A Model for the Growth of Hydrogen Attack Cavities in Carbon Steels

A detailed analysis of precision dilatometry data of McKimpson on plain carbon steel during the incubation stage of hydrogen attack has been carried out in the light of both the surface and grain boundary diffusion limited cavity growth models. The results are consistent with cavity growth by grain boundary vacancy diffusion provided the methane pressure in the cavity exceeds a threshold pressure. The threshold pressure observed by the analysis decreases with an increase in temperature from ~400 MPa at 658 K to ~200 MPa at 723 K. The magnitude of the threshold pressure is consistent with a constrained cavity growth model in which the strain due to atom plating is accommodated by local plastic deformation around the precipitates in the boundary. Quantitative agreement between model predictions and experimental data on expansion rates is good.