Phase transformation and recrystallization of cold-rolled AISI 304L austenitic stainless steel during annealing

Abstract

Refining the heat-affected zone (HAZ) microstructure of thermomechanically welded cold-worked AISI 304L austenitic stainless steels (ASSs) improves the weld quality. This study explored the annealing behavior of cold-rolled AISI 304L ASSs through heat treatments over temperatures ranging from 600 °C to 1200 °C and holding times ranging from 2 min to 480 min. The microstructure was analyzed using optical microscopy and EBSD, and the deformation-induced martensite (DIM) content was evaluated using the ferrite scope. Vickers hardness values were correlated with the microstructure evolution following the Hall-Patch relationship. The grain size distribution and the kinetics of grain coarsening were analyzed. Results show that during annealing, the reverse transformation of DIM occurred, followed by static recrystallization of the γ-austenite phase. After recrystallization, grains coarsen with an activation energy of 133.8 kJ/mol, and grain size distribution fits a log-normal function. Nanoscale grains (< 180 nm) were achieved in cold-rolled samples (67 % thickness reduction) annealed at 700 °C for 4 h. The δ-ferrite, primarily located at γ-grain boundaries, retarding their movement during recrystallization and coarsening. Finally, the δ-ferrite partially transformed into austenite and globularized during annealing. These findings show that the processes of phase transformation and recrystallization in cold-worked dual-phase steels are coupled.