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相联系。微小的缺陷不能保证焊接结构在承受应力时的安全完整性。在这种情况下，微米范围内的微裂纹是一个严重的危险，因为通过非破坏性检测进行检测可能是不可能的，或者是有限的，因为裂纹的尺寸大多小于传统X射线技术的分辨率。由于镍基合金具有很高的安全意义和非常高的采购和加工成本，因此分析微裂纹形成的原因并采取纠正措施以避免裂纹的产生是非常有意义的。在焊缝冷却过程中，在再结晶以下，变形性降低。当超过临界应变时，固态会产生沿晶裂纹。微裂纹形成于焊料或热影响区。裂纹形成的原因目前尚不清楚，但在文献中，晶粒度和晶界上的析出条件被称为主要影响因素。以往的研究主要集中在母材的填充金属和热影响区上。然而，目前还没有对不同析出条件下的热影响区的实际组织进行分析，因此，研究的目的是确定焊缝热影响区微裂纹的创新评价标准。利用高分辨率的同步折射计算机层析成像技术对微裂纹进行了表征和量化。应变断裂(STF)试验用于在基材的热机械载荷过程中产生裂缝。比较了两种合金母体相同但析出条件不同的镍基合金。此外，热影响区的物理模拟结构使得微观组织、析出动力学和晶粒结构之间能够相互关联。基于这些结果，将绘制热影响区不同区域的温度-应变曲线。最后，本研究的目标是建立一个扩展的微裂纹形成模型，该模型包括热机械和微观组织特有的影响因素以及包括大量裂纹网络的评价标准。在给定材料因素的情况下，应说明热影响区的抗微裂纹敏感性。在更新方案中，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.