Development of a 9Cr-0.5Mo-1.8W Steel for Boiler Tubes

At present, high strength ferritic steel is required as material for the superheater and reheater tubes of super-critical and advanced power plant, instead of austenitic stainless steels. Based on the finding that the replacement of a part of Mo of 912%Cr-Mo steels by W is very effective for increase in creep rupture strength, a 9Cr-0.5Mo-1.8W-Nb-V steel whose 105 h creep rupture strength at 600°C reached 157 MPa was developed for boiler tube. This steel also possesses excellent weldability and notch toughness, and its allowable tensile stress at 600°C is 85 MPa, which is significantly greater than that of 9Cr-Mo steels and 18Cr-8Ni austenitic steels. The creep rupture strength of TIG weld joint is nearly the same as that of the mother tube. With MITI's approval, several tertiary superheater tubes of SUS321HTB of a power plant were replaced by the boiler tubes made of the new steel. The boiler has subsequently operated without any trouble for more than 13000 h at a tube temperature of 600°C and a design stress of 85 MPa. These investigations demonstrate that 9Cr-0.5Mo-1.8W-Nb-V steel can replace the 18-8 series austenitic steels and that it is suitable for headers and piping as well as tubes.