抄録
High strength steel sheets have been developed for both reducing automobile weight and keeping passengers’ safety. Delayed fracture due to diffusible hydrogen is known as one of problems in arc welds of high strength steel plates, which is often prevented by applying pre/post heating processes. However, there is little reported regarding hydrogen diffusion behavior during rapid temperature change in resistance spot welds of steel sheets. Hydrogen diffusion behavior is affected by thermal and stress fields. Therefore the effect of rapid change in these fields needs to be properly considered for elucidation of hydrogen diffusion behavior in resistance spot welds. This study aims to clarify the delayed fracture mechanism of resistance spot welds. The hydrogen diffusion simulation model considering the effects of thermal and stress changes during welding is constructed. It is assumed in the model that hydrogen solubility and diffusivity in the nugget are larger than those of base metal. A change in fracture pattern dependent on hydrogen environment can be well explained using this model. The numerical simulation predicts that high residual stress leads to hydrogen concentration at the edge of the nugget after welding. This concentration would be the main factor of the delayed fracture in the nugget when the nugget diameter is small. In this situation, hydrogen is introduced from antirust oil, water and contamination on sheet surfaces to the nugget during welding. Numerical simulation assumed in actual use is also performed, in which the hydrogen is introduced from the sheet surfaces. Hydrogen concentrates at the edge of the nugget in relatively mild hydrogen environment, whereas in severe environment it highly concentrates in the HAZ around the nugget before concentrating in the nugget. These calculated results can well explain the transition of fracture pattern from the nugget to the HAZ in the experiment.
