Abstract
Recent years high strength 9Cr1MoNbV steel developed in USA has been major material in boiler high temperature components with the increase of steam parameters of coal fired thermal power plants. As the microstructure of this steel 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 TypelV cracking. To develop an effective method to improve the rupture strength of welded joint, advanced welding procedure and normalizing-tempering heat treatment after weld was proposed. 9Cr1MoNbV plates with thickness of 40-50mm were welded by 10mm width automatic narrow gap MAG welding procedure using specially modified welding material. After normalizing at 1,050°C and tempering at 780°C, material properties of the welded joints were examined. Microstructure of 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 was confirmed in the test exceeded 30,000hours. This welding procedure has been applied to seam weld of hot reheat piping and headers in USC boilers successfully.
