Influence of Coarsening Microstructure in Weld Metal on Creep Strength of a Long-Term Used Mod.9Cr-1Mo Welded Joint

Mod.9Cr-1Mo steels are widely used for steam pipes in ultra-super critical power plants. The creep strength of Mod.9Cr-1Mo steel welded joint is significantly lower than that of the base metal. It is necessary to clarify influence of long-term operation on creep strength of welded joints. In this study, a long-term used boiler welded joint which contained coarsening microstructure (CM) in weld metal was used. Creep tests were performed using tensile loading type miniature specimens with 1mm diameter and 5mm parallel length of constituent materials, standard and miniature size welded joint specimens taken from the welded joint. Minimum creep strain rate of the weld metal containing CM was 1000 times faster than that of new weld metal indicating significant reduction of creep deformation resistance and, was 10 times faster than a long-term used weld metal with only dendrite microstructure (DM). Creep rupture time of the standard size welded joint specimens, which failed at the CM in weld metal, was 4 times shorter than that failed at the DM. The miniature welded joint specimens failed at either HAZ or weld metal depending on influence of the CM containing in weld metal. A finite element creep analysis of both standard and miniature welded joint specimens with and without the CM in weld metal indicated that creep strain accumulation in the CM was larger than that in the DM causing shorter rupture time of the welded joints with the CM in weld metal. ©2024 The Society of Materials Science, Japan.