Unraveling the Precipitation Tailoring Strategy and Hardening Mechanism of ε-Cu Phase in 9Cr Martensite Ferritic Steel during Aging

The Cu alloying strategy can substantially enhance the creep rupture strength of martensite ferritic steel (MFS), yet the precipitation tailoring strategy and hardening mechanisms remain not fully understood. This study examines ε-Cu precipitation mechanism and phase modulation in 9Cr MFS with varying Cu concentrations of 0.5, 1.0, and 1.5 wt% during aging at 650 °C. The results indicate that solute Cu atoms hardly contribute to solid solution hardening, whereas the precipitation of nano scale ε-Cu particles could contribute to aging hardness. As Cu content increases, the austenite transformation temperature A₁ decreases, coinciding with the tempering temperature range designed to acquire hierarchical tempered martensitic lath structures with desirable toughness. Numerical simulations reveal that significant element partitioning takes place between austenite (γ) and ferrite (α) during the tempering process when the Cu content exceeds 1 wt%, promoting the heterogeneous nucleation of ε-Cu particles and formation of ferrite. Moreover, the ferrite phase is subjected to rapid static recovery softening during subsequent aging processes, which negate the strengthening effect of nano-sized ε-Cu precipitates. This study provides insights into the potential of Cu alloying strategies for improving the performance of martensite ferritic steels.