Hydrogen trapping behavior of copper precipitates in steel

The hydrogen trapping behavior at Cu precipitates was investigated in martensitic Fe-0.18%C-1.5%Mn-4.0%Cu steel having various precipitation conditions. A hydrogen desorption peak was observed around 373 K in all samples. Total amount of desorped hydrogen decreased first with aging period but increased later. The initial decrease has been attributed to the recovery of lattice defects, and the increase in the later stage to trapping at Cu precipitates. The amount of desorped hydrogen increased even after peak hardening. The crystal structure of Cu precipitates was observed by SR-XAFS to be basically bcc; however, indication of fcc structure was also observed. The intensity of fcc structure increases with aging time. This suggests that incoherency of matrix-precipitate interface increases with aging time especially after peak strength, and that hydrogen trapping is more pronounced at incoherent interface than coherent one at Cu precipitates. Since misfit strain of coherent Cu precipitate is smaller than other precipitates such as TiC, the ability of hydrogen trapping at coherent Cu precipitates is estimated to be small.