缺口对渗氮CrNiW钢超高周疲劳特性的影响

 为了研究缺口敏感性对渗氮CrNiW钢疲劳特性的影响,设计了一种光滑试样和两种缺口试样(应力集中系数K_t=1.20、1.55),开展了渗氮CrNiW钢的超高周疲劳试验,使用SEM观测疲劳断口的微观结构,分析了疲劳缺口系数K_f、疲劳敏感系数q和循环周次N_f之间的关系。结果表明,渗氮对钢材疲劳性能的提升不大,光滑试样和K_t=1.20的缺口试样存在表面和内部两种起裂模式,诱发内部起裂的缺陷均位于内部非渗氮区,K_t=1.55时发现一种新的渗氮层内不均匀基体组织诱发的失效模式。光滑试样未出现双重S-N特性,K_t=1.20时S-N曲线出现表面与内部起裂双重S-N特性,K_t=1.55时出现短寿命区和超高周寿命区表面起裂双重S-N特性。随着K_t的增大,多个内部缺陷同时萌生裂纹的现象被单裂纹源诱发失效的现象取代,并且诱发失效的内部缺陷位置呈现向试样表面靠近的趋势,鱼眼形状呈近似横向扁形→圆形→纵向椭圆形变化。缺口应力集中对表面起裂疲劳寿命影响较大,对内部起裂疲劳寿命影响很小。

Influence of notch on fatigue properties of nitrided CrNiW steel in very high cycle regime

In order to study the influence of notch sensitivity on fatigue properties of nitrided CrNiW steel,a smooth specimen and two notched specimens with concentration factor K_t=1.20 and K_t=1.55 were designed,respectively.The very high cycle fatigue tests were carried out by using nitrided CrNiW steel.The microstructure of the fatigue specimen was observed by SEM.The relationship of notch fatigue factor K_f,notch sensitivity index qand number of cycles to failure Nfwere analyzed.The results show that,the case-nitriding has little effect on the improvement of fatigue strength of steel.For the smooth specimen and the notch specimen with K_t=1.20,the failure modes can be classified as surface induced failure and the interior induced failure where the defects are limited in the non-nitriding area.For the notch specimen with K_t=1.55,interior failure induced by the inhomogeneous matrix area within the nitrided layer is observed,which is a new failure characteristic.The duplex S-Nproperty of smooth specimen is not obvious.However,there is a duplex S-N property with surface induced failure and interior induced failure as K_t=1.20,and there is a duplex S-N property only with surface induced failure which occurs in the short life regime and the VHCF regime as K_t=1.55.With the increase of K_t,the phenomenon of multiple interior crack sites is replaced by the single interior crack site,and the interior defect position gradually moves towards the surface of specimen,and the shape of fisheye first approximately exhibits the oblate with major axis at xdirection,followed by the circular shape,and then the elliptical shape with major axis at ydirection.The K_thas a large effect on fatigue life with surface failure,but has a little effect on fatigue life with interior failure.