Elasto-Plastic Properties in Dissimilar Metal Weld Junctions: Stress-Strain Curves Determination for Constitutive Materials

Abstract

Within the framework of European project MULTIMETAL (Structural performance of multi-metal component), several fracture tests on different types of multi-material specimens have been performed. Present fracture toughness standard methods, e.g. ASTM E 1820 are not directly intended for Dissimilar Metal Weld (DMW). Therefore further investigations are needed in order to define the best practice in fracture mechanical tests and their analysis for DMWs. Specimens are taken from welded plates: a narrow gap Inconel DMW junction between ferritic and austenitic stainless steels, designed and delivered by AREVA France. The aim of this work is to provide guidelines for the determination of DMW fracture properties. For that purpose, fracture specimen needs to be modelled by FE. The first task, which is the purpose of that paper, is the determination of the mechanical properties in terms of stress-strain curve of all DMW constitutive materials: austenitic stainless steel, ferritic steel, heat affected zone of the ferritic steel zone and Nickel alloy zone properties. Conventional techniques for tensile test are not able to provide the tensile curve of the different materials constituting a weld joint. Image correlation techniques are well suitable but imply too long and difficult work for the images analysis. Therefore CEA has developed an intermediate solution based on laser sensors which provides a complete profile of the specimen during the tensile test. Using Bridgman equations, the stress and strain can be deduced from the measurement of the shape of the specimen (reduction of section but not only…). This innovative device has been used with new developments using local Bridgman equations in the post-processing of measurements. This allows to access to the material behaviour of several materials with only one specimen. Numerical interpretation using FE methods is presented and confirms the material behaviour determined from the experimental work using Bridgman equations assumptions. Finally, this combined experimental and numerical work has provided material data relative to all constitutive materials of the DMW junction. A hardened area in stainless steel material due to the welding process has been pointed out, and the heat affected zones of the ferritic material have been characterized in terms of stress-strain curves. The next stage of the project is to carry out tests on fracture specimens and to model these multi-materials specimens by FE. The gradient of elasto-plastic properties is now available for this next step.