Low temperature cross-rolling to modify grain boundary character distribution and its effect on sensitization of SS304

Abstract

Sensitization of austenitic stainless steels is well-known in the temperature range of 400–800°C. This effect can be lowered by the introduction of low-Σ coincident site lattice (CSL) boundaries in the material. In present work, 304 stainless steel was thermo-mechanically processed to obtain microstructures containing varied fractions of low-Σ CSL boundaries. Processing consisted of multi-step cross rolling at −41°C followed by a short heat treatment at 1000°C. In order to understand the effect of grain boundary character distribution (GBCD) on sensitization behavior, firstly, heating condition corresponding to the maximum degree of sensitization (DOS) was found by comparing double loop electro-chemical potentiodynamic reactivation (DL-EPR) plots for all the samples, which were given sensitization heat treatment at 600°C for various time. Later, a relation between the GBCD and DOS was established by conducting DL-EPR test on all the processed samples. Low-temperature cross rolling was found to be effective in keeping the fraction of low angle grain boundaries (LAGBs) low and ensuring equi-axed grain structure. The sample, which was given low strain followed by short heat treatment, showed highest fraction of CSL boundaries among all other conditions. Grain size was found to decrease with increase in pre-strain. The results indicated that sensitization was inversely related to the fraction of CSL boundaries and grain size, while it was directly related to the fraction of random high angle grain boundaries (HAGBs). It was also found that sensitization was weakly related to the fraction of LAGBs indicating that these boundaries did not show “special” behavior, as they do in many other grain boundary related phenomenon.