Fracture mechanics and surface chemistry studies of subcritical crack growth in AISI 4340 steel

Abstract

Coordinated fracture mechanics and surface chemistry experiments were carried out to develop further understanding of environment enhanced subcritical crack growth in high strength steels. The kinetics of crack growth were determined for an AISI 4340 steel (tempered at 204°C) in hydrogen and in water, and the kinetics for the reactions of water with the same steel were also determined. A regime of rate limited (Stage II) crack growth was observed in each of the environments. Stage II crack growth was found to be thermally activated, with an apparent activation energy of 14.7 ±2.9 kJ/mole for crack growth in hydrogen, and 33.5 ± 5.0 kJ/mole in water. Fractographic evidence indicated that the fracture path through the microstructure was the same for these environments, and suggested hydrogen to be the embrittling species for environment enhanced crack growth in hydrogen and in water/water vapor. A slow step in the surface reaction of water vapor with steel was identified, and exhibited an activation energy of 36 ± 14 kJ/ mole. This reaction step was identified to be that for the nucleation and growth of oxide. The hydrogen responsible for embrittlement was presumed to be produced during this reaction. On the basis of a comparison of the activation energies, in conjunction with other supporting data, this slow step in the water/metal surface reaction was unambiguously identified as the rate controlling process for crack growth in water/water vapor. The inhibiting effect of oxygen and the influence of water vapor pressure on environment enhanced subcritical crack growth were considered. The influence of segregation of alloying and residual impurity elements on crack growth was also considered.