Influence of slow strain rate tensile deformation on creep–fatigue endurance of 20Cr–25Ni–Nb stainless steel at 593°C

Abstract

Tests have been conducted on a 20Cr–25Ni–Nb stabilized stainless steel at 593°C during which a creep component was introduced into the tensile half of a fully plastic fatigue cycle. This was achieved either by the introduction of hold period at the maximum tensile strain of a rapid fatigue cycle or by the reversal of slow strain rate tensile deformation by rapid compressive straining. These cycles result in a significant reduction in fatigue endurance from that obtained during rapid fatigue cycling tests. The present results indicate that the most damaging cycle is one in which slow strain rate tensile deformation is reversed by rapid compressive straining. Here an order of magnitude reduction in life from that in cycles with equal rates of tensile and compressive straining was observed over a wide range of applied strain levels. Detailed metallographic examination of failed specimens has indicated that these life reductions occur because failure takes place by the interaction between a surface nucleated fatigue crack and internal grain boundary creep cavitation. A theoretical model based on this damage mechanism is presented which describes the experimental data obtained from the different cycle shapes.