Effect of post weld heat treatment on grain boundary character distribution and corrosion resistance of friction stir welded armour-grade nickel and molybdenum-free high-nitrogen austenitic stainless steel

This study examines the effects of friction stir welding (FSW) and post-weld heat treatment (PWHT) on the microstructure and corrosion resistance of nickel- and molybdenum-free high-nitrogen austenitic stainless steel (HNASS). FSW at 400 rpm and 30 mm/min resulted in finer grains (4.03 μm) and higher Σ3 twin boundaries (32.3%) at the top of the stir zone (SZ) due to dynamic recrystallization (DRX). PWHT at 900 °C for 1 hour led to grain coarsening (12.91 μm at the bottom SZ) but enhanced Σ3 boundaries from 53% to 57.7%, improving grain boundary stability. PWHT reduced the kernel average misorientation (KAM) by 14.9% in the SZ-top layer and 20.4% in the SZ-bottom layer, accompanied by a 25% decrease in hardness in the SZ-top layer and 26.7% in the SZ-bottom layer, indicating strain recovery and reduced dislocation density. Potentiodynamic polarization tests (PDP) showed a 18% increase in pitting potential and a 76% reduction in corrosion rate after PWHT. The improvement in corrosion resistance is attributed to the increase in Σ3 twin boundaries, which enhance grain boundary stability and reduce susceptibility to localized corrosion. These findings highlight the role of PWHT in refining the microstructure and strengthening corrosion resistance, making HNASS a promising material for demanding applications.