Development of low thermal expansion nickel base superalloy for steam turbine applications

Abstract

The target operating temperature of ultrasupercritical power plants is increasing and is planned to reach 700°C. Austenitic superalloys are promising materials for these applications to replace ferritic heat resistant steels, because of their high strength at 650–700°C. In general, austenitic nickel base superalloys show higher creep rupture strength than ferritic heat resistant steels; however, they have higher coefficients of thermal expansion, lower creep rupture ductilities, and higher costs. The effect of the Mo and Co content, amount of γ' phase, and Al/Ti ratio in the γ' phase on the thermal expansion behaviour of a Mo containing superalloy has been investigated by use of the conventional Mo containing Alloy 252 as a reference. Tensile and creep rupture properties were also measured. Following a modified heat treatment, the Co free superalloy developed on the basis of these tests showed higher creep rupture ductility than Alloy 252, while retaining comparable low thermal expansion and high creep rupture strength. Creep rupture properties at 700°C for up to 20 000 h were satisfactory, suggesting that the alloy is suitable for long term applications. Initial assessments of the weldability and mechanical properties of weld joints at 750°C are encouraging for boiler tube applications.