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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 127 records · Page 7
All sourcesNASA NTRSDOE OSTI
DOE OSTI2021

Marine Atmospheric Corrosion of Additively Manufactured Stainless Steels

Additively manufactured (AM) stainless steels exhibit numerous microstructural differences compared to their wrought counterparts, such as Cr enriched dislocation cell structures. The influence these unique features have on a SSs corrosion resistance are still under investigation with most current works limited to laboratory experiments. The work herein shows the first documented study of AM 304L and 316L exposed to a severe marine environment on the eastern coast of Florida with comparisons made to wrought counterparts. Coupons were exposed for 21 months and resulted in significant pitting corrosion to initiate after 1 month of exposure for all conditions. At all times, the AM coupons exhibited lower average and maximum pit depths than their wrought counterparts. After 21 months, pits on average were 4 μm deep for AM 316L specimen and 8 μm deep for wrought specimen. Pits on the wrought samples tended to be nearly hemispherical and polished with some pits showing crystallographic attack while pits on AM coupons exhibited preferential attack at melt pool boundaries and the cellular microstructure.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Interrogating the Effects of Hydrogen on the Behavior of Planar Deformation Bands in Austenitic Stainless Steel

The effects of internal hydrogen on the deformation microstructures of 304L austenitic stainless steel have been characterized using electron backscattered diffraction (EBSD), transmission Kikuchi diffraction (TKD), high-resolution scanning transmission electron microscopy (HRSTEM), and nanoprobe diffraction. Samples, both thermally precharged with hydrogen and without thermal precharging, were subjected to tensile deformation of 5 and 20 pct true strain followed by multiple microscopic interrogations. Internal hydrogen produced widespread stacking faults within the as-forged initially unstrained material. While planar deformation bands developed with tensile strain in both the hydrogen-precharged and non-precharged material, the character of these bands changed with the presence of internal hydrogen. As shown by nanobeam diffraction and HRSTEM observations, in the absence of internal hydrogen, the bands were predominantly composed of twins, whereas for samples deformed in the presence of internal hydrogen, ε -martensite became more pronounced and the density of deformation bands increased. For the 20 pct strain condition, α '-martensite was observed at the intersection of ε -martensite bands in hydrogen-precharged samples, whereas in non-precharged samples α '-martensite was only observed along grain boundaries. We hypothesize that the increased prevalence of α '-martensite is a secondary effect of increased ε -martensite and deformation band density due to internal hydrogen and is not a signature of internal hydrogen itself.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Oxidation kinetics and microstructure evolution of high Mn stainless-steel alloy in CO 2 at 700 °C

Oxidation kinetics and scale formation were examined in 21–2N Stainless Steel alloys in a CO 2 environment at 700 °C. Several characterization techniques were used to identify the morphology, crystallographic structure, and chemical composition of the oxide scale formation during exposures up to 1925 h. High manganese content played an important role in homogenous oxide scale formation of Mn 3 O 4 and MnCr 2 O 4 at the scale/gas and scale/alloy interfaces and as an FCC stabilizer in the alloy. Depletion of Mn resulted in an FCC to BCC phase transformation at the scale/alloy interface. No carburization was observed, indicating that the oxide layer is preventing carbon uptake.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Effects of niobium and tantalum on the microstructure and strength of ferritic-martensitic steels

Herein, detailed investigations of two ferritic-martensitic steels, i.e., Nb- and Ta-steel, indicate that the higher Nb diffusivity and the lower nucleation driving force of Nb-carbide promote preferential nucleation of Nb-carbides at grain boundaries and thus a higher boundary occupancy than Ta-carbides, which favored greater creep resistance of Nb-steel than that of Ta-steel despite their similar yield strength.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Microstructural Engineering of Mn-Alloyed Austenitic Steel for Hydrogen Storage and Delivery

Austenitic stainless steels are commonly used for hydrogen storage and transportation. These alloys have a high nickel (Ni) content, which increases alloy cost. In this study, high manganese (Mn) austenitic alloys were evaluated as potential lower cost alternatives. Two heats of high Mn alloys with different stacking fault energies (SFE) of ~29 mJ·m -2 and 49 mJ·m -2 were acquired. Additionally, a new vanadium (V)-microalloyed high Mn alloy was designed to achieve a SFE of 47 mJ·m -2 to minimize planar slip deformation mechanisms. Post-processing via cold working in conjunction with aging was also performed on the V-microalloyed high Mn steel. Hydrogen embrittlement sensitivity was investigated using circumferential notch tensile specimens cathodically charged with hydrogen in a 0.05M NaOH electrolytic solution. The alloys were compared to a cold-worked 316L stainless steel, which exhibited no strength loss due to hydrogen. The high Mn alloys with SFE of ~29 mJ·m 2 and 49 mJ·m -2 had notch strength losses of 11 and 6 pct, respectively. The V-microalloyed high Mn steel in the as-hot-rolled condition had a notch strength loss of 17 pct. Furthermore, the V-microalloyed high Mn steel in the cold worked and aged condition indicated no notch strength loss in hydrogen, which was comparable to the performance of the 316L stainless steel.

36 MATERIALS SCIENCE↗
DOE OSTI2026

Effect of deposition rate on microstructure and mechanical properties of 17-4 PH stainless steel fabricated by laser engineered net shaping

Laser engineered net shaping (LENS) is an additive manufacturing technique for fabricating and repairing metallic components. However, the relationships between its process parameters and the resulting mechanical properties of alloys such as 17-4 precipitation-hardening (PH) stainless steel require further investigation to enable reliable application. This study examines the specific effect of LENS deposition rate on the microstructure and mechanical properties of 17-4 PH stainless steel. Specimens were fabricated at two different deposition rates (8.47 and 9.31 mm s−1), subjected to subsequent solution and H900 aging treatment, and then evaluated via tensile testing, hardness measurements, and microscopy. A higher deposition rate results in a finer grain structure but increased porosity, leading to greater ultimate tensile strength (∼1294 MPa) yet lower ductility (strain at failure ∼5.8%) compared to the slower deposition rate (∼1266 MPa, ∼9.0% strain). Hardness follows the same trend as tensile strength. Tensile fracture surfaces for both conditions exhibited a mixed mode of ductile dimples and brittle quasi-cleavage regions, with chromium/silicon oxide particles identified within dimples. Complementary finite element modeling indicates that small void fractions primarily reduce ductility by enhancing localized plasticity, with only a marginal decrease in tensile strength. These integrated experimental and numerical results elucidate the mechanical properties linked to deposition rate, providing insight into tailoring LENS processes to achieve desired properties of 17-4 PH stainless steel.

17-4 PH stainlesssteel↗
DOE OSTI2020

Microstructure and fracture toughness characterization of three 9Cr ODS EUROFER steels with different thermo-mechanical treatments

Ferritic martensitic ODS steels are one of the candidate structural materials for future Gen-IV nuclear fission and fusion reactors. Here, the dependence of fracture toughness on microstructure was investigated by comparing three 9Cr ODS EUROFER steels manufactured through different thermo-mechanical processing routes. Quasi-static fracture toughness testing was performed with sub-sized C(T) specimens and microstructural characterization was carried out using scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy. It was found that at lower test temperatures (-100 – 22 °C), the fracture toughness was primarily controlled by crack initiation at sub-micron particles and by production of secondary cracks during fracture. At higher temperatures (above 100 °C), fracture toughness was predominantly controlled by the matrix ductility and the grain boundary strength with a relatively ductile coarse-grained alloy demonstrating higher fracture toughness compared to high-strength fine-grained alloys. These results and discussion show that variations in thermomechanical treatments can produce significant differences in microstructure and fracture toughness behavior of ferritic martensitic ODS steels.

36 MATERIALS SCIENCE↗
DOE OSTI2022

Process and feedstock driven microstructure for laser powder bed fusion of 316L stainless steel

Here in the pursuit of improving additively manufactured (AM) component quality and reliability, fine-tuning critical process parameters such as laser power and scan speed is a great first step toward limiting defect formation and optimizing the microstructure. However, the synergistic effects between these process parameters, layer thickness, and feedstock attributes (e.g. powder size distribution) on part characteristics such as microstructure, density, hardness, and surface roughness are not as well-studied. In this work, we investigate 316L stainless steel density cubes built via laser powder bed fusion (L-PBF), emphasizing the significant microstructural changes that occur due to altering the volumetric energy density (VED) via laser power, scan speed, and layer thickness changes, coupled with different starting powder size distributions. This study demonstrates that there is not one ideal process set and powder size distribution for each machine. Instead, there are several combinations or feedstock/process parameter ‘recipes’ to achieve similar goals. This study also establishes that for equivalent VEDs, changing powder size can significantly alter part density, GND density, and hardness. Through proper parameter and feedstock control, part attributes such as density, grain size, texture, dislocation density, hardness, and surface roughness can be customized, thereby creating multiple high-performance regions in the AM process space.

316L stainless steel↗
DOE OSTI2021

Through-Thickness Microstructure Characterization in a Centrifugally Cast Austenitic Stainless Steel Nuclear Reactor Primary Loop Pipe Using Time-of-Flight Neutron Diffraction

The US code of Federal Regulations mandates regular inspection of centrifugally cast austenitic stainless steel pipe, commonly used in primary cooling loops in light-water nuclear power plants. These pipes typically have a wall thickness of ~8 cm. Unfortunately, inspection using conventional ultrasonic techniques is not reliable as the microstructure strongly attenuates ultrasonic waves. Work is ongoing to simulate the behavior of acoustic waves in this microstructure and ultimately develop an acoustic inspection method for reactor inspections. In order to account for elastic anisotropy in the material, the texture in the steel was measured as a function of radial distance though the pipe wall. Experiments were conducted on two 10 × 12.7 × 80 mm radial sections of a cast pipe using neutron diffraction scans of 2 mm slices using the HIPPO time-of-flight neutron diffractometer at the Los Alamos Neutron Science Center (LANSCE, Los Alamos, NM, USA). Strong textures dominated by a small number of austenite grains with their (100) direction aligned in the radial direction of the pipe were observed. ODF analysis indicated that up to 70% of the probed volume was occupied by just three single-grain orientations, consistent with grain sizes of almost 1 cm. Texture and phase fraction of both ferrite and austenite phases were measured along the length of the samples. These results will inform the development of a more robust diagnostic tool for regular inspection of this material.

42 ENGINEERING↗
DOE OSTI2024

The Influence of Residual Stress on Fatigue Crack Growth Rates in Stainless Steel Processed by Different Additive Manufacturing Methods

The properties and microstructure of Type 304L stainless steel produced by two additive manufacturing (AM) methods—directed energy deposition (DED) and powder bed fusion (PBF)—are evaluated and compared. Localized heating and steep temperature gradients of AM processes lead to significant residual stress and distinctive microstructures, which may be process-specific and influence mechanical behavior. Test data show that materials produced by DED and PDF have small differences in tensile strengths but clear differences in residual stress and microstructural features. Measured fatigue crack growth rates (FCGRs) for cracks propagating parallel to and perpendicular to the build directions differ between the two AM materials. To separate the influences of residual stress and microstructure, K-control test procedures with decreasing and constant stress intensity factor ranges are used to measure FCGRs in the near-threshold regime (crack growth rates ≤ 1 × 10 −8 m/cycle). Residual stress is quantified by the residual stress intensity factor, K res , measured by the online crack compliance method. Correcting the FCGR data for differences in K res brings results for specimens of the two AM materials into agreement with each other and with results for wrought specimens, when the latter are corrected for crack closure. Differences in microstructure and tensile strength have an insignificant influence on FCGRs in these tests.

additive manufacturing↗
NASA NTRS1986

Analytical ultrasonics for structural materials

The application of ultrasonic velocity and attenuation measurements to characterize the microstructure of structural materials is discussed. Velocity measurements in cast stainless steel are correlated with microstructural variations ranging from equiaxed (elastically isotropic) to columnar (elastically anisotropic) grain structure. The effect of the anisotropic grain structure on the deviation of ultrasonic waves in cast stainless steel is also reported. Field-implementable techniques for distinguishing equiaxed from columnar grain structures in cast strainless steel structural members are presented. The application of ultrasonic velocity measurements to characterize structural ceramics in the green state is also discussed.

Kupperman, D. S.↗
DOE OSTI2025

Welding investigation of a wrought FeMnAl steel for armor application

A new wrought FeMnAl steel contains high Mn content and high Al content. This steel has high strength. When it is compared to conventional steels with the similar strength, its density is lower. FeMnAl steels can be used in armor vehicle manufacturing, in which welding is a common practice to join the steel plates together. The welding of FeMnAl steels has not been sufficiently studied before. Here, this work studied the weldability and weld microstructure of an FeMnAl steel and provided useful data for the welding practice of FeMnAl steels. The influences of weld filler metals and shielding gases were studied, using single-pass bead-on-plate gas metal arc welding (GMAW) and gas tungsten arc welding (GTAW) two processes. The optimal filler metals and shielding gases have been determined to achieve the better weldability. Autogenous GTAW and electron beam welding were also conducted. Cracks were found in the HAZ in the GMAW welds and electron beam welds. Fewer or no cracks were found in the HAZ of GTAW welds. SEM, SEM/EDX, TEM, TEM/EDX, and EBSD were used to analyze the grain boundaries in the HAZ and the cracks. The grain boundaries of FeMnAl base metal are mostly high angle grain boundaries (HAGBs), which attract impurity and solute atoms and cause intergranular cracking. The proposed cracking mechanisms are liquation cracking and ductility-dip cracking. It was found that GTAW provides much better weldability for the FeMnAl steel than GMAW and EBW.

FeMnAl steel↗
NASA NTRS2020

Corrosion Behavior of Stainless Steel 304 and Nickel 625 Under Iodine Vapor at 300 °C

The chemical, structural, and microstructural behavior of stainless steel 304 (SS 304) and nickel 625 (Ni 625) were probed after exposure to iodine vapor laminar flow at 300 °C. This work was conducted in support of the development of in-space propulsion utilizing iodine as a propellant. The kinetics of corrosion was measured in a custom-built iodine-compatible thermogravimetric analyzer rig at 300 °C in which samples were exposed to an iodine laminar vapor flow of 1 mg/min (carried by 20 mL/min argon gas) for up to 31 days. Samples were characterized after the experiment by X-ray diffraction, electron microscopy, energy dispersive X-ray spectroscopy, and dynamic secondary ion mass spectrometry. All samples formed scales consisting mainly of metal oxyiodides showing different chemistry, microstructure, and crystalline phases. The paralinear rate law best describes the kinetics of oxidation of the samples. SS 304 exhibited slightly slower kinetics of oxidation (106±3·10(exp –5) mg/sq.cm/day) than Ni 625 (122±3·10(exp –5) mg/sq.cm/day) in the low-oxygen and low-water environment.

Chemistry and Materials↗