With the implementation of more stringent emissions standards, ultra-high strength steel has been increasingly used in vehicle body to reduce the carbon emissions, but softening in the heat affected zone is one of the most serious issues faced with in welding of this steel. In this paper, a finite element model (FEM) was developed to estimate temperature distribution in laser welding of ultra-high strength steel M1500 and a carbon diffusion model was then developed to estimate the martensite tempering transformation in the softening zone based on the simulated temperature distribution results. Maximum softening degree, minimum hardness point position and boundary of the softening zone were estimated and validated by hardness measurement experiments. This work provides a better understanding of the mechanism for heat affected zone softening in laser welding of ultra-high strength steel.

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