Prediction of microsegregation and pitting corrosion resistance of austenitic stainless steel welds by modelling: Dissertation

The present study focuses on the ability of several computer models to accurately predict the solidification, microsegregation and pitting corrosion resistance of austenitic stainless steel weld metals.Emphasis was given to modelling the effect of welding speed on solute redistribution and ultimately to the prediction of weld pitting corrosion resistance.Calculations were experimentally verified by applying autogenous GTA- and laser processes over the welding speed range of 0.1 to 5 m/min for several austenitic stainless steel grades.Analytical and computer aided models were applied and linked together for modelling the solidification behaviour of welds.The combined use of macroscopic and microscopic modelling is a unique feature of this work.This procedure made it possible to demonstrate the effect of weld pool shape and the resulting solidification parameters on microsegregation and pitting corrosion resistance.Microscopic models were also used separately to study the role of welding speed and solidification mode in the development of microsegregation and pitting corrosion resistance.These investigations demonstrate that the macroscopic model can be implemented to predict solidification parameters that agree well with experimentally measured values. The linked macro-micro modelling was also able to accurately predict segregation profiles and CPT-temperatures obtained from experiments.The macro-micro simulations clearly showed the major roles of weld composition and welding speed in determining segregation and pitting corrosion resistance while the effect of weld shape variations remained negligible.The microscopic dendrite tip and interdendritic models were applied to welds with good agreement with measured segregation profiles.Simulations predicted that weld inhomogeneity can be substantially decreased with increasing welding speed resulting in a corresponding improvement in the weld pitting corrosion resistance.In the case of primary austenitic solidification, the dendrite cores were predicted to be the weakest link with respect to weld pitting corrosion resistance.In primary ferritic solidification, the second phase austenite in the vicinity of d/g interfaces was predicted to show lowest pitting corrosion resistance.Solidification parameters used in the modelling were verified by cooling rate and dendrite arm spacing measurements as well as by analytical calculations.Experimental investigations using electron probe microanalyses (EPMA, CMA), electron microscopy (SEM, FEG-STEM), microstructural investigations and pitting corrosion tests were used in assessing the calculated microsegregation and CPT-temperatures and showed a reasonably good compatibility with the results of modelling.