窒化処理を施した構造用鋼SCM435の異物混入潤滑下における転がり疲労特性評価

抄録

In this study, the rolling contact fatigue (RCF) life of nitrided Cr-Mo steel, which has three types of different nitride compound layers, was investigated under contaminated lubrication. Specimen-γ’ has nitride compound layer which crystal structure is adjusted to γ’-Fe₄N. Specimen-ε and specimen-ε_por have the ε-Fe_2-3N type nitride compound layer, however specimen-ε_por has a porous layer on its outermost surface. To investigate the crucial factors on RCF life, material characteristics of the surface such as Vickers hardness, residual stress, resistance to indentations were evaluated. Changes of surface topography around Vickers indentation were measured through the RCF test under clean lubrication. As a result, the specimens with ε-Fe_2-3N type nitride compound layer showed higher RCF fatigue life than that of specimen with γ’-Fe₄N. The RCF life of specimen-ε_por was the longest among the specimens, which was about 3-5 times longer than the RCF life of specimen-γ’. The rate of reduction in unevenness around the indentations was dramatically improved by the wear of the friable porous layer on the surface of specimen-ε_por. Thus, the specimen-ε_por with the hard ε-Fe_2-3N type nitride compound layer and the friable porous layer kept the RCF life long under contaminated lubrication, because the stress concentration around the indentations decreased due to smooth indentation edge on the ε_por surface.