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At least 289 records · Page 16
All sourcesNASA NTRSDOE OSTI
DOE OSTI2023

Improving mechanical properties of carbon and tool steels via chromizing

Steels are commonly used in high-performance demanding applications due to their favourable mechanical properties. Various surface engineering techniques have been developed for steels, among which chromizing is an affordable high-throughput case-hardenig process for improved surface hardness and wear resistance while retaining the substrate ductility and toughness. In this work, tribological testing along with nano- and micro-indentation and morphological and composional characterisation were used to understand the effects of the chromizing process on the AISI 1095 carbon steel, 52100 bearing steel and A2, D2 and M2 tool steels. The results of this study demonstrate that the chromizing treatment of low-cost 1095 and 52100 steels significantly improves their wear and hardness properties to a level comparable to the more costly tool steels. While chromizing also increased the hardness of the tool steels, it had little improvement on the wear resistance for the D2 and M2 tool steels.

36 MATERIALS SCIENCE↗
DOE OSTI2019

Steel–Concrete Composite Pressure Vessels for Hydrogen Storage at High Pressures

The need to store large quantities of hydrogen in large diameter steel vessels under high pressures results in shell thicknesses that are too large to produce by most steel mills and not practical to fabricate. Accordingly, a research program was undertaken by Oak Ridge National Laboratory to develop a new concept of combining steel with concrete to construct such vessels economically and practically. The concept is to fabricate vessels where the steel shell thickness is approximately one half that required to resist the hoop forces due to internal pressure. As such, the steel shell is designed to carry the full amount of the longitudinal forces in the vessel but only one half of the hoop loads due to internal pressure. The other half of the hoop loads is carried by a prestressed and reinforced concrete shell. In large diameter vessels, the cost of the shell can further be reduced by using layered steel shell construction rather than solid-wall construction. Such shell construction has also the added advantage of easily venting the hydrogen that permeates through the steel shell directly to the atmosphere through vent holes. This mechanism prevents the hydrogen from damaging the steel shell. The theoretical formulation of the steel concrete shell design is presented in this paper. Finally, in addition, details of a full-scale mock up vessel designed, fabricated, and tested to prove the proposed methodology are given.

42 ENGINEERING↗
DOE OSTI2020

Investigating the Linear Thermal Expansion of Additively Manufactured Multi-Material Joining between Invar and Steel

This work investigated the linear thermal expansion properties of a multi-material specimen fabricated with Invar M93 and A36 steel. A sequence of tests was performed to investigate the viability of additively manufactured Invar M93 for lowering the coefficient of thermal expansion (CTE) in multi-material part tooling. Invar beads were additively manufactured on a steel base plate using a fiber laser system, and samples were taken from the steel, Invar, and the interface between the two materials. The CTE of the samples was measured between 40 °C and 150 °C using a thermomechanical analyzer, and the elemental composition was studied with energy dispersive X-ray spectroscopy. The CTE of samples taken from the steel and the interface remained comparable to that of A36 steel; however, deviations between the thermal expansion values were prevalent due to element diffusion in and around the heat-affected zone. The CTEs measured from the Invar bead were lower than those from the other sections with the largest and smallest thermal expansion values being 10.40 μm/m-K and 2.09 μm/m-K. In each of the sections, the largest CTE was measured from samples taken from the end of the weld beads. An additional test was performed to measure the aggregate expansion of multi-material tools. Invar beads were welded on an A36 steel plate. The invar was machined, and the sample was heated in an oven from 40 °C and 160 °C. Strain gauges were placed on the surface of the part and were used to analyze how the combined thermal expansions of the invar and steel would affect the thermal expansion on the surface of a tool. There were small deviations between the expansion values measured by gauges placed in different orientations, and the elongation of the sample was greatest along the dimension containing a larger percentage of steel. On average, the expansion of the machined Invar surface was 42% less than the expansion of the steel surface. The results of this work demonstrate that additively manufactured Invar can be utilized to decrease the CTE for multi-material part tooling.

36 MATERIALS SCIENCE↗
DOE OSTI2021

Hydrogen Embrittlement of Medium Mn Steels

Recent research efforts to develop advanced–/ultrahigh–strength medium-Mn steels have led to the development of a variety of alloying concepts, thermo-mechanical processing routes, and microstructural variants for these steel grades. However, certain grades of advanced–/ultrahigh–strength steels (A/UHSS) are known to be highly susceptible to hydrogen embrittlement, due to their high strength levels. Hydrogen embrittlement characteristics of medium–Mn steels are less understood compared to other classes of A/UHSS, such as high Mn twinning–induced plasticity steel, because of the relatively short history of the development of this steel class and the complex nature of multiphase, fine-grained microstructures that are present in medium–Mn steels. The motivation of this paper is to review the current understanding of the hydrogen embrittlement characteristics of medium or intermediate Mn (4 to 15 wt pct) multiphase steels and to address various alloying and processing strategies that are available to enhance the hydrogen-resistance of these steel grades.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Characterization and Remediation of the Hanford 618-10 Burial Ground Steel-pipe Vertical Pipe Units - 18238

The 618-10 Burial Ground is approximately 485 feet by 470 feet and consisted of 12 trenches and 94 VPUs. It is located approximately 5 miles north of Richland, Washington in the 600 Area of the Hanford Site. The 618-10 Burial Ground received low- to high-activity waste (e.g., fission products and some plutonium-contaminated waste). Most of the waste originated at the 300 Area laboratory facilities. Lower activity radioactive material and shielded drums were disposed in trenches, while laboratory wastes with higher levels of radiation were disposed in Vertical Pipe Units (VPUs). The 618-10 Burial Ground contained approximately 94 VPUs (uncertain mix of first and second generation). There are different variants of VPUs. The first generation of VPUs was simply corrugated culverts or standard pipes, 14-in diameter and 10- to 15-ft long, buried vertically. The second generation of VPUs was 22-in diameter, 15-ft long waste receptacles constructed by welding five 55-gallon bottomless drums together end-to-end and burying them vertically, separated by approximately 10 feet on center in rows approximately 25 feet apart. A subset of the first generation VPUs consisted of the steel pipe VPUs represented by carbon-steel pipes, 10 to 24 in. in diameter with various wall thicknesses. This paper will present the remediation of the steel-pipe VPUs, which were the last of the VPUs remediated in this project. The remediation process developed for steel pipe VPUs included exposing a section of the VPU within an excavated trench, placing a mix box around the VPU, filling the mix box with grout, shearing the VPU and contents, and then removing the grout waste mixture into a waste box for disposal at the Hanford Environmental Restoration Disposal Facility (ERDF). Prior the start of remedial activities extensive mock-ups were performed on surrogate steel-pipe VPUs to ensure the effectiveness of the process before introducing the hazards of radioactive VPU waste. The critical steps in the process of size reducing steel-pipe VPUs and mixing the contents with grout was successfully demonstrated. This process was completed for each steel-pipe VPU in successive lifts until the entire set of steel-pipe VPUs were mixed in grout and removed from the excavation. The remediation of the steel-pipe VPUs proved be successful in that all the VPUs were removed and transported to ERDF without a release to the local environment. (authors)

12 MANAGEMENT OF RADIOACTIVE AND NON-RADIOACTIVE W↗
DOE OSTI2016

In-Situ High Energy X-ray Characterization of Tensile Deformation of Advanced Austenitic Stainless Steels

A deep understanding of the deformation mechanisms for next-generation structural materials of advanced nuclear reactors is critical to their successful applications. For this purpose, high energy synchrotron X-ray characterization techniques in combination with in situ thermo-mechanical loading have distinctive advantages in correlating the mechanical responses of the specimens with their microstructure evolution in real time. In this study, this method is employed to compare the deformation characteristics of two advanced austenitic stainless steels, NF709 steel a High-Temperature Ultrafine-Precipitate Strengthened (HT-UPS) steel, with a traditional 316 austenitic stainless steel (SS). The NF709 and HT-UPS steels are both promising candidates for next generation nuclear energy systems. The NF709 is a 20Cr-25Ni austenitic stainless steel stabilized with Nb/Ti(C,N) precipitates and possesses excellent high temperature and corrosion resistance. The HT-UPS steel (Fe-14Cr-16Ni-Ti,Nb,V) also has much improved creep resistance and high temperature performance than 316SS through the careful control of minor alloying elements to form creep resistant precipitates. While both have shown improved mechanical properties over 316SS, their deformation characteristics and underlying mechanisms are yet to be fully explored. The recent applications of simultaneous Wide-Angle X-ray Scattering (WAXS) and Small-Angle X-ray Scattering (SAXS) techniques under thermo-mechanical loading of steel samples have revealed valuable insights of the microstructural evolution during deformation. This work is to apply these techniques to characterize the deformation behaviors and also understand their relationship with the microstructures of advanced austenitic stainless steels. (authors)

36 MATERIALS SCIENCE↗
DOE OSTI2011

Energy Saving Melting and Revert Reduction (E-SMARRT): Precision Casting of Steel

This project addresses improvements in metal casting processes by reducing scrap and reducing the cost of production, due to scrap reduction from investment casting and yield improvement offered by lost foam casting as compared to no-bake or green sand molding. The objectives for the investment casting portion of the subtask are to improve knowledge of fracture toughness of mold shells and the sources of strength limiting flaws and to understand the effects of wax reclamation procedures on wax properties. Applying 'clean steel' approaches to pouring technology and cleanliness in investment casting of steel are anticipated to improve incoming materials inspection procedures as they affect the microstructure and toughness of the shell. This project focused on two areas of study in the production of steel castings to reduce scrap and save energy: (1) Reducing the amount of shell cracking in investment cast steel production; (2) Investigate the potential of lost foam steel casting The basic findings regarding investment casting shell cracking were: (1) In the case of post pouring cracking, this could be related to phase changes in silica upon cooling and could be delayed by pouring arrangement strategies that maintained the shell surface at temperature for longer time. Employing this delay resulted in less adherent oxidation of castings since the casting was cooler at the time o fair exposure. (2) A model for heat transfer through water saturated shell materials under steam pressure was developed. (3) Initial modeling result of autoclave de-waxing indicated the higher pressure and temperature in the autoclave would impose a steeper temperature gradient on the wax pattern, causing some melt flow prior to bulk expansion and decreasing the stress on the green shell. Basic findings regarding lost foam casting of steel at atmospheric pressure: (1) EPS foam generally decomposes by the collapse mode in steel casting. (2) There is an accumulation of carbon pick-up at the end of the casting opposite the gate. (3) It is recommended that lost foam castings in steel be gated for a quiescent fill in an empty cavity mold to prevent foam occlusion defects from the collapse mode. The energy benefit is primarily in yield savings and lower casting weight per function due to elimination of draft and parting lines for the larger lost foam castings. For the smaller investment casting, scrap losses due to shell cracking will be reduced. Both of these effects will reduce the metal melted per good ton of castings. There will also be less machine stock required per casting which is a yield savings and a small additional energy savings in machining. Downstream savings will come from heavy truck and railroad applications. Application of these processes to heavy truck castings will lighten the heavy truck fleet by about ten pounds per truck. Using ten years to achieve full penetration of the truck fleet at linear rate this will result in a fuel savings of 131 trillion BTU over ten years.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗
DOE OSTI2020

Investigation of the irradiation effects in additively manufactured 316L steel resulting in decreased irradiation assisted stress corrosion cracking susceptibility

While additive manufacturing (AM) is an area of increasing interest to the nuclear industry, there are significant research and development needs prior to successful implementation of additively manufactured components into reactors. AM steel behaves differently than traditional wrought material, as has been observed in numerous mechanical testing studies. The nuclear industry must also understand the material response to irradiation. This work examines the irradiation damage and the irradiation assisted stress corrosion cracking behavior (IASCC) of both proton irradiated AM and wrought 316L steel. For IASCC testing, specimens were strained to 4% plastic strain in a simulated boiling water reactor environment (288°C, 0.2 µS/cm). Transmission and scanning electron microscopy, confocal microscopy, and X-ray computed tomography were used to examine coupons of the material as well as tensile bars used in the IASCC testing. This work found that the wrought 316L was more susceptible to IASCC than the AM steel, however, some IASCC was observed in the AM steel when tested with the tensile axis parallel to the build direction. Electron microscopy has shown significantly fewer radiation induced voids formed in the AM steel. While the pre-existing pore network in the AM steel may act as a sink to point defects, this is not expected to be the major factor in the reduced swelling observed in the AM steel. The lower density of radiation induced voids was expected to contribute to the IASCC resistance by decreasing the localization of deformation in the irradiated specimens, however, severity of localized deformation in AM steel, measured in dislocation channel spacing and height, was similar or more severe to that found in wrought. In conclusion, the presence of severe cracking in the wrought specimens did not allow for a direct comparison of localized deformation severities between the AM and wrought steel.

Additive manufacturing↗
DOE OSTI2023

Explosive fragmentation of additively manufactured stainless steel

Properties of fragmentation from an explosively driven 316L stainless steel spherical shell section fabricated by a laser powder bed additive manufacturing process with minimal surface finishing are investigated. This shell is driven by an insensitive high explosive, resulting in high strain rate deformation (>8 × 10 3 s –1 ) and failure of the stainless steel. Photonic Doppler velocimetry measures the expansion rate; dynamic radiography and high-speed imaging capture the fracture behavior of the stainless steel. The fracture response of the additively manufactured stainless steel shell is compared to published experimental results on additively manufactured 316L stainless steel and conventionally manufactured wrought 316L and 304 stainless steel shell fragmentation. Despite preferred crack orientation, suggesting the influence of surface grooves on fracture time, fragment size is identical to that measured in a similar experiment on wrought 304 stainless steel. Further analysis indicates that the 316L additively manufactured stainless steel shell exhibits comparable spall strength and fragmentation toughness to conventionally manufactured stainless steel yet lower failure strain due to surface stress concentrations.

36 MATERIALS SCIENCE↗
DOE OSTI2016

A Bottom-up Energy Efficiency Improvement Roadmap for China’s Iron and Steel Industry up to 2050

Iron and steel manufacturing is energy intensive in China and in the world. China is the world largest steel producer accounting for around half of the world steel production. In this study, we use a bottom-up energy consumption model to analyze four steel-production and energy-efficiency scenarios and evaluate the potential for energy savings from energy-efficient technologies in China’s iron and steel industry between 2010 and 2050. The results show that China’s steel production will rise and peak in the year 2020 at 860 million tons (Mt) per year for the base-case scenario and 680 Mt for the advanced energy-efficiency scenario. From 2020 on, production will gradually decrease to about 510 Mt and 400 Mt in 2050, for the base-case and advanced scenarios, respectively. Energy intensity will decrease from 21.2 gigajoules per ton (G/t) in 2010 to 12.2 GJ/t and 9.9 GJ/t in 2050 for the base-case and advanced scenarios, respectively. In the near term, decreases in iron and steel industry energy intensity will come from adoption of energy-efficient technologies. In the long term, a shift in the production structure of China’s iron and steel industry, reducing the share of blast furnace/basic oxygen furnace production and increasing the share of electric-arc furnace production while reducing the use of pig iron as a feedstock to electric-arc furnaces will continue to reduce the sector’s energy consumption. We discuss barriers to achieving these energy-efficiency gains and make policy recommendations to support improved energy efficiency and a shift in the nature of iron and steel production in China.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗
DOE OSTI2016

Articles comprising ferritic stainless steels

An article of manufacture comprises a ferritic stainless steel that includes a near-surface region depleted of silicon relative to a remainder of the ferritic stainless steel. The article has a reduced tendency to form an electrically resistive silica layer including silicon derived from the steel when the article is subjected to high temperature oxidizing conditions. The ferritic stainless steel is selected from the group comprising AISI Type 430 stainless steel, AISI Type 439 stainless steel, AISI Type 441 stainless steel, AISI Type 444 stainless steel, and E-BRITE.RTM. alloy, also known as UNS 44627 stainless steel. In certain embodiments, the article of manufacture is a fuel cell interconnect for a solid oxide fuel cell.

30 DIRECT ENERGY CONVERSION↗
DOE OSTI2019

Aluminothermic Reduction Process Under Nitrogen Gas Pressure for Preparing High Nitrogen Austenitic Steels

The aluminothermic reduction casting process under nitrogen gas pressure to make austenitic Cr-Mn-N (Ni-free) and Cr-N (Ni/Mn-free) high nitrogen stainless steels was investigated. Thermodynamic simulation of the redox reaction depending on process parameters was performed. As a result, the optimal ratio of aluminum to oxygen in the initial powder mixture to obtain the highest yield of metal product with minimal aluminum nitride contamination was predicted to be slightly greater than the stoichiometric ratio of 1.125. Microstructures of aluminothermic 26Cr1N and 23Cr9Mn1N steels, prepared taking into account the results of thermodynamic simulation, were investigated by X-ray diffraction, metallography, and transmission electron microscopy. The as-cast microstructure was a pseudo-pearlite (layered ferrite-nitride mixture) in 26Cr1N steel and a ferrite-austenite with signs of discontinuous austenite decomposition in 23Cr9Mn1N steel. After hot forging and subsequent water quenching from 1200 °C, the microstructure was fully austenitic in both steels. Tensile tests of quenched 23Cr9Mn1N steel showed a combination of high strength (ultimate strength of 1324 MPa) and ductility (elongation of 27 pct). The results illustrate that the aluminothermic casting process for producing high nitrogen steel is competitive with the commonly used methods, such as pressure electroslag remelting, both in terms of cost and mechanical properties of manufactured steel.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Experimental Study on the Hot Deformation Characterization of Low-Carbon Nb-V-Ti Microalloyed Steel

X80 steel, as typical low-carbon Nb-V-Ti microalloyed steel, is getting more and more attention in oil and gas transmission pipeline manufacturing. In this paper, the hot deformation behavior of X80 steel has been investigated. Hot compression tests of the steel were conducted under different temperatures and strain rates. Based on the experimental data, the flow stress constitutive equations were established. It is found that the hot deformation activation energy of this steel is higher than C-Mn and low-carbon steel. Then, the kinetics model and grain size model of dynamic recrystallization were developed according to the flow curves and the optical microstructures. In addition, the processing maps were developed to analyze the workability of X80 steel at elevated temperature. The analysis results show that the optimum processing window is at the temperature of 1300-1473 K and the strain rate of 0.01-10 s{sup −1}. The microstructure observation indicates that the optimum processing parameters are applicable to the tested steel.

36 MATERIALS SCIENCE↗
DOE OSTI2019

The High-Temperature Mechanical Properties of EN 1.4509 Stainless Steel Cast Using Investment Casting

The high-temperature (600-900 °C) mechanical properties of EN 1.4509 stainless steel made by investment casting were studied by changing the Nb content based on this stainless steel composition standard. The results indicate that with an increase in tensile temperature, the high-temperature strength shows a downward trend and the high-temperature plasticity increases. The results also indicate that with an increase in Nb content, the high-temperature strength rises continually and the high-temperature plasticity decreases; for example, the tensile strengths at 900 °C of Steel A, Steel B and Steel C are 21, 25 and 31 MPa, respectively, and the yield strengths are 15, 20 and 22 MPa, respectively, and the elongations are 97.7, 85.3 and 73.7%, respectively. The high-temperature tensile strength of the investment casting EN 1.4509 stainless steel sample is closely related to the bulk (Ti, Nb) (C, N) and the Laves phase (Fe{sub 2}Nb) at the ferritic grain boundaries and in the ferritic grains. These are the main factors for improvement in the high-temperature strength of the tested steels; additionally, the coarse ferritic grains also play a certain role in improving the mechanical properties. The Nb content of the investment casting EN 1.4509 stainless steel can be reduced to 0.25 wt.%, and at this content, a combination of good high-temperature mechanical properties and higher room-temperature plasticity is obtained.

36 MATERIALS SCIENCE↗
DOE OSTI2023

Creep-Resistant Ferritic-Martensitic Steels for Power Plant Applications

Creep-resistant ferritic-martensitic steels are essential materials used in power plants for electricity generation. The efficiency and emissions of the plant are closely related to the operating temperature. Materials used in the hottest sections are one of the main limiting factor for plant operation at 650 °C. This review describes the recent development of creep-resistant ferritic-martensitic steels from a large dataset of in-house research. The findings are tied to available literature for discussion. A first steel, CPJ-7, was developed with an operating temperature approaching 650 °C, and a refined design, JMP, showed the potential to operate at 650 °C. Approximately fifty different ingots, each weighing ~ 7 kg, were vacuum induction melted and processed. Overall, wrought and cast versions of CPJ-7 present superior creep properties when compared to wrought and cast versions of COST alloys for steam turbine and P91/92 for boiler applications. The prolonged creep life was attributed to slowing down the process of the destabilization of the MX and M 23 C 6 precipitates at 650 °C. The cast version of CPJ-7 also revealed superior mechanical performance, above commercially available cast 9% Cr martensitic steel or derivatives. Following the work on CPJ-7, the JMP steels were designed with higher Co for increased solid solution strengthening, Si for oxidation resistance and increased W for matrix strength and stability. The JMP steels showed increases in creep life compared to CPJ-7 between 118 and 150% at 650 °C for testing at various stresses between 138 and 207 MPa. On a Larson–Miller plot, the performance of the JMP steels surpasses that of state-of-the-art MARBN steel. The influence of various elements within the composition of the alloys on the microstructure and mechanical properties is discussed. This review presents approximately 420,000 h of in-house creep testing, the equivalent of almost 50 years of cumulative creep tests.

36 MATERIALS SCIENCE↗
DOE OSTI2017

Tensile and high cycle fatigue behaviors of high-Mn steels at 298 and 110 K

Tensile and high cycle fatigue behaviors of high-Mn austenitic steels, including 25Mn, 25Mn0.2Al, 25Mn0.5Cu, 24Mn4Cr, 22Mn3Cr and 16Mn2Al specimens, were investigated at 298 and 110 K. Depending on the alloying elements, tensile ductility of high-Mn steels either increased or decreased with decreasing temperature from 298 to 110 K. Reasonable correlation between the tendency for martensitic tranformation, the critical twinning stress and the percent change in tensile elongation suggested that tensile deformation of high-Mn steels was strongly influenced by SFE determining TRIP and TWIP effects. Tensile strength was the most important parameter in determining the resistance to high cycle fatigue of high-Mn steels with an exceptional work hardening capability at room and cryogenic temperatures. The fatigue crack nucleation mechanism in high-Mn steels did not vary with decreasing tempertature, except Cr-added specimens with grain boundary cracking at 298 K and slip band cracking at 110 K. The EBSD (electron backscatter diffraction) analyses suggested that the deformation mechanism under fatigue loading was significantly different from tensile deformation which could be affected by TRIP and TWIP effects. - Highlights: •The resistances to HCF of various high-Mn steels were measured. •The variables affecting tensile and HCF behaviors of high-Mn steels were assessed. •The relationship between tensile and the HCF behaviors of high-Mn steels was established.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Effects of Sn microalloying on cold rolling and recrystallization textures and microstructure of a ferritic stainless steel

Highlights: • Sn microalloying contributes to the enhancement in cold rolling texture intensity and the acceleration in cold rolling texture transformation. • Sn microalloying promotes the recrystallization process, and recrystallization occurs preferentially in steel with Sn compared with Sn-free steel. • Sn microalloying inhibits the growth of recrystallized grains, and the finer grains are obtained in steel with Sn. - Abstract: We investigated the effects of Sn microalloying on cold rolling and recrystallization textures and microstructure of a ferritic stainless steel. The as-received hot rolled sheets were cold rolled at 10%–90% reductions; the sheets cold rolled up to an 80% reduction were annealed at 740–880 °C for 5 min. For comparison, the cold rolling and recrystallization textures of the steel sheets with and without Sn were determined with the conventional X-ray diffraction method. The cold rolling and recrystallization metallographic microstructures were measured by scanning electron microscope and optical microscope, and the crystal orientation maps of the cold rolled and annealed samples were observed by electron backscattered diffraction. The micro-hardness was determined using a digital micro-hardness tester. Our results show that Sn microalloying in a medium chromium ferritic stainless steel plays multiple roles in the cold rolling and recrystallization processes. Sn micrcoalloying contributes to the enhancement in cold rolling texture intensity and the acceleration in cold rolling texture transformation. Sn microalloying is beneficial in promoting the recrystallization process. Recrystallization occurs preferentially in the ferritic stainless steel with Sn. Sn microalloying plays an important role in inhibiting the growth of recrystallized grains. The finer grains are obtained in ferritic stainless steel with Sn when annealing at high temperatures.

36 MATERIALS SCIENCE↗
DOE OSTI2018

Yttrium Oxide Concentration Effect on Helium Porosity Formation in Oxide-Dispersion-Hardened Ferrite-Martensite Steel

Studies are presented of helium porosity in EP-450 oxide-dispersion-hardened yttrium steel, obtained by electopulse sintering, as a function of the Y{sub 2}O{sub 3} content in comparison with EP-450 matrix steel and dispersion-hardened steel fabricated by hot extrusion. It is found that multiple zones with different types of helium porosity and different zone distribution develop in steel with 1 wt.% Y{sub 2}O{sub 3}; in steel with 0.3 wt.% Y{sub 2}O{sub 3}, there are fewer such zones than in matrix steel and steel obtained by hot extrusion. It is proposed that the extremely nonuniform distribution of porosity over volume and size in steel fabricated by electropulse sintering is associated with the initially strongly defective structure, including residual porosity, as well as with the chromium redistribution between ferrite grains and tempered-martensite grains during the sample preparation process.

36 MATERIALS SCIENCE↗