Tempering of a martensitic stainless steel: Investigation by in situ synchrotron X-ray diffraction

Tempering of a martensitic stainless steel is investigated by means of in situ X-ray diffraction using high-energy synchrotron radiation. The simultaneous evolutions of the fractions of martensite/ferrite, retained austenite, M23C6 and M2X (X = N,C) precipitates are determined during a continuous heating, and show that precipitation of M2X can be observed directly only above 650 °C, concomitantly to the transformation of austenite into ferrite. However, a careful cross-analysis of the evolution of the lattice parameters of all phases shows the precipitation of metastable carbides/nitrides at low temperatures, followed by some stress relaxation. Between 500 and 650 ° C, the lattice parameters of M2X feature changes associated with the evolution of their composition, as supported by the analysis of additional isothermal agings, and consistently with the literature. Finally, the complex variations of the lattice parameters of austenite are analyzed thanks to a simple micromechanical model, supporting the previous conclusions and highlighting the subtle interplay between precipitation and stress relaxation.