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

• Applicant: Professor Dr.-Ing. Thomas Kannengießer
• Subject Area: Production Automation and Assembly Technology ; Metallurgical, Thermal and Thermomechanical Treatment of Materials
• Term: since 2020
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 442001176

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.

• DFG Programme: Research Grants
• Co-Investigator: Dr.-Ing. Axel Griesche