Evolution of the microstructure of a 15-5PH martensitic stainless steel during precipitation hardening heat treatment

Abstract

The mechanical properties of precipitation-hardened stainless steels rely on a complex multi-scale microstructure developed during a sequence of quenching after austenitization, followed by a precipitation heat treatment. Important features of the resulting microstructure include the microstructure of martensite, retained and reverted austenite, nanoscale precipitation and the homogeneity of the Cr concentration. In this paper, the microstructure of a Cu-bearing 15-5PH steel is thoroughly characterized along the precipitation heat treatment, using a combination of transmission electron microscopy with phase and orientation mapping, atom probe tomography, in-situ small-angle X-ray scattering and X-ray diffraction. The fraction of austenite is observed to increase during the ageing treatment, together with the precipitation of the Cu precipitates, which present a core-shell structure with a shell enriched in Ni, Mn and Si. After heat treatment, the Cr concentration is found to be slightly inhomogeneous in the matrix, with some segregations at the dislocations.