High temperature fracture toughness properties of Alloy-625

The fracture toughness of Alloy-625 in the cast and welded conditions has been determined as part of an overall evaluation of the properties of candidate materials for advanced steam turbine applications. The fracture characteristics of the solution treated alloy change due to long time ageing at 650°C, with metallurgical precipitation being responsible for changes to both the critical toughness and fracture resistance profiles. Determination of the toughness properties of the cast nickel base alloy provided a number of challenges. At high temperatures, in particular 750°C, there were test measurement difficulties to overcome. In the solution treated condition, the observed fracture resistance profile of Alloy-625 could not be interpreted according to existing standard practice. Of particular significance, was the variability in fracture characteristics arising from the large grain size of the cast alloy. The nature of these challenges, the ways in which they were addressed and the practical implications are the subject of the following paper.