Crack growth from internal hydrogen—temperature and microstructural effects in 4340 steel |
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Authors: | W W Gerberich T Lvne X F Chen M Kaczorowski |
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Affiliation: | (1) Department of Chemical Engineering and Materials Science, University of Minnesota, 55455 Minneapolis, MN;(2) RAFEL, Ministry of Defense, Haifa, Israel;(3) Warsaw Technical University, Narbutta 85, 02-524, Warsaw, Poland |
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Abstract: | Internal hydrogen effects on stage II crack growth rates in AISI 4340 steel have been studied as a function of test temperature.
A model is developed that is physically based in that classical thermodynamics relates to solubility and trapping and Fick’s
second law controls hydrogen transport. Both of these are microstructurally related to how trapping affects both the crack
initiation site and diffusion to it. For two tempered conditions of 4340 steel, it is shown that there is a test temperature,T
0, for stage II crack growth, above which the crack does not grow. The fractography associated with test temperatures approachingT
0 tends toward 100 pct intergranular for both 1340 MPa and 1620 MPa strength levels. At lower test temperatures, there is as
much as 50 pct microvoid coalescence or 30 pct quasi-cleavage. In the lower strength condition, hydrogen traps at oxysulfide
particles with a binding energy near 75 kJ/mol. Where these intersect the prior austenite grain boundaries, this promotes
fingers of intergranular fracture which later triggers tearing of 100 μm size ligaments by microvoid coalescence. For the
higher strength material, it is proposed that hydrogen traps along martensite lath intersections with prior austenite grain
boundaries, the binding energy being near 27 kJ/mol. This promotes 1 μm size striations along intergranular facets. In both
cases the fractography is consistent with a proposed model of stress field concentration of hydrogen, further concentration
along trap sites, fracture nucleation at trap sites, and local, discontinuous fracture instabilities. |
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