Microstructure evolution in HAZ and suppression of Type IV fracture in advanced ferritic power plant steels

The effect of boron and nitrogen on the microstructure evolution in heat affected zone (HAZ) of 9Cr steel during simulated heating and on the Type IV fracture in welded joints has been investigated at 650 °C. Gr.92 exhibits a significant decrease in time to rupture after thermal cycle to a peak temperature near A_C3, while the creep life of Gr.92N, subjected to only normalizing but no tempering, and 9Cr-boron steel is substantially the same as that of the base metals. In Gr.92 after A_C3 thermal cycle, very few precipitates are formed along PAGBs in the fine-grained microstructure. In the P92N and 9Cr-boron steel after A_C3 heat cycle, on the other hand, not only PAGBs but also lath and block boundaries are covered by M₂₃C₆ carbides in the coarse-grained microstructure. It is concluded that the degradation in creep life in Gr.92 after the A_C3 thermal cycle is not caused by grain refinement but that the reduction of boundary and sub-boundary hardening is the most important. Soluble boron is essential for the change in α/γ transformation behavior during heating and also for the suppression of Type IV fracture in welded joints. Newly alloy-designed 9Cr steel with 160 ppm boron and 85 ppm nitrogen exhibits much higher creep rupture strength of base metal than P92 and also no Type IV fracture in welded joints at 650 °C.