Microstructural Evolution and Toughening Mechanisms of DIFT in 15Cr–5Ni Steel with Banded Ferrite-Martensite Heterogeneous Structure

A key challenge in the design of high-performance martensitic stainless steels is enhancing toughness without significant loss in strength. In this study, we address this by leveraging a banded ferrite-martensite heterostructure in a 15Cr-5Ni steel. Such a structure is achieved via an industrial hot extrusion and normalization process during which deformation-induced ferrite transformation is facilitated. The as-extruded sample exhibits tensile strength comparable to that of the as-forged sample, yet its impact toughness is improved by an order of magnitude (from 10.5 J to 101.4 J). Detailed microstructural analyses reveal that hot extrusion promotes the accumulation of high-density geometrically necessary dislocations (GNDs), so that HDI stress concentrates significantly at the ferrite-martensite interfaces. This subsequently leads to strengthening of ferrite, promotion of interfacial cracking, and alleviation of stress concentration in martensite. As a consequence, crack tip is blunted and crack propagation is deflected in the as-extruded material, giving rise to exceptional impact toughness. This study provides a new microstructure design approach for developing high-toughness martensitic stainless steels.