Study on development of filler metal for Ni-Cr-W superalloy (joint research)

Development of filler metal is the important item to be solved in study on development of Ni-Cr-W superalloy for high temperature gas-cooled reactors with coolant outlet temperatures around 1000degC. Effects of minor alloying elements on weldability were studied for the alloy with basic composition of Ni-18.5Cr-21W proposed as an optimum chemical composition for the base metal. On this basis filler metal for the Ni-Cr-W superalloy was developed. The main results obtained were as follows: (1) Minor alloying elements and condition of heat-treatment had large effects on susceptibility to cracking in the weldments of Ni-Cr-W alloy. For this reason, good weldability could not be obtained constantly using alloys within optimum range of chemical composition for the base metal. (2) Improvement effect to reduce susceptibility to cracking in the weldments was little obtained by means of increase in contents of elements which had deoxidizing or desulfurizing effect. Addition of optimum contents of Ti or Mg reduced the susceptibility to cracking in the weldments slightly. (3) It was found that elements such as Zr, Y and B, which were added to the base metal of Ni-Cr-W alloy to maintain high temperature properties, made susceptibility to cracking high significantly when accumulating basic data on weldability of this alloy. Contents of these minor alloying elements were adjusted in the lower ranges than those of the base metal. Filler metal with good weldability for Ni-Cr-W alloy was developed through the investigation of effect on weldability. (author)