In Situ Studies of Phase Transformation and Residual Stresses in LTT Alloys During Welding Using Synchrotron Radiation

Novel martensitic filler materials with specially adjusted martensite start temperatures (M_s) can counteract the cooling specific shrinkage due to expansion effects of the weld metal associated with phase transformations. That can be exploited to create compressive residual stresses in the weld and adjacent areas, i.e. beneficial for increasing the fatigue strength. The M_s temperature is shifted via the chemical composition, mainly by the alloying elements nickel and chromium, resulting as well in different retained austenite contents. Comparative investigations were made using a Low Transformation Temperature (LTT) alloy and a conventional high strength steel. The resulting phase transformation temperatures were—for the first time—detected using high energy synchrotron diffraction. Compared to angle dispersive diffraction, energy dispersive diffraction offers the possibility to measure residual stresses of the martensite and austenite phase parallel fast in one experiment. Furthermore, the high energy allows for obtaining information from the material volume by measuring in transmission geometry. For that purpose a special welding setup was designed applicable at different beam-lines and diffraction setups, allowing for diffraction experiments under realistic welding conditions. In particular the setup gives the possibility to observe and correlate localized phase transformations and thermo-mechanical stress/strain evolution during and after welding specific, rapid heat treatments. Additionally, due to local melting, solidification processes can be investigated. First results, presented here, show the correlation of local residual stress distributions affected by lowered transformation temperature.