Microstructure and transient thermal stress analysis of austenitic stainless steel weldments using finite element method

Abstract

<p>A micro/macro-structural analysis is undertaken to determine the effects of welding thermal cycles on austenitic stainless steel. The analysis is subdivided into three parts; heat flow analysis, micro-structural analysis, and thermal elastic visco-plastic analysis.</p> <p>A three-dimensional, finite element program based on heat conduction equation is developed and also incorporates a model for heat input due to welding to approximate the welding thermal cycles. The program is capable of handling the nonlinear material properties according to the temperature field. A two-point recurrence scheme is employed to obtain the temperature field for every time step. A weighted residual iterative method is employed to minimize drifting.</p> <p>A micro-structural model based on the Avrami Equation and the grain growth law is developed to predict the grain growth due to welding. The heat affected zone is assumed to be composed mainly of the grain growth zone for the austenitic steel and ignores the recrystallization process.</p> <p>A coupled thermo-elastic viso-plastic formulation including the micro-structural changes is developed to predict the overall deformation and residual stresses caused by a welding thermal cycle. The macro-structural behaviour is modelled by using the nine-node isoparametric shell element. A twenty seven integration point scheme per element is used to transfer the temperature and the thermal strain field from the heat flow analysis and the austenite grain size from the micro-structural analysis to the thermal elastic visco-plastic analysis. An incremental load method is used along with the Newton-Raphson iterative method to solve for the incremental displacement and hence obtain the macro-structural response.</p> <p>After the development of all segments of the finite element programs, the accuracy and the stability of the solutions are tested. Various studies are conducted to check the modelling of the heat flow for both low carbon steel and austentic stainless steel, and the results are compared with the experimental data available in the literature. The micro-structural model is also checked with experimental data available in the literature. Moreover, a micro/macro-structural analysis is performed on heat on edge weld and head on edge weld for austenitic stainless steel and the results are compared with the experimental and analytical results available in the literature.</p>