Hot Deformation Characteristics of 9Cr-1.5Mo-1.25Co-VNb Ferritic-Martensitic Steel

The hot workability of 9Cr-1.5Mo-1.25Co-VNb ferritic-martensitic steel was investigated through hot compression tests using Gleeble simulator over a temperature range of 1223 K (950 °C) to 1573 K (1300 °C) and strain rates of 0.001 to 0.5 per second. Through an analysis of the stress(σ)–strain(ε) curves obtained during the hot compression tests, a processing map was developed to predict the hot workability, based on a dynamic material model (DMM). The processing map indicated that the hot workability deteriorates with an increase in the strain rate and a decrease in the temperature. During hot compression tests at temperatures exceeding 1473 K (1200 °C), δ-ferrite was formed along austenite grain boundaries and fine austenite grains then nucleated at the austenite/δ-ferrite interphase due to dynamic recrystallization (DRX), which was beneficial to the hot workability. At a temperature below 1223 K (950 °C), intergranular cracking occurred, after which a transmission electron microscopy (TEM) analysis revealed that this intergranular cracking resulted from the precipitation of intergranular M₂₃C₆ particles.