Vergleich der Mikrostruktur ausgewählter hochfester Schweißungen

In this diploma thesis different fusion welding processes were used for joining 8 mm thick sheets of S960MC structural steel. The following welding methods with or without welding additives were used: laser beam welding without filler metal, laser hybrid welding with filler metals EMK 8, plasma welding without filler metal, plasma welding with filler metal EMK 8, gas metal arc welding with filler metal EMK 8 and gas metal arc welding with filler metal 960 IG. Various etching methods (Nital, LePera, picric acid) were applied to these samples in order to allow an extensive microstructural characterization of the welded joints. By using high-resolution imaging techniques, like electron backscatter diffraction, scanning electron microscopy and light microscopy, a deeper understanding of the microstructure of the weld metal was obtained and the individual structural components were characterized in detail. In addition, the microstructure of the weld metal was compared with the mechanical properties. The chemical composition of the fusion zone depends on the seam preparation, the filler material (if present), the base material and the welding process. The resulting microstructure is influenced by the t8/5 time and the critical cooling rate, which depends on the chemical composition. The weld metals of at laser beam and plasma welding without and with filler metal EMK 8 have a similar chemical composition. As a result of the high dilution of these welding processes, the mechanical properties in the weld metal can also be achieved with the low-alloyed EMK 8. In plasma welding, the weld metal and the t8/5 time are significantly larger than in laser beam welding. This, however, hardly affects the mechanical characteristics and the structure (fine needle and needle packets). This is not the case using gas metal arc welding with the filler metal EMK 8, since the dilution with the base material is less. The weld metal therefore has a lower chemical. This shifts the critical cooling rate for the martensitic transformation to shorter times and globular grains are recognizable. The use of the highly alloyed filler metal 960 IG in gas metal arc welding increases the chemical composition of the fusion zone and reduces the critical cooling rate. The structure is made of fine needles and this weld meets the requirements.