Thermal simulation on double-pass welding of a high Cr ferritic steel

The heat affected zone (HAZ) after double-pass welding in a high Cr ferritic steel was simulated by using a thermomechanical simulator. The microstructural observation and mechanical properties tests were conducted to investigate the effect of double-pass thermal cycle on the microstructure evolution and mechanical properties. The results show that the double-pass thermal cycle leads to the decrease of martensitic fraction, and the refinement of prior austenite. M23C6 precipitates is coarsened, and dissolved subsequently, by increasing the peak temperature during the second-pass thermal cycle, while MX with the higher thermal stability is undissolved until at 1200 °C. Thermal cycle at low temperature (as 900 °C) deteriorates the impact toughness, due to the high δ-ferrite fraction. Short-term creep tests suggest that, only the second-pass thermal cycle temperate of 1200 °C could eliminate the unfavorable effect on creep life resulted from the first-pass thermal cycle. Coarse M23C6 particles and the heterogeneous alloy elements distribution maybe the main reason.