Investigation of the Conditions for Crack Propagation and Arrest Under Cathodic Polarization by Rising Step Load Bend Testing

Abstract

The effects of potential step polarization and hydrogen overpotential on crack propagation and crack arrest are investigated for T-250 maraging steel (yield strength = 1708 MPa [248 ksi]) and PH 13-8 (UNS(1) S13800) steel (yield strength = 1467 MPa [213 ksi]). The critical stress intensity threshold for hydrogen-assisted cracking, KISCC, measured as a function of potential using a rising step load bend (RSL-B) technique, is compared to KISCC vs potential curves by other investigators. Both materials showed an increase in KISCC with increasing potential. A fractographic examination of specimens tested at different potentials revealed distinct changes in the fracture mechanism as a function of potential that are explained in terms of the decohesion model for hydrogen-assisted cracking. The crack arrest and propagation experiments reveal that the crack tip responds instantly to changes in the hydrogen activity; i.e., step polarization. The ability to arrest and restart crack propagation by stepping the applied potential at a constant applied stress intensity is demonstrated, proving that KISCC is a direct function of the hydrogen overpotential at the crack tip.