Control strategy of reducing hydrogen enrichment in heat-affected zone during welding for ultra-high strength steels

Abstract

The hydrogen diffusion behaviors in welded joints of ultra-high strength steel (UHSS), which are related to hydrogen-induced cracks (HIC) in welding, are not yet fully understood due to the lack of in-situ observation techniques for hydrogen during welding. An improved microphotography technique is proposed to visualize the hydrogen distribution within the as-welded joint. When UHSS is welded using traditional welding materials, the transformation temperature from γ to α phase (TF) of the weld metal (WM) is higher than that (TB) of the heat-affected zone (HAZ), resulting in the observed hydrogen enrichment in the HAZ. We developed a low transformation temperature (LTT) welding material. This material is designed so that the TF of WM is lower than the TB of HAZ, facilitating hydrogen back-diffusion from the HAZ to the WM. This back-diffusion effectively reduces the hydrogen concentration in the HAZ. Additionally, a thermo-mechanical hydrogen diffusion sequentially coupled finite element procedure was utilized to simulate hydrogen concentration evolution during welding, revealing the control process of phase transformation temperature differences (ΔT = TF - TB) on hydrogen diffusion behavior and the influence of residual stress on the accumulation of hydrogen. The microphotography technique has successfully validated these findings, which can help to mitigate the risk of HIC in the HAZ during welding of UHSS.