Hot crack assessment during welding using novel LTT weld filler materials

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The use of Low Transformation Temperature (LTT) welding consumables is an innovative way of reducing tensile residual stresses during welding of high-strength steels.In a previous research project, investigations on weldability, transformation kinetics and residual stress formation of LTT welding consumables have already been carried out. It turned out that some of the investigated LTT alloys show an increased hot crack sensitivity compared to conventional welding consumables due to their solidification and segregation behavior. In the first funding period of the research project, tomographic examinations were used for a comprehensive hot crack evaluation considering the entire material volume to evaluate the results of the established MVT hot crack test. During the investigations it was successfully shown that in comparison to the conventional MVT test, in which only the cracks near the surface are evaluated, the 3D information for the development of the hot crack networks represent an extended evaluation basis. For a comprehensive evaluation of the LTT alloys the consideration of the material volume is necessary. The approach of tomographic analysis using µCT has proven to be effective. Nevertheless, several questions appeared which have encouraged the applicants to submit a two-year follow-up application to achieve the objective of systematic description of hot crack formation in the LTT system using tomographic methods. Within the continuation, it is to be investigated to what extent the residual stresses that have been proven to relax during the necessary sample preparation affect the result of the µCT analyses and how the reproducibility of the selected procedure is. The local orientation of the 3D hot crack networks as a function of the welding parameters is to be analyzed as an extended quantity of the µCT and used as an evaluation instrument for the hot crack susceptibility. From a materials science point of view, it must be clarified how the opposite hot crack sensitivity of the two LTT alloy systems CrNi and CrMn is caused with increasing heat input. By means of solidification tests in a quenching and forming dilatometer, the relevant influencing variables material and load are isolated from the welding process specifications. Critical parameters are to be identified to ultimately enable the safe utilization of LTT alloys.