HCM12A (ASME Grade 122) is used for boiler components in thermal power plants because of its high creep strength. However, a type IV creep damage formed in the heat-affected zone can cause a considerable decrease in the creep strength of the weldment and a failure of large diameter piping due to this damage took place recently in a thermal power plant. In order to update the design method and develop life estimation method for this kind of piping system with axial weld, the creep rupture data of base metal and welded joints have been collected and analyzed by the Strength of High-Chromium Steel Committee in Japan. In the present paper, the creep rupture data of over 400 points for welded joint specimens of HCM12A offered from six Japanese organizations are analyzed. These data clearly indicate that the long-term creep strength of the welded joints becomes weaker than that of the base metal at above $600°C$ due to the type IV fracture in the fine grain heat-affected zone. After the discussions on the effects of product form, welding procedure, specimen sampling procedure, etc., on the creep strength, the master creep life equation for the welded joints is developed. The so-called region decomposition technique was adopted to fit the data in both high and low stress regimes with a reasonable accuracy. The creep strength reduction factor obtained from 100,000 h creep strength of the welded joints and the base metal is given as a function of temperature.

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