Sequential nucleation of phases in a 17-4PH steel: Microstructural characterisation and mechanical properties

A major concern for 17-4PH steels operating at elevated temperatures is embrittlement due to Fe-rich (α) and Cr-enriched (α′) phase separation, along with precipitation of other detrimental phases. In this study the sequence of microstructural changes at the atomic scale in a 17-4PH steel is characterized by atom probe tomography (APT) at two different ageing temperatures, 480 °C and 590 °C. In the earliest stages of heat treatment at both temperatures, APT reveals that dislocations and matrix defects are highlighted by the segregation of NbN/CrN ionic species, providing heterogeneous nucleation sites for subsequent Cu-rich precipitates (CRPs) and Nb-rich precipitates, respectively. At the lower temperature, Cr-rich α′-phase also nucleates and ultimately a Mn, Ni, and Si-rich (MNS) phase was observed to form. The evolution in number density and fraction of CRPs and Cr-rich α′-phase, the latter of which was not observed at the higher temperature, has been quantified and their respective contributions to the overall precipitation hardening of the material has been estimated.