Dynamic transformation as a tool to enhance the cold-working response of a 9% Cr steel through stress and strain partitioning

Dynamic transformation in a 9% Cr steel is shown to improve the room-temperature workability through microstructural modification. The dynamically transformed microstructure contains 10–18% of ductile ferrite in a brittle martensitic matrix. The two-phase microstructure results in 40% higher uniform elongation and 25% lower tensile strength, which facilitate cold working. Further, the presence of ductile ferrite is shown to increase the strain hardening exponent, further enhancing the workability. It is also shown that the dynamic transformation alters the martensitic matrix, and creates a secondary source of softening. These softening effects and overall mechanical response are accurately modelled by a robust modified rule-of-mixtures formulation with prediction error of ~ 0.3%. This investigation reveals the potential of dynamic transformation to create intermediate microstructures suitable for cold working during the manufacturing process and establishes the limits within which deformation can be safely imparted.