Microstructural refinement by spontaneous recrystallization without prior deformation of a 15-5 PH steel alloy and its mechanism

Abstract

The recrystallization without previous deformation is reported in literature for a small, selected group of alloys. The present work provides evidence for the first time that the commercial stainless steel 15-5 PH also shows this recrystallization phenomenon during austenitization. A set of in-situ and ex-situ high-temperature techniques reveal that, on heating of the martensitic microstructure, recrystallization takes place after phase transformation between 900 and 1000 ◦C, causing a distinct reduction of the austenite grain size. This work also shows that the recrystallization correlates with the mechanisms involved in the prior martensite to austenite transformation. It is observed that increasing heating rates lead to decreasing grain sizes. This is attributed to increased defect density in the reverted austenite and increased driving pressure for the nucleation of recrystallized grains. It is proposed that, during martensite to austenite reversion, a defect arrangement of highly stable low-angle grain boundaries and, with increased heating rate, an increased density of internal, grown-in dislocations is inherited from martensite laths. This highly defect-loaded microstructure, formed without external plastic deformation, leads to a recrystallization at increased temperatures. The experimental results agree well with thermokinetic calculations based on the proposed defect arrangement, underpinning the mechanism of spontaneous recrys- tallization in 15-5 PH.