Brittle fracture analysis of Dissimilar Metal Welds

Abstract

This paper deals with a fracture mechanics analysis of a narrow-gap Dissimilar Metal Weld (DMW) in the brittle fracture domain. The considered case is a Ni base alloy weld joint between a ferritic component and an austenitic pipe and the aim of the present study is to show that in the same loading conditions, the weld joint is less sensitive to the brittle fracture than the nearby ferritic part of the component.The bases of the proposed qualitative study rely on a stress-based criterion model, using a threshold stress (σth) below which the cleavage cannot occur. From that criterion, brittle fracture risk is evaluated, then compared between two positions of a postulated crack: one in the Heat Affected Zone (HAZ) of the ferritic material, the other in Base Metal (BM) far from the weld joint.For the experimental part of this comparison, one mock-up containing a DMW is used. From this mock-up, a large number of specimens of various types and with various crack locations was tested at low temperature. In parallel to that extensive experimental program, Finite Element Analysis has been done. The main result of that F.E.A. is that, due to the mismatch between the different materials, the brittle fracture risk is lower in the HAZ because of the stress relaxation with plasticity in the weld material and loss of constraint in the cracked section.This paper presents firstly a survey of DMW integrity evaluation, then the criterion used for the comparison, the experimental work and the F.E.A. made to show qualitatively the highest risk of brittle fracture of the homogeneous ferritic material compared to the DMW one.