Accession Number:

ADA045532

Title:

Fracture Mechanics and Surface Chemistry Studies of Subcritical Crack Growth in AISI 4340 Steel.

Descriptive Note:

Technical rept.,

Corporate Author:

LEHIGH UNIV BETHLEHEM PA INST OF FRACTURE AND SOLID MECHANICS

Personal Author(s):

Report Date:

1977-09-01

Pagination or Media Count:

48.0

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 steeel 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 or - 2.9 kJmol for crack growth in hydrogen, and 33.5 or - 5.0 kJmol 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 specie for environment enhanced crack growth in hydrogen and in waterwater vapor. A slow step in the surface reaction of water vapor with steel was identified, and exhibited an activation energy of 36 or - 14 kJmol. 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 watermetal surface reaction was unambiguously identified as the rate controlling process for crack growth in waterwater 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. Author

Subject Categories:

  • Properties of Metals and Alloys

Distribution Statement:

APPROVED FOR PUBLIC RELEASE