Post weld-treatment of laser welded AHSS by application of quenching and partitioning technique

Two-step quenching and partitioning (Q&P) treatment was applied on specimens of an advanced high strength steel (AHSS) after laser welding, for post welding treatment. In order to avoid formation of brittle martensite phase, which usually form due to very high cooling rate of laser welding. To simulate the effect of different Q&P parameters after welding in the most critical part of HAZ, several cycles were performed in Gleeble simulator and analyzed in advance. Subsequently some of the cycles were repeated after laser welding by using an induction heater close to the weld. Different techniques including SEM, EBSD and XRD were used to analyze the microconstituents of the structure and mechanical properties were investigated by micro-hardness measurements across the weld, tensile and impact toughness tests. The final structure consists of controlled amount of tempered martensite with precipitates, bainite laths and small amount of fresh martensite depending on the thermal cycles. In addition, samples heated at a temperature between Ms and Bs (in this case 540 °C) showed the best mechanical properties. Therefore, this technique not only improves the microstructure and mechanical properties of the fusion zone (FZ) and heat affected zone (HAZ) but gives also a quick industrial processing method for post welding treatments.