1、Designation: D8093 16 An American National StandardStandard Guide forNondestructive Evaluation of Nuclear Grade Graphite1This standard is issued under the fixed designation D8093; the number immediately following the designation indicates the year oforiginal adoption or, in the case of revision, the
2、 year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope1.1 This guide provides general tutorial information regard-ing the application of conventional nondestructive ev
3、aluationtechnologies (NDE) to nuclear grade graphite. An introductionwill be provided to the characteristics of graphite that definesthe inspection technologies that can be applied and the limita-tions imposed by the microstructure. This guide does notprovide specific techniques or acceptance criter
4、ia for end-userexaminations but is intended to provide information that willassist in identifying and developing suitable approaches.1.2 The values stated in SI units are to be regarded as thestandard.1.2.1 ExceptionAlternative units provided in parenthesesare for information only.1.3 This standard
5、does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM S
6、tandards:2C709 Terminology Relating to Manufactured Carbon andGraphiteD7219 Specification for Isotropic and Near-isotropicNuclear GraphitesE94 Guide for Radiographic ExaminationE1025 Practice for Design, Manufacture, and MaterialGrouping Classification of Hole-Type Image Quality In-dicators (IQI) Us
7、ed for RadiologyE1441 Guide for Computed Tomography (CT) Imaging3. Summary of Guide3.1 This guide describes the impact specific material prop-erties have on the application of three nondestructive evalua-tion technologies: Eddy current/electromagentic testing (ET)(surface/near surface interrogation)
8、, ultrasonic testing (UT)(volumetric interrogation), radiographic (X-ray) testing (RT)(volumetric interrogation), to nuclear grade graphite.4. Significance and Use4.1 Nuclear grade graphite is a composite material madefrom petroleum or a coal-tar-based coke and a pitch binder.Manufacturing graphite
9、is an iterative process of baking andpitch impregnation of a formed billet prior to finalgraphitization, which occurs at temperatures greater than2500 C. The impregnation and rebake step is repeated severaltimes until the desired product density is obtained. Integral tothis process is the use of iso
10、tropic cokes and a forming process(that is, isostatically molded, vibrationally molded, or ex-truded) that is intended to obtain an isotropic or near isotropicmaterial. However, the source, size, and blend of the startingmaterials as well as the forming process of the green billet willimpart unique
11、material properties as well as variations withinthe final product. There will be density variations from thebillet surface inward and different physical properties with andtransverse the grain direction. Material variations are expectedwithin individual billets as well as billet-to-billet and lot-to
12、-lot.Other manufacturing defects of interest include large pores,inclusions, and cracks. In addition to the material variationinherent to the manufacturing process, graphite will experiencechanges in volume, mechanical strength, and thermal proper-ties while in service in a nuclear reactor along wit
13、h thepossibility of cracking due to stress and oxidation resultingfrom constituents in the gas coolant or oxygen ingress.Therefore, there is the recognized need to be able to nonde-structively characterize a variety of material attributes such asuniformity, isotropy, and porosity distributions as a
14、means toassure consistent stock material. This need also includes theability to detect isolated defects such as cracks, large pores andinclusions, or distributed material damage such as material lossdue to oxidation. The use of this guide is to acquire a basicunderstanding of the unique attributes o
15、f nuclear grade graphite1This guide is under the jurisdiction of ASTM Committee D02 on PetroleumProducts, Liquid Fuels, and Lubricants and is the direct responsibility of Subcom-mittee D02.F0 on Manufactured Carbon and Graphite Products.Current edition approved Dec. 1, 2016. Published March 2017. DO
16、I: 10.1520/D8093-16.2For referenced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.Copyright ASTM International, 100 Barr Ha
17、rbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United StatesThis international standard was developed in accordance with internationally recognized principles on standardization established in the Decision on Principles for theDevelopment of International Standards, Guides and Recommenda
18、tions issued by the World Trade Organization Technical Barriers to Trade (TBT) Committee.1and its application that either permits or hinders the use ofconventional eddy current, ultrasonic, or X-ray inspectiontechnologies.5. Graphite Properties5.1 Table 1 provides a summary of pertinent materialprop
19、erties for a limited selection of commercial nuclear graph-ite types.5.2 The composite nature of graphite results in a multipartmicrostructure with variably shaped and sized porosity (seeFig. 1). The innate porosity in essence forms a flaw populationthat, in part, dictates not only material properti
20、es, but theminimum size limit of isolated flaws that conventional NDEtechnologies can and or should differentiate. However, this isnot to overlook the potential need to detect and characterizedistributed flaw populations such as oxidation or radiationdamage that may be dimensionally smaller than the
21、 inherentporosity. The nature of the microstructure along with thematerial properties of low electrical conductivity, low acousticvelocity, and limited material constituents will dictate how thevarious NDE technologies can be applied and limit theinformation available from the examinations.6. Eddy C
22、urrent Examinations6.1 Eddy current testing (ET) is an established inspectiontechnology well suited for surface/near surface inspection ofelectrically conductive components. ET is based on generatingeddy currents in an electrically conductive test sample throughinductive coupling with a test coil. T
23、he characteristics anddepth of the interrogating eddy currents are governed by thebulk electromagnetic properties of the test piece, test piecegeometry, test frequency, and degree of electromagnetic cou-pling. The primary electromagnetic properties of interest areelectrical conductivity and magnetic
24、 permeability. Any mate-rial or physical condition (for example, cracks, porosity,changes in grain structure, or different phases) that locallyaffects one or both of these properties can be detected andcharacterized. Typically, material anomalies are sensed throughchanges in the drive coil impedance
25、 when coupled to the testpiece but can also be detected by means of secondary pickupinduction coils or other magnetic field measurementtechnologies, for example, Hall or giant magnetoresistive(GMR) devices. The approaches and test probes that can beimplemented are diverse and dependent on the type,
26、size, andlocation of the material anomaly or condition of interest as wellas the test piece electromagnetic properties, geometry, surfacecondition, microstructure, temperature, and so forth.6.2 Eddy currents can be used to inspect nuclear graphite forthe presence of surface/near surface cracks, void
27、s, and inclu-sions as well as to characterize the distribution of porosity orother distributed flaw populations that affect the bulk electricalconductivity. Aspects to consider when applying eddy currentsto nuclear graphite include its low electrical conductivity,microstructure, and test conditions.
28、 The measured electricalconductivity of nuclear graphite is in the range of 0.1 106to0.9106Sm, making it less than or nearly equal inconductivity to low conductivity metal alloys such as Ti-6Al-4V titanium (0.58 106S/m), Inconel 600 (1.02 106Sm), and stainless steel 304 (1.39 106Sm) (1).3Forlow cond
29、uctivity materials such as this, the dominance of skineffect (the exponential decay of eddy current density in testsample) will be significantly reduced compared to that of probecoil diameter to control depth sensitivity. The plane-waveapproximation of eddy current density, jx, in a test piece yield
30、sEq 1 (2):jx5 j0e2 x = f!(1)where:jx= current density in test piece at depth x (A/m2),j0= current density at test piece surface (A/m2),x = depth into test piece (m), = 3.1416,f = test frequency (Hz), = magnetic permeability in free space (4 x107H/m),and = test piece electrical conductivity (S/m).6.3
31、 The standard depth of penetration, =1/(f), isdefined as the depth at which the eddy current density drops to1/e or 36.8 % of the value of j0.Although eddy currents will begenerated past 1, they attenuate rapidly. Eddy current densityat 2 is only 13.5 % of j0. It should also be noted that the phaseo
32、f the eddy currents progressively lags with depth into the test3The boldface numbers in parentheses refer to a list of references at the end ofthis standard.TABLE 1 Graphite PropertiesAGraphiteDesignation/ManufacturerDensity (kg/m3)Electrical Resistivity(-m)L-Wave AcousticVelocity (km/s)S-Wave Acous
33、tic Velocity(km/s)MaximumAverageParticle (Grain)Size (mm)FormingProcessWG AG WG AG WG AG WG AGPCEA/ GrafTechInternational1.775e+003 1.781e+003 7.49 8.01 2.65 2.56 1.59 1.58 0.7 ExtrudedNBG-17/ SGLGroup1.850e+003 1.843e+003 9.51 9.84 2.77 2.76 1.61 1.61 0.8VibrationallymoldedNBG-18/ SGLGroup1.871e+00
34、3 1.872e+003 9.57 9.16 2.87 2.93 1.67 1.68 1.6VibrationallymoldedIG-110/ ToyoTanso USA Inc.1.777e+003 1.778e+003 11.24 10.98 2.46 2.51 1.56 1.57 0.01IsostaticallymoldedIG-430/ ToyoTanso USA Inc.1.812e+003 1.814e+003 9.78 8.62 2.40 2.57 1.54 1.58 0.01IsostaticallymoldedAIdaho National LaboratoryAGC 2
35、 sample measurements:Average values for small, evenly distributed samples sectioned from a single billet, against grain (AG) and withgrain (WG) directions are determined by orientation of the primary sample axis when sectioned from billet.D8093 162piece which is used to differentiate the source of t
36、he signal.Each standard depth produces 1 radian (57.3) of phase lag.High current density yields good detectability and the standarddepth of penetration is typically adjusted by means of manipu-lation of the test frequency to optimize current density andsignal phase for defect detection or the measur
37、ement ofinterest. For example, selecting a test frequency that yields a 1at the depth where defects are expected to be located for aspecific test piece should provide sufficient current density(approximately 37 % of surface current density) to detectdefects at that depth and provide an approximate d
38、efect signalphase shift of 115 compared to a surface lift-off response (2).Lift-off is the response obtained from decoupling of the probecoil from the test piece due to increased probe coil to test pieceseparation or surface roughness, and higher test frequencies arerequired to get an equivalent for
39、 graphite compared to ametal. To get a = 0.005 m in SS304 ( =1.39x106Sm), atest frequency of approximately 7.3 kHz is required. Forgraphite with a conductivity of 0.5 x 106Sm, a test frequencyof approximately 20.3 kHz is required. However, to obtainadequate eddy current density at the calculated ski
40、n depth, theinduction probe coil must be able to project a sufficientlystrong magnetic field to that depth.6.4 A factor determining depth of penetration of the mag-netic field into the test piece and thus the production of eddycurrents will be the extent and magnitude of the axial fieldprojected by
41、a probe coil. The extent of the axial fieldprojection is directly proportional to the diameter of the coilwindings with a magnitude that decreases rapidly down theaxis away from the coil.At an axial distance equal to13 the coildiameter the field strength is approximately 50 % of the fieldstrength at
42、 the coil face, and at a distance equal to 1 diameteronly 10 % of the field strength remains (2). Compared to highconductivity metals, the projection limit of the axial field of thetest coil may control the depth sensitivity in graphite versusskin depth. Therefore, proper selection of probe coil siz
43、ecombined with suitable low test frequencies will permit muchthicker sections of graphite to be interrogated compared to anequivalent probe coil and a high conductivity metal combina-tion. This provides the capability to perform limited volumetricexaminations to detect large internal defects or char
44、acterizevariations in bulk microstructural features such as porosity.Note that the area interrogated by the probe coil is proportionalto its size and orientation. To improve detection of smallersurface defects, that is, concentrate eddy currents near thesurface in a confined space, high test frequen
45、cies and smallerprobe coils should be implemented (see Fig. 2). However, thecoarse microstructure of some graphite types may introducesignificant material noise. In this case, the 0.8 mm diameter by0.8 mm deep flat bottom hole is equivalent in size to surfacebreaking porosity inherent to the graphit
46、e.6.5 Per Specification D7219 and Terminology C709, grainsizes of the starting material in the mix for nuclear graphite canrange from a maximum of 1.68 mm (medium grained) down toless than 2 micron (microfine grained).The size of the resultingmicrostructural features within the graphite (“grains” an
47、dporosity) will also range in a similar manner, as will thematerial noise recorded during inspections. That will limit thesize of an anomaly or material variation that can be detected tosomething larger than the inherent microstructure. Mediumgrain materials will produce significantly more material
48、noisethan a fine grain material (see Fig. 3). This is especially true forthe examination of machined surfaces using smaller diameterprobes at high test frequencies. The rough, as-manufacturedFIG. 1 Micrograph of SGL Group, NBG-18 GraphiteD8093 163surface of a billet will present a similar problem. T
49、heworkmanship, finish, and appearance criteria in SpecificationD7219 only require a billet to be brushed clean after removalfrom the graphitization furnace resulting in rough, potentiallyuneven surfaces that will introduce significant material andlift-off noise into the signal. In both cases, probe diameter,design, test frequencies, or filtering can be adjusted to helpmitigate the noise, assuming the defect or material anomaly ofinterest is of a nature to provide a relevant indication.7. Ultrasonic Examinations7.1 Ultrasonic inspection is
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