SOME INVESTIGATIONS ON FRACTURE BEHAVIOUR OF BIMETALLIC WELD

Abstract

Dissimilar metal joints between plates of mild steel and austenitic steel are widely used in various fields like nuclear power plant. It is seen from literature available that a significant number of operation failures in power plants have occurred due to occurrence of fracture in the dissimilar welded joints. To prevent any possible failures extensive fracture mechanics based studies are required for mechanical characterization of these dissimilar welds. Nowadays various methodologies are being used to evaluate the fracture behaviour and one among these is damage mechanics. Micromechanical analysis is an extensively used tool in damage mechanics based studies to evaluate the fracture resistance as it conquers the problem of geometric dependency observed, while carrying out fracture tests on different types of specimens. Micromechanical analysis is done considering nucleation, growth and coalescence of voids in a material following large scale plasticity, which is usually exhibited by materials before final fracture. Various types of models form the core of micromechanical analysis and among these one is Gurson Tvergaard Needleman (GTN) model. The Primary Heat Transport (PHT) piping of Pressurized Heavy W ater Reactor (PHWR) is a critical system - any failure of this system will lead to very grave consequences, not speaking of huge monetary losses resulting from nonutilization of the reactor setup. Hence, it is necessary to have an accurate and reliable method for predicting the fracture behaviour in such components. The constitutive model introduced by Gurson and modified by Tvergaard and Needleman is used in this work to predict ductile fracture of bimetallic welded plates. In this (GTN model) the model parameters are determined using a hybrid methodology of metallographic study, different tensile tests subsequent numerical analysis and curve fitting an finally prediction of components fracture behaviour of bimetallic welds. A parametric study on different Gurson parameters was carried out to experience the effect of different parameters on the material behaviour (Load-AD) curves o f NRTB and J-R curves o f CT specimen. Later, the model parameters were calibrated by fitting the analytical results in the experim ental results. The present w ork deals with the experim ental evaluation of GTN model parameters of weld zone (SS 308L) and base materials (SA 516 Grade 65 and SS 304) of bimetallic weld joint. These assessed parameters form the basis of the study of mechanical fracture behaviour of different zones i.e. weld zone and base materials. SEM m icrographs have been taken fo r the various specim ens and various loading conditions using the Scanning Electron M icroscope (SEM) and image analysis o f these SEM m icrographs has been carried out evaluaite to the various GTN param eters experimentally. The residual stress mapping of the dissim ilar welds have also been assessed by performing the residual stress m easurem ent o f weld zone and base metal regions o f the dissim ilar joint. The m icrostructural study, evaluation of basic tensile properties (stress-strain relations) and basic fracture properties (J-R curves) have been earlier determined at W elding: Research Laboratory, IIT Roorkee. In this current work different /Notched Round. Tensile. Bars were tested at Institute Instrumentation Centre, IITrRoorkeei to obtain the Load-AD curves. A param etric study on different Gurson parameters was carried out to experience the effect of different param eters on the material behaviour (Load- : AD) curves of NRTB and J-R curves of CT specimen). Later; the model parameters have been c a lib ra te by fitting the analytical results in the'expeirirriehtal resultsland^ fracture characteristics.