Austenitic ductile iron castings have traditionally been used for gas turbine exhaust components that require castability, good machinability, low thermal expansion, and high strength at elevated temperatures. The achievement of optimum properties in austenitic ductile irons hinges on the ability of the foundry to produce nodular graphite in the microstructure throughout the component. In large, complex components, consistently producing nodular graphite is challenging. A high-nickel steel alloy that is suitable for sand castings has been recently developed for industrial gas turbine engine applications. The alloy exhibits similar mechanical and physical properties to austenitic ductile irons, but with improved processability and ductility. This alloy is weldable and exhibits no secondary graphite phase. This paper presents the results of a characterization program conducted on a 35% nickel, high-alloy steel. The results are compared with an austenitic ductile iron of similar composition. Tensile and creep properties from ambient temperature to 760°C (1400°F) are included, along with fabrication experience gained during the manufacture of several sand cast components at Solar Turbines Incorporated. The alloy has been successfully adopted for gas turbine exhaust system components and other applications where austenitic ductile irons have traditionally been utilized. The low carbon content of austenitic steels permits improved weldabilty and processing characteristics over austenitic ductile irons. The enhancements provided by the alloy indicate that additional applications, as both austenitic ductile iron replacements and new components, will arise in the future.
General Design Considerations for Gas Turbine Waste Heat Steam Generators
