Physical simulation aided process optimisation aimed sufficient HAZ toughness for quenched and tempered AHSS (original) (raw)

Physical simulation aided process optimisation aimed sufficient HAZ toughness for quenched and tempered AHSS

The outstanding mechanical properties of quenched and tempered high strength steels cannot be adequately preserved during the welding due to the irreversible microstructural changes in the heat affected zone (HAZ). There is a general statement that the area of weldability lobe decreases by the application of higher strength categories. According to numerous experimental results, by the application of low heat input welding the disadvantageous microstructural changes, and therefore the decrease of strength and toughness properties can be minimized, although other welding difficulties can be noticed (e.g. cold cracking). HAZ properties can be limitedly analysed by conventional material tests, therefore physical simulators (i.e. Gleeble) were developed for the examination of different HAZ areas. In our research work three technological variants 5, 15 and 30 s t8.5/5 were applied during the HAZ simulation of S960QL steel (EN 10025-6), and the effect of cooling time on the critical HAZ areas of single and multipass welded joints was analysed. Heat cycles were determined according to Rykalin 3D model, where the whole heat cycle, including heated part as well, was described by time-temperature points instead of the automatic software settings. The properties of the selected coarse grained (CGHAZ), intercritical (ICHAZ) and intercritically reheated coarse grained (ICCGHAZ) zones were investigated by scanning electron microscopes, macro and micro hardness tests and instrumented Charpy V-notch impact tests. The industrial utilization possibilities of experimental results are also presented by an industrial partner in this research work.