Nature of hydrogen trapping sites in steels induced by plastic deformation

Thermal desorption spectroscopy (TDS) has been used to reveal the nature of defects acting as trapping sites of hydrogen. Hydrogen was charged to ferr itic and eutectoid steels deformed to various degrees and then given anneal ing treatment. Desorption with a single peak appeared between roam temperat ure and 600 K from ferritic steels. Under constant hydrogen charging condit ions, the amount of desorption increased with strain. However, when the def ormed samples were subjected to annealing at temperatures as low as 500 K, the increase of desorbed hydrogen no longer appeared. Vacancy clusters, whi ch themselves annihilate in the course of TDS measurement, are the probable source of hydrogen desorption. When heavy deformation was given to ferriti c steels, a two-step decrease of hydrogen desorption took place with increa sing annealing temperature, corresponding to annihilation of vacancy cluste rs and decrease of dislocation density, respectively. The desorption with a single peak has two origins, one due to the annihilation of the trapping s ites themselves and the other to desorption from stable sites. For heavily deformed eutectoid steel, an additional desorption peak centered at around 640 K appeared. The peak likely results from deformation-induced defects wi thin the cementite phase or supersaturated carbon in ferrite. Two types of desorption, one due to the annihilation of trapping sites in the course of measurement and the other due to desorption from stable sites, should be di scriminated. TDS using hydrogen as a tracer can be applied as a tool to inv estigate the various defects induced by plastic deformation. (C) 1999 Elsev ier Science S.A. All rights reserved.