The effect of hydrogen source on crack initiation in 4340 steel
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I.
INTRODUCTION
A
popular theory of hydrogen cracking, proposed by Troiano 1-4 and later developed by Gerberich,5-8 Steigerwald,3'9 and VanLeeuwen, 1~ assumes that hydrogen cracking is controlled by the long-range diffusion of hydrogen in the stress field associated with a notch. Consideration of a plastic enclave ahead of a crack or notch leads to the expectation that the hydrogen concentration will reach a maximum at or near the elastic-plastic boundary where lattice dilatation and hydrostatic pressure is a maximum. In strain hardening materials, this maximum might be reached inside the elasticplastic boundary. If the combination of stress level and hydrogen concentration is sufficient to cause cracking, crack initiation will be at the edge of the plastic enclave. The process is then repeated as the crack grows in a series of large, discontinuous jumps. In a study of internally charged 4340, Troiano and coworkers 2 demonstrated that hydrogen cracking did indeed originate a significant distance in front of a machined notch. They also determined that as the radius of the notch increased, the distance between the notch and the initial crack also increased. The effect of hydrogen source on the crack nucleation site in Troiano's model, however, is not yet understood. Furthermore, the need to define the nucleation site in order to formulate realistic models cannot be overemphasized. In this paper we investigate the effect of internal and external hydrogen sources and notch root radius on the crack nucleation site. These data are then compared to the elastic-plastic boundary location calculated using fracture mechanics techniques. Measurements of crack incubation times are also presented and interpreted.
II.
EXPERIMENTAL
The specimens used in this study were machined from a 50 m m square bar of 4340 steel having the following composition:
R. A. PAGE, Senior Research Metallurgist, is with Southwest Research Institute, San Antonio, TX 78284. W.W. GERBERICH, Professor and Associate Head, is with the Department of Chemical Engineering and Materials Sciences, University of Minnesota, Minneapolis, MN 55455. Manuscript submitted April 20, 1981. METALLURGICAL TRANSACTIONS A
Composition, Wt Pct C
Mn
0.42 0.83
P
S
0.008
0.002
Si
Cu
Ni
Cr
Mo
0.21 0.18 1.78 0.84 0.26
Both tensile and compact tension (CT) specimens were machined from the 4340 steel bar. As shown in Figure 1, the CT specimens were machined with three different notch root radii. After machining, these radii were measured to be 0.21 mm, 0.95 mm, and 2.6 nun. All CT specimens were machined in the transverse direction such that crack propagation took place perpendicular to the rolling direction. Following machining, the specimens were austenitized in argon at 843 ~ for two hours, followed by an oil quench to room temperature and then tempered at 500 ~ for one hour. The tempering treatment was also followed by an oil quench to room temperature. This heat treatment resulted in a tempered martensite microstructure with a yield strength of 1314 MPa, a ha
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