1、Designation: A262 10A262 13Standard 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 revisio
2、n, the 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 Department of Defense.1. Scope*1.1 These practices cover
3、 the following five tests:1.1.1 Practice AOxalic Acid Etch Test for Classification of Etch Structures of Austenitic Stainless Steels (Sections 3 to 7,inclusive),1.1.2 Practice BFerric SulfateSulfuric Acid Test for Detecting Susceptibility to Intergranular Attack in Austenitic StainlessSteels (Sectio
4、ns 8 to 14, inclusive),1.1.3 Practice CNitric Acid Test for Detecting Susceptibility to Intergranular Attack in Austenitic Stainless Steels (Sections15 to 21, inclusive),1.1.4 Practice ECopperCopper SulfateSulfuric Acid Test for Detecting Susceptibility to Intergranular Attack in AusteniticStainless
5、 Steels (Sections 22 to 31, inclusive), and1.1.5 Practice FCopperCopper Sulfate50 % Sulfuric Acid Test for Detecting Susceptibility to Intergranular Attack inMolybdenum-Bearing Austenitic Stainless Steels (Sections 32 to 38, inclusive).1.2 The following factors govern the application of these practi
6、ces:1.2.1 Susceptibility to intergranular attack associated with the precipitation of chromium carbides is readily detected in all sixtests.1.2.2 Sigma phase in wrought chromium-nickel-molybdenum steels, which may or may not be visible in the microstructure, canresult in high corrosion rates only in
7、 nitric acid.1.2.3 Sigma phase in titanium or columbium stabilized alloys and cast molybdenum-bearing stainless alloys, which may or maynot be visible in the microstructure, can result in high corrosion rates in both the nitric acid and ferric sulfate-sulfuric acid solutions.1.2 The oxalic acid etch
8、 test is a rapid method of identifying, by simple etching, those specimens of certain stainless steel gradesthat are essentially free of susceptibility to intergranular attack associated with chromium carbide precipitates. These specimenswill have low corrosion rates in certain corrosion tests and t
9、herefore can be eliminated (screened) from testing as “acceptable.”The etch test is applicable only to those grades listed in the individual hot acid tests and classifies the specimens either as“acceptable” or as “suspect.”1.3 The ferric sulfatesulfuric acid test, the coppercopper sulfate50 % sulfur
10、ic acid test, and the nitric acid test are based onweight loss determinations and, thus, provide a quantitative measure of the relative performance of specimens evaluated. Incontrast, the coppercopper sulfate16 % sulfuric acid test is based on visual examination of bend specimens and, therefore,clas
11、sifies the specimens only as acceptable or nonacceptable.NOTE 1See Appendix X1 for information regarding test selection.1.5 In most cases either the 15-h coppercopper sulfate16 % sulfuric acid test or the 120-h ferric sulfatesulfuric acid test,combined with the oxalic acid etch test, will provide th
12、e required information in the shortest time. All stainless grades listed inthe accompanying table may be evaluated in these combinations of screening and corrosion tests, except those specimens ofmolybdenum-bearing grades (for example 316, 316L, 317, and 317L), which represent steel intended for use
13、 in nitric acidenvironments.1 These practices are under the jurisdiction ofASTM Committee A01 on Steel, Stainless Steel and RelatedAlloys and are the direct responsibility of Subcommittee A01.14on Methods of Corrosion Testing.Current edition approved April 1, 2010May 1, 2013. Published April 2010Jun
14、e 2013. Originally approved in 1943. Last previous edition approved in 20082010 asA262 02a (2008).A262 10. DOI: 10.1520/A0262-10.10.1520/A0262-13.This document is not an ASTM standard and is intended only to provide the user of an ASTM standard an indication of what changes have been made to the pre
15、vious version. Becauseit may not be technically possible to adequately depict all changes accurately, ASTM recommends that users consult prior editions as appropriate. In all cases only the current versionof the standard as published by ASTM is to be considered the official document.*A Summary of Ch
16、anges section appears at the end of this standardCopyright ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States11.6 The 240-h nitric acid test must be applied to stabilized and molybdenum-bearing grades intended for service in nitric acidand to all
17、stainless steel grades that might be subject to end grain corrosion in nitric acid service.1.7 Only those stainless steel grades are listed in Table 1 for which data on the application of the oxalic acid etch test and ontheir performance in various quantitative evaluation tests are available.1.8 Ext
18、ensive test results on various types of stainless steels evaluated by these practices have been published in Ref (1).21.4 The values stated in SI units are to be regarded as standard. The inch-pound equivalents are in parentheses and may beapproximate.1.5 This standard does not purport to address al
19、l of the safety problems, if any, associated with its use. It is the responsibilityof the user of this standard to establish appropriate safety and health practices and determine the applicability of regulatorylimitations prior to use. (Specific precautionary statements are given in 5.6, 11.1.1, 11.
20、1.9, and 35.1.)2. Referenced Documents2.1 ASTM Standards:2A370 Test Methods and Definitions for Mechanical Testing of Steel Products2.2 ISO Standard:3ISO 3651-2 Determination of Resistance to Intergranular Corrosion of Stainless SteelsPart 2: Ferritic, Austenitic, andFerritic-Austenitic (Duplex) Sta
21、inless SteelsCorrosion Test in Media Containing Sulfuric AcidPRACTICE AOXALIC ACID ETCH TEST FORCLASSIFICATION OF ETCH STRUCTURES OF AUSTENITIC STAINLESS STEELS (21)43. Scope3.1 The oxalic acid etch test is used for acceptance of material but not for rejection of material. This may be used in connec
22、tionwith other evaluation tests to provide a rapid method for identifying those specimens that are certain to be free of susceptibilityto rapid intergranular attack in these other tests. Such specimens have low corrosion rates in the various hot acid tests, requiringfrom 4 to 240 h of exposure. Thes
23、e specimens are identified by means of their etch structures, which are classified according tothe following criteria:3.2 The oxalic acid etch test may be used to screen specimens intended for testing in Practice BFerric Sulfate-Sulfuric AcidTest, Practice CNitric Acid Test, Practice ECopper-Copper
24、Sulfate16 % Sulfuric Acid Test, and Practice FCopper-CopperSulfate-50 % Sulfuric Acid Test.3.2.1 Each practice contains a table showing which classifications of etch structures on a given stainless steel grade areequivalent to acceptable, or possibly nonacceptable suspect performance in that particu
25、lar test. Specimens having acceptable etchstructures need not be subjected to the hot acid test. Specimens having nonacceptablesuspect etch structures must be tested in thespecified hot acid solution.3.3 The grades of stainless steels and the hot acid tests for which the oxalic acid etch test is app
26、licable are listed in Table 2.3.3 Extra-low-carbon grades, and stabilized grades, such as 304L, 316L, 317L, 321, and 347, are tested after sensitizing heattreatments at 650 to 675C (1200 to 1250F), which is the range of maximum carbide precipitation. These sensitizing treatmentsmust be applied befor
27、e the specimens are submitted to the oxalic acid etch test. The most commonly used sensitizing treatmentis 1 h at 675C (1250F).4. Apparatus4.1 Source of Direct CurrentBattery, generator, or rectifier capable of supplying about 15 V and 20 A.4.2 AmmeterRange 0 to 30 A (Note 12).4.3 Variable Resistanc
28、e (Note 12).4.4 CathodeA cylindrical piece of stainless steel or, preferably, a 1-qt (0.946-L) stainless steel beaker.4.5 Large Electric ClampTo hold specimen to be etched.4.6 Metallurgical MicroscopeFor examination of etched microstructures at 250 to 500 diameters.4.7 Electrodes of the Etching Cell
29、The specimen to be etched is made the anode, and a stainless steel beaker or a piece ofstainless steel as large as the specimen to be etched is made the cathode.2 For referencedASTM standards, visit theASTM website, www.astm.org, or contactASTM Customer Service at serviceastm.org. For Annual Book of
30、 ASTM Standardsvolume information, refer to the standards Document Summary page on the ASTM website.3 Available from International Organization for Standardization (ISO), 1 rue de Varemb, Case postale 56, CH-1211, Geneva 20, Switzerland, http:/www.iso.ch.4 The boldface numbers in parentheses refer t
31、o a list of references at the end of this standard.A262 1324.8 ElectrolyteOxalic acid, (H2C2O42H2O), reagent grade, 10 weight % solution.NOTE 2The variable resistance and the ammeter are placed in the circuit to measure and control the current on the specimen to be etched.5. Preparation of Test Spec
32、imens5.1 CuttingSawing is preferred to shearing, especially on the extra-low-carbon grades. Shearing cold works adjacent metaland affects the response to subsequent sensitization. Microscopical examination of an etch made on a specimen containing shearededges, should be made on metal unaffected by s
33、hearing. A convenient specimen size is 25 by 25 mm (1 by 1 in.).5.2 The intent is to test a specimen representing as nearly as possible the surface of the material as it will be used in service.Therefore, the preferred sample is a cross section including the surface to be exposed in service. Only su
34、ch surface finishing shouldbe performed as is required to remove foreign material and obtain a standard, uniform finish as described in 5.3. For very heavysections, specimens should be machined to represent the appropriate surface while maintaining reasonable specimen size forconvenient testing. Ord
35、inarily, removal of more material than necessary will have little influence on the test results. However, inthe special case of surface carburization (sometimes encountered, for instance, in tubing or castings when lubricants or binderscontaining carbonaceous materials are employed) it may be possib
36、le by heavy grinding or machining to completely remove thecarburized surface. Such treatment of test specimens is not permissible, except in tests undertaken to demonstrate such effects.5.3 PolishingOn all types of materials, cross sectional surfaces should be polished for etching and microscopical
37、examination.Specimens containing welds should include base plate, weld heat-affected zone, and weld metal. Scale should be removed fromthe area to be etched by grinding to an 80- or 120-grit finish on a grinding belt or wheel without excessive heating and thenpolishing on successively finer emery pa
38、pers, No. 1, 12, 10, 20, and 30, or finer. This polishing operation can be carried out in arelatively short time since all large scratches need not be removed. Whenever practical, a polished area of 1 cm2 or more isdesirable. If any cross-sectional dimension is less than 1 cm, a minimum length of 1
39、cm should be polished. When the availablelength is less than 1 cm, a full cross section should be used.5.4 Etching SolutionThe solution used for etching is prepared by adding 100 g of reagent grade oxalic acid crystals(H2C2O42H2O) to 900 mL of distilled water and stirring until all crystals are diss
40、olved.5.5 Etching ConditionsThe polished specimen should be etched at 1A/cm2 for 1.5 min. To obtain the correct current density:5.5.1 The total immersed area of the specimen to be etched should be measured in square centimetres, and5.5.2 The variable resistance should be adjusted until the ammeter r
41、eading in amperes is equal to the total immersed area of thespecimen in square centimetres.5.6 Etching Precautions:5.6.1 WarningEtching should be carried out under a ventilated hood. Gas, which is rapidly evolved at the electrodes withsome entrainment of oxalic acid, is poisonous and irritating to m
42、ucous membranes.5.6.2 Ayellow-green film is gradually formed on the cathode. This increases the resistance of the etching cell. When this occurs,the film should be removed by rinsing the inside of the stainless steel beaker (or the steel used as the cathode) with an acid suchas 30 % HNO3.5.6.3 The t
43、emperature of the etching solution gradually increases during etching. The temperature should be kept below 50Cby alternating two beakers. One may be cooled in tap water while the other is used for etching. The rate of heating depends onthe total current (ammeter reading) passing through the cell. T
44、herefore, the area etched should be kept as small as possible whileat the same time meeting the requirements of desirable minimum area to be etched.5.6.4 Immersion of the clamp holding the specimen in the etching solution should be avoided.5.7 RinsingFollowing etching, the specimen should be thoroug
45、hly rinsed in hot water and in acetone or alcohol to avoidcrystallization of oxalic acid on the etched surface during drying.5.8 On some specimens containing molybdenum (AISI 316, 316L, 317, 317L), which are free of chromium carbidesensitization, it may be difficult to reveal the presence of step st
46、ructures by electrolytic etching with oxalic acid. In such cases, anelectrolyte of a 10 % solution of ammonium persulfate, (NH4)2S2O8, may be used in place of oxalic acid. An etch of 5 or 10 minat 1 A/cm2 in a solution at room temperature readily develops step structures on such specimens.6. Classif
47、ication of Etch Structures6.1 The etched surface is examined on a metallurgical microscope at 250 to 500 for wrought steels and at about 250 forcast steels.6.2 The etched cross-sectional areas should be thoroughly examined by complete traverse from inside to outside diameters ofrods and tubes, from
48、face to face on plates, and across all zones such as weld metal, weld-affected zones, and base plates onspecimens containing welds.6.3 The etch structures are classified into the following types (Note 23):6.3.1 Step Structure (Fig. 1)Steps only between grains, no ditches at grain boundaries.A262 133
49、6.3.2 Dual Structure (Fig. 2)Some ditches at grain boundaries in addition to steps, but no single grain completely surroundedby ditches.6.3.3 Ditch Structure (Fig. 3)One or more grains completely surrounded by ditches.6.3.4 Isolated Ferrite (Fig. 4)Observed in castings and welds. Steps between austenite matrix and ferrite pools.6.3.5 Interdendritic Ditches (Fig. 5)Observed in castings and welds. Deep interconnected ditches.6.3.6 End-Grain Pitting I (Fig. 6)Structure contains a few deep end-grain pits along with som
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