Dominating driven factors of hydrogen diffusion and concentration for the weld jointcoupled analysis of heat transfer induced thermal stress driven hydrogen diffusion

Abstract

Hydrogen embrittlement cracking caused at a weld joint is considered to be dominated by hydrogen diffusion and concentration driven by thermal stress induced by heat transfer during cooling process. The gradient of hydrostatic stress component is considered to be a driven force of hydrogen transportation. However, this problem concerns the occurrence phenomenon during cooling process. Therefore, diffusion coefficient, yield stress and Young’s modulus are changed corresponding with temperature change. Especially, diffusion coefficient shows the space gradient corresponding with space gradient of temperature caused by heat transfer. This affects the diffusion equation of hydrogen as a driven force of hydrogen diffusion. Under these backgrounds, to clarify not only the effect of local thermal stress but also that of space gradient of diffusion coefficient on hydrogen release and trap, the hydrogen diffusion analysis based on our proposed ¡ multiplication and FEM-FDM methods was conducted by introducing the terms of gradients of diffusion coefficient and temperature into the diffusion equation. The following results were obtained. The space gradient of diffusion coefficient was found to contribute the release of hydrogen from the site of stress concentration when the gradient of local hydrogen concentration takes the same sign as that of diffusion coefficient. Concerning the prevention of hydrogen embrittlement cracking at weld joint, these results show that not only Pre-Heat Treatment (PHT) which is a mechanical factor, but also the space gradient of diffusion coefficient which is a factor of material science was found to be one of effective factor of release of hydrogen from a site of stress concentration.