高钴钼不锈轴承钢疲劳裂纹萌生及扩展行为

Fatigue crack initiation and propagation behavior of high cobalt molybdenum stainless bearing steel

采用高钴钼不锈轴承钢光滑圆柱形试样和缺口试样(理论应力集中系数Kt=3)进行旋转弯曲疲劳测试,研究了高合金轴承钢的裂纹萌生及裂纹扩展行为。用升降法和成组法分别测得轴承钢的疲劳极限和S-N曲线,利用扫描电镜对轴承钢旋转弯曲疲劳试样断口进行观察。结果表明,光滑试样起裂类型为单源萌生起裂,起裂源为表面缺陷和次表面夹杂物,表面缺陷为表面粗糙度、驻留滑移带和加工凹痕,次表面夹杂物为Al2O3-CaO-MgO-SiO2复合夹杂;缺口试样疲劳极限显著下降,起裂类型为多源萌生起裂,计算得轴承钢的缺口敏感系数qf为1.18。光滑试样疲劳破坏从以高应力幅粗糙度萌生表面裂纹的破坏向低应力幅驻留滑移带、加工凹痕、夹杂缺陷萌生裂纹转移。疲劳裂纹萌生寿命占整个疲劳寿命的94.1%以上。

The crack initiation and propagation behavior of high cobalt molybdenum stainless bearing steel was studied by rotating bending fatigue test with smooth cylindrical specimens and notched specimens (theoretical stress concentration factor Kt =3).The fatigue limit and S-N curve of bearing steel were measured by up-and-down method and group method,respectively.The fractures of the specimens were observed by scanning electron microscopy.The results show that the cracking type of the smooth specimens is single source initiation.The crack source is surface defects and subsurface inclusion.The surface defects are surface roughness,persistent slip band and machining dent,while the subsurface inclusion is Al2O3-CaO-MgO-SiO2 composite inclusion.The fatigue limit of notched specimens is significantly decreased.The cracking type of the notched specimens is multi-source initiation.The notch sensitivity factor qt of bearing steel is 1.18.The fatigue failure of the smooth specimens is transferred from the surface roughness with high stress amplitude to the persistent slip bands,the machining dents and the inclusions with low stress amplitude.The fatigue crack initiation life accounts for more than 94.1% of the whole fatigue life.