The effects of martensitic transformation and (Fe, Cr)23C6 precipitation on the properties of transformable ferritic stainless steel

Abstract

Hot-rolled 430 ferritic stainless steel samples were annealed at 840–1150 °C together with water or air cooling to study their microstructural evolutions, mechanical properties and corrosion resistances via a scanning electron microscopy (SEM), electron back scattering diffraction (EBSD), tensile tests and immersion tests. The experimental results demonstrate that after annealing below 880 °C, the microstructure consists of ferrite grains and (Fe, Cr)23C6 particles at the grain boundaries, whereas abundant martensitic phase is observed at the grain boundaries after annealing above 950 °C. The ferrite grains coarsen as the annealing temperature increases. Martensitic transformation after high-temperature annealing results in a sharp increase in both the tensile strength and Vickers hardness and a substantial decrease in the elongation. Both the formation of (Fe, Cr) 23C6 particles and martensitic transformation at the grain boundaries are detrimental to the corrosion resistance in Cl-rich environments due to the Cr depletion in the ferrite among the (Fe, Cr) 23C6 particles. The annealed samples with water quenching show higher hardness and better corrosion resistance than that with air cooling. Annealing at intermediate temperatures (900–950 °C) and then cooling in air is an appropriate method for fabricating 430 ferritic stainless steel with good mechanical properties and high corrosion resistance.