Influence of Frequency on the High-Temperature Fatigue Crack Growth Behavior of 17-4 PH Stainless Steels

The influence of frequency on fatigue crack growth (FCG) behavior was investigated for three differently heat-treated 17-4 PH stainless steels, namely unaged “Condition A,” peak-aged “Condition H900,” and overaged “Condition H1150,” at 673 and 773 K, and at frequencies ranging from 0.002 to 20 Hz. At 773 K, all heat-treated conditions exhibited similar FCG behavior in which no frequency effect was observed at frequencies higher than 2 Hz and the fatigue crack growth rates (FCGRs) increased with decreasing frequency below 2 Hz. The increase in FCGR at a lower frequency at 773 K was thought to be caused by a time-dependent, oxidation-assisted cracking mechanism. At 673 K, for a given heat-treated condition, the FCGRs increased with decreasing frequency at higher frequencies and leveled off at lower frequencies. Such an anomalous FCG behavior was attributable to a dynamic strain aging (DSA) effect. At a given frequency, when the temperature was increased from 673 to 773 K, the FCGR increased in Condition H1150, but decreased in Conditions A and H900 due to an in-situ overaging and precipitate-coarsening effect during the FCG test for the latter.