Research by Fraunhofer ILT looked at new lightweight construction solutions, joining technology and end face seams for martensitic chromium steels.
Martensitic chromium steels are one of the steel grades with a future: they are steels that are ideal for automotive applications since they are both lightweight and corrosion resistant. These materials are particularly in demand for the design of collision-safe battery boxes for electric cars. For this reason, the Fraunhofer Institute for Laser Technology (ILT) uses these sophisticated components as demonstration components for laser welding and heat treatment.
As part of the AiF research project FAAM, supported by Forschungsvereinigung Stahlanwendungen e.V. (FOSTA), experts from industry and research took a close look at the current status of those grades. The final online conference in summer 2020 focused on new lightweight construction solutions, joining technology and end face seams, among other things, for martensitic chromium steels.
In detail, Fraunhofer ILT investigated how suitable it is to weld a press-hardened chromium steel with martensitic microstructure X46Cr13 (1.4034) in similar and dissimilar joints for assembly applications; this steel is considered difficult to weld due to its high carbon content. The dissimilar joints were combinations with work-hardened high-manganese steel (1.4678), press-hardened manganese-boron steel (1.5528), high-strength dual-phase steel (1.0944) and cold-rolled fine-grained structural steel (1.0984).
Martin Dahmen of the Macro Joining and Cutting Group at Fraunhofer ILT explains, “The main focus was on the mixing of the different materials, on the metallurgy and the resulting property profiles.”
The joining quality can be improved by heat treatment. For this purpose, linear seams of a 1.4034 joint of the same type were heat-treated in the lap joint from 300 to 700 deg C outside the process (ex-situ); the seams had to prove their quality in the subsequent shear tensile test.
“At 400 to 500 deg C, the highest strengths and lowest hardnesses were obtained,” explains Dahmen. “Remarkable is the high proportion of ductile failure on the fracture surface already at 400 deg C.”
The researchers aimed at reaching short holding times in order to use laser radiation for heat treatment.
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