Hydrogen diffusion in thick-walled S690 saw joints: part 1—experimental determination of microstructure-dependent diffusion coefficients

Abstract High-strength low-alloyed steel S690 is widely used in heavy-duty applications, such as structural components, mobile cranes, and industrial plant construction, owing to their high strength and weldability. However, thick-plate submerged arc welding (SAW) can introduce elevated hydrogen levels and residual stresses that promote time-delayed hydrogen-assisted cold cracking (HACC). Accurate, microstructure-specific diffusion data are scarce, limiting predictive HACC assessments. This study presents an experimental determination of hydrogen diffusion coefficients ( D H ) in two S690 variants: thermomechanically rolled (S690MC) and quenched and tempered (S690Q). Multi-layer SAW welds were produced from 30-mm-thick plate material at three heat input levels, and diffusion membranes were extracted from weld metal (WM), heat-affected zone (HAZ), and base material (BM). Hydrogen permeation tests, conducted in accordance with DIN En ISO 17081, yielded flux curves normalized in time from which D H was derived using the inflection-point method. At room temperature, D H values ranged from 6 × 10 −5 mm 2 /s to 9 × 10 −5 mm 2 /s across all regions and heat inputs, with no significant difference between S690MC and S690Q. Weld metal exhibited marginally lower D H , attributed to enhanced hydrogen trapping, while base material measurements showed greater variability. These microstructure-resolved diffusion coefficients add quantitative data for modern S690 SAW welds and provide internally consistent input parameters for the numerical simulations presented in part 2. The results support improved parameterization for subsequent assessments of HACC risk through the optimization of welding parameters.