Thermal Treatment Simulation for Each Subregions in Heat Affected Zone of Domestic P91 Steel Welded Joints

The different subregions in heat affected zone of P91 steel welded joints were simulated by different temperature (800, 850, 950, 1 300 ℃)×25 min normalizing+755 ℃×60 min tempering, and the 50 000 h service situation was simulated by 650 ℃×200 h high temperature accelerated aging heat treatment process. The microstructure and properties of different subregions in heat affected zone of the P91 steel welded joint after 50 000 h service were studied by comparing those of the heat simulation soumples and the actural joint samples. The results show that after service the morphology of martensitic lath, and the quantity, distribution and particle size of MX phase, M₂₃C₆ carbide and Laves phase of the thermal simulation samples of coarse grained and fine grained regions were very close to those of the corresponding subregions of actural welded joint. In addition to the coarse grained region, the average grain size of the each subregion thermal simulation samples and the corresponding subregions of welded joint after service was very close, and the relative error was no more than 12.5%. The microhardness and small punch mechanical properties were also similar, and the strength, toughness and fracture deflection rate relative errors were less than 5%. The fracture had typical microporous aggregation ductile fracture characteristics. The microhardness was the smallest in the partial phase change region of heat affected zone, which was 191 HV. The hardness increased greatly in the fine grained and coarse grained region, and reached the maximum at the fusion line, which was 314 HV. The small punch strength and plasticity were the highest and the plasticity was the worst in coarse grained region, while the strength and plasticity were the lowest and the plasticity was the best in partial phase change region. Compared with coarse grained region, partial phase change region had higher necking degree and larger and deeper dimple.