Cracking resistance enhancement of high strength welded joints through the application of modern weld filler materials with low transformation temperatures (LTT)

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Welding using low transformation temperature (LTT) filler materials is an innovative method to mitigate welding residual stresses. In particular due to the selective producing of compressive residual stresses in the weld and in the heat affected zone (HAZ) a significant enhancement of the cold cracking resistance of highly stressed welded components can be expected. For the effective usage of these materials an in-depth comprehension of the microstructural developments during welding are necessary to determine the complex processes that occur during residual stress formation. Hitherto results of various research groups, i.e. measured and simulated residual stress distributions, show that the underlying models are valid in principle, but accurate prediction of the magnitude and the distribution of residual stresses are not possible by this means. The superior objective of the proposed follow-up application is the improvement of the cold cracking resistance of high strength weld constructions through an optimized combination of welding metallurgy (by using specific LTT weld filler materials) and processing dependent cooling conditions. Within the current cooperative work the fundamental comprehension of the interaction between modern LTT alloy constitutions and the resulting residual stress states with the joint characteristics under mechanical load will be gained. With respect to an improved transferability to real components the realization will be carried out under consideration of constructive constraint conditions as they act in real parts. Within the first funding phase of the current project for the first time in-situ diffraction experiments using specially designed samples were realized to analyze the local phase transformation kinetics and the evolution of local strains as a consequence of thermal and constructive boundary conditions. A realistic MAG welding process was monitored in-situ under consideration of mechanical constraints. Over the course of the studies various questions occurred, which require a careful repeated evaluation of experimental data based on results of complementary investigations in order to ensure a meaningful valuation of the in-situ diffraction studies. Within the scope of the continuation of the successful project further, the transferability of the results determined within the first funding period on realistic multilayer welds under constructive constraint (butt joint) will be investigated.