Publications

Abstract not provided.

Abstract not provided.

Austenitic stainless steel microstructures produced by directed energy deposition (DED)are analogous to those developed during welding, particularly high energy density welding. To better understand microstructural development during DED, theories of microstructural evolution,which have been established to contextualize weld microstructures, are applied in this study to microstructural development in DED austenitic stainless steels. Phenomenological welding models that describe the development of oxide inclusions, compositional microsegregation, ferrite,matrix austenite grains, and dislocation substructures are utilized to clarify microstructural evolution during deposition of austenitic stainless steels. Two different alloys, 304L and 316L, arecompared to demonstrate the broad applicability of this framework for understanding microstmctural development during the DED process. Despite differences in grain morphology and solidification mode for these two alloys (which can be attributed to compositional differences),similar tensile properties are achieved. It is the fine-scale compositional segregation and dislocation structures that ultimately determine the strength of these materials. The evolution of microsegregation and dislocation structures is shown to be dependent on the rapid solidification and thermomechanical history of the DED processing method and not the composition of the starting material.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

The selection of austenitic stainless steels for hydrogen service is challenging since there are few intrinsic metrics that relate alloy composition to hydrogen degradation. One such metric, explored here, is intrinsic stacking fault energy. Stacking fault energy has an influence on the character and structure of dislocations and on the formation of secondary crystalline phases created during mechanical deformation in austenitic alloys. In this work, a data-driven model for the intrinsic stacking fault energy of common austenitic stainless steel alloys is applied to compare the relative degradation of tensile performance in the presence of hydrogen. A transition in the tensile reduction of area of both 300-series and manganese stabilized stainless steels is observed at a calculated stacking fault energy of approximately 43 mJ m^-2, below which pronounced hydrogen degradation on tensile ductility is observed. The model is also applied to suggest alloying strategies for low nickel austenitic stainless steels for hydrogen service. Lastly, through this investigation, we find that calculated intrinsic stacking fault energy is a high-throughput screening metric that enables the ranking of the performance of a diverse range of austenitic stainless steel compositions, as well as the identification of new alloys, with regard to hydrogen compatibility.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Following the ASME codes, the design of pipelines and pressure vessels for transportation or storage of high-pressure hydrogen gas requires measurements of fatigue crack growth rates at design pressure. However, performing tests in high pressure hydrogen gas can be very costly as only a few laboratories have the unique capabilities. Recently, Code Case 2938 was accepted in ASME Boiler and Pressure Vessel Code (BPVC) VIII-3 allowing for design curves to be used in lieu of performing fatigue crack growth rate (da/dN vs. ?K) and fracture threshold (KIH) testing in hydrogen gas. The design curves were based on data generated at 100 MPa H2 on SA-372 and SA-723 grade steels; however, the data used to generate the design curves are limited to measurements of ?K values greater than 6 MPa m^1/2. The design curves can be extrapolated to lower ?K (<6 MPa m^1/2), but the threshold stress intensity factor (?Kth) has not been measured in hydrogen gas. In this work, decreasing ?K tests were performed at select hydrogen pressures to explore threshold (?Kth) for ferritic-based structural steels (e.g. pipelines and pressure vessels). The results were compared to decreasing ?K tests in air, showing that the fatigue crack growth rates in hydrogen gas appear to yield similar or even slightly lower da/dN values compared to the curves in air at low ?K values when tests were performed at stress ratios of 0.5 and 0.7. Correction for crack closure was implemented, which resulted in better agreement with the design curves and provide an upper bound throughout the entire ?K range, even as the crack growth rates approach ?Kth. This work gives further evidence of the utility of the design curves described in Code Case 2938 of the ASME BPVC VIII-3 for construction of high pressure hydrogen vessels.

Following the ASME codes, the design of pipelines and pressure vessels for transportation or storage of high-pressure hydrogen gas requires measurements of fatigue crack growth rates at design pressure. However, performing tests in high pressure hydrogen gas can be very costly as only a few laboratories have the unique capabilities. Recently, Code Case 2938 was accepted in ASME Boiler and Pressure Vessel Code (BPVC) VIII-3 allowing for design curves to be used in lieu of performing fatigue crack growth rate (da/dN vs. ?K) and fracture threshold (KIH) testing in hydrogen gas. The design curves were based on data generated at 100 MPa H2 on SA-372 and SA-723 grade steels; however, the data used to generate the design curves are limited to measurements of ?K values greater than 6 MPa m1/2. The design curves can be extrapolated to lower ?K (<6 MPa m1/2), but the threshold stress intensity factor (?Kth) has not been measured in hydrogen gas. In this work, decreasing ?K tests were performed at select hydrogen pressures to explore threshold (?Kth) for ferritic-based structural steels (e.g. pipelines and pressure vessels). The results were compared to decreasing ?K tests in air, showing that the fatigue crack growth rates in hydrogen gas appear to yield similar or even slightly lower da/dN values compared to the curves in air at low ?K values when tests were performed at stress ratios of 0.5 and 0.7. Correction for crack closure was implemented, which resulted in better agreement with the design curves and provide an upper bound throughout the entire ?K range, even as the crack growth rates approach ?Kth. This work gives further evidence of the utility of the design curves described in Code Case 2938 of the ASME BPVC VIII-3 for construction of high pressure hydrogen vessels.

Additive manufacturing (AM) includes a diverse suite of innovative manufacturing processes for producing near-net shape components, typically from powder or wire feedstock. Reported mechanical properties of AM materials vary significantly depending on the details of the manufacturing process and the characteristics of the processing defects (namely, lack of fusion defects). However, an excellent combination of strength, ductility and fracture resistance can be achieved in AM type 304L and 316L austenitic stainless steels by minimizing processing defects. It is also important to recognize that localized solidification processing during AIVI produces microstructures more analogous to weld microstructures than wrought microstructures. Consequently, the mechanical behavior of AM austenitic stainless steels in harsh environments can diverge from the performance of wrought materials. This report gives an overview of the fracture and fatigue response of type 304L materials from both directed energy deposition (DED) and powder bed fusion (PBF) techniques. In particular, the mechanical performance of these materials is considered for high-pressure hydrogen applications by evaluating fatigue and fracture resistance after thermally precharging of test specimens in high-pressure gaseous hydrogen. The mechanical behaviors are considered with respect to previous reports on hydrogen-assisted fracture of austenitic stainless steel welds and the unique characteristics of the AM microstructures. Fatigue crack growth can be relatively insensitive to processing defects, displaying similar behavior as wrought materials. Fracture resistance of dense AM austenitic stainless steel, on the other hand, is more consistent with weld metal than with compositionally-similar wrought materials. Hydrogen effects in the AM materials are generally more severe than in wrought materials, but comparable to measurements on welded austenitic stainless steels in hydrogen environments. While hydrogenassisted fracture manifests differently in welded and AM austenitic stainless steel, the fracture process appears to have a common origin in the compositional microsegregation intrinsic to solidification processes.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Tensile properties, fatigue crack initiation, fatigue crack growth rate, and fatigue life are evaluated in 304L austenitic stainless steel fabricated by directed energy deposition (DED). Large lack of fusion (LoF) defects (often >1 mm in length) significantly reduce ultimate tensile strength and ductility, as well as accelerate fatigue crack initiation and reduce fatigue life. In comparison, small spherical defects (<100 μm in diameter) have less effect on tensile and fatigue properties. Fatigue crack growth rate is less severely affected by defects than other properties, showing only local acceleration in the proximity of LoF defects. Therefore, shorter fatigue life is attributed to the role of LoF defects on facilitating fatigue crack initiation and to a lesser extent fatigue crack propagation. Additionally, the fatigue life can be normalized for defects by considering their effect on ultimate tensile strength, suggesting that in the limit of low defect population, the fatigue strength of additively manufactured stainless steel is similar to conventional wrought materials.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.

Abstract not provided.