Powder metallurgy with hot isostatic pressing (PM-HIP) is a leading candidate to replace forging as a manufacturing method for structural components in future nuclear reactors. Understanding PM-HIP material performance as compared to conventional forgings under realistic reactor operating conditions is therefore essential for certification and deployment. In this study, PM-HIP and forged Ni-based Alloys 625 and 690 are investigated under neutron irradiation at target damage levels of ~1 and ~3 dpa. Uniaxial tensile tests evaluate the irradiation-induced changes in mechanical behavior, while the irradiation-induced microstructural changes are investigated using transmission electron microscopy and atom probe tomography. Overall, PM-HIP Alloy 625 presents superior mechanical properties compared to forged Alloy 625 under irradiation. This is primarily attributed to an order-of-magnitude lower void population in PM-HIP Alloy 625 compared to its forged counterpart at all damage levels. Minimal differences in irradiation-induced microstructures are observed between PM-HIP and forged Alloy 690, resulting in suppressed differences in mechanical properties. These findings demonstrate comparable or greater irradiation tolerance in PM-HIP Alloys 625 and 690 than in their forged counterparts, providing crucial data to support the qualification of PM-HIP manufacturing of Ni-based alloys for future generation nuclear structural components.

The objective of this study is to understand structure-property relationships across electron beam (EB) welds on nuclear reactor pressure vessel (RPV) steel SA508, Grade 3. Modern nuclear reactor designs typically rely on single-forging RPVs in an effort to eliminate arc welds that require costly in-service inspection. By comparison, advanced EB welds are being considered for RPV applications as they can be rapidly produced, quality heat treated, and often exhibit fewer impurities and narrower heat-affected zones (HAZ). But little is known about microstructure evolution during EB welding and its implications on mechanical properties. This work identifies key structure-property relationships in electron beam welds on forged SA-508 and on a powder metallurgy with hot isostatic pressing (PM-HIP) compact produced to match the SA508 Grade 3 chemical composition. In both the forging and PM-HIP compact, the EB weld fusion zone and HAZ exhibit significant hardening due to martensite nucleation during rapid cooling, while the base metal retains a dual-phase ferrite-bainite microstructure. An appropriately designed heat treatment can eliminate hardness gradients by homogeneously recrystallizing a ferrite-bainite microstructure across the weldment. While PM-HIP and forged SA508 exhibit similar EB weldinduced microstructure evolution, the more extensive porosity in the PM-HIP specimen promotes grain growth and stabilizes ferrite and martensite. Hardness is governed by ferrite phase contiguity, wherein dislocation transmission is inhibited at dissimilar phase interfaces. But if the microstructure is dominated by bainite and/or martensite, hardness is instead governed by their phase fractions. This work illustrates the potential for combining EB welding and PM-HIP with an appropriate quality heat treatment to create RPV welds having negligible microstructure gradients and consistent hardening micro-mechanisms.

Powder metallurgy with hot isostatic pressing (PM-HIP) is an advanced manufacturing process that is envisioned to replace forging for heavy nuclear components, including the reactor pressure vessel (RPV). But PM-HIP products must at least demonstrate comparable irradiation tolerance than forgings in order to be qualified for nuclear applications. The objective of this study is to directly compare PM-HIP to forged SA508 Grade 3 Class 1 low-alloy RPV steel at two neutron irradiation conditions: ~0.5-1.0 displacements per atom (dpa) at ~270C and ~370C. PM-HIP SA508 experiences greater irradiation hardening and embrittlement (total elongation) than forged SA508. However, uniform elongation and approximate toughness are comparable across all irradiated materials, suggesting irradiated PM-HIP SA508 exhibits superior ductility at maximum load-bearing capacity. The irradiation hardening mechanism is linked to composition rather than fabrication method. Since PM-HIP SA508 has higher Mn and Ni concentration, it is more susceptible to irradiation-induced nucleation of Mn-Ni-Si-P (MNSP) nanoprecipitates and dislocation loops, which both contribute to hardening. Conversely, the forged material nucleates fewer MNSPs, causing dislocation loops to control irradiation hardening. These results show promise for the irradiation performance of PM-HIP SA508 and can motivate future nuclear code qualification of PM-HIP fabrication for RPVs.

As part of a Department of Energy (DOE) funded programme assessing advanced manufacturing techniques for SMR applications, the Nuclear AMRC and EPRI have been developing Electron Beam Welding (EBW) parameters and procedures based upon SA508 Grade 3 Class 1 base material. For linear EB welds, the start and stop regions can be managed by using sacrificial run on/off blocks. However, for circumferential welds, such as joining shell to flange components, the use of sacrificial material is not possible. As such an effective method of closing the keyhole must be developed to ensure that weld defects are not entrained. This process is termed ‘slope out’ welding. This paper presents results from the steady state welding in the 80–90 mm material thickness range, showing that weld properties meet specification requirements. Subsequently the paper describes the steps in developing an effective slope out welding procedure for circumferential welds. Weld quality was assured by Phased Array Ultrasonic Testing (PAUT) in conjunction with weld sectioning. All welds were assessed against ASME V requirements. The results presented in this study clearly indicate that defect free and repeatable electron beam welds, including the slope out region, can be produced on thick section pressure vessel steel.

Power metallurgy hot isostatic pressing (PM-HIP), as a versatile manufacturing process, can produce net-shape or near-net-shape components with complicated geometries from materials that are not easily cast, deformed, or welded. In PM-HIP, capsule (or die) filling is a critical step to get dimensionally and microstructurally sound outputs. Particularly, capsule filling controls the initial relative density (homogeneity) of the PM-HIP compact. In this study, the pre-consolidation capsule filling process is simulated by the discrete element method (DEM), to capture the impact of vibration parameters, including the vibration frequency, amplitude, duration, and direction, on the initial relative density (RD). The output of the DEM model was imported into a user subroutine-based finite element of PM-HIP containing a combined constitutive model of compressive and consolidative mechanical behavior of powder. The simulation model was used to quantitatively study the relationships between the vibration parameters and the initial RD of the product. The ultimate results of this work have shown an optimal vibration frequency for maximizing the initial density. The study also quantitatively tested the relationship between the other vibration parameters and the initial relative density and provided the analysis.

Modeling Powder Metallurgy Hot Isostatic Pressing (PM-HIP) is of paramount importance due to its critical role in modern manufacturing. Accurate modeling of PM-HIP and understanding the associated deformation behavior are essential for optimizing the process and advancing the design and production of high-performance materials in critical applications. To accurately simulate the deformation behavior during PM-HIP, this paper presents a comprehensive investigation of modeling PM-HIP processes using various creep models incorporating rate-dependent plasticity embedded into Finite Element Analysis (FEA). This study experimentally and computationally compares four distinct creep models: Kuhn McMeeking, power law creep, unified creep law, the rate-dependent creep that was modified by Abouaf et al. and with modification of Van Nguyen et al. for density dependency. The results showed the modified Abouaf's Model, uniquely designed to account for both primary and secondary creep, consistently exhibits the closest alignment with experimental outcomes compared to the other three creep models tested.

This article presents the comprehensive mechanical testing data archive from a neutron irradiation campaign of nuclear structural alloys fabricated by powder metallurgy with hot isostatic pressing (PM-HIP). The irradiation campaign was designed to facilitate a direct comparison of PM-HIP to conventional casting or forging. Five common nuclear structural alloys were included in the campaign: 316L stainless steel, SA508 pressure vessel steel, Grade 91 ferritic steel, and Ni-base alloys 625 and 690. Irradiations were carried out in the Advanced Test Reactor (ATR) at Idaho National Laboratory (INL) to target doses of 1 and 3 displacements per atom (dpa) at target temperatures of 300 and 400 °C. This article contains the data collected from post-irradiation uniaxial tensile tests following ASTM E8 specifications, fractography of these tensile bars, and nanoindentation. By making this systematic and valuable neutron irradiated mechanical behavior dataset openly available to the nuclear materials research community, researchers may now use this data to populate material performance databases, validate material performance and hardening models, design follow-on experiments, and enable future nuclear code-qualification of PM-HIP techniques.

This article presents neutron irradiation and post-irradiation examination (PIE) capabilities available to the nuclear materials research community through the US Department of Energy’s Nuclear Science User Facilities (NSUF). The pressing need to deploy advanced nuclear reactors to combat climate change requires qualification of new fuels and materials. Among advanced manufacturing processes, powder metallurgy with hot isostatic pressing (PM-HIP) is nearest to becoming qualified for nuclear applications. This article provides examples from a recent irradiation and PIE program on a series of structural alloys fabricated by PM-HIP to illustrate how NSUF capabilities can be used to generate qualification data. The neutron irradiation experiments are described, and a sampling of results from tensile testing, nanoindentation, transmission electron microscopy, and atom probe tomography are presented, showing the favorable performance of PM-HIP alloys compared to their cast or forged counterparts under irradiation. This article provides a perspective on leveraging NSUF for future nuclear fuels and materials testing and qualification.

Power metallurgy hot isostatic pressing (PM-HIP) is a manufacturing technique capable of producing net shape or near-net shape components with complicated geometries from materials that are difficult to melt and cast, mechanically deform or weld. However, the process and soundness of the outcome are extremely sensitive to the geometric design of the capsule (also known as the die or can) that is used in the process. The capsule design for each new component involves several trial–error iterations to achieve the desired geometry and shape of the component. For each iteration, costly HIP experiments need to be conducted and new capsules need be manufactured with small modifications. In this study, a robust finite element analysis (FEA) model of the HIP process is developed, then wrapped in a multi-objective genetic algorithm (MOGA) optimization framework to obtain the optimal pre-HIP capsule design, which yields the desired post-HIP component geometry in one HIP run. The FEA-based optimization algorithm is validated by HIP experiments, showing excellent agreement between the experiment and the model.

This article describes the design of an Advanced Test Reactor (ATR) drop-in neutron irradiation experiment aiming to directly compare the performance of nuclear structural alloys fabricated by powder metallurgy with hot isostatic pressing (PM-HIP) against conventional casting or forging. There is growing interest in PM-HIP alloys for nuclear applications because of their microstructural uniformity, superior mechanical properties, and reduced dependence on welding and machining, compared to cast/forged alloys. Nuclear code-qualification of PM-HIP alloys requires neutron irradiation testing to demonstrate performance under relevant conditions. In this experiment, six nuclear structural alloys were irradiated: Ni-based alloys 625 and 690, Grade 91 ferritic steel, SA508 pressure vessel steel, and 304L and 316L austenitic stainless steels. The experiment is assembled into seven capsules in four test trains and irradiated in three ATR inboard A positions. Both the PM-HIP and cast/ forged versions of each alloy were irradiated under nearly identical conditions for comparative purposes, to target doses of 1 +/- 0.2 and 3 +/- 0.2 dpa at temperatures of 300 +/- 50 degrees C and 400 +/- 50 degrees C. A thorough description of the experiment design and thermal, structural and neutronic analyses performed to ensure the targeted irradiation conditions are met is provided. Specimens were configured as small disks, compact tension specimens and tensile bars to facilitate post-irradiation examination (PIE) that will include mechanical testing, micro-structure characterization, and fracture toughness testing. Given the considerations for ASTM standardized mechanical testing, comparative fluence and temperature across specimen pairs, and comprehensive PIE plan-ning herein, this work serves as a template for future nuclear materials qualification experiment designs.

There has been a significant movement in advancing the use of large hot-isostatically pressed (HIP) powdered metal parts in various industries recently. This is driven by a desire to reduce lead times, generate near-net shaped components, eliminate welds, improve inspection characteristics, and to address other constraints for applications where large forgings and castings are currently used in several industries. Much of this interest in large HIP technology is currently being driven by the near-term commercialization of small modular reactors (SMRs) and advanced reactor (ARs) throughout the world, but particularly within the USA[1]. A consortium within the USA has been investigating the design of a large, 160 inches (4.05m) diameter × 160 inches (4.05 m) length HIP for several years now and hope to move toward manufacture of a large HIP unit within the next 18 months. Similarly, in the UK a slightly larger HIP unit (∼4.5m) referred to as Titan is also being considered. Interest is also growing in other countries including Korea, Sweden, and Japan towards design and production of a large HIP unit to support nuclear and other applications. This paper is an attempt to summarize recent advancements in developing an ultra-large HIP system and discuss challenges in developing HIP equipment of this scale.

Joining ferritic SA508 low alloy steel (LAS) and austenitic 316 L stainless steel (SS) via powder metallurgy hot isostatic pressing (PM-HIP) was evaluated as an alternative method to welding. This study investigated the mechanical and microstructural evolutions of the bimetallic interface under different joint designs and heat treatments. The direct joining of dissimilar metal alloys by PM-HIP method resulted in two designs: 1) powder SA508 to solid bar 316 L (P508-B316L) and 2) powder 316 L solid bar SA508 (P316L-B508). In both cases, P508-B316L and P316L-demonstrated satisfactory tensile strength, however, high hardness and severe depreciation in toughness were located on the bimetallic interface. The mechanisms responsible for the detrimental mechanical properties were verified. Large oxides were observed in P508-B316L due to the prior powder boundary (PPB) oxides present in SA508 powder. The intense sensitization occurred from the formation of M23C6 carbides, consequently from the slow cooling after PM-HIP in P316L-B508. Post-HIP heat treatments were explored to reduce the distance of carbide formation; however, the heat treatment could not eliminate the carbides. The experimental results were compared to the diffusion couple simulation as a function of carbide formation with distance. The analysis also showed the high hardness at the bimetallic interface was primarily contributed by solid solution strengthening. In conclusion, the direct joining of P316L-B508 and P508-B316L via PM-HIP was deemed to be unfeasible, and a transitional material is necessary to impede the diffusion of carbon.

The nuclear power industry has growing interest in qualifying powder metallurgy with hot isostatic pressing (PM-HIP) to replace traditional alloy fabrication methods for reactor structural components. But there is little known about the response of PM-HIP alloys to reactor conditions. This study directly compares the response of PM-HIP to forged Ni-base Alloy 625 under neutron irradiation doses-0.5-1 displacements per atom (dpa) at temperatures ranging-321-385 degrees C. Post-irradiation examination involves microstructure characterization, ASTM E8 uniaxial tensile testing, and fractography. Up through 1 dpa, PM-HIP Alloy 625 appears more resistant to irradiation-induced cavity nucleation than its forged counterpart, and consequently experiences significantly less hardening. This observed difference in performance can be explained by the higher initial dislocation density of the forged material, which represents an interstitial-biased sink that leaves a vacancy supersaturation to nucleate cavities. These findings show promise for qualification of PM-HIP Alloy 625 for nuclear applications, although higher dose studies are needed to assess the steady-state irradiated microstructure.

The nuclear industry has growing interest in replacing forgings with structural components fabricated by powder metallurgy with hot isostatic pressing (PM-HIP), owing to their chemical homogeneity, uniform grain structure, and near-net shape production. This study compares the ion irradiation response of PM HIP and forged Alloy 625, over 50 and 100 dpa, 400 degrees C and 500 degrees C. Microstructure is characterized using down-zone bright-field scanning transmission electron microscopy (DZBFSTEM), and hardening is characterized using nanoindentation. PM-HIP Alloy 625 has a lower initial dislocation line density, resulting in a more rapid onset of dislocation loop growth and unfaulting than the forged material. But the total defect population (i.e. loop line length plus dislocation density) is insensitive to fabrication method. This finding shows promise for the eventual qualification of PM-HIP alloys for nuclear applications. (c) 2021 Elsevier B.V. All rights reserved.

Reduced-pressure electron beam (EB) plate butt welds were manufactured in two low-alloy pressure-vessel steels, SA508 Gr 3 Cl 1 and SA508 Gr 2, at two thicknesses in both steels, 30 mm and 130 mm. Transient temperatures during welding were recorded using thermocouple arrays. Residual stresses in the as-welded condition and after post-weld heat treatment were measured using diverse methods: neutron diffraction and the contour method at 30 mm thickness; and deep hole drilling and the contour method at 130 mm. Incremental centre hole drilling measurements were performed at 130 mm thickness to better understand near-surface stresses. Weld and heat-affected zone microstructures and microconstituents were evaluated using a combination of hardness mapping, optical microscopy and electron microscopy. The as-welded residual stresses exhibit the characteristic M-shaped distribution for hardenable steels, reaching 500–600 MPa in tension in both steels at both thicknesses. However, the modest changes to the chemical composition and the change in plate thickness both significantly influenced microstructures, mechanical properties and residual stress distributions. These sensitivities underline the need for physically faithful models. This extensive characterisation study enables the development and validation of models that predict the development of microstructures and residual stresses in EB welds in low alloy pressure vessel steels.

The Modular In-Chamber Electron Beam Welding (MIC-EBW) system which is being designed and assembled under DOE Projects DE-NE0008846 (Phase 1--completed) and DE-NE0009039 (Phase 2—in progress) includes several major components: four modules (the lower vacuum module, electron beam module, a spacer module, and a lid), a robust rotary table, large vacuum pumps/system, electron beam welding system controls/power supply, and an electron beam generator/gun. Many of these components and their functions were described in the earlier EPRI-DOE report 3002018146. This report provides an overview of one of the primary components, the rotary table, that has been designed and will be manufactured by Rusach International in Phase 2 of the project.

This research paper evaluated three pathways for qualification of 316 L stainless steel components made by laser powder bed fusion additive manufacturing (AM). Comprehensive and consistent process flows with computational modeling, in-situ measurements, ex-situ characterization and mechanical testing with simple-and complex-geometries were explored. The role of post-process hot isostatic pressing (HIP), and solution anneal treatment were evaluated. By using HIP, the scatter in 316 L steel AM properties within single and complex components was minimized to meet the requirement of existing industry standards. For applications where HIP may not be feasible and with some extent of defect tolerance, alternative qualification methodologies of deploying L-PBF AM parts were also explored with samples made with and without engineered porosities. The data generated in this research will be relevant to deployment of AM components for emerging nuclear energy applications. (c) 2021 Elsevier B.V. All rights reserved.