1、Designation: A262 14Standard Practices forDetecting Susceptibility to Intergranular Attack in AusteniticStainless Steels1This standard is issued under the fixed designation A262; 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.This standard has been approved for use by agencies of the U.S. Department of Defense.1. Scope*1.1 These practices cover t
3、he following five tests:1.1.1 Practice AOxalic Acid Etch Test for Classificationof Etch Structures of Austenitic Stainless Steels (Sections 4 to13, inclusive),1.1.2 Practice BFerric Sulfate-Sulfuric Acid Test for De-tecting Susceptibility to Intergranular Attack in AusteniticStainless Steels (Sectio
4、ns 14 to 25, inclusive),1.1.3 Practice CNitric Acid Test for Detecting Suscepti-bility to Intergranular Attack in Austenitic Stainless Steels(Sections 26 to 36, inclusive),1.1.4 Practice ECopperCopper SulfateSulfuric AcidTest for Detecting Susceptibility to Intergranular Attack inAustenitic Stainles
5、s Steels (Sections 37 to 46, inclusive), and1.1.5 Practice FCopperCopper Sulfate50 % SulfuricAcid Test for Detecting Susceptibility to Intergranular Attackin Molybdenum-Bearing Austenitic Stainless Steels (Sections47 to 58, inclusive).1.2 The oxalic acid etch test is a rapid method ofidentifying, by
6、 simple etching, those specimens of certainstainless steel grades that are essentially free of susceptibilityto intergranular attack associated with chromium carbideprecipitates. These specimens will have low corrosion rates incertain corrosion tests and therefore can be eliminated(screened) from te
7、sting as “acceptable.” The etch test isapplicable only to those grades listed in the individual hot acidtests and classifies the specimens either as “acceptable” or as“suspect.”1.3 The ferric sulfate-sulfuric acid test, the coppercoppersulfate50 % sulfuric acid test, and the nitric acid test are bas
8、edon weight loss determinations and, thus, provide a quantitativemeasure of the relative performance of specimens evaluated. Incontrast, the coppercopper sulfate16 % sulfuric acid test isbased on visual examination of bend specimens and, therefore,classifies the specimens only as acceptable or nonac
9、ceptable.1.4 The presence or absence of intergranular attack in thesetests is not necessarily a measure of the performance of thematerial in other corrosive environments. These tests do notprovide a basis for predicting resistance to forms of corrosionother than intergranular, such as general corros
10、ion, pitting, orstress-corrosion cracking.NOTE 1See Appendix X1 for information regarding test selection.1.5 The values stated in SI units are to be regarded asstandard. The inch-pound equivalents are in parentheses andmay be approximate.1.6 This standard does not purport to address all of thesafety
11、 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. Some specifichazards statements are given in 10.1, 20.1.1, 20.1.9, 31.3, 34.
12、4,53.1.1, and 53.1.10.2. Referenced Documents2.1 ASTM Standards:2A370 Test Methods and Definitions for Mechanical Testingof Steel ProductsA380/A380M Practice for Cleaning, Descaling, and Passi-vation of Stainless Steel Parts, Equipment, and SystemsD1193 Specification for Reagent WaterE3 Guide for Pr
13、eparation of Metallographic Specimens2.2 ASME Code:3ASME Boiler such use is outside the scope of this practice.4.2 This test is intended to be used in connection with otherevaluation tests described in these practices to provide a rapidmethod for identifying qualitatively those specimens that arecer
14、tain to be free of susceptibility to rapid intergranular attackin these other tests. Such specimens have low corrosion rates inthe various hot acid tests which require from 15 to 240 h ofexposure. These specimens are identified by means of theiretch structures, which are classified according to the
15、criteriagiven in Section 11.4.3 The oxalic acid etch test may be used to screenspecimens intended for testing in Practice BFerric Sulfate-Sulfuric Acid Test, Practice CNitric Acid Test, PracticeECopper-Copper Sulfate16 % SulfuricAcid Test, and Prac-tice FCopper-Copper Sulfate50 % Sulfuric Acid Test.
16、4.4 Each of these other practices contains a table showingwhich classifications of etch structures on a given stainlesssteel grade are equivalent to acceptable or suspect performancein that particular test. Specimens having acceptable etchstructures need not be subjected to the hot acid test. Specim
17、enshaving suspect etch structures must be tested in the specifiedhot acid solution.4.5 There are two classes of specimens to be considered:base metal, and process-affected metal.4.5.1 Process-affected metal contains any condition thataffects the corrosion properties of the material in a non-uniformw
18、ay, such as (but not limited to) welds; carburized. nitrided, oroxidized surfaces; mechanical deformation; and areas affectedby heat. Base metal has none of these conditions.4.5.2 Because Practices B, C, and F involve immersing theentire specimen and averaging the mass loss over the totalspecimen ar
19、ea, and because welding, carburization, mechanicaldeformation, and the like affect only part of a specimen, thepresence of process-affected metal in a specimen can affect thetest result in an unpredictable way depending on the propor-tions of the area affected.4.5.3 If the presence of these or other
20、 localized conditions isa concern to the purchaser, then tests that do not average themass loss over the total specimen surface area, such as PracticeA, the OxalicAcid EtchTest, or Practice E, the CopperCopperSulfateSulfuric Acid Test for Detecting Susceptibility toIntergranular Attack in Austenitic
21、 Stainless Steels, should beconsidered.5. Summary of Practice5.1 A specimen representative of the material to be evalu-ated is polished to a specified finish and over-etched usingoxalic acid electrolytically. The etched specimen is thenexamined using a metallurgical microscope. The etched struc-ture
22、 is compared with reference photographs to determinewhether the material is acceptable or suspect. Suspect materialis then subjected to the specified hot acid immersion test.6. Significance and Use6.1 Use of the etch test allows rapid acceptance of specificlots of material without the need to perfor
23、m time-consumingand costly hot acid immersion tests on those lots.7. Apparatus7.1 Etching Cell:7.1.1 An etching cell may be assembled using componentsas described in this section. Alternatively, a commercialelectropolisher/etcher (as used for metallographic samplepreparation) may be used for small s
24、pecimens provided thecurrent density requirement of 10.2 is met.7.1.2 Source of Direct CurrentBattery, generator, or rec-tifier capable of supplying about 15 V and 20 A.7.1.3 AmmeterFor direct current; used to measure thecurrent on the specimen to be etched.7.1.4 Variable ResistanceUsed to control t
25、he current onthe specimen to be etched.7.1.5 CathodeA stainless steel container, for example, a1-L (1-qt) stainless steel beaker.7.1.5.1 Alternate CathodeA piece of flat stainless steel atleast as large as the specimen surface.7.1.6 Electrical ClampTo hold the specimen to be etchedand to complete th
26、e electrical circuit between the specimen andthe power source such that the specimen is the anode of thecell.5Available from International Organization for Standardization (ISO), 1, ch. dela Voie-Creuse, CP 56, CH-1211 Geneva 20, Switzerland, http:/www.iso.org.6For suggestions on the testing of reag
27、ents not listed by the American ChemicalSociety, see Analar Standards for Laboratory Chemicals, BDH Ltd., Poole, Dorset,U.K., and the United States Pharmacopeia and National Formulary, U.S. Pharma-copeial Convention, Inc. (USPC), Rockville, MD.7The boldface numbers in parentheses refer to a list of
28、references at the end ofthis standard.A262 1427.1.7 The power source, resistor, and ammeter must be sizedappropriately for providing and controlling the current asspecified in 10.2 of this practice.7.1.8 As described, the electrolyte container is the cathode;it may be a stainless steel beaker or fab
29、ricated from stainlesssteel such as by welding a section of tube or pipe to a flat plateor sheet. Alternatively, the electrolyte container may be glass(or other non-conducting, corrosion resisting material) in lieuof a stainless steel container, and the cathode may be a flat plateor sheet of a corro
30、sion resisting alloy. In this latter case, the flatsurface of the cathode must be at least as large as, facing, andapproximately centered on, the prepared surface of the speci-men. Other configurations of the electrodes might not provideuniform etching over the specimen surface. In any case, thesize
31、 and shape of the specimen dictate the size and constructionof the etching cell and of the power source and controls. Theoverriding principle is that the etch needs to be uniform overthe surface to be examined.7.2 Metallurgical MicroscopeFor examination of etchedmicrostructures at 250 to 500 diamete
32、rs.8. Reagents and Materials8.1 Etching Solution (10 %)Dissolve 100 g of reagentgrade oxalic acid crystals (H2C2O42H2O) in 900 mL ofreagent water. Stir until all crystals are dissolved.8.1.1 Alternate Etching Solution (See 10.7)Dissolve 100 gof reagent grade ammonium persulfate (NH4)2S2O8)in900mL of
33、 reagent water. Stir until dissolved.9. Sampling and Test Specimens9.1 The specified hot acid test provides instructions forsampling and for specimen preparation such as a sensitizationheat treatment. Additional instructions specific to Practice Afollow:9.2 The preferred specimen is a cross-section
34、including theproduct surface to be exposed in service. Only such finishing ofthe product surface should be performed as is required toremove foreign material.9.3 Whenever practical, use a cross-sectional area of 1 cm2or more. If any cross-sectional dimension is less than 1 cm,then the other dimensio
35、n of the cross-section should be aminimum of 1 cm. When both dimensions of the product areless than 1 cm, use a full cross section.9.4 PolishingOn all types of materials, polish cross sec-tional surfaces through CAMI/ANSI 600 FEPA/ISO P1200 inaccordance with Guide E3 prior to etching and examination
36、.Not all scratches need to be removed.10. Procedure10.1 (WarningEtching should be carried out under aventilated hood. Gas, which is rapidly evolved at the electrodeswith some entrainment of oxalic acid, is poisonous andirritating to mucous membranes.)10.2 Etch the polished specimen at 1 A/cm2for 1.5
37、 min.10.2.1 To obtain the correct specified current density:10.2.1.1 Measure the total immersed area of the specimen tobe etched in square centimetres.10.2.1.2 Adjust the variable resistance until the ammeterreading in amperes is equal to the total immersed area of thespecimen in square centimetres.
38、10.3 A yellow-green film is gradually formed on the cath-ode. This increases the resistance of the etching cell. When thisoccurs, remove the film by rinsing the inside of the stainlesssteel beaker (or the steel used as the cathode) with an acid suchas 30 % HNO3.10.4 The temperature of the etching so
39、lution graduallyincreases during etching. Keep the temperature below 50C.This may be done by alternating two containers. One may becooled in tap water while the other is used for etching.10.4.1 The rate of heating depends on the total current(ammeter reading) passing through the cell. Therefore, kee
40、pthe area to be etched as small as possible while at the same timemeeting the requirements of desirable minimum area to beetched.10.5 Avoid immersing the clamp holding the specimen inthe etching solution.10.6 RinsingFollowing etching, rinse the specimen thor-oughly in hot water and then in acetone o
41、r alcohol to avoidcrystallization of oxalic acid on the etched surface duringdrying.10.7 It may be difficult to reveal the presence of stepstructures on some specimens containing molybdenum (AISI316, 316L, 317, 317L), which are free of chromium carbidesensitization, by electrolytic etching with oxal
42、ic acid. In suchcases, an alternate electrolyte of ammonium persulfate may beused in place of oxalic acid. (See 8.1.1.) An etch for 5 or 10min at 1 A/cm2in a solution at room temperature readilydevelops step structures on such specimens.11. Classification of Etch Structures11.1 Examine the etched su
43、rface on a metallurgical micro-scope at 250 to 500 for wrought steels and at about 250 forcast steels.11.2 Examine the etched cross-sectional areas thoroughlyby complete traverse from inside to outside diameters of rodsand tubes, from face to face on plates.11.2.1 Microscopical examination of a spec
44、imen shall bemade on metal unaffected by cold-working, carburization,welding, and the like. If any of these conditions are found, notetheir presence in the report.11.3 Classify the etch structures into the following types(Note 2):11.3.1 Step Structure (Fig. 1)Steps only between grains,no ditches at
45、grain boundaries.11.3.2 Dual Structure (Fig. 2)Some ditches at grainboundaries in addition to steps, but no single grain completelysurrounded by ditches.11.3.3 Ditch Structure (Fig. 3)One or more grains com-pletely surrounded by ditches.11.3.4 Isolated Ferrite (Fig. 4)Observed in castings andwelds.
46、Steps between austenite matrix and ferrite pools.11.3.5 Interdendritic Ditches (Fig. 5)Observed in castingsand welds. Deep interconnected ditches.A262 14311.3.6 End-Grain Pitting I (Fig. 6)Structure contains afew deep end-grain pits along with some shallow etch pits at500. (Of importance only when t
47、he nitric acid test is used.)11.3.7 End-Grain Pitting II (Fig. 7)Structure containsnumerous, deep end-grain pits at 500. (Of importance onlywhen nitric acid test is used.)NOTE 2All photomicrographs were made with specimens that wereetched under standard conditions: 10 % oxalic acid, room temperature
48、, 1.5min at 1 A/cm2.11.4 The evaluation of etch structures containing only stepsand of those showing grains completely surrounded by ditchesin every field can be carried out relatively rapidly. In cases thatappear to be dual structures, more extensive examination isrequired to determine if there are
49、 any grains completelyencircled. If an encircled grain is found, classify the steel as aditch structure.11.4.1 On stainless steel castings (also on weld metal), thesteps between grains formed by electrolytic oxalic acid etchingtend to be less prominent than those on wrought materials orFIG. 1 Step Structure (500) (Steps between grains, no ditchesat grain boundaries)FIG. 2 Dual Structure (250) (Some ditches at grain boundariesin addition to steps, but no one grain completely surrounded)FIG. 3 Ditch Structure (500) (One or more grain
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