TRANSIENT MODEL FOR SUB-CRITICAL CRACKING IN BCC ALLOYS.

A proposed model for K//I-independent crack growth rates, as induced by either external or internal hydrogen, is proposed. It is based upon Fick's second law, a desorption isotherm and a fracture stress criterion which decreases linearly with hydrogen concentration. As such, it potentially involves at least three thermally activated steps, adsorption/desorption, diffusion and trapping with the latter affected by the available 'bulk' hydrogen in the dilated lattice at the crack tip. The model is consistent with existing data in 4130 and maraging steels for crack growth in hydrogen partial pressures. Here, the model reproduces the observed behavior that the crack growth rate tends toward zero above some critical elevated temperature T//o. If weakly trapped hydrogen is involved, the site for cracking is 50 to 150 nm below the crack tip in the materials evaluated. The pressure and temperature dependencies are interpreted partially in terms of a critical coverage to produce cracking. For internal hydrogen, the experimental data are qualitatively similar to the external data for 4340 steel.