High strain rate superplasticity of a 25 Wt Pct Cr-7 Wt Pct Ni-3 Wt Pct Mo-0.14 Wt Pct N duplex stainless steel

Abstract

Effects of prior thermomechanical treatments on the superplasticity of a 25 wt pct Cr-7 wt pct Ni-3 wt pct Mo-0.14 wt pct N δ/γ duplex stainless steel have been studied by means of hot tensile testing with constant crosshead speeds. The objective is to increase the strain rate suitable for superplasticity. The strain rate is found to be markedly increased by a special prior treatment,i.e., solution treatment at temperatures in the δ single-phase region with subsequent heavy cold-rolling. In hot tensile tests at 1273 K, elongations greater than 1000 and 300 pct were observed at initial strain rates (έ) of 10⁻³ to 10⁻¹ s⁻¹ and 1 x 10⁰ s⁻¹, respectively. The results for strain rates 〈10⁻¹ s⁻¹ can be explained in terms of a structural superplastic effect due to grain refinement. In the case of έ 〉 10⁻¹ s⁻¹, transformation superplastic effects due to γ-phase precipitation from the σ-ferrite matrix are also important, especially in the early stages of deformation. In the equiaxedδ/γ microduplex structures during stable superplastic deformation, there exists a mixture of two different structures,i.e., dislocated and recovered/ recrystallized δ grains with a homogeneous dispersion of dislocation-free γ particles. This result shows that dynamic recrystallization ofδ grains occurs locally and intermittently due to the dispersion of relatively hardγ particles. The apparent average grain growth rate during deformation is small compared to static grain growth, because grain refinement due to dynamic recrystallization reduces the superplasticity-enhanced grain growth.