ML22164A439

NRC Technical Assessment of Powder Metallurgy - Hot Isostatic Pressing

ML22164A439
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Issue date:	06/21/2022
From:	Mark Yoo NRC/RES/DE
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NRC Technical Assessment of Powder Metallurgy Hot Isostatic Pressing

1. Introduction and Purpose This document provides a U.S. Nuclear Regulatory Commission (NRC) technical assessment of the process considerations and knowledge gaps related to the application of powder metallurgyhot isostatic pressing (PM-HIP) in the nuclear power industry. This assessment is primarily based upon the technical information and gap analysis developed by Oak Ridge National Laboratory (ORNL) in technical letter report (TLR) entitled The Use of Powder Metallurgy (PM) and Hot Isostatic Pressing (HIP) for Fabricating Components of Nuclear Power Plants (NPPs), (Agencywide Documents Access & Management System (ADAMS) Accession No. ML22164A438) (hereafter referred to as the ORNL TLR). This assessment, combined with the ORNL TLR, highlights key technical information related to the implementation of PM-HIP in nuclear facilities and fulfills the deliverable for PM-HIP under Subtask 1A of the Action Plan for Advanced Manufacturing Technologies (AMTs), Revision 1, dated June 23, 2020 (ADAMS Accession No. ML19333B973).

2. NRC Identification and Assessment of Differences This section describes the differences between PM-HIP and traditionally manufactured heavy section large components, assesses the impact that the identified differences have on component performance, and identifies specific technical considerations related to PM-HIP components. The overall impact to plant safety (e.g., safety significance) of these identified differences is a function of component performance and the specific component application (e.g., its intended safety function). This report does not include impact on plant safety, as such an assessment would not be possible without considering a specific component application.

The staff identified the differences between PM-HIP and traditional manufacturing processes by reviewing the information and gap analysis rankings from the ORNL TLR, as well as other relevant technical information (e.g., from NRC regulatory and research experience, technical meetings and conferences, codes and standards activities, Electric Power Research Institute and U.S. Department of Energy products and activities). The identified differences and their significance originated either as important aspects or gaps of the PM-HIP process or component performance as defined here:

important aspect: part of the AMT fabrication process or component performance that needs to be considered and carefully controlled during manufacturing gap: part of the AMT fabrication process or component performance that is not well known or understood due to limited information and data The results of this technical assessment are provided in two tables. Table 1 includes the powder production and PM-HIP process considerations. Table 2 includes additional material-specific considerations for producing PM-HIP components using American Society for Testing Materials (ASTM) A508, Standard Specification for Quenched and Tempered Vacuum-Treated Carbon and Alloy Steel Forgings for Pressure Vessels, Grade 3, Class 1 low-alloy steel (A508), which is the alloy of primary interest from the nuclear industry for producing heavy section large components. Components produced by PM-HIP using 316L stainless steel are generally smaller

in size and weight and have undergone an extensive development effort that has led to the American Society of Mechanical Engineers (ASME) Boiler and Pressure Vessel Code (ASME Code),Section III, Rules for Construction of Nuclear Facility Components, Division 1, Code Case N-834, ASTM A988/A988M-11 UNS S31603, Subsection NB, Class 1 ComponentsSection III, Division 1, for Class 1 components. While Table 2 is based on the available information in the open literature for A508-type low-alloy steels, the differences identified in Table 2 involving material-specific properties and performance would likely need to be considered for any new material to be fabricated using PM-HIP. In general, an important need for any nuclear PM-HIP component is material-specific data for the proposed processing and post-processing parameters to ensure adequate component performance in its environment, including applicable properties (e.g., fracture toughness, tensile strength) and aging mechanisms (e.g., thermal aging, irradiation effects, and stress corrosion cracking (SCC)).

The following columns in Tables 1 and 2 identify and provide technical information for the PM-HIP process and component performance:

Difference: Identification of corresponding gaps from Section 3.2 of the ORNL TLR.

Definition: Brief description of the difference with the PM-HIP process.

NRC Ranking of Significance: Discussion of two considerations:

o Importance: Impact on final component integrity considering the likelihood of occurrence or magnitude of degradation in conjunction with the ease of detection or ability to mitigate.

A high ranking signifies that the difference has a significant impact on component performance.

A medium ranking signifies that the difference has a moderate impact on component performance.

A low ranking signifies that the difference has a minimal impact on component performance.

o Knowledge/Manageability: Description of how well understood and manageable the difference is.

Key Technical Information: Technical information for the consideration of PM-HIP for use in nuclear power plants.

Discussion of the corresponding ORNL gaps can be found in Section 3.2 of the ORNL TLR.

3. Codes and Standards Section 3.3 of the ORNL TLR provides an overview of the ASME Code, including an analysis to identify any gaps pertaining to using PM-HIP to fabricate heavy section, low-alloy-steel components. For heavy section, low-alloy-steel components, there is a need to develop an ASTM specification for PM-HIP low-alloy steel to use as a basis for an ASME Code,Section III, Code Case and eventual inclusion in ASME Code,Section II, Materials, Part A, Ferrous Material Specifications, and Part D, Properties. Key challenges include consistently fabricating materials with sufficient starting and irradiated fracture toughness as described in Tables 1 and 2.

4. Summary and Conclusion In Tables 1 and 2 of this report, the staff has identified and assessed the material-generic differences for the PM-HIP process and component performance as well as the material-specific differences for A508 low-alloy steel compared to conventional manufacturing. The staff has also discussed gaps in existing codes and standards that should be addressed to support PM-HIP use in nuclear applications, including the need to develop an ASTM specification and ASME Code Case for PM-HIP of low-alloy steel to support eventual inclusion of this material in ASME Section II.

Table 1 Technical InformationPM-HIP Generic Difference Definition NRC Ranking of Significance Key Technical Information Metal powder Low Producing metal powders by atomization is the preferred method composition and since any composition can be produced accurately to particle size The composition and particle specifications.

distribution covers size distribution of gas atomized - Depending on the reactivity of the metal powders, the proper the production and metal powders must be carefully choice of atomization process must be considered since an management of the controlled within specifications to increase in reactivity will potentially lead to an increase in important physical ensure adequate properties and contamination of the metal powder.

characteristics of the performance of the final product. Optimal particle size should balance densification with the powder before the However, commercial production likelihood of contamination.

Metal Powder build process. of gas atomized metal powders - Generally, smaller particle sizes help to achieve higher packing Composition and is very mature and well density and greater densification after HIP but can also increase Particle Size established. the likelihood of contamination due to their higher percentage of Distribution1 surface area to volume.

The particle size and size distribution characteristics of metal powders are influenced by the atomization method and the associated process parameters used in powder production.

Sieving is an essential activity to effectively control the particle size and size distribution.

- Sieving also ensures a proper size range for filling and packing the inside of the steel can, which improves the densification and microstructure of consolidated component after HIP.

This covers the Low Inert gas atomization methods are superior to air and water important atomization methods since the surface of the molten metal droplets considerations of The method of melting the are protected by the inert gas atmosphere during solidification to using gas elemental constituents of form powder with varying sizes with low oxygen content.

atomization for the stainless steels and low-alloy - Water atomization produces irregular-shaped particles with commercial steels and the atomization higher oxygen levels compared to gas atomization.

Metal Powder production of high- environment can significantly Gas atomization using primarily argon and nitrogen has emerged Production by Gas quality metal powder affect the quality of the powder as the most popular method for producing high-quality powders Atomization for manufacturing and resulting component that have spherical morphology and accurate and reproducible PM-HIP components. properties and performance. compositions of a wide range of stainless steels, low-alloy steels, However, these powder and nickel-based alloys.

production methods have been Important gas atomization parameters that are optimized in order studied carefully and are well to produce desirable particle size and size distribution include high established. gas-to-metal ratio, melt superheat, and gas recirculation.

Difference Definition NRC Ranking of Significance Key Technical Information The can serves as Medium ASTM PM-HIP specifications require that the can material be the pressure vessel selected to ensure that it has no deleterious effect on the final containing the metal Scaling up the fabrication of product.

powder and must steel cans from design The materials used for fabricating the can must have high strength form an impenetrable specifications for producing for dimensional stability and ductility for plastic deformation during seal between the heavy section large components the dimensional reduction of the can to the fully dense product.

metal powder and by PM-HIP has been - The materials typically used for fabricating the can are stainless pressurized gas successfully demonstrated in steel and low-carbon steels due to the combination of strength during HIP. initial applications. Design and ductility properties.

Producing heavy models for predicting the volume The wall thickness of the can is important to consider. The can section large steel shrinkage of heavy section large must be thick enough not to crack or fail during the HIP process requires fabricating steel cans have been but not too thick since a thick wall will resist plastic deformation cans of successfully demonstrated to be compared to a thinner wall.

corresponding size. accurate with low or no The steel can must also be weldable to ensure mechanical integrity This covers the distortions. However, the design of the can during volumetric shrinkage associated with HIP.

important and fabrication of large cans is The design and geometric dimensions of the component are Design and considerations of dependent on component- important factors that need to be optimized for predicting the Fabrication of designing and specific properties and volume shrinkage and minimizing post-HIP machining of the can Heavy Section fabricating these characteristics (e.g., complexity from the product.

Large Steel Cans2 cans, including the of the component geometry). - For example, differences in the thickness of sections should be use of modeling to considered in the design and fabrication of the cans as they will predict the impact the densification of the can and powder due to uneven densification of the cooling rates.

can and metal For the complex can designs, models are required for predicting powders during HIP the volumetric shrinkage.

to achieve the - This especially applies to very large can designs since less dimensional experience and knowledge are available due to the limited tolerances required number of large products that are typically produced by for a component. PM-HIP.

The accuracy of modeling to predict the volume shrinkage of the large can will influence the can design, including selection of material, complexity of the component geometry, and HIP parameters.

Large complex-shaped components will likely need a demonstration run to verify the HIP procedure, including shrinkage prediction and achievement of dimensional tolerances.

Difference Definition NRC Ranking of Significance Key Technical Information Dimensions of the steel can may be intentionally oversized due to uncertainties in precisely predicting the volume shrinkage of the powder and deformation behavior of the steel can during HIP.

Most models that have been developed are based on continuum mechanics, such as micromechanical simulations and constitutive relations, using the finite element method for calculating the distributions of stress and density of the metal powder enclosed can. These approaches consist of plasticity models to understand the mechanical properties, such as yielding and hardening of the metal powder during compaction and continuum models for predicting the effects of sintering on changes in grain size, densification, and plastic deformation of the metal powders.

This covers the High Proper degassing conditions must be used for successful important desorption of all adsorb molecules associated with contamination.

considerations Improper filling, degassing, and - Degassing effectiveness is influenced by the combination of the related to the process vacuum annealing can cause vacuum pump properties (pressure, flow rate, etc.) and the for filling, degassing, contamination of the metal design and placement of the degassing ports and lines and vacuum powder and can adversely affect connecting to the pump.

annealing of metal densification. This can have a - The temperature during degassing is an important consideration powders before HIP. significant impact on final to prevent contaminant gases from forming stronger bonds product performance. There is (e.g., oxides and nitrides) with the metal powder particles.

Filling, Degassing, limited industry experience with - Air is the most common contributor to contamination of metal and Vacuum properly filling large cans for powder.

Annealing of Metal PM-HIP, especially for Improper filling may result in non-uniform packing of stainless steel Powders3 components with complex and low-alloy-steel powders, particularly in large, complex-shaped shapes. Also, vacuum steel cans.

degassing has not been - This may result in different local tap densities of the powder effectively demonstrated on the inside and cause non-uniform consolidation and local density large cans needed for heavy variations of the HIP component. These issues may section PM-HIP components, subsequently impact component properties and performance.

and degassing will be more Scaling up the HIP process to larger components makes degassing challenging as the component more challenging due to the greater volume of gas to remove.

size increases.

Difference Definition NRC Ranking of Significance Key Technical Information This covers the Medium Understanding and optimizing HIP process parameters (e.g., time, important temperature, pressure) is important for producing components with considerations Improper HIP parameters may the required microstructure and mechanical properties. For regarding the HIP affect the densification and example, the HIP process parameters should be optimized to process parameters microstructure and can impact reduce and eliminate pores to achieve high theoretical density of that are used mechanical properties of the the metal.

specifically to large component. Although Non-uniform densification can occur, depending on the HIP manufacture heavy optimized HIP parameters have parameters for applied pressure, temperature, heating rate, and section large been developed for various component size. Rapid heating rate combined with large HIP Parameters for components. other applications, optimization component sizes can cause preferential densification of the metal Heavy Section for producing heavy section powder, especially near the surface of the can, which can result in Large Components large components has not been distortions.

consistently demonstrated. For cans with complex geometries, differences in the thickness of Post-HIP heat treatment can sections will enhance the distortions due to uneven cooling rates help obtain the required caused by gas flow disturbances on the surface of the component.

microstructure and properties. - Reducing the cooling rate can reduce the temperature differences but will add time to the cooling cycle and may affect the resulting mechanical properties.

Proper HIP parameters combined with rapid cooling rates through the use of argon cooling systems can eliminate the need for post-process heat treatment.

Witness specimens High The use of witness specimens and protrusions may be the only and protrusions practical method to measure the density of heavy section large are test specimens Protrusions and witness components.

that are fabricated specimens can be used to Material for conducting microstructure, mechanical properties, and concurrently with measure the density and corrosion tests can be machined directly from a separately end-use mechanical properties of heavy produced witness specimen or from a protrusion on the components and section large components to component(s) that have undergone HIP.

Witness Specimens used to provide demonstrate process control. Use of witness specimens and protrusions should demonstrate that and Protrusions confirmation of However, it has not been they are representative of the components performance.

build quality and demonstrated that protrusions or - Dimension and size relative to the component are two important product witness specimens can be factors for witness specimens.

performance. acceptably relied upon to verify - The dimension, size, number, location, and orientation relative material properties that are to the steel can are important factors for protrusions.

representative or bounding of - For producing heavy section large components by PM-HIP, the the entire component, or at least dimensions of the witness specimens and protrusions may need to match the largest thickness of the component. This can

Difference Definition NRC Ranking of Significance Key Technical Information those critical locations that ensure that microstructural inhomogeneities that may occur in govern the design requirements. the heavy section large component are captured in the witness specimens and protrusions for correlation between mechanical properties measurements.

- Microstructural inhomogeneities can be due to variations with tap density of the metal powder and variations in localized densification rates and cooling rates between the surface and internal location.

This refers to any Low Depending on the can removal method, a fairly rough surface may combination of be left. Lack of post-HIP machining or surface processing (such as machining, finish Post-HIP surface finish can be a peening or grinding) may lead to greater susceptibility to SCC, grinding, and other potential concern, depending on corrosion, and fatigue.

surface treatments the can removal method. If can removal is done by acid pickling, the effects of the acid on that are employed However, machining to the final the component surface should be carefully considered and both to remove the dimensions and surface post- mitigated as needed.

Can Removal/ can after completing processing steps can make For low-alloy-steel materials, the can removal should be done by Surface Finish/ HIP and to meet the surface finish similar to machining rather than acid pickling due to the susceptibility of Processing final dimensional conventionally manufactured low-alloy steel to acid.

requirements for the components. The machinability Because machining can be used to meet the final dimensional component. of the PM-HIP component would tolerances for a component, the dimensions of the steel can are not be expected to differ sometimes intentionally oversized. Due to uncertainties in precisely significantly from conventionally predicting the volume shrinkage of the powder and deformation manufactured components. behavior of the steel can during HIP, oversizing and machining may be more practical for producing heavy section large components for nuclear reactors by PM-HIP.

Note 1: Difference combines the Composition assurance of the gas atomized metal powders and Powder particle size distribution ORNL gaps from Section 3.2 of the ORNL TLR.

Note 2: Difference combines the Fabrication of heavy section large steel cans and Predicting the volume shrinkage of heavy section large components during HIP ORNL gaps from Section 3.2 of the ORNL TLR.

Note 3: Difference combines the Filling the large Steel Can with the metal powders, Degassing and vacuum annealing of metal powders, and Scale Up of the Vacuum System for Degassing Powders in Heavy Section Large Steel Can ORNL gaps from Section 3.2 of the ORNL TLR.

Table 2 Technical InformationPM-HIP A508 Material-Specific Difference Definition NRC Ranking of Significance Key Technical Information Impact toughness can be High To date, large components produced by PM-HIP have correlated to fracture not been able to produce sufficient impact toughness toughness, which is a Producing heavy section large consistently across the component.

measure of the materials low-alloy-steel components by A number of microstructural or composition variations ability to resist the PM-HIP with consistent and may contribute to reduced impact toughness values propagation of flaws. acceptable impact toughness is a as well as increased toughness variability.

significant challenge. Consistent - Oxygen contamination or varying oxygen levels process control through the may contribute to reduced impact toughness in fabrication process (powder PM-HIP low-alloy-steel components.

production, handling, storage, Control of the oxygen and other contamination in Impact Toughness1 degassing, and conducting HIP) PM-HIP produced large components needs to be is needed to achieve managed effectively through the powder production, densification, minimize handling, storage, and degassing stages.

contamination, and produce Optimizing HIP process parameters and post-HIP optimal toughness properties. heat treatment would be expected to improve impact toughness values and consistency.

Given that PM-HIP A508 is a new product form that may behave differently from that of forgings, the correlation between acceptable Charpy impact and fracture toughness properties of PM-HIP A508 should be demonstrated.

SCC refers to stress crack Low PM-HIP is not expected to significantly change the initiation and subsequent SCC performance in these materials, but test data crack growth of Low-alloy-steel materials would be helpful to confirm this expectation.

susceptible materials generally do not come into Lack of post-HIP machining or surface processing operating under contact with the light-water may lead to greater susceptibility to SCC.

Stress Corrosion approximately constant reactor (LWR) environment due SCC is a common failure mode in nuclear power plant Cracking (SCC) stress in a corrosive to the use of stainless steel applications but has generally not been a significant environment. cladding, so SCC is not likely to issue in low-alloy-steel components due to the use of occur. stainless steel cladding to separate the low-alloy steel from the coolant. In addition, water chemistry is tightly controlled to reduce the corrosion potential of the system.

Difference Definition NRC Ranking of Significance Key Technical Information SCC in medium-strength unclad low-alloy steels, such as A508, has not been observed in primary pressure boundary LWR environments.

Fatigue refers to the Medium Very few publications were found that discuss fatigue initiation and propagation of PM-HIP low-alloy steels.

of cracks due to cyclic Fatigue is an important design Lack of post-HIP machining or surface processing loading with or without requirement for some nuclear may lead to greater susceptibility to fatigue.

environmental effects components. Limited data Optimizing HIP process parameters, post-HIP heat playing a significant role in suggest that fatigue performance treatment, and post-HIP surface finish processing Fatigue the process. may be similar to wrought would be expected to improve fatigue susceptibility.

materials. PM-HIP materials should demonstrate sufficient strength to mitigate high-cycle fatigue and sufficient ductility to mitigate low-cycle fatigue.

Data in representative environments are important to support fatigue calculations, including environmentally assisted fatigue in PM-HIP materials.

Irradiation effects refer to High Low-alloy steels in the wrought condition are the impact of neutron susceptible to irradiation effects, including irradiation on various Irradiation effects are a significant embrittlement, at LWR-relevant temperatures and aspects of material aging effect for low-alloy-steel neutron doses.

properties and reactor pressure vessel PM-HIP-produced low-alloy steels are expected to Irradiation Effects performance, including, components. Very limited data exhibit similar irradiation responses to wrought steels but not limited to, loss of are available on neutron- having similar chemical compositions and fracture toughness, irradiated PM-HIP low-alloy-steel microstructures, but test data are needed to confirm irradiation assisted SCC, materials. this expectation.

and void swelling.

Difference Definition NRC Ranking of Significance Key Technical Information Other material aging Medium There are limited data on other material aging effects, effects include corrosion, such as corrosion, wear, and thermal aging on wear, and thermal aging. There are limited data on these low-alloy-steel PM-HIP materials in LWR Thermal aging refers to other aging effects that can environments.

the change in impact component performance. Corrosion and wear are not expected aging microstructure after The applicability and importance mechanisms for stainless-steel-clad low-alloy-steel Other Material Aging significant time at elevated of mechanisms will depend on reactor pressure vessel components but may be (Corrosion, Wear, temperature, which can the design requirements and important for other low-alloy steel components.

Thermal Aging) alter mechanical plant-specific application of the Thermal aging in particular can be highly sensitive to properties, including component. microstructure and chemical composition, which may reductions in fracture be different in a PM-HIP material.

toughness and ductility and increases in hardness and strength.

Tensile properties refer to Low The tensile properties of PM-HIP components have the materials ultimate been observed to be comparable to or better than tensile and yield strength A reasonable body of data to date those of components produced by traditional casting, as well as ductility shows that tensile properties of forging, drawing, and rolling methods.

Tensile Properties measures such as percent PM-HIP A508 components elongation and percent should meet or exceed those of reduction of area at forged A508.

failure.

Note 1: Difference corresponds to the Impact toughness variability of heavy-section large components ORNL gap from Section 3.2 of the ORNL TLR.

ML22164A37; ML22164A439 OFFICE RES/DE/CIB RES/DE/CMB RES/DE LLund NAME MYoo MY MHiser MH JMcKirgan for JM DATE Jun 14, 2022 Jun 16, 2022 Jun 17, 2022