Thermal Processing Design of Cast INCONEL^® Alloy 740H for Improved Mechanical Performance

The increasing operating temperatures of turbine designs for power generation applications present challenges in selecting materials. With steam temperatures approaching 700 °C, and beyond, ferritic or martensitic steels are inadequate. Ni-based superalloys perform much better under these conditions and possess mechanical properties suitable for turbine and boiler applications. However, only limited Ni-based superalloys are available for large turbine castings where a combination of high-temperature strength (in particular, yield stress), long-term creep resistance, toughness, and weldability are essential properties. Research was undertaken at the National Energy Technology Laboratory to produce a cast INCONEL^® 740H alloy with mechanical properties comparable to those of the wrought product. Since thermo-mechanical processing is not an option in castings, aging trials were performed to alter the grain boundary morphology after a computationally designed homogenization heat treatment. Thermodynamic simulations were used throughout to produce various grain boundary microstructures. Early investigations on cast INCONEL^® 740/740H revealed less than desirable ductility. The modified microstructures altered the tensile and creep properties, and reinforced the possible use of cast INCONEL^® 740H in power plants. The benefits associated with the alternate heat treatments were found to originate from the grain boundary phases present in the alloy controlled through the targeted aging treatments. Those results and the relations between thermal processing, microstructure, mechanical properties and failure mechanisms of cast INCONEL^® 740H will be discussed.