A Numerical Analysis of the Diffusion and Trapping of Hydrogen in Steels and its Application to Weldments

Abstract

It is known that the apparent diffusion coefficient and the occlusible amount of hydrogen in steels show anomaly under the temperature of 200°C, depending upon the amount of the plastic strain. If the hydrogen diffusion in steels is influenced by the plastic strain fields, steels cannot be uniform mecdia for hydrogen diffusion. The hydrogen diffusion, therefore, does not follow the Fick's second law but does the general law of mass transfer. in which a driving force for the diffusion is exerted by the chemical potential gradient.
The diffusion and local accumuration of hydrogen in steels were quantitatively analized the finite difference method under the condition holding the law of mass transfer. The analysis was made for steels with distributed hydrogen trapping sites which are assumed to be plastically strained regions. As a-result of the numerical analysis, the apparent diffusion coefficient in steels plastically strained corresponding to 0.1% void volume below 150°C was found to be 0.12 exp (-7800/RT) which coincides with the experimental result by Johnson and Hill.
This analytical method was attempted to solve quantitatively the hydrogen diffusion and accumulation in the weld fusion boundary of a bead on a plate. Findings in this analysis are as follows:
1. The hydrogen concentation at the weld fusion boundary attains to several times the initial concentration 4 days after the completion of welding;
2. The preheat and postheat treatment are effective to prevent the hydrogen accumulation in the weld fusion boundary and the hydrogen concentration never exceeds the initial level when applying 150°C postheat treatment.