Effect of microstructure and cycling frequency on the torsional fatigue properties of 17-4PH stainless steel

Abstract Torsional fatigue tests in the high and very high cycle regime were performed with martensitic stainless steel 17-4PH. Tests were conducted with a servo-hydraulic machine and an ultrasonic equipment at cyclic frequencies of 15-35 Hz and 19 kHz, respectively. Fatigue lifetimes were significantly longer for smooth specimens tested at 19 kHz. Fatigue limits were not affected by testing frequency. This behaviour was explained by the presence of δ ferrite grains, in which crack initiation at conventional frequency is accelerated, but their size is too small to significantly decrease the fatigue limit. In the presence of detrimental defects – i.e. small drilled holes that were introduced into test specimens –, no frequency effect could be observed. The geometry of non-propagating shear cracks emanating from δ ferrite grains and in the martensitic matrix was measured by stepwise polishing of a run-out specimen. Fracture-mechanics considerations suggest that the threshold condition for the propagation of Mode-I cracks determines the fatigue limit under torsional loading.