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)填充材料焊接是一种减少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，预计可以显着提高高应力焊接构件的抗冷裂纹能力。为了有效地使用这些材料，有必要对焊接过程中的微观组织发展进行深入的了解，以确定在残余应力形成过程中发生的复杂过程。到目前为止，不同研究小组的结果，即测量和模拟残余应力分布，表明基本模型在原则上是有效的，但用这种方法无法准确地预测残余应力的大小和分布。建议的后续应用的更好目标是通过优化焊接冶金(通过使用特定的LTT焊接填充材料)和依赖于工艺的冷却条件来提高高强度焊接结构的抗冷裂性能。在目前的合作工作中，将对现代LTT合金成分之间的相互作用以及在机械载荷下产生的具有接头特征的残余应力状态有一个基本的了解。关于改进向实部的可转移性，实现将在考虑构造性约束条件的情况下进行，因为它们在实部中起作用。在本项目的第一个资助阶段，首次利用专门设计的样品进行了原位衍射实验，以分析局部相变动力学和局部应变随热边界条件和建设性边界条件的演变。在考虑机械约束的情况下，对一个真实的MAG焊接过程进行了现场监测。在研究过程中出现了各种问题，这些问题需要根据补充调查的结果仔细地反复评估实验数据，以确保对原位衍射研究进行有意义的评估。在继续成功项目的范围内，将调查在第一个资助期内确定的结果在施工约束(对接)下的实际多层焊接中的可转移性。

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.