42CrMo exhibits good fatigue strength, typically in the range of 400-500 MPa for a smooth specimen at 10^7 cycles (fatigue limit), which is roughly 40-50% of its tensile strength. Key influencing factors are:
Surface Finish: Critical. Machining marks, grinding burns, or decarburized surfaces act as stress raisers and initiate fatigue cracks. A smooth, polished surface significantly improves fatigue life.
Residual Stresses: Compressive residual stresses on the surface (induced by shot peening, surface rolling, or nitriding) greatly enhance fatigue strength by counteracting applied tensile stresses that drive crack propagation.
Material Defects: Non-metallic inclusions, especially at or near the surface, can be crack initiation points.
Design: Sharp corners (poor fillet radii) create stress concentrations.
Core Strength: A strong, tough core (achieved through proper through-hardening) supports the surface and prevents crack propagation.
For cyclic loading applications, specifying a good surface finish and considering surface enhancement processes is crucial.