Strain dependent constitutive model and microstructure evolution of a novel 9Cr martensitic steel during high-temperature deformation

Abstract

The flow deformation behavior and microstructure evolution of a novel tempered martensite ferritic steel G115 were systematically investigated under temperatures ranging from 625 to 675 °C and stain rates of 5.2 × 10−5–5.2 × 10−3 s−1. The strain dependent constitutive model was established to reflect the flow deformation behavior of G115 steel. The corresponding material constants and the activation energies for this steel were determined under the different strains (0.005–0.07). The flow stresses predicted by the exponential equation show a good agreement with the experimental data, which suggests the stress-strain relationship of G115 steel can be accurately described using the exponential equation. Fine M23C6 carbides and Cu-rich phase particles have been characterized for G115 steel under different deformation conditions. The size of M23C6 carbide increases by a factor of 1.6 while that of Cu-rich phase particles increases in 2.4 times when the temperature increases from 625 °C to 675 °C. In addition, the number density of Cu-rich phase particles decreased drastically at 675 °C. Regular martensite lath structure still existed at 625 and 650 °C, but breakup of the martensite laths occurred at 675 °C. This was mainly strongly related to the evolution of M23C6 carbides and Cu-rich phase particles. The fine Cu-rich phase particles with a high number density contributed to slowing down the degradation of microstructure in G115 steel.