Multi-pass ferritic steel weld modelling: Phase transformation and residual stress

The solid state phase transformation (SSPT) occurring during welding thermal cycles gives rise to
distinctive microstructures across the fusion zone and heat affected zone (HAZ), as well as significant
effects on the residual stress generated in the weldment. We have developed a numerical model to simulate
multi-pass welding in low alloy ferritic steel with consideration of SSPT. In this study, we applied a semiempirical modelling approach to three-pass gas tungsten arc welding in a grooved plate made of SA508
steel (widely used in nuclear power plants). The microstructure, hardness and residual stress were
predicted using a 2D finite element model and the predictions were compared with experimental results.
We examined the sensitivity of the predicted hardness and stress to austenitisation kinetics and weld-metal
plasticity. Two sets of empirical parameters were considered in the kinetic model of austenitisation to
represent different levels of the heating-rate dependence of the critical temperatures (i.e. Ac1 and AC3) for
austenite formation. A rule-of-mixtures method based on dilution and hardness was proposed to estimate
the plastic properties of weld metal for each pass, using the predicted phase fractions and the yield stress
dataset for each transformation product of base material. The modelling results show that the extent of the
inter-critical HAZ, hardness and residual stress are affected by the austenitisation kinetics. The use of
weld-metal plastic properties estimated by the rule-of-mixtures method can improve the residual stress
prediction for the weld metal.