HAZ characterization of welded 42SiCr steel treated by quenching and partitioning

An experimental steel 42SiCr was heat treated with a quenching and partitioning treatment in order to achieve high strength and at the same time acceptable ductility. This crucial combination requires the formation of a martensitic microstructure with a small volume fraction of retained austenite embedded on a microscopic scale. The objective of this work was to investigate the evolution of microstructure and hardness in the heat affected zone (HAZ) of welds in this 42SiCr and its major influence factors. Thermo-physical simulations with a dilatometer yielded samples of each zone in the HAZ and allowed a detailed investigation of HAZ microstructures and properties. A decrease in hardness from 580 HV in the quenched and partitioned base material to the HAZ, with minimum hardness values of 370 HV in a region exposed to temperatures close to the Ac3 temperature, was revealed. Simulation showed that the experienced peak temperature is predominantly determining the HAZ properties. The microstructure after quenching and partitioning treatment consisted of fine martensite with presumed retained austenite. The HAZ regions that were heated above Ac3 temperature can be characterized to have a fine bainitic/martensitic microstructure, while the regions heated to more than 1100 °C show coarser grain, with bainitic/martensitic features. To support the HAZ simulation results sheets of this steel were rolled and resistance spot welds were produced. The welds were characterised with respect to the microstructure in the heat affected zone, as well as the mechanical properties of the joint. Results proved that the data obtained by thermo-physical simulation is in very good agreement with the spot welding results. On a macroscopic scale the welding results indicate very good joint formation but very brittle fracture behaviour using a conventional resistance spot welding process for the quenching and partitioning steel. However, despite the relatively high carbon equivalent of the material, also ductile aspects of fracture were observed