Effect of tempering temperature on very high cycle fatigue properties of high strength steel

To investigate the effect of tempering temperature on the very high cycle fatigue properties, uniaxial tension-compression fatigue tests of Ni-Cr-Mo steel (SNCM439, corresponding to AISI 4340 steel) which had been tempered, either at 433K (640Hv) or 573K (543Hv) were carried out. The fracture origins in 104 - 105 cycle range located at the specimen surface, whereas in the 105 - 108 cycle range the fracture origins were sub-surface. The fatigue limits for the surfaceinitiated failures were in good agreement with the conventional expression σw0 ≈ 1.6·Hv. However, in the the fatigue limit in the sub-surface crack-initiation range was lower for the harder condition. SEM examination showed that the type of inclusion at the failure origin depended upon the tempering temperature. This dependency may be the cause of the unusual relation between hardness and fatigue strength in the very high cycle fatigue range.