Effect of Annealing Temperature on Mechanical Behavior, Pitting Resistance and Grain Boundary Character of a 2304 Lean Duplex Stainless Steel

Abstract

The effect of the annealing temperature on mechanical behavior, pitting resistance, and grain boundary character of 2304 lean duplex stainless steel was studied. For this purpose, 4 mm thick hot-rolled sheets were cold-rolled up to a thickness of 1 mm, followed by isochronal annealing from 900 °C to 1100 °C for 180 seconds. Microstructural and CSL boundary analysis were carried out by electron backscatter diffraction technique. The SIM formation was examined by transmission electron microscopy and the phase distribution was quantified by X-ray diffraction. Mechanical properties were evaluated based on microhardness measurements and tensile tests, and the electrochemical analyses were performed by potentiodynamic polarization studies. The results revealed a Nishiyama–Wasserman relationship between α′-martensite laths and the metastable austenite after cold rolling. Increasing temperature enhanced the softening and ductility but decreased yield and tensile strengths of specimens. The deformation-induced martensitic transformation took place less actively and the strain-hardening rate decreased at higher temperatures. The improvement in ductility was related to the increasing fraction of $\sum 3$ boundaries in the austenite, whereas the strength worsening was associated with the increasing fraction of the $\sum {13b}$ boundaries in the ferrite as temperature increased. The nucleation of metastable pits changed from austenite to the ferrite phase and the pitting potential shifted to nobler values with increasing annealing temperature. The increasing pitting resistance of austenite was associated with the increase in the fraction of $\sum 3$ boundaries, while the decreasing pitting resistance of ferrite was related to the increased fraction of the $\sum {13b}$ boundaries at higher annealing temperatures.