Investigation of the softening mechanism during welding in the heat-affected zone of micro-alloyed high-strength steels

• Applicant: Professor Dr.-Ing. Thomas Kannengießer
• Subject Area: Metallurgical, Thermal and Thermomechanical Treatment of Materials ; Production Automation and Assembly Technology
• Term: from 2019 to 2024
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 435083436

There is a great divergence between the suggested welding parameters applied to a certain steel grade and the practical use of parameters for individual high strength steels. This is ascribed to the fact that currently different alloying design concepts with micro-alloy addition have been utilized to create diversely micro-alloyed steel. As a consequence of such a difference, the steels even having similar level of mechanical properties exhibit different weldability. However, the knowledge about the role of micro-alloy elements during weld microstructure evolution and for the mechanical properties is not advanced enough for allowing reliable predictions of the performance of welded micro-alloyed steel. This fundamental comprehension of the relationship between addition of micro-alloy elements, weld metallurgy and mechanical properties shall be worked out within the scope of the present research project. For various designed micro-alloy steels, it is crucial to build an understanding of the mechanism that governs the HAZ softening properties. The reason for this significant issue is that the strength and ductility of welds with severe local softening region are lower than that of the base metal. This reduction in mechanical properties is problematic when structural integrity is considered. To determine the influence of softening zone on the fracture behavior will be realized by using digital image correlation technique during tensile test. Physical simulation of HAZ by dilatometer will be performed to understand the role of different micro-alloy addition in affecting the softening behaviour. A quantitative analysis will be given to the phase transformation products composition, complex carbides precipitation and coarsening effect. The simulation of carbides coarsening by thermodynamic and kinetic modeling will be made. Both experimental and simulation results will be evaluated to reveal the softening mechanisms. The expected results will provide guidelines for micro-alloyed steels design with an aim to ensure excellent weldability and integrity properties of weldments.

• DFG Programme: Research Grants
• Co-Investigator: Professor Dr.-Ing. Michael Rhode