Clarification of Damage Extension Mechanism on 2.25 Cr-1Mo Steel under Creep-Fatigue Loading Condition.

Abstract

High temperature components are subjected to creep-fatigue loading where creep cavities initiate and grow on grain boundaries. Development of a quantitative evaluation method of cavity growth is important for reliable meintenance. In this study, a creep-fatigue test was carried out at 600°C on 2.25 Cr-1Mo steel in a scanning electron microscope and continuous observation of cavity growth behavior was made. Based on the cavity growth observation, previously proposed cavity growth models were modified and a simulation result by the modified model was discussed by comparing with observed cavity. From the observation, spherical shape cavities initiate and grow up to their length of 2 μm on the grain boundary at initial stage of damage and these cavites change their shape to crack-like to grow until their length reaches around 10 μm. Both spherical and crack-like cavity growth equations, which consider diffusion and power low creep as driving force under constrained condition, were derived. Measured cavity growth rate is well expressed by the derived equations and a cavity growth simulation result corresponds to the change in the maximum cavity size with cycles.