高速工具钢的超长寿命S-N曲线特征和内部裂纹萌生行为

S-N CURVE CHARACTERISTICS AND SUBSURFACE CRACK INITIATION BEHAVIOUR IN ULTRA-LONG LIFE FATIGUE OF A HIGH-SPEED TOOL STEEL

为了研究高速工具钢超长寿命S-N曲线特征和内部裂纹萌生行为,使用沙漏形状试样在室温空气环境下进行旋转弯曲的疲劳试验。结果表明,材料的S-N曲线均由表面裂纹萌生型和内部裂纹萌生型组成。表面裂纹萌生型的S-N曲线位于高应力幅短寿命区,内部裂纹萌生型的S-N曲线位于低应力幅长寿命区。前者的S-N曲线向后者的转移几乎是连续的,回火温度对疲劳强度和寿命没有明显的影响。分别使用扫描电子显微镜、扫描探针显微镜和电子探针X射线微区分析仪对内部裂纹萌生位置和材料组织进行对比观察,在萌生内部裂纹的夹杂物周围存在一个颗粒区域,该区域内凸起的颗粒是组织中的球形碳化物。颗粒区是在内部裂纹扩展的应力强度因子门槛值范围以下形成的,对超长寿命疲劳起着重要的作用。

In order to investigate S-Ncurve characteristics and subsurface crack initiation behaviour of high speed tool steel tempered at two temperatures in ultra-long life fatigue, cantilever-type rotating-bending fatigue tests are performed in laboratory air at room temperature using hour-glass-shaped specimen. The results show that the testing material can be classified into two S-N curves corresponding to the surface induced fracture and the internal inclusion induced fracture, respectively. The S-N curve for surface induced fracture is at high stress amplitudes and low cycles, while the S-N curve for internal inclusion induced fracture is at low stress amplitude and high cycles. The transition of S-N curve from the former to the later is almost continuous. The S-N curve characteristics are not affected by the tempering temperatures. The fracture origin in the vicinity of an internal inclusion and the microstructure of the material are observed and compared in detail by using SPM, SEM and EPMA, respectively. A granular area exists in the vicinity of the inclusion. Some convex particles in the granular area are the spherical carbide in microstructure. The granular area forms under the threshold stress intensity factor of internal crack propagation, and plays an important role in ultra-long life fa- tigue process.