Thermal Stability of Rolled Metastable Austenitic Stainless Steel 1.4307 Studied Using Positron Annihilation

Abstract

Positron annihilation spectroscopy (PAS) was used in the study of cumulative isochronal and isothermal annealing of rolled 1.4307 (EN) stainless steel (SS). Due to different rolling temperatures, the SS samples varied in the α′-martensite volume fraction from 0.09 to 0.91. The measurements of positron annihilation characteristics, i.e., Doppler broadening (DB) of the annihilation line, showed a gradual annealing of vacancies in the temperature range between 200 °C and 400 °C, which indicated the first stage of recovery. This first stage of recovery did not change the microhardness. In the temperature range from 475 °C to 600 °C, a decrease in the microhardness and generation of new open volume defects accompanying the reversion of α′-martensite to austenite was observed. The amount of these defects correlated with the initial α′-martensite volume fraction. Their formation could be related to the volume contraction occurring during bcc/fcc transformation. The different chemical surroundings suggested that the positron trapping defects were associated with the metal carbide precipitates.