Studies of Creep Damage in the Heat-Affected Zone of 9Cr-1Mo-VNb Steel Weldments

Creep strength enhanced ferritic (CSEF) steels are used in power plants for high temperature, pressure-containing welded components such as steam headers. One CSEF steel, 9Cr-1Mo-VNb or Grade 91, often exhibits localized creep deformation and cavitation in the weld heat-affected zone (HAZ), identified as Type IV creep damage. Three Grade 91 weldments were evaluated by creep testing and microstructural characterization using optical and scanning electron microscopy, combined with image analysis, to investigate microstructure evolution and creep damage. Two weldments fabricated from dissimilar base metals were used for direct comparison of creep damage in two materials simultaneously during cross-weld testing. Longitudinal creep specimens from a third weldment, with the specimen axis oriented parallel to the welding direction, were evaluated in a preliminary investigation of the effect of iso-strain loading on creep damage for two heat treatment conditions.

Dissimilar cross-weld testing showed a difference in damage susceptibility in the HAZs of three base metals with different sulfur contents and nitrogen:aluminum (N:Al) ratios. Type IV rupture in material with low N:Al ratio exhibited a significantly higher extent of creep cavitation than material with high N:Al ratio; in the low N:Al ratio material, a potential correlation was observed between greater extent of creep damage and a reduced density of fine precipitates in the base metal and intercritical HAZ, a larger subgrain size after creep exposure, and higher sulfur and aluminum contents. However, greater extent of Type IV creep damage was not unequivocally associated with lower N:Al ratio since another weldment showed greater creep damage in the material with high N:Al ratio. Base metal inclusion contents did not show a clear correlation with extent of creep damage. Longitudinal creep testing showed differences in creep damage between subcritical PWHT and N&T heat treatments; the N&T condition showed a longer rupture life despite increased creep damage and reduced creep ductility. Based on these results, Type IV creep damage in the dissimilar cross-weld tests was proposed to depend both on base metal composition and on microstructural differences. Longitudinal creep testing is a promising technique to evaluate the effect of iso-strain loading on damage development in Grade 91 welds.