Effects of prior deformation on precipitation behavior and mechanical properties of super-ferritic stainless steel

The effects of the initial microstructure on the precipitation behaviors and related mechanical properties of 27Cr-4Mo-2Ni steel were investigated by X-ray diffraction, scanning electron microscopy, electron backscatter diffraction (EBSD), and transmission electron microscopy techniques and tensile experiments, as well as by Vickers hardness tests. Experimental results demonstrated the occurrence of inhomogeneous intragranular and intergranular deformation, e.g., the formation of shear bands because of differences in the grain orientations in the rolled specimens, especially in the specimen cold-rolled to a reduction of 70 %. EBSD revealed the presence of ferritic regions with large plastic deformation around TiN particles because of stress concentration during the rolling deformation. The sub-grain boundaries, shear bands, and regions near TiN particles formed during the hot/cold rolling and aging processes served as additional nucleation sites and the high stored deformation energy provided a large driving force for intermetallic-phase precipitation, which caused the rolling deformation to have an acceleration effect on the precipitation process, especially on that of the sigma phase. The sigma-phase precipitation caused transition of the tensile fracture mode from ductile to brittle fracture and an increase in the Vickers hardness to as high as 587 +/- 7 HV. The large cooling rate during annealing process for the alloy should be controlled to suppress the brittle phase precipitation.