The effect of weld metal anisotropy on the creep of a pressurized, circumferentially welded pipe

Abstract

A finite element implementation of the Hill anisotropic model has been applied to the problem of the determination of the steady-state creep behaviour of an internally-pressurised, two-material, axisymmetric P91 welded pipe. The anisotropic model results have been validated against an available analytical solution using homogeneous steady-state creep properties. In the two-material model, the parent (base) metal is assumed to be isotropic while the weld metal is assigned different creep properties in the welding direction (circumferential) and the direction transverse to welding, consistent with observations on the creep behaviour of P91 weld metal. The anisotropic material behaviour of the pipe was characterised using two constants, p and S, which respectively represent (a) the ratio of transverse to longitudinal creep strain rates in the weld metal and (b) the ratio of weld metal longitudinal to parent metal creep strain rates. The effects of these parameters on the stresses at critical positions were studied parametrically, along with the Norton stress exponent, n, and different end loading conditions.

Development and use of an anisotropic damage model for the high-temperature life assessment of a P91 weldment

“…equation ( 10) and the associated scalar rupture terms in equation ( 11), and on the other hand to possible assumptions about the relative propensities of void or cavitation damage growth under tension-dominated and compression-dominated loading. The present material model is developed primarily for application to pressurized welded connections, which are typically tensile hoop and tensile axial stress dominated; see reference [8], for example, for a detailed parametric analysis of the stress distributions in a circumferentially welded pipe using an anisotropic steady state creep material model based on equations (17) and (18). The particular choice of stress dependence of equations ( 11) is well established for isotropic damage-based life predictions for notched specimens and for pressurized piping applications (see, for example, reference [4]).…”

“…Another objective of the work presented in this paper is to investigate the effects of weld material anisotropy on creep life predictions of realistic P91 pipe weldments using the developed anisotropic creep damage methodology. Previous work on the secondary creep behaviour of P91 welds under internal pressure and axial loads by Hyde et al [8] has shown that anisotropy has a significant effect on the steady state stresses under high end load conditions. Therefore, FE analyses of an axisymmetric pipe weld containing four material zones [10,11] i.e.…”

“…Previous work [7] on anisotropic weld metal has focused on understanding how the stress state in multiaxial stress situations is affected by the initial material anisotropy. Recent work [8] has used realistic constitutive equations, based on those for P91 weld metal (WM) and parent metal (PM), in finite element (FE) analyses of the stationary creep behaviour of welded pipes. The effects of the degree of anisotropy in the WM, the relative creep strengths of the WM and PM, the stress exponents in the creep equations, and the magnitude of the axial (system) loading (relative to the mean diameter hoop stress) on the peak stress levels and peak stress positions were investigated.…”