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) 23 C 6 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) 23 C 6 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) 23 C 6 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.