Publications

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The capabilities in the Hydrogen Effects on Materials Laboratory (HEML) at Sandia National Laboratories and the related materials testing activities that support standards development and technology deployment are reviewed. The specialized systems in the HEML allow testing of structural materials under in-service conditions, such as hydrogen gas pressures up to 138 MPa, temperatures from ambient to 203 K, and cyclic mechanical loading. Examples of materials testing under hydrogen gas exposure featured in the HEML include stainless steels for fuel cell vehicle balance of plant components and Cr[sbnd]Mo steels for stationary seamless pressure vessels.

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Direct energy deposition (DED) is an additive manufacturing process that can produce complex near-net shape metallic components in a single manufacturing step. DED additive manufacturing has the potential to reduce feedstock material waste, streamline manufacturing chains, and enhance design flexibility. A major impediment to broader acceptance of DED technology is limited understanding of defect populations in the novel microstructures produced by DED and their relationship to process parameters and resultant mechanical properties. A design choice as simple as changing the build orientation has been observed to result in differences as great as ∼25% in yield strength for type 304L austenitic stainless steel deposited with otherwise identical deposition parameters. To better understand the role of build orientation and resultant defect populations on fatigue behavior in DED 304L, tension-tension fatigue testing has been performed on circumferentially notched cylindrical test specimens extracted from both vertical and horizontal orientations relative to the build direction. Notched fatigue behavior was found to be strongly influenced by the manufacturing defect populations of the material for different build orientations.

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Austenitic stainless steels are typically used in hydrogen environments due to their resistance to hydrogen embrittlement; however, the behavior of welds is not as well understood and can vary from wrought base materials due to chemical composition differences and the presence of ferrite in the fusion zone of the weld. Applications of welded austenitic stainless steels exposed to hydrogen are not limited to room temperature but also include sub-ambient environments, which can have an additional effect on the degradation. In this study, fracture thresholds were measured of three different austenitic stainless steel welds in the hydrogen-precharged condition. Forged 304L, 316L, and 21Cr-6Ni-9Mn stainless steels were gas tungsten arc welded with 308L filler metal and machined into 3-pt bend bars for fracture testing. Crack growth resistance (J-R) curves were measured of the three welds in the hydrogen-precharged condition at ambient (293 K) and sub-ambient (223 K) temperatures to determine the effects of temperature on fracture threshold. Fracture thresholds were determined using elastic-plastic fracture mechanics through development of J-R curves to determine the stress intensity factor following standard practice for determination of fracture toughness. Fracture threshold tests for the welds revealed significant susceptibility to subcritical cracking when tested in the hydrogen-precharged condition. The 21-6-9/308L and 304L/308L welds exhibited some variability in fracture thresholds that did not appear to trend with temperature, while the 316L/308L weld exhibited a reduction of over 50% in fracture threshold at the lower temperature compared to room temperature. In addition to fracture testing, mini-tensile specimens were extracted from the weld region and tested at 293 K and 223 K in the hydrogen-precharged condition. Hydrogen-precharging slightly increased the yield strength relative to the as-welded condition for all three welds at both temperatures. For all three welds, hydrogen reduced the total elongation by 3-11% at 293 K, whereas reductions in total elongation from 50-64% were observed at 223 K (relative to room temperature without hydrogen). The role of slip planarity on hydrogen-induced degradation of ductility and fracture resistance is discussed as a function of temperature, nickel content, and hydrogen. The fracture surfaces were examined to elucidate the observed differences and similarities in mechanical properties.

Circumferentially notched specimens of several austenitic stainless steel alloys subjected to positive load ratio, load-controlled fatigue have been cycled to failure in high pressure hydrogen gas. The number of cycles to failure for a given applied stress amplitude varies among the alloys tested indicating that control of fatigue life in hydrogen environments may be attained through informed alloy selection. The number of cycles to initiate a crack does not vary significantly among the alloys tested, however the total life to failure varied by over an order of magnitude. This difference in life is attributed to variations of the stress and strain fields ahead of the blunt notch. These fields are influenced by the strength and strain hardening characteristics of each alloy and they dictate the driving force for fatigue crack growth while the crack is small.

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