Three-pass gas tungsten arc welding in a 20-mm thick SA508 steel plate is modelled to predict the thermal, metallurgical and mechanical behaviour. 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 predicted fusion zone. The temperature prediction 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 then 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 affects the metallurgical and mechanical properties of weld metal (either as-deposited or reheated), and its consideration notably improves the microstructure and residual stress predictions for the multi-pass steel weldment. Furthermore, 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 adopted in this study, coupled with delayed austenite decomposition on cooling.

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