Influence of heat treatment on the HIC susceptibility in the process pipe steel was investigated with respect to the hydrogen diffusion behavior by employing the electrochemical permeation technique. In order to examine the change in morphology of precipitates in acicular ferrite after the heat treatment, extraction replica technique for TEM analysis was utilized. This study clearly shows that, contrary to other microstructures, in acicular ferrite, iron carbide (Fe3C) particles are newly precipitated mostly along the grain boundaries by the simulated PWHT conducted at 620°C for 1 hour. In addition, decrease in hydrogen diffusivity attributed to the hydrogen trapping effect by the newly generated Fe3C leads to an increase in apparent hydrogen solubility in the steel. As a result, the heat treated process pipe steel is more susceptible to HIC.

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