Anisotropic Creep and Fracture Behaviour of a 9CrMoNbV Weld Metal at 650°C

Welds form an integral part of most power and chemical plant structures. At elevated temperature, the service lives of these structures are often governed by the creep behaviour in localised regions of the welds. Efforts have been made to understand the characteristic features of the creep stress distributions in and the deformation behaviour of welds and to predict the failure life. One of the major factors that affects the life of a weldment is the creep behaviour of the weld metal and the related failures due to cracking within the weld metal which has been identified in practice. The intrinsic macroscopic anisotropy of the weld metal is related to the nature of the welding process.In this paper, the high temperature creep and creep rupture behaviours of a 9CrMoNbV weld metal, at 65°C, are reported. Uniaxial creep and creep rupture and Bridgman notched bar creep rupture tests were performed, using test specimens removed from two directions, i.e. longitudinal and transverse, with respect to the welding direction. From the test results obtained, the differences in the creep ductility, minimum creep strain rate, rupture strength and notch strength sensitivity behaviour of the material in the two directions, are identified. Material constants, in creep and damage constitutive equations, were obtained from the test data. Metallurgical studies were conducted with the aim of gaining an understanding of the difference in the creep failure mechanisms in the two directions which cause the anisotropy. The results obtained clearly indicate that anisotropy of the weld metal creep properties exists and that this anisotropy will need to be considered in numerical modelling of P91 welds, joined by the same or similar weld steels, if accurate predictions are to be obtained.