Carbides and possible hydrogen irreversible trapping sites in ultrahigh strength round steel

The carbides in ultrahigh strength round steel have been investigated by using laser-assisted atom probe tomography (APT) and high resolution transmission electron microscopy (HRTEM) in this paper. Two kinds of carbides are found and one is iron carbide M₆C, where carbide formation elements Cr, Mn and Mo replace partial Fe, while the other is niobium carbide MC, where M includes V and Mo besides Nb. These two carbides, due to their different evaporation field, have various densities in reconstructed image of APT. After correction, the hydrogen content within these two carbides illustrates that M₆C cannot trap hydrogen, while MC can. The different behaviors in trapping hydrogen between these two carbides may result from elements Fe or Cr in M₆C carbide having weaker affinity for hydrogen than Nb and V have in MC.