Influence of microalloying elements on the mechanical properties of ultra high strength welds

Abstract

eng. Today's tendencies to reduce the overall weight of technical constructions create a need for ultra-high strength filler materials. The main goal of this thesis is to assist the voestalpine Böhler Welding GmbH in their search for the optimum composition of this new welding consumable. After giving an overview about the necessary theoretical foundations like precipitation, the simulation of solidification, grain growth kinetics and of course strengthening mechanisms, an adequate simulation framework for MatCalc has been designed in order to perform calculations for microalloyed ultra-high strength welds based upon their composition and to assess the strength (total yield stress) of the final material. From the large amount of simulated alloys (over 720), eight have been actually produced and tested, and the numerical values for the calculated yield stress are in overall good agreement with the results of the measurement, the values are off by about 3,8% in the mean. This deviation should be considered appropriate when keeping in mind the numerous effects that influence the strength of the material. When a working model for the given welding process was found, it was also examined for its sensitivity to changes in the given heat treatment. This analysis indicated that especially the peaks of reheating after the initial cooling - due to the passing of additional weld beads in proximity to the original one - are crucial to the outcome of the strength calculation and, therefore, the model should be considered very sensitive to changes in heat treatment. Nevertheless, the work at hand provides a good insight into the processes that most likely take place in the material during the welding process and shows useful tendencies for the final development of the right chemical composition of the new filler material to be developed. Also the implementation with the script and Visual Basic for Applications make the simulation easy to maintain for future reference purposes.