HCM12A (ASME Grade 122) is used for boiler components in thermal power plants because of its high creep strength. However, type IV creep damage formed in heat affected zone brings about considerable decrease in creep strength of the weldment and a failure of large diameter piping in a thermal power plant due to this damage took place recently. In order to update the design method and develop life estimation method for this kind of piping system with axial weld, creep rupture data of base metal and welded joints has been collected and analyzed in the SHC (Strength of High-Chromium Steel) committee in Japan since 2004. In the present paper, the creep rupture data of over 400 points for welded joint specimens of HCM12A offered from six Japanese organizations were analyzed. These data clearly indicated that the long-term creep strength of welded joints becomes weaker than that of base metal at above 600C due to Type IV fracture in fine grain heat-affected zone. After discussing the effects of product form, welding procedure and specimen sampling etc. on the creep strength, the master creep life equation for the welded joints was developed. So-called region decomposition technique was adopted to fit the data both in high and low stress regimes with a reasonable accuracy. The creep strength reduction factor obtained from 100,000 hours creep strength of welded joints and base metal was given as a function of temperature.

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