在常温下采用逐级降载法进行不同缺口位置、不同应力比下25Cr2Ni2MoV钢焊接接头的疲劳裂纹扩展试验，研究疲劳裂纹扩展曲线上近门槛值区转折点的变化规律。结果表明，焊接接头的疲劳裂纹扩展曲线在近门槛值区内存在一个转折点，其对应的应力强度范围随应力比的增大而减小，其大小为疲劳门槛值的0.5～0.8。近门槛值区疲劳裂纹扩展转折点前后扩展曲线斜率发生变化表明裂纹扩展机理的改变，热影响区缺口处的转折行为对疲劳裂纹扩展影响较小。应力比R低于0.5时，循环塑性区尺寸总体接近于材料特征组织尺寸；应力比R高于0.5后，两者偏差加大。应力比低于0.5时转折点对应的循环塑性区尺寸同特征组织尺寸接近，驱动力由K控制向Kmax主导的转变是引起转折点随应力比变化的原因。

Near-threshold fatigue crack growth tests are conducted at various stress ratios and different pre-cracking locations of a 25Cr2Ni2MoV welded joint by using load-shedding procedure at room temperature to investigate the transition behavior of fatigue crack growth curve. Results show that there exists a transition point in the fatigue crack growth curve in the near-threshold regime, and the stress intensity range at the transition point which is 0.5 to 0.8 times of the fatigue threshold decreases with the increasing of stress ratio. The different slope values of the fatigue crack growth curve before and after the transition point indicates the change of crack propagation mechanisms. The transition behavior with pre-cracks in the heat-affected zone shows slight influence on the fatigue crack growth behavior. The cyclic plastic zone size at the transition point is found close to the characteristic microstructure size when load ratio is below 0.5. It is indicated that the transition of crack growth driving force from stress intensity range K to maximum stress intensity Kmax is responsible for the load ratio influence on the transition point.