced slip deformation, and this microstructural change occurrence due to the hydrogen is detrimental when compared with a surface inclusion for the fatigue fracture life. The slip band-like microstructural change was observed by the electron backscattering diffraction (EBSD), scanning ion microscopy (SIM) and transmission electron microscopy (TEM). Numerous Mode U fatigue cracks and slip band-like microstructural changes were observed only in the hydrogen-precharged specimens. The EBSD analysis revealed that the blocks in the martensite structure in the vicinity of the slip band-like microstructural change were arranged along the slip band-like microstructural change. The SIM and TEM analyses revealed that the slip band-like microstructural change consists of band structures and that structure differed from base tempered martensite. Therefore, it is presumed that cyclic shear stresses act on the base tempered martensite and hydrogen enhanced slip deformation in the hydrogen-precharged specimens, as a result base tempered martensite structure is deformed, and eventually produces slip band-like microstructural change along the two shear stress directions. Key Words:Hydrogen, Shear stress, Mode U fatigue crack growth, Bearing steel, Microstructural change