Influence of annealing temperature on the strain-hardening behavior of a lean duplex stainless steel

The influence of varying the annealing temperature from 1000 °C to 1200 °C on the strain-hardening behavior of a lean duplex stainless steel with a metastable austenite phase was investigated by tensile deformation. The results indicated that the tensile properties of the test steel were sensitive to the annealing temperature. The test steel showed enhanced ultimate tensile strength (UTS) and elongation due to the transformation-induced plasticity (TRIP) or/and the twinning-induced plasticity (TWIP) effects. The optimum combination of UTS and ductility with approximately 60 GPa% was obtained at an annealing temperature of 1050 °C. The shape of the strain-hardening curve for the selected annealing temperature range could be divided into two cases: (i) a typical three-stage strain-hardening from 1000 to 1050 °C and (ii) a multiple-stage strain-hardening from 1100 to 1200 °C. Microstructural observations revealed that the typical three-stage hardening was mainly related to a strain-induced martensitic transformation with a sequence of γ→ε→α', i.e., the TRIP effect. In addition to the strain-induced martensites, mechanical twins were observed in the deformed austenite of the specimens annealed at temperatures above 1100 °C. This result indicated that TRIP and TWIP occurred concurrently in the austenite at higher annealing temperatures. The synergy and mutual competition from the coexistence of TRIP and TWIP caused the multiple-stage strain-hardening.