Damage Evaluation of a Welded Joint in a Long-term Service-exposed Boiler by Using a Small Punch Creep Test

A small punch (SP) creep test was applied to the welded joint of a boiler header (1.0Cr–0.3Mo steel), which had been actually used for around 250 000 h, to evaluate the changes in creep properties. A miniaturized plate-type specimen of 10×10×0.5 mm was taken from the base metal, weld metal and the heat affected zone (HAZ), respectively, for the SP creep test and microstructure observation. Experimental results revealed that the creep rupture strength of each portion had decreased by long-term service exposure, especially in the high stress (load) conditions. The creep rupture time of the HAZ was shorter than that of the base and weld metals in the low stress (load) levels, suggesting the possibility of Type IV creep damage. TEM observation also revealed that film-like (Mo, Fe)6C carbides precipitated along the grain boundaries in the HAZ, as well as the coarsening of the M23C6 carbides. These microstructural changes were likely to cause the above-mentioned reduction in the creep rupture strength at the HAZ.