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

E Dryzek,M Wróbel,M Sarnek

doi:10.1007/s11661-018-5053-x

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.

Highlights

AUSTENITIC stainless steel (SS) is plastically deformed by a slip of dissociated dislocations because of low stacking fault energy (SFE)
The purpose of the present study was to investigate the use of the Positron annihilation spectroscopy (PAS) technique to study 1.4307 (EN) SS samples deformed by rolling at different temperatures and to a much higher degree of deformation compared to our previous studies
Peaks from the c phase are predominant in comparison to two other samples deformed at lower temperatures, i.e., samples A and B deformed close to the liquid nitrogen (LN) and room temperature (RT), respectively

Summary

Introduction

AUSTENITIC stainless steel (SS) is plastically deformed by a slip of dissociated dislocations because of low stacking fault energy (SFE). This effect produces hardening due to an increase in dislocation density (dislocation hardening), and due to stacking faults and mechanical twins formation.[1,2] a large part of strain hardening results from the creation of deformation-induced martensite (DIM) caused by thermodynamic metastability of austenite at, or just below, room temperature (RT).[3,4] Annealing of cold-worked austenitic SS causes martensite reversion along with other phenomena including recovery, recrystallization, and carbide precipitation.[5,6] Several experimental techniques, such as microscopic examination, X-ray diffraction (XRD), and magnetic or mechanical properties measurements, have been applied to study the properties of cold-worked austenitic SS during annealing.

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