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HB7, an ultra-clean, high strength pressure vessel steel manufactured in France, is compared to A723 steel. This steel, suggested as an improved pressure vessel material is currently being proposed for critical applications, and will likely be used more frequently as design engineers discover its capabilities. This paper includes comparisons of strength, fracture toughness, fatigue properties and composition of the two steels, followed by an in-depth comparison and modeling of environmental cracking resistance, Bauschinger-modified residual stresses and fatigue lives. Results indicate that in all critical areas, with the exception of Bauschinger-reduced residual stress, the HB7 is superior to the A723 steel. Particularly for small amounts of autofrettage, near-bore residual stresses are reduced for HB7 steel compared to those for A723 steel at the same strength level. The greatest improvement of the HB7 over the A723 is in environmental cracking resistance. The HB7, when tested in concentrated sulfuric acid, exhibits five orders of magnitude longer crack incubation times and three orders of magnitude slower crack growth rates, when compared to A723 steel at the same strength level.
HB7, an ultra-clean, high strength pressure vessel steel manufactured in France, is compared to A723 steel. This steel, suggested as an improved pressure vessel material is currently being proposed for critical applications, and will likely be used more frequently as design engineers discover its capabilities. This paper includes comparisons of strength, fracture toughness, fatigue properties and composition of the two steels, followed by an in-depth comparison and modeling of environmental cracking resistance, Bauschinger-modified residual stresses and fatigue lives. Results indicate that in all critical areas, with the exception of Bauschinger-reduced residual stress, the HB7 is superior to the A723 steel. Particularly for small amounts of autofrettage, near-bore residual stresses are reduced for HB7 steel compared to those for A723 steel at the same strength level. The greatest improvement of the HB7 over the A723 is in environmental cracking resistance. The HB7, when tested in concentrated sulfuric acid, exhibits five orders of magnitude longer crack incubation times and three orders of magnitude slower crack growth rates, when compared to A723 steel at the same strength level.
Hydrogen-induced cracking tests were conducted on high-strength steels and nickel-iron base alloys using the constant displacement bolt-loaded compact specimen. The bolt-taaded specimen was subjected to both acid and electrochemical cell environments to produce hydrogen. The materials tested were A723, Maraging 200, PH 13-8 Mo, Alloy 718, Alloy 706, and A286, ranging in yield strength from 760-1400 MPa. The effects of chemical composition, refinement, heat treatment, and strength on hydrogen-induced crack growth rates and thresholds were examined. In general, all high strength steels tested exhibited similar crack growth rates and threshold levels. In comparison, the nickel-iron base alloys tested exhibited crack growth rates up to three orders of magnitude lower than the high-strength steels tested. It is widely known that high-strength steels and nickel base alloys exhibit different crack growth rates, in part, because of their different crystal cell structure. In the high-strength steels tested, refinement and heat treatment had some effect on hydrogen-induced cracking, although strength was the predominant factor influencing susceptibility to cracking. When the yield strength of one of the high-strength steels tested was increased moderately, from 1130 MPa to 1275 MPa, the incubation times decreased by over two orders of magnitude, the crack growth rates increased by an order of magnitude, and the threshold sttess intensity was slightly lower.
APPROVED FOR PUBLIC RELEASE; DISTRIBUTION UNLIMITED REPORT DOCUMENTATION PAGE Form Approved OMB No. 0704-018SPublic reDorting burden for this collection of information is estimated to at/eraae 1 hour per response, including the time for reviewing instructions, searching existing data sources, qathenna and mamtainina the data needed, and completing and reviewma the collection of information. Send comments regarding this burden estimate or any other aspect of this collection" of information "including suaoestions for reducing this burden, High strength alloy steels typically used for gun steel were investigated to determine their susceptibility to hydrogen embrittlement. Although AISI grade 4340 was quite susceptible to hydrogen embrittlement, ASTM A723 steel, which has identical mechanical properties but slightly different chemistries, was not susceptible to hydrogen embrittlement when exposed to the same conditions. The degree of embrittlement was determined by conducting notched tensile testing on uncharged and cathodically charged specimens. Chemical composition was modified to isolate the effect of alloying elements on hydrogen embrittlement susceptibility. Two steels-Modified A723 (C increased from 0.32% to 0.40%) and Modified 4340 (V increased from 0 to 0.12%)-were tested. X-ray diffraction identified the presence of vanadium carbide, V 4 C 3 , in A723 steels, and subsequent hydrogen extraction studies evaluated the trapping effect of vanadium carbide. Based on these tests, it was determined that adding vanadium carbide to 4340 significantly decreased hydrogen embrittlement susceptibility because vanadium carbide traps ties up diffusible hydrogen. The effectiveness of these traps is examined and discussed in this paper.
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