Manufacturers of vehicle and machine components often use high-strength steels to save material in lightweight construction and for crash-relevant structural components that require exceptionally high durability. When welding these components, various factors may lead to the unwanted formation of fine cracks, which may spread and even lead to component failure. Unfortunately it is very hard or impossible to assess these factors with experiments, for example hydrogen concentration would have to be measurable in highly localized points inside the component during welding. Another difficulty: the period of time during which a break may forms is relatively long – it can form within seconds during the welding process or after a few days. This is why the reject rate of laser welded high-strength steel components to this day remains uncomfortably high.
Illustrating the influence of hydrogen traps
To enable component manufacturers to reduce the reject rate of high-strength steels, Dr. Frank Schweizer, a member of the »Microstructure, Residual Stress« group at the Fraunhofer IWM, refined industrially used methods of numerical welding simulation in his dissertation. Now he can recreate any events at highly localized component points on his computer. This even functions for very quick changes between room and melting temperatures, as they occur during welding. »Now we can calculate the temporal development of influencing factors and their intersections accurately and monitor them virtually«, explains Schweizer. These factors include hardening structures, residual stress and localized hydrogen concentration, which can lead to crack formation in the welded component. Alongside the hydrogen which is introduced during the welding process, previously existing hydrogen in the steel is loosened up by the welding heat and rendered movable and diffusible.
»The special feature of this new method is that it also takes into account the effect of so-called hydrogen traps«, says Schweizer. He discovered that hydrogen traps greatly influence the »movable« hydrogen occurring in low hydrogen concentration for different laser welding connections. With higher hydrogen content the thermomechanical behavior of the material grows more relevant for the formation of breaks. »The hydrogen atoms slowly collect in the narrow area of the heat-affected zone where tensile residual stress is especially high«, says Schweizer. Even after the steel has cooled down, hydrogen can collect at these points and the steel grows brittle. »Thus even after hours or days cracks may form which must lead to the component’s rejection«, explains Schweizer.