Graded Creep Deformation Properties of 316H Welds and its Impact on Creep

Abstract

The creep damage evolution in multi-pass welds is believed to be influenced by the variation of creep rates from the weld to the base metal and through the HAZ. Material heterogeneity in a multi-pass weld leads to a non-uniform stress distribution resulting in non-uniform evolution of creep strains with strain localisation. Also, a non-uniform stress distribution may lead to highly multiaxial stress states in the weld resulting in a lower creep ductility. Since creep damage in metallic components is influenced by creep strain rate and creep ductility of the material amongst other factors, creep inhomogeneity in a weldment may significantly affect creep damage accumulation. Therefore, in order to predict creep behaviour of a multi-pass weld, it is important to take into account the gradation of creep deformation properties through the weld HAZ. Impression creep tests are useful in revealing localised creep properties in a material, where test results can be directly correlated to uniaxial creep tests. In this paper, a 2D finite element model of a multipass 316H weld with three different material sections (weld, HAZ, parent) is used to demonstrate the effects of creep deformation mismatch on stress and strain distributions. The paper also describes a series of impression creep tests planned and being conducted on an ex-service 316H weldment from a power plant steam generator with specimens taken from locations in the HAZ and at varying proximities to the weld fusion line. One specimen from the far away base metal and one from the weld centerline were also taken to serve as reference since the uniaxial creep deformation properties for the weld and the base material are known from uniaxial creep tests. By comparing the minimum creep rates for the HAZ specimens against the reference specimens from the weld and the base metal, Norton’s law creep coefficients and stress exponents will be derived for the HAZ specimens thereby revealing the gradation of creep deformation properties as a function of distance from the weld fusion line.