Cyclic softening behaviors of modified 9–12%Cr steel under different loading modes: Role of loading levels

Abstract

Cyclic softening behavior of modified 9–12%Cr steel was investigated at 873 K under different loading modes and stress levels. Results indicated that two domains were identified depending on the plastic strain level, in which cyclic deformations and control modes were different. Significant different cyclic softening was only observed in the macroplasticity domain at high loading levels between stress and strain controlled fatigue, companied with a relatively homogenous coarsening of subgrain. By contrast, in the microplasticity domain at low loading levels, slight cyclic softening was attributed to the fact that back stress remained a relative stable value (or decreased very slightly). An additional back stress component was produced due to the heterogeneous subgrain coarsening at the inter-packet scale. As a result, the cyclic softening was retarded due to the competition between the hardening of inter-packet back stress and the softening of intragranular back stress.