Dorian K. Balch

Diffraction strain measurements in a partially crystallized bulk metallic glass composite containing ductile particles

In situ diffraction experiments were performed with high-energy synchrotron X-rays to examine load partitioning and high-stress relaxation during uniaxial compression of a bulk metallic glass composite containing both ductile tantalum particles and crystallized matrix material. The tantalum particles yielded at an applied stress of )800 MPa, while the matrix precipitates remained elastic up to the maximum applied stress of )1250 MPa. The von Mises effective stress in the tantalum particles at yielding was 1500 MPa, well in excess of typical tantalum yield stresses, which is attributed to a combination of solid-solution strengthening and the inhibition of dislocation motion in the 1–2 lm particles. A series of constant crosshead-position measurements made at )1250 MPa suggested the possibility of roomtemperature matrix relaxation under high applied loads. 2003 Elsevier Science B.V. All rights reserved.

Effect of High-Pressure Hydrogen Gas on Fracture of Austenitic Steels

Applications requiring the containment and transport of hydrogen gas at pressures greater than 70 MPa are anticipated in the evolving hydrogen economy infrastructure. Since hydrogen is known to alter the mechanical properties of materials, data are needed to guide the selection of materials for structural components. The objective of this study is to characterize hydrogen-assisted fracture in two austenitic steels, 21Cr-6Ni-9Mn (21-6-9) and 22Cr-13Ni-5Mn (22-13-5), as well as explore the role of yield strength and small concentrations of ferrite on hydrogen-assisted fracture. The testing methodology involves exposure of uniaxial tensile specimens to high-pressure hydrogen gas in order to precharge the specimens with hydrogen, then subsequently testing the specimens to measure strength and ductility. In all cases, the alloys remained ductile despite precharging to hydrogen concentrations >1 at%, this is substantiated by reduction in area of >50% and fracture surfaces dominated by microvoid coalescence. Low concentrations of ferrite and moderate changes in yield strength did not affect the hydrogen-assisted fracture of 21-6-9 and 22-13-5 respectively.Copyright

Measurement of Sustained-Load Cracking Thresholds for Steels in Hydrogen Delivery and Storage

Threshold stress intensity factors at crack arrest have been measured for three commercial low alloy pressure vessel steels, SA372 grade J, DOT 3AAX and DOT 3T as well as X100 line pipe steel using sustained load testing. Testing conditions were consistent with the recently published Article KD-10 from Section VIII, Division 3 of the ASME Boiler and Pressure Vessel Code. Measured threshold values for these steels suggest a higher resistance to hydrogen-assisted fracture than previously expected, however some improvements to the methodology of Article KD-10 may be required to ensure conservative results are measured. Specifically, conservative measurements of threshold stress intensity factor for hydrogen-assisted fracture cannot be determined without crack propagation.Copyright

Nondestructive detection and assessment of high temperature hydrogen attack damage in carbon steel pressure vessels

A nondestructive testing approach capable of evaluating high temperature hydrogen attack (HTHA) damage in carbon steel pressure vessels is presented. The approach, involving non-collinear wave mixing of ultrasonic waves, is applied to a test sample extracted from a retired pressure vessel. Nonlinear ultrasonic results are consistent with tensile test results obtained using specimens extracted throughout the thickness of the pressure vessel, and with damage observed using scanning electron microscopy micrographs. Results show that the nonlinear ultrasonic approach has the potential of being capable to detect and assess HTHA damage through the thickness of pressure vessels. The method only requires access to the vessels’ outside surface, which makes it very attractive for field inspections.

SNL/SRNL Joint Project on degradation of mechanical properties in structural metals and welds for GTS reservoirs.

This project was intended to enable SNL-CA to produce appropriate specimens of relevant stainless steels for testing and perform baseline testing of weld heat-affected zone and weld fusion zone. One of the key deliverables in this project was to establish a procedure for fracture testing stainless steel weld fusion zone and heat affected zones that were pre-charged with hydrogen. Following the establishment of the procedure, a round robin was planned between SNL-CA and SRNL to ensure testing consistency between laboratories. SNL-CA and SRNL would then develop a comprehensive test plan, which would include tritium exposures of several years at SRNL on samples delivered by SNL-CA. Testing would follow the procedures developed at SNL-CA. SRNL will also purchase tritium charging vessels to perform the tritium exposures. Although comprehensive understanding of isotope-induced fracture in GTS reservoir materials is a several year effort, the FY15 work would enabled us to jump-start the tests and initiate long-term tritium exposures to aid comprehensive future investigations. Development of a procedure and laboratory testing consistency between SNL-CA and SNRL ensures reliability in results as future evaluations are performed on aluminum alloys and potentially additively-manufactured components.

Effect of Hydrogen on Tensile Strength and Ductility of Multi-Pass 304L / 308L Austenitic Stainless Steel Welds

Austenitic stainless steels such as 304L are frequently used for hydrogen service applications due to their excellent resistance to hydrogen embrittlement. However, welds in austenitic stainless steels often contain microstructures that are more susceptible to the presence of hydrogen. This study examines the tensile strength and ductility of a multi-pass gas tungsten arc weld made on 304L cross-rolled plate using 308L weld filler wire. Sub-sized tensile specimens were used to ensure the entire gage section of each tensile specimen consisted of weld metal. Specimens were extracted in both axial and transverse orientations, and at three different depths within the weld (root, center, and top). Yield strength decreased and ductility increased moving from the root to the top of the weld. A subset of specimens was precharged with hydrogen at 138 MPa (20,000 psi) and 300°C prior to testing, resulting in a uniform hydrogen concentration of 7700 appm. The presence of hydrogen resulted in a slight increase in yield and tensile strength and a roughly 50% decrease in tensile elongation and reduction in area, compared to the hydrogen-free properties.Copyright

Fracture Threshold Measurements of Hydrogen Precharged Stainless Steel Weld Fusion Zones and Heat Affected Zones

Austenitic stainless steels are used in hydrogen environments because of their generally accepted resistance to hydrogen embrittlement; however, hydrogen-assisted cracking can occur depending on the microstructures or composition of the stainless steel. One area that has not been well researched is welds and in particular heat affected zones. The goal of this work was to measure the subcritical cracking susceptibility of hydrogen precharged gas tungsten arc (GTA) welds in forged stainless steels (21Cr-6Ni-9Mn and 304L). Welds were fabricated using 308L filler metal to form 21-6-9/308L and 304L/308L weld rings, and subsequently three-point bend specimens were extracted from the fusion zone and heat affected zone and precharged in high-pressure hydrogen gas. Crack growth resistance curves were measured in air for the hydrogen precharged fusion zones and heat affected zones under rising-displacement loading, revealing significant susceptibility to subcritical cracking. Fracture thresholds of 304L/308L welds were lower than 21-6-9/308L welds which was attributed to higher ferrite fractions in 304L/308L since this phase governed the crack path. Fracture thresholds for the heat affected zone were greater than the fusion zone in 21-6-9/308L which is likely due to negligible ferrite in the heat affected zone. Modifications to the weld joint geometry through use of a single-J design were implemented to allow consistent testing of the heat affected zones by propagating the crack parallel to the fusion zone boundary. Despite low hydrogen diffusivity in the austenitic stainless steels, effects of displacement rates were observed and a critical rate was defined to yield lower-bound fracture thresholds.Copyright

High-energy rate forgings of wedges. Characterization of processing conditions

The wedge geometry is a simple geometry for establishing a relatively constant gradient of strain in a forged part. The geometry is used to establish gradients in microstructure and strength as a function of strain, forging temperature, and quenching time after forging. This geometry has previously been used to benchmark predictions of strength and recrystallization using Sandias materials model for type 304L austenitic stainless steel. In this report, the processing conditions, in particular the times to forge and quench the forged parts, are summarized based on information recorded during forging on June 18, 2013 of the so-called wedge geometry from type 316L and 21Cr-6Ni-9Mn austenitic stainless steels.

Fabrication and Testing of Electron Beam Welded Alloy AA2219 Aluminum Pressure Vessels for High-Pressure Hydrogen Service

Aluminum alloys offer significant advantages for hydrogen service such as low weight, improved uniformity of properties relative to forged austenitic stainless steels, and immunity to embrittlement in the presence of dry hydrogen. For these reasons aluminum alloys are now being considered for high-pressure hydrogen isotope pressure vessel applications where forged stainless steels have been the standard materials of construction for decades. In particular, alloy AA2219 is being evaluated due to its excellent weldability, microstructural stability, and good mechanical and fracture toughness properties. Prototype AA2219 pressure vessels have been fabricated and tested, including electron beam weld development, weld hardness and tensile testing prior to and after post-weld heat treatment, and burst testing. The design, manufacture, and testing of AA2219 pressure vessels will be discussed, including an ongoing long-term shelf storage program where pressure vessels are loaded with gaseous hydrogen at pressure of 103 MPa (85% of the burst pressure for these vessels).Copyright

HYDROGEN-ASSISTED FRACTURE IN FORGED TYPE 304L AUSTENITIC STAINLESS STEEL

Austenitic stainless steels generally have good resistance to hydrogen-assisted fracture; however, structural designs for high-pressure gaseous hydrogen are constrained by the low strength of this class of material. Forging is used to increase the low strength of austenitic stainless steels, thus improving the efficiency of structural designs. Hydrogen-assisted racture, however, depends on microstructural details associated with manufacturing. In this study, hydrogen-assisted fracture of forged type 304L austenitic stainless steel is investigated. Microstructural variation in multi-step forged 304L was achieved by forging at different rates and temperatures, and by process annealing. High internal hydrogen content in forged type 304L austenitic stainless steel is achieved by thermal precharging in gaseous hydrogen and results in as much as 50% reduction of tensile ductility.