σ-Phase precipitation mechanism of 15Cr–15Ni titanium-modified austenitic stainless steel during long-term thermal exposure

Abstract

The σ-phase precipitation in 20% cold-worked 15Cr–15Ni titanium-modified austenitic stainless steel was studied during long-term aging treatment. During the isothermal aging stage, the amount of σ-phase was significantly increased at beginning and gradually became constant. The aging time only slightly affected the size and morphology of the σ-phase. Conversely, during the isochronal aging stage, the amount of σ-phase grew rapidly with the increase in the aging temperature. The σ-phase with a large amount of stacking faults was prone to nucleate around the (Ti, Mo)C particles or at the grain boundaries. The (Ti, Mo)C particles can act as effective nucleation sites, where the σ-phase directly precipitates from the austenitic matrix. From this work, the growth of σ-phase is found to be influenced by the aging temperature, and a new mechanism of σ-phase precipitation from austenite has been proposed.