Evaluation of Microstructure and Mechanical Properties in a Thick Plate of G91 Steel and its Weld for High Temperature Nuclear System

Abstract

The purpose of this study is to investigate the microstructure and mechanical properties in 9Cr-1Mo-V steel and its weld. A 220 mm thick forged plate with a typical composition of G91based on ASME A336 was used. Narrow gap welds were produced by submerged arc welding (SAW) with two different welding speeds. The microstructures in the base metal were typical tempered martensite at all locations, and the sizes of prior austenite grains were increased with the depth from the surface. The yield and tensile strengths tend to decrease with an increase in test temperature, especially at temperatures higher than 500oC. The upper shelf energy of the specimen from center was lower than that from the surface, and it also showed a higher index temperature. The toughness deterioration at the center might be caused by the larger size of the prior austenite grains and the existence of the delta ferrite. In the case of the weld, the larger sizes of the weld beads were observed in the upper region with a lower welding pass speed. There was no significant difference in the strength of the upper and lower welds, but the elongations of the upper weld were slightly larger than those of the lower weld. In the Charpy impact test, the lower weld showed a better impact toughness than the upper weld caused by the relatively smaller size of the weld beads and finer microstructures owing to a lower heat input by the increased welding speed.