Evaluation of Microstructure and Mechanical Properties on Post-Weld Heat Treatment in the Heat Affected Zone of SA508 Gr.4N Ni-Mo-Cr Low Alloy Steel for Reactor Pressure Vessel

摘要

The heat-affected zone (HAZ) of SA508 Gr.4N Ni-Mo-Cr low alloy steel, which has higher, Ni and Cr contents than SA508 Gr.3 Mn-Mo-Ni low alloy steel, was investigated on the microstructure and mechanical properties. The HAZ was categorized into seven characteristic zones (CGCG, FGCG, ICCG, SCCG, FGFG, ICIC and SCSC-HAZ) according to the peak temperature from the thermal cycle experienced during multi-pass welding. Post Weld Heat Treatment (PWHT) was conducted in the temperature range of 550-610 deg C for 30 hours to evaluate the effect of PWHT conditions on the microstructure and mechanical properties. Before PWHT, CGHAZ and FGFGHAZ showed high yield strength (YS) ranging from 1000 to 1250 MPa, while YS of SCSCHAZ decreased from 607 MPa (observed for base metal) to 501 MPa. The Charpy impact energies of sub-HAZs fell below 100J at -29 deg C, except in the SCSCHAZ. By applying PWHT to sub-HAZ specimens, YS decreased as the PWHT temperature increased. In the case of CGHAZs and FGFGHAZ heat-treated at 610 deg C, YS dropped drastically to the range of 654-686 MPa. From the Charpy impact test, the upper-shelf energy (USE) increased to approximately 250J and Index temperature (T_(68J) decreased below -50 deg C. Specifically, in FGFG, ICIC and SCSC-HAZ, T_(68J) was below -110, which was lower than the case of base metal.