Hydrogen embrittlement of steels

Hydrogen is a ubiquitous contaminant in all arc welding processes. Hydrogen has a much higher solubility in molten iron and its solubility decreases with temperature in the solid state. The rates of weld metal solidification are very high and, as a result, the hydrogen that has dissolved in the fused weld metal is retained. Given the right conditions, these highly mobile atoms seek rifts and discontinuities in the metal lattice and concentrate at those points. In concert with the residual stresses in the lattice that are due to external restraint and volume changes caused by solidification and solid - state transformation, the hydrogen enlarges the discontinuities to form micro cracks. The localized stresses are relieved abruptly as the atoms penetrate the fissures and become trapped as molecules. The resulting micro cracks which have sharp tips, also are associated with high stress concentrations at which additional atoms collect. Those stresses build up until they, too, are relieved as the cracks is extended. This process of stress build up and relief by cracking continues until failure occurs. Approaches to prevent the occurrence of HIC and examples of prevention of HIC are presented in this paper. (author)