Enhancing strength and ductility in the nugget zone of friction stir welded 7Mn steel via tailoring austenitic stability

Abstract

The martensite often appears in the nugget zone (NZ) of friction stir welding (FSW) 7 wt.% Mn steel due to low austenite stability, deteriorating ductility and toughness. In this work, a 7 wt.% Mn steel was subjected to FSW, and preheating was used to tailor the austenitic stability to greatly improve the strength–ductility combination of the NZ. The austenitic deformation behavior and strain hardening mechanism in the NZ were systematically investigated. The microstructure of the as-welded NZ was composed of ultrafine blocky ferrite, austenite, and small amounts of martensite, whereas the as-preheated NZ contained ultrafine blocky ferrite and austenite, and the concentration of Mn in austenite was increased from 8.4 wt.% to 10.7 wt.%. This enhanced the austenitic stability, resulting in a significant increase in the volume fraction of austenite in the as-preheated NZ from 37.3% to 66.4%. The product of strength and elongation (PSE) in the as-preheated NZ increased dramatically from 42.6 GPa% to 67.1 GPa%, depending on a persistent high strain hardening rate (SHR). Multiple strain-hardening mechanisms were revealed. The austenite with enhanced stability can provoke sustained transformation-induced plasticity (TRIP) and twinning-induced plasticity (TWIP) effects, and massive dislocation multiplication occurs during tension, resulting in strong strain hardening.