fine-grained duplex stainless steel by cold rolling and reversion/recrystallization annealing

Alloy 2205 is the most widely used duplex stainless steel (dss), which is composed of austenite and ferrite phases in about equal amounts. superior mechanical properties and acceptable corrosion resistance are attainable by adjusting the grain size in the absence of detrimental sigma phase. in the present work, this aim was achieved by cold rolling followed by annealing at elevated temperatures corresponding to the equal amounts of ferrite and austenite. cold rolling led to the formation of deformation-induced martensite and work-hardening of austenite, which manifested itself into an increased hardness of 433 hv compared to the hardness of 254 hv for the as-received sample. the formation of martensite was investigated based on the x-ray diffraction (xrd) analysis, where the reduction in thickness of 50% resulted in the formation of only 6.2 vol% martensite, which reveals the high mechanical stability of the austenite phase in this material when compared to the metastable austenitic grades such as aisi 304l stainless steel. annealing at 1120 °c resulted in a fine grain grained microstructure with an average grain size of 4 µm after 1 min. this fine grained alloy showed a higher hardness of 266 hv compared to the as-received alloy with an average grain size of 11 µm and hardness of 254 hv. shorter annealing times resulted in the appearance of partially recrystallized microstructures with pancaked grains surrounded by some fine and freshly formed recrystallized grains; while longer annealing times led to the grain coarsening in both austenite and ferrite phases with the resulting hardness reduction. for instance, after annealing for 5 and 120 min, the average hardness values of 249 and 238 hv were obtained, respectively. this was resulted from the average grain sizes of 7 and 14 µm, respectively. the cold rolled sample showed high strength and low ductility based on the tensile stress–strain curves. however, annealing at 1120 °c for 1 min led to strength drop but enhanced total elongation due to the occurrence of primary recrystallization. accordingly, the yield stress of 582 mpa, ultimate tensile strength of 847 mpa, and elongation to failure of 64% were obtained. longer annealing times resulted in the drop of yield stress and ultimate tensile strength. the hall–petch law for the yield stress (ys) was used to quantify the effect of the average grain size (d). accordingly, the hall–petch plot for the duplex 2205 based on the average grain size resulted in the equation of ys (mpa) = 349.2/√d + 409.2, in which ys can successfully be correlated to d.