Modelling of creep rupture of ferritic/austenitic dissimilar weld interfaces under mode I fracture

Abstract

Dissimilar metal welded structures (DMWs) have been used extensively in conventional and nuclear power generation plants. Evaluation of creep rupture properties of DMWs is critical to the structural integrity assessment. Failure of DMWs can occur in the base metal, the heat-affected zone (HAZ), or the dissimilar interface between the two welded materials, depending on the operating stress and temperature. The primary focus of this work is on interface failure in systems consisting of a ferritic steel (P91 or P22) and an Inconel filler material, which has an austenitic structure. A planar damage zone is introduced within a finite element (FE) framework to model the response of the interface. A traction-separation constitutive law with a Kachanov-type damage accumulation relationship is employed to describe the interface response, with the material parameters calibrated against available creep rupture data in which failure occurred at the dissimilar weld interface. It is found that the difference in damage accumulation along the interface of different DMW systems can be attributed to the mismatch in creep properties of the continuum materials either side of the interface. Diversion of the crack path into the HAZ is also captured as a result of damage accumulation in the heat affected zone (HAZ). The relationship between the empirical damage accumulation model and the major microstructural features that are responsible for interface failure is also discussed.

Effect of weld angle on the creep rupture life of ferritic/austenitic dissimilar weld interfaces under remote mode I fracture

“…base (parent) metal(s), heataffected zone (HAZ), filler metal, and the interface between the two dissimilar materials (i.e. the ferritic steel and INCONEL) [4,8,24]. As our main focus is on creep cavity growth at the interface zone, deformation models for other regions have been simplified and are briefly introduced here.…”

“…As our main focus is on creep cavity growth at the interface zone, deformation models for other regions have been simplified and are briefly introduced here. One can refer to [8] for a more detailed description.…”

“…In a simple uniaxial creep rupture test on a cross-weld component, failure can occur in the weld metal (Type I), originate in the weld metal but propagate through the coarse-grain heat affected zone (CGHAZ, Type II), occur entirely within the coarse-grain heat affected zone (CGHAZ) (Type III), in the fine-grain heat affected zone (FGHAZ) (Type IV), in the base (parent) metal (Type V), or at the dissimilar weld interface (Type VI or VII, e.g. between HAZ and the filler metal), with the last of these representing a unique failure mode for DMWs [4,5,8]. For the interface failure mode, a crack initiates from a feature that gives rise to a local stress concentration, usually…”

“…where the interface meets the free surface, and Type VII refers to the subsequent propagation of the crack strictly along the interface, while Type VI refers to an eventual diversion into the HAZ, which can be regarded as a combination of Type VII and Type III-IV [8]. Factors that determine the dominant failure mode include externally applied load, operating temperature, weld geometry, and environmental conditions [3][4][5]9].…”

“…2). In our previous work [8,21], we have developed an analytical and computational Kachanov-type interface damage zone approach in which the damage growth along the dissimilar interface is a function of only the normal traction and separation perpendicular to the interface. Here we extend our work by developing a physical creep cavity growth interface zone model based on the work conducted in [22] to take into account the influence of the component of the traction tangential to the interface on the development of damage.…”