Kohji Tamura

Long Term Creep Rupture Strength of 9Cr1MoNbV Narrow Gap Welded Joints Improved by Normalizing and Tempering After Welding

With the increase of steam parameters of coal-fired thermal power plants, high strength 9%Cr steel containing niobium and vanadium became major material in high temperature boiler components. As the microstructure of these steels is tempered martensite, it is known that the softening occurs in HAZ of the weldment. In the creep rupture test of these welded joints the rupture strength is lower than that of the parent metal, and sometimes this reduction of strength is caused by Type IV cracking. To develop an effective method to improve the rupture strength of welded joints, a normalizing-tempering heat treatment after weld was proposed. 9Cr1MoNbV plates with a thickness of 40–50 mm were welded by 10 mm width automatic narrow gap MAG welding procedure using specially modified welding material. After normalizing at 1050C and tempering at 780C, material properties of the welded joints were examined. Microstructure of the HAZ was improved as before weld, and rupture strength of the welded joints was equal to that of the parent metal. The long term rupture strength of the welded joints as confirmed in the test exceeded 30,000 hours. This welding procedure has been applied to the seam weld of boiler hot reheat piping in USC plants successfully.Copyright