Thermal cycling, microstructure and mechanical properties of 9Cr2WVTa steel welds

Samples of enlarged heat affected zones (HAZs) submitted to single and double welding thermal cycles were fabricated by a thermal-mechanical physical simulator (Gleeble 1500). Typical welding thermal cycle curves were extracted based on the measurement of the non-equilibrium phase transformation of 9Cr2WVTa steel, and the simulated and experimental thermal cycles were compared. Observation of the microstructure revealed that the blocky δ-ferrite that existed in the coarse grained heat affected zone (CGHAZ) was the main cause of the decrease in impact toughness. Each of the areas retained the δ-ferrite with different sizes and shapes after the CGHAZ underwent the second welding thermal cycle. Hardness tests, tensile tests, and instrumented impact tests were carried out to investigate the corresponding mechanical properties. When the peak temperature of the second welding thermal cycle was 1315°C, grain and structure coarsening was the main causes of the decrease in impact toughness. When the peak temperature of the second welding thermal cycle was decreased to 1100°C, the toughness increased the most due to the fine grains formed by phase transformation recrystallization.