Cyclic hardening, softening, and crack growth during high temperature fatigue

Abstract

High temperature total endurance tests were conducted on two alloys (316L steel and 9Cr–1Mo (wt-%) steel) at 550°C in high strain fatigue under continuous cycling at total strains in the range 0·6–2·0%. The austenitic steel cyclically hardened, whereas the ferritic steel cyclically softened. From striation spacing measurements on fracture surfaces (and to a lesser extent from observations of tensile load drop), it has been possible to correlate progress of the major crack with strategic points on the hardening/softening curves at each strain range. The minimum depth at which striations were measured with confidence was ~0·04 mm and by establishing growth laws it is demonstrated that this depth was typically achieved by the ‘plateau’ regions for both alloys. By extrapolation of the growth relationships it is concluded that cracking ~4–20 μm deep has occurred by the end of the earlier rapid hardening and softening phases.

MST/1813