Primary creep of TiN dispersion hardened 20%Cr-25%Ni stainless steel at 160 MPa and 1123 K—I. Deformation behaviour

Abstract

The development of the microstructure of nitrided 20%Cr 25%Ni 1.8% Ti stainless steel through primary creep at 160 MPa and 1173 K is investigated. It is found that up to ~3% strain the dislocation density in the matrix increases, but thereafter falls. Dislocation tangles are generated around TiN particles as strain proceeds, and are to some extent replaced by subgrain boundaries towards the end of primary creep at ~11%. It is proposed that at less than 3% strain the flow stress of the material is increasing with work hardening components provided both by the increasing matrix dislocation content and also by the tangle structure. The contribution of the latter is modelled using parabolic work hardening theory, which, however, overestimates the hardening rate. It is therefore suggested that recovery is also occurring during the hardening régime, a hypothesis supported by observations of the behaviour of the matrix dislocation network. The “unmixing” of the microstructure at higher strains, with the matrix dislocation content falling, the matrix network tending to increase in perfection and the tangles at TiN particles intensifying, is interpreted in terms of a redistribution of the components of the macroscopic flow stress driven by a reduction in the stored energy of the system (recovery). The Garofalo equation is used to describe the primary creep curve, and displays excellent fit with the experimental data again above 3–3.5% strain. It is argued that this expression may therefore have a physically real foundation, but that the latter is limited to the recovery-controlled part of primary creep. At lower strains, a phenomenological equation based on thermally activated glide provides a better description of the strain/time behaviour.