The effects of thermal treatments on the γ′ precipitate morphology and creep-rupture properties of Nimocast Alloy 738

The application of the thermal cycle associated with a turbine blade coating technique was found to cause an excessive drop in the creep rupture life of thinwall aerofoil blades. This was most severe at 760°C under relatively high stress conditions. The changes in the gamma prime (γ′) morphology during the thermal cycle were followed using thin-wall blades, solid aerofoil blades, and cast ‘carrot’ testpieces. Differences in the dimensions of the castings produced variations in the response of the γ′ precipitates to the thermal cycle. This resulted in variations in the relative size and amounts of the primary and secondary γ′ particles which correlated with the changes in creep rupture properties. Modifications to the thermal cycle are suggested for application to thin-wall turbine blades to reduce this problem.