Microstructure Evolution and Mechanical Behaviors of P92 Steel Welded Joint with Repetitive Post Tempering

The microstructure evolution and mechanical properties of P92 steel welded joint have been systematically investigated relating to the number of tempering cycles. Martensite laths, M₂₃C₆ precipitates, and tiny MX phase have been characterized in P92 base metal (BM). Likewise, the heat-affected zone (HAZ), fusion zone (FZ), and weld metal (WM) show the similar feature of phase composition to the BM. However, δ-ferrite patches can be found in FZ. With the increasing number of tempering cycles, the area fraction and average size of M₂₃C₆ precipitates gradually increase in HAZ and FZ despite that microstructure has little change. Consequently, the hardness, ultimate tensile strength, and yield strength tend to decrease, while the elongation can be improved in some degree. The reduction in solution strengthening, grain refinement strengthening, and precipitation strengthening is responsible for the strength degeneration of P92 steel welded joint with reduplicative tempering.