The effect of thermal ageing and mechanical exposure on low cycle creep-fatigue strength of 316 steel at 625°C

Abstract

Comparisons have been made on evolutionary and stabilised cyclic stress-strain behaviour following different prior ageing treatments in a 316 steel. Prior stress-free ageing at a temperature higher than the test temperature reduces the ability of this steel to cyclically harden and consequently the material does not achieve maximum strength. An apparent saturation of cyclic hardening at room temperature can however, be continued by further cyclic hardening at 625°C. The rate of precipitation during low cycle fatigue can be two orders of magnitude greater than that occurring during stress-free ageing at 625°C. Quantitative measurements based on an extraction technique are presented and compared with particle size measurements in the literature. Prior high temperature low cycle fatigue exposure has been shown to reduce subsequent creep rates with a concomitant reduction in ductility: the same effect is found by prestrain at room temperature. Neither cyclic stress-strain properties nor creep behaviour can be fully restored by annealing after such prior deformation has taken place. On the other hand, once the material is fully cyclically hardened, stress-free ageing appears to have little further effect on properties. Other experiments on prestrain effects are reviewed and equations derived for their prediction.

Keywords:

• stress-free/fatigue-induced precipitation
• particle diameter
• cyclic stress-strain
• solution treating
• prior cold/hot work
• ductility