DEVELOPMENT OF A NI-CR-W SUPERALLOY FOR APPLICATION TO HIGH-TEMPERATURE GAS-COOLED REACTOR SYSTEMS :CREEP PROPERTIES AND WELDABILITY

摘要

Creep tests in air and weldability tests were carried out in a research program on development of the new Ni-Cr-W superalloy as a component material for high-temperature gas-cooled reactors(HTGRs) with coolant outlet temperatures up to 1000℃. The optimum chemical composition has already been proposed on the basis of a series of experiments. Creep tests were performed at 900, 1000 and 1050℃ using the new Ni-Cr-W superalloy manufactured in an industrial scale, i. e. 2000 kg. Creep rupture lives of the Ni-Cr-W superalloy were dependent on grain size. It was confirmed that the creep rupture strength of the Ni-Cr-W superalloy with grain sizes above ASTM No. 2 wasrnabove 9.8 MPa for a 1×10~5 h life at 1000℃, which was the final target of the program. Creep rupture lives of this superalloy could be estimated in terms of Larson-Miller parameter. Weldability tests were carried out using several heats of the Ni-Cr-W superalloy with different content levels of minor alloying elements as filler metals on the TIG welding process. Although most of heats showed high susceptibility to cracking in the weldments, some heats showed good weldability. According to the weldability tests, it was estimated that minor alloying elements such as Zr, B, Y and Ti influenced susceptibility to cracking in the weldments.Two heats of the Ni-Cr-W superalloy showing good weldability were used to fabricate trial welded joints. Creep rupture strength of the trial welded joints was a little lower than that of the base metal with grain size of ASTM No.2.