在100℃含氧量小于1 mg·L^-1的饱和蒸汽环境中，对Custom 450沉淀硬化型不锈钢在三种应力比（-1，-0.6，0.1）下进行了轴向等幅力控制的超高周疲劳试验，观察了疲劳断口形貌并研究了其疲劳断裂机制。结果表明：试验钢的S-N曲线没有水平段，始终保持下降趋势，其疲劳极限消失；疲劳断口呈现从表面缺陷、内部夹杂物和内部结构不连续三个位置处形成裂纹源的起裂模式；随着应力比的提高，表面形成裂纹源的概率增大，内部形成裂纹源的概率降低。

In the saturated steam with oxygen content less than 1 mg·L^-1 at 100℃, axial constant-amplitude force controlled fatigue tests were conducted on the precipitation hardening stainless steel Custom 450 with three stress ratios (-1, -0.6, 0.1). The fatigue fracture morphology was observed, and the ultra-high cycle fatigue fracture mechanism was investigated. The results show that the S-N curves of the tested steel presented a continuous decline shape without a horizontal asymptote, indicating that the tested steel had no fatigue limit. Three crack initiation patterns, namely forming crack sources at surface defects, internal inclusions and internal defects, were observed on the fracture surfaces. With the increase of stress ratio, the probability of the surface crack initiation increased while that of the internal crack initiation decreased.