Prediction of hydrogen content in long products based on a new mathematical model and an innovative hydrogen solid-state sensor

Background Hydrogen can be dissolved in steel during steelmaking owing to its high enough solubility in the liquid phase. Hydrogen can be picked-up during steelmaking and casting from the moisture of atmosphere, refractories, alloys, process gases, recarburizers and slag former additives. The solubility of hydrogen in the solid steel decreases, but it is high enough to produce surface and internal defects, which can be unacceptable for some applications. Owing to its small atomic radius, hydrogen shows a high diffusivity in the solid phase, therefore it tends to escape during cooling, reducing its concentration. The amount of hydrogen removed during cooling depends on initial concentration, size of the product, steel grade and cooling rate. Also at low concentrations (some ppm) hydrogen can severely affects steel quality as residual hydrogen tends to concentrate in the discontinuities inside the material (cavities, grain boundaries, interface between non-metallic inclusions and matrix) where it can recombine from atomic to molecular state, generating very strong localised pressures which may exceed the resistance of the steel and cause fractures (so-called flakes), typical defects found in great size, non-degassed semifinished-products. Moreover, high hydrogen contents may cause cracks when in the presence of hardening structures and can lead to a significant decrease of ductility. For this reason, upper specification limits are often requested as function of the material grade and final size. Vacuum degassing, stirring by inert gas and controlled cooling patterns after solidification are the most commonly used methods to remove hydrogen in steelmaking; nevertheless, the problem to detect the real residual hydrogen content in steel products remains still a topical subject, as its high diffusivity is responsible for significant loss of the element which can affect the reliability of subsequent detection.