Type IV creep cracking of welded joints: numerical study of the grain size effect in HAZ

A reliable creep design for very long lives, capable to account for both diffusion and dislocation mechanisms, was used to explain the onset of type IV fracture in weldments. An unexpected decreased creep rupture strength at low stress is the typical consequence of this failure. The fine-grained heat affected zone (FGHAZ) is the most sensitive region to the type IV failure. In this paper the effect of the grain size distribution was numerically investigated. The creep rupture occurs at the FGHAZ as a result of localized creep strain accumulation supported by high stress triaxiality which is known to promote nucleation and growth of cavities and reduce the material ductility. The stress triaxiality is mostly related to the grain size gradient in HAZ, thus an optimized grain size distribution could result in a longer creep life of the joints.