Influence of frequency on high-temperature fatigue behavior of 17-4 PH stainless steels

The effect of frequency (2 and 20 Hz) on the high-cycle fatigue and fracture behavior was investigated at 573-773 K for 17-4 PH stainless steels in three conditions: Condition A (unaged), H900 (peak-aged) and H1150 (overaged). S-N results indicated that at 573 and 673 K, there was generally no difference in fatigue strength between 2 and 20 Hz, except for H900 at 673 K where the fatigue strength at 2 Hz was lower than that at 20 Hz. At 773 K, the fatigue strength of each condition at 2 Hz was lower than that at 20 Hz due to the occurrence of creep mechanism at this low frequency. At 773 K and 2 Hz, the fatigue fracture mode exhibited a mixed mode involving transgranular and intergranular cracking and the grain boundary cavities were also observed. At a given temperature and frequency, the fatigue strength for the three conditions generally took the following order: H900 > Condition A > H1150, except for Condition A at 773 K in the long life regime where the fatigue strength was close to that of H1150 due to a precipitate-coarsening effect. With the exception of Condition A tested at 673 K, the fatigue strength of each condition was decreased with increasing temperature as a result of a reduction in yield strength. At both frequencies, the fatigue strength of Condition A at 673 K was greater than that at 573 K as a result of an in-situ precipitation-hardening effect. Fractography observations indicated that the fatigue crack initiation site, crack propagation path and fracture surface morphology were functions of testing temperature, loading frequency and applied cyclic stress level.