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

采用低相变温度（LTT）填料进行焊接是一种降低焊接残余应力的创新方法。特别是由于在焊缝和热影响区（HAZ）中选择性地产生压缩残余应力，可以预期高应力焊接部件的抗冷裂性的显著增强。为了有效地使用这些材料，有必要深入了解焊接过程中的微观结构发展，以确定残余应力形成过程中发生的复杂过程。各种研究小组的结果，即测量和模拟的残余应力分布，表明基本模型在原则上是有效的，但通过这种方法不可能准确预测残余应力的大小和分布。所提出的后续应用的上级目的是通过焊接冶金学（通过使用特定的LTT焊接填充材料）和工艺相关冷却条件的优化组合来改善高强度焊接结构的抗冷裂性。在当前的合作工作中，将获得现代LTT合金成分之间的相互作用的基本理解和由此产生的残余应力状态与机械载荷下的接头特性。关于改进的可转移性到真实的组件，实现将在考虑构造性约束条件的情况下进行，因为它们作用在真实的部件中。在当前项目的第一个资助阶段，首次实现了使用专门设计的样品进行的原位衍射实验，以分析局部相变动力学和局部应变的演变，作为热和建设性边界条件的结果。一个现实的MAG焊接过程中进行了现场监测下考虑机械约束。在研究过程中出现了各种问题，需要根据补充调查的结果对实验数据进行仔细的反复评估，以确保对原位衍射研究进行有意义的评估。在成功项目的继续范围内，将进一步研究在第一个资助期内确定的在结构约束下的实际多层焊缝（对接接头）的结果的可转移性。

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.