The precipitation behavior and its effect on mechanical properties of cold-rolled super-ferritic stainless steels during high-temperature annealing

Abstract

The precipitation behavior, microstructure evolution and mechanical properties of a cold-rolled 27.6Cr–3.7Mo–2Ni ferritic stainless steel during high temperature annealing treatment were investigated. Experimental results demonstrated that the cold rolled microstructure was characterized by orientation-dependent heterogeneity, and many shear bands were found in the //ND (normal direction) orientation grains. The elongated deformation microstructure gradually changed into equiaxed grains and three kinds of interphases including σ-phase, χ-phase and Laves phase were observed as the annealing temperature increased from 850 to 1000 °C. After annealing below 1000 °C, the σ-phase and χ-phase were observed to be located at grain boundaries, near the TiN particles as well as along with shear bands in grains, whereas the Laves phase distributed dispersedly in grains. After annealing at 1000 °C, both the σ-phase and χ-phase were almost completely dissolved, while the nanoscale Laves phases at grain boundaries were dominant. The grain boundary Laves phase precipitation at high temperature was attributed to the Nb segregation induced by cold rolling deformation. The σ-phase precipitation kinetics was calculated based the Johnson–Mehl–Avrami equation, and the fastest precipitation kinetics appeared at 850 °C with the activation energy of ~176.7 kJ·mol−1. The Vickers hardness, ultimate tensile strength and yield strength values decreased while the elongation value increased with increasing annealing temperature. The breakage pattern transformed from brittle to ductile fracture as the annealing temperature increases from 850 to 1000 °C.

Keywords

• Ferritic stainless steel
• Precipitation kinetics
• σ-phase
• Laves phase
• Mechanical properties