When steel is exposed to hydrogen, the uptake of hydrogen atoms can lead to a substantial negative change in material properties. By enhancing the plasticity of the material as well as the decohesion, an unpredictable, delayed brittle failure of technical parts in service is possible, which is known as hydrogen embrittlement. Especially high strength steels with a body-centred (bcc) crystal structure are most affected due to a faster diffusion of hydrogen as in face-centred (fcc) steels. The susceptibility of steel to hydrogen-induced failure depends not only on the crystal structure but also on the hydrogen concentration, as a higher amount of hydrogen intensifies the microstructural damage. A common method to analyse the hydrogen concentration is the thermal desorption spectroscopy (TDS), where a small probe is heated to extract the hydrogen. An advantage of this method is the differentiation between diffusible and trapped hydrogen. On the other hand, only a global hydrogen concentration can be determined, whereby local concentration can be significantly higher. Additionally, the extraction of a probe damages the technical part, which cannot be reused again.
With this motivation, a non-destructive method to analyse the hydrogen concentration is currently being investigated. For this, cylindrical specimens of quenched and tempered as well as normalized AISI 4140 steel are electrochemically charged and then analyzed by using different measurement techniques to characterise a hydrogen uptake directly after charging as well as after several hours of hydrogen effusion. With the help of micromagnetic sensors, the influence of hydrogen on the ferromagnetic properties of the steel is investigated. Residual stresses are obtained by cos(α) X-ray diffraction measurements. A quantitative hydrogen concentration is determined by an electrochemical measurement cell called local hydrogen analysis (LHA). When correlating both, the residual stresses and the hydrogen concentration, increasing compressive residual stresses can be assessed with an increasing hydrogen concentration. An exponential decline of the hydrogen concentration due to effusion can be observed at the same specimen when using the LHA.