Measurement and Modeling of Hydrogen Environment-Assisted Cracking of Ultra-High-Strength Steel

Modern precipitation-hardened ultra-high-strength AERMET 100 steel (Fe-Co-Ni-Cr-Mo-C) is susceptible to severe transgranular hydrogen environment-assisted cracking (HEAC) in neutral 3.5 pct NaCl solution. The threshold stress intensity for HEAC, K^sub TH^, is reduced to as low as 10 pct of K^sub IC^, and the stage II subcritical crack growth rate, da/dt^sub II^, is up to 0.5 µm/s. Low K^sub TH^ and high da/dt^sub II^ are produced at potentials substantially cathodic, as well as mildly anodic, to free corrosion. However, a range exists at slightly cathodic potentials (-0.625 to -0.700 V^sub SCE^), where the crack growth rate is greatly reduced, consistent with reduced crack-tip acidification and low cathodic overpotential for limited H uptake. Short crack size (250 to 1000 µm) does not promote unexpectedly severe HEAC. High-purity AERMET 100 is susceptible to HEAC because martensite boundary trapping and high crack-tip stresses strongly enhance H segregation to sites that form a transgranular crack path. The stage II da/dt is H diffusion rate limited for all potentials examined. A semiquantitative model predicts the applied potential dependence of da/dt^sub II^ using reasonable input parameters, particularly crack-tip H uptake reverse calculated from measured K^sub TH^ and a realistic critical distance. Modeling challenges remain. [PUBLICATION ABSTRACT]