In situ Untersuchung der Phasenverteilung und der Schmelzbaderstarrung von Duplexstählen mittels laserinduzierter Plasmaspektroskopie (LIBS)

双相不锈钢（DSS）由于其显著的高强度性能和耐腐蚀性，近年来受到越来越多的关注。最近的创新发展，全球对可持续和环境适用的工程应用的需求，在过去的20年里，DSSs的需求和利用迅速增长。由铁素体和奥氏体相平衡组成的DSSs在焊接过程中表现出奥氏体重构和显微组织变化，导致相比失衡和化学成分改变。这些多相材料的主要结构变化会导致凝固开裂、易腐蚀、延性降低和临界强度值降低。因此，为了保持理想的焊接性能，在焊接过程中必须密切监测焊缝金属的奥氏体/铁素体分布，甚至微小的化学成分变化，从而有可能通过改变焊缝金属成分来控制焊缝金属的相比。此前，已经开发了焊缝金属预测工具，即Schaeffler、De Long、WRC-1992图。最新开发的，特别是WRC-1992图表，已被证明具有良好的预测能力，但它需要进一步发展。该图仅涵盖了化学成分组合的有限区域，忽略了重要的合金元素，并且完全排除了冷却速率，尽管这是最终焊接结构的主要参数。激光诱导击穿光谱（LIBS）不仅提供了一种新的焊接过程中原位化学成分测量方法，而且实现了时间和空间分辨的电子温度计算。利用电子温度数据，首次可以在不使用热电偶等传统方法的情况下计算焊接金属的冷却速率并分析其冷却曲线。研究目的是利用LIBS在线化学成分测量来研究双相不锈钢的原位焊接凝固和随后的相变，以确定焊接和凝固过程中的铁素体-奥氏体相变/分布。此外，它将用于将测量的相变与焊缝的机械性能相关联。最后，将测量的冷却速率和冷却曲线与相分布信息相结合，利用WRC-1992图研究相应的凝固修正量。详细的潜在凝固和固态转变机制将由材料模型描述。

Duplex stainless steels (DSS) have been drawing increasing attention in recent years due to their significant high-strength performance and corrosion resistance. Recent innovative developments, global need for sustainable and environmentally applicable engineering applications brought about a rapid growth in demand and utilization of DSSs in the last 20 years. DSSs, composed of ca. 50-50% balanced ferrite and austenite phases, show austenite reformation and microstructural changes during welding, which leads to unbalanced phase ratios and chemical composition modifications. Such major structural changes in these multiphase materials cause solidification cracking, corrosion susceptibility, lower ductility and critical strength values. Therefore, to maintain the desired welding performance, the austenite/ferrite distribution and even the minor chemical composition change of the weld metal must be closely monitored during welding, thus it is possible to control the phase ratio of the weld metal by modification of the weld metal compositions. Previously, weld metal prediction tools have been developed, namely Schaeffler, De Long, WRC-1992 diagrams. The latest developed, specifically the WRC-1992 diagram, has been proven to have a good predictive capability, however, it needs further development. The diagram covers only a limited area of chemical composition combinations, important alloying elements are ignored, and the cooling rate is completely excluded, although this is a major parameter for the resulting weld structure. Laser-induced breakdown spectroscopy (LIBS) not only enables a new in situ chemical composition measurement method during welding, but also implements time and space resolved electron temperature calculations. Using the electron temperature data, for the first time it is possible to calculate the cooling rate and analyze the cooling curves of the solidifying weld metal without conventional methods e.g. thermocouples. The research objective is to investigate the in situ weld solidification and subsequent phase transitions of duplex stainless steels using online chemical composition measurements with LIBS, to determine ferrite-austenite phase change/distribution during welding and solidification. Additionally, it will be used to correlate the measured phase changes with mechanical properties of the welds. Finally, adding the measured cooling rate and cooling curves to the phase distribution information, the WRC-1992 diagram will be used to investigate the corresponding solidification modii. The detailed underlying solidification and solid-state transformation mechanisms will be described by material models.