Modelling of Dilution Effects on Microstructure and Residual Stress in a Multi-Pass Weldment

Three-pass gas tungsten arc welding in a 20-mm thick SA508 steel plate is modelled using a sequentially coupled thermal-metallurgical-mechanical model. The dilution for each pass is estimated as the proportion of base material in the weld metal, based on an analysis of the cross-sectional area of each fusion zone. The thermal solution of the weld model is validated using thermocouple measurement data and cross-weld macrographs. The predicted microstructure is qualitatively compared with that observed in cross-weld optical micrographs. The measured hardness distribution is used to quantitatively validate the post-weld ferritic phase distribution (e.g. the ferrite, bainite and martensite fractions), based on a hardness-microstructure correlation. The predicted residual stresses are compared with those measured by neutron diffraction. The results show that dilution significantly influences the metallurgical and mechanical properties of weld metal (either as-deposited or reheated), and its consideration notably improves microstructure and residual stress predictions for a multi-pass steel weldment. For the weldment considered, an increase in dilution promotes the formation of martensite, enhances the hardness and leads to lower tensile stresses (or higher compressive stresses) in the weld metal. Such behaviour arises due to the higher hardenability of the base material, coupled with delayed austenite decomposition on cooling.