Influence of Specimen Thickness on Thermal Desorption Spectrum of Hydrogen in High Strength SCM435 Steel

Thermal desorption analysis (TDA) of hydrogen was carried out in high strength SCM435 steel in which the strain field of dislocation presumably is a major hydrogen trap site. Cylindrical specimens of radius 0.5-5 mm were cathodically charged until saturated and were heated at a rate ranging from 25 to 200 degrees C/hr, aiming to vary the condition of hydrogen desorption from detrapping- to diffusion-control. For specimens of radius 0.5 mm the trap energy of hydrogen evaluated from the Choo-Lee (C-L) plot was as high as 33.8 kJ/mol, while for thicker specimens it was significantly smaller, i.e. 25.6-27.5 kJ/mol. The possible causes for the dependence of the trap energy on specimen thickness are discussed in terms of the influence of initial hydrogen distribution on the peak temperature and the deviation from local equilibrium of hydrogen during desorption. If pre-exposure is carried out for a sufficiently long time prior to TDA, the C-L plot seems to give a correct detrap energy even in the mixed-control desorption.