Type IV creep cavity accumulation and failure in steel welds

One region of concern for creep damage accumulation in steel welds is a narrow region adjacent to the parent material. This narrow region, called Type IV, consists of partially transformed material having a relatively small grain size. In this region creep arises from crack growth and creep cavitation. The conditions where one mechanism dominates failure are not clear. In this paper, results are presented from an extensive experimental programme using both cross-weld and simulated Type IV specimens, first to determine the mechanisms of strain accumulation in the narrow region, and second to determine creep crack growth characteristics. Creep cavitation occurred across the net section, and the numbers of cavities per unit area were quantified as a function of fractions of rupture life. When sufficient constraint was provided by cross-weld specimens, creep crack growth was promoted in the Type IV region. When constraint was low, the presence of a crack did not promote localised cavity accumulation and creep crack growth. It is suggested that cavitation is a consequence of grain boundary sliding leading to relaxation of constraint and multiaxial rupture governed by the von Mises stress