Importance of cold rolling and tempering on the microstructure evolution, precipitation behavior and mechanical responses of 9Cr3Co3W1Cu ferritic/martensitic steel

The effects of thermomechanical treatment (cold rolling + tempering) on the microstructure evolution, precipitation behavior, and mechanical properties of the 9Cr3Co3W1Cu martensitic steel (G115) were investigated, by adjusting the cold rolling reduction (0, 20% and 40%), tempering temperature (750 °C and 800 °C) and time (30 min and 120 min). The results indicate that the G115 steel with 20% cold-rolling reduction rate followed by tempering at 800 °C for 30 min exhibits the optimum mechanical response (yield strength (σ_s) 899 MPa, elongation (δ) 26%), compared with that of 40% cold-rolled (σ_s 812 MPa, δ 17%) and undeformed steels (σ_s 670 MPa, δ 15%). The optimum yield strength in 20%-800 °C-30 min steel is attributed to the dislocation strengthening and precipitation strengthening, for the relatively low recrystallization rate and high density Cr₂₃C₆ and NbC precipitates, and the good ductility is attributed to the low recrystallization ratio and the uniform microstructure. As a comparison, in 40%-800 °C-30 min steel, a ferrite martensite dual phase structure formed. Due to the hardness difference between the two phases, there may be significant inharmony during the deformation process, leading to premature fracture.