Determination of novel assessment criteria for the Evaluation of welding-related micro cracks using high resolution synchrotron-refraction-computer-tomography

创新材料的焊接加工总是与高要求联系在一起。当受到应力作用时，细小的缺陷不能保证焊接结构的安全完整性。在这种情况下，微米范围内的微裂纹呈现出严重的危险，因为由于裂纹的尺寸大多小于传统x射线技术的分辨率，通过无损检测可能不可能或受到限制。由于具有很高的安全意义，购买和加工成本非常高，特别是镍基合金，因此分析微裂纹形成的原因并制定避免裂纹的纠正措施是非常有兴趣的。镍基合金在焊接过程中容易形成延性倾裂（DDC）型微裂纹。在焊缝冷却过程中再结晶后，变形能力下降。当超过临界应变时，在固态中产生晶间裂纹。微裂纹主要在填充金属或热影响区形成。裂纹形成的原因目前尚不清楚，但在文献中，晶粒尺寸和晶界上的沉淀条件被认为是主要影响因素。以往的研究主要集中在填充金属和基材的热影响区。然而，对不同降水条件下HAZ的实际结构，即不同晶粒结构的分析尚未开展。因此，研究目标是确定焊接热影响区微裂纹的创新评价标准。采用高分辨率同步加速器-折射-计算机断层扫描技术对微裂纹进行了表征和量化。应变断裂（STF）测试用于在基材的热机械载荷下产生裂纹。比较了两种合金基体相同但析出条件不同的镍基合金。此外，HAZ的物理模拟结构使微观结构、析出动力学和晶粒结构之间具有相关性。在此基础上，建立了热影响区不同区域的温度-应变曲线。最后，研究的目标是创建微裂纹形成的扩展模型，该模型包括热力学和微观结构特定的影响因素以及包括大量裂纹网络在内的评估标准。在给定材料因素的情况下，应对热影响区中微裂纹的敏感性进行说明。在更新建议中，结果将在约束下的多层焊缝上进行验证。

The welding processing of innovative materials is always connected with high requirements. By minor imperfections a safe integrity of welding constructions can not be guaranteed when subjected to stresses. In this context, micro-cracks in µm-range present a serious danger because a detection by non-destructive testing may not be possible or is limited because the size of the cracks is mostly smaller than the resolution of conventional X-ray techniques. Due to the high safety significance and the very high costs for purchasing and processing, specifically for nickel-base alloys, it is of high interest to analyse reasons for micro-crack formation and to create corrective actions for avoiding cracks.Nickel-base alloys tend to form micro-cracks of the type ductility dip cracking (DDC) during welding. Below recrystallization during cooling of the weld seam a decrease of deformability appears. When critical strains are exceeded intergranular cracks occur in the solid state. The micro-cracks are formed in the filler metal or in the heat affected zone (HAZ). Causes for crack formation are currently unknown, but in literature grain size and precipitation condition on grain boundaries are referred to as main influences. Previous investigations concentrated on filler metal and HAZ of the base material. However, analyses on real structures of a HAZ with different precipitation conditions and, therefore, different grain structures, have not taken place yet.Therefore, the research objective is the determination of innovative evaluation criteria of micro-cracking in the HAZ of welds. Micro-cracks are characterised and quantified by means of high-resolution synchrotron-refraction-computer-tomography. The strain-to-fracture (STF) test is used to create cracks during thermo-mechanical loads of the base material. Two nickel-base alloys with the same alloy matrix but different precipitation conditions are compared. Moreover, physical simulated structures of a HAZ enable the correlation between microstructure, precipitation kinetics and grain structure. Based on these results temperature-strain curves will be developed for different areas of the HAZ.Finally, it is the goal of the studies to create an extended model for micro-crack formation, which includes both thermo-mechanical and microstructure specific influence factors as well as evaluation criteria including a voluminous network of cracks. With given material factors a statement about the susceptibility against micro-cracking in the HAZ should be made. In a renewal proposal the results will be verified on multi-layer welds under restraint.