Microstructural Evolution, Precipitation and Mechanical Properties of 27Cr-4Mo-2Ni Super-Ferritic Stainless Steels

Abstract

Good mechanical properties and excellent pitting corrosion resistance for 0.8 mm-thick 27Cr-4Mo-2Ni super-ferritic stainless steels produced by one-stage cold rolling or two-stage cold rolling together with intermediate annealing processes are achieved. The microstructural evolution, precipitation and their effects on mechanical properties and corrosion resistance are investigated in terms of optical microscopy, scanning electron microscopy, electron backscattered diffraction pattern and transmission electron microscopy. The results demonstrated that the as-received hot-rolled plates consist of single ferrite grains characterized by α-fiber and γ-fiber orientations. A few Laves phases close to Nb(C, N) are formed in the recrystallized sheets solution-treated at 1050°C. After cold-rolling and finally annealing, fine recrystallized grains characterized by weaken γ-fiber orientation, are accomplished. The formation of Laves phases near the spherical Nb(C, N) makes large Nb(C, N) particles change into small granules. Corrosion resistance is more sensitive to Laves phases than mechanical properties. Small grain size improves strength and ductility, while it has a negative influence on resistance to pitting corrosion. Finer grains and a few more Laves phases are gained in steels processed by a one-stage cold-rolling process. The percentage elongation, yield strength (0.2% proof stress), ultimate tensile strength and average corrosion rate of final sheets produced by a one-stage cold-rolling process are 27.3%, 520 MPa, 641 MPa and 0.033 mm/a, respectively, and the values for two-stage cold-rolling process are 24.4%, 494 MPa, 610 MPa and 0.022 mm/a, respectively.