A262-2002a_奥氏体不锈钢晶间浸蚀敏感性的检测

Designation: A 262 – 02a Standard Practices for Detecting Susceptibility to Intergranular Attack in Austenitic Stainless Steels1 This standard is issued under the fixed designation A 262; the number immediately following the designation indicates the year of original adoption or, in the case of revision, the year of last revision. A number in parentheses indicates the year of last reapproval. A superscript epsilon (e) 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 the following five tests: 1.1.1 Practice A—Oxalic Acid Etch Test for Classification of Etch Structures of Austenitic Stainless Steels (Sections 3 to 7, inclusive), 1.1.2 Practice B—Ferric Sulfate–Sulfuric Acid Test for Detecting Susceptibility to Intergranular Attack in Austenitic Stainless Steels (Sections 8 to 14, inclusive), 1.1.3 Practice C—Nitric Acid Test for Detecting Suscepti- bility to Intergranular Attack in Austenitic Stainless Steels (Sections 15 to 21, inclusive), 1.1.4 Practice E—Copper–Copper Sulfate–Sulfuric Acid Test for Detecting Susceptibility to Intergranular Attack in Austenitic Stainless Steels (Sections 22 to 31, inclusive), and 1.1.5 Practice F—Copper–Copper Sulfate–50 % Sulfuric Acid Test for Detecting Susceptibility to Intergranular Attack in Molybdenum-Bearing Cast Austenitic Stainless Steels (Sec- tions 32 to 38, inclusive). 1.2 The following factors govern the application of these practices: 1.2.1 Susceptibility to intergranular attack associated with the precipitation of chromium carbides is readily detected in all six tests. 1.2.2 Sigma phase in wrought chromium-nickel- molybdenum steels, which may or may not be visible in the microstructure, can result in high corrosion rates only in nitric acid. 1.2.3 Sigma phase in titanium or columbium stabilized alloys and cast molybdenum-bearing stainless alloys, which may or may not be visible in the microstructure, can result in high corrosion rates in both the nitric acid and ferric sulfate- –sulfuric acid solutions. 1.3 The oxalic acid etch test is a rapid method of identify- ing, by simple etching, those specimens of certain stainless steel grades that are essentially free of susceptibility to intergranular attack associated with chromium carbide precipi- tates. These specimens will have low corrosion rates in certain corrosion tests and therefore can be eliminated (screened) from testing as “acceptable.” 1.4 The ferric sulfate–sulfuric acid test, the copper–copper sulfate–50 % sulfuric acid test, and the nitric acid test are based on weight loss determinations and, thus, provide a quantitative measure of the relative performance of specimens evaluated. In contrast, the copper–copper sulfate–16 % sulfuric acid test is based on visual examination of bend specimens and, therefore, classifies the specimens only as acceptable or nonacceptable. 1.5 In most cases either the 24-h copper–copper sul- fate–16 % sulfuric acid test or the 120-h ferric sulfate–sulfuric acid test, combined with the oxalic acid etch test, will provide the required information in the shortest time. All stainless grades listed in the accompanying table may be evaluated in these combinations of screening and corrosion tests, except those specimens of molybdenum-bearing grades (for example 316, 316L, 317, and 317L), which represent steel intended for use in nitric acid environments. 1.6 The 240-h nitric acid test must be applied to stabilized and molybdenum-bearing grades intended for service in nitric acid and to all 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 on their performance in various quantitative evaluation tests are available. 1.8 Extensive test results on various types of stainless steels evaluated by these practices have been published in Ref (1).2 1.9 The values stated in SI units are to be regarded as standard. The inch-pound equivalents are in parentheses and may be approximate. 1.10 This standard does not purport to address all of the safety problems, if any, associated with its use. It is the responsibility 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. (Specific precau- tionary statements are given in 5.6, 11.1.1, 11.1.9, and 35.1.) 1 These practices are under the jurisdiction of ASTM Committee A01 on Steel, Stainless Steel and Related Alloys and are the direct responsibility of Subcommittee A01.14 on Methods of Corrosion Testing. Current edition approved Nov. 10, 2002. Published December 2002. Originally approved in 1943. Last previous edition approved in 2002 as A 262 – 02. 2 The boldface numbers in parentheses refer to the list of references found at the end of these practices. 1 *A Summary of Changes section appears at the end of this standard. Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States. 2. Referenced Documents 2.1 ASTM Standards: A 370 Test Methods and Definitions for Mechanical Testing of Steel Products3 2.2 ISO Standard: ISO 3651-2 Determination of Resistance to Intergranular Corrosion of Stainless Steels—Part 2: Ferritic, Austenitic, and Ferritic-Austenitic (Duplex) Stainless Steels— Corrosion Test in Media Containing Sulfuric Acid4 PRACTICE A—OXALIC ACID ETCH TEST FOR CLASSIFICATION OF ETCH STRUCTURES OF AUSTENITIC STAINLESS STEELS 2 3. Scope 3.1 The oxalic acid etch test is used for acceptance of material but not for rejection of material. This may be used in connection with other evaluation tests to provide a rapid method for identifying those specimens that are certain to be free of susceptibility to rapid intergranular attack in these other tests. Such specimens have low corrosion rates in the various hot acid tests, requiring from 4 to 240 h of exposure. These specimens are identified by means of their etch structures, which are classified according to the following criteria: 3.2 The oxalic acid etch test may be used to screen specimens intended for testing in Practice B—Ferric Sulfate- –Sulfuric Acid Test, Practice C—Nitric Acid Test, Practice E—Copper–Copper Sulfate–16 % Sulfuric Acid Test, and Practice F—Copper–Copper Sulfate–50 % Sulfuric Acid Test. 3.2.1 Each practice contains a table showing which classi- fications of etch structures on a given stainless steel grade are equivalent to acceptable, or possibly nonacceptable perfor- mance in that particular test. Specimens having acceptable etch structures need not be subjected to the hot acid test. Specimens having nonacceptable etch structures must be tested in the specified hot acid solution. 3.3 The grades of stainless steels and the hot acid tests for which the oxalic acid etch test is applicable are listed in Table 2. 3.4 Extra-low–carbon grades, and stabilized grades, such as 304L, 316L, 317L, 321, and 347, are tested after sensitizing heat treatments at 650 to 675°C (1200 to 1250°F), which is the range of maximum carbide precipitation. These sensitizing treatments must be applied before the specimens are submitted to the oxalic acid etch test. The most commonly used sensitiz- ing treatment is 1 h at 675°C (1250°F). 4. Apparatus 4.1 Source of Direct Current—Battery, generator, or recti- fier capable of supplying about 15 V and 20 A. 4.2 Ammeter—Range 0 to 30 A (Note 1). 4.3 Variable Resistance (Note 1). 4.4 Cathode—A cylindrical piece of stainless steel or, preferably, a 1-qt (0.946-L) stainless steel beaker. 3 Annual Book of ASTM Standards, Vol 01.03. 4 Available from International Organization for Standardization (ISO), 1, rue de Varembé, Case postale 56 CH-1211 Geneva 20, Switzerland. TABLE 1 Application of Evaluation Tests for Detecting Susceptibility to Intergranular Attack in Austenitic Stainless Steels NOTE 1—For each corrosion test, the types of susceptibility to intergranular attack detected are given along with the grades of stainless steels in which they may be found. These lists may contain grades of steels in addition to those given in the rectangles. In such cases, the acid corrosion test is applicable, but not the oxalic acid etch test. NOTE 2—The oxalic acid etch test may be applied to the grades of stainless steels listed in the rectangles when used in connection with the test indicated by the arrow. OXALIC ACID ETCH TEST ↓ ↓ ↓ ↓ ↓ AISIA: 304, 304L AISI: 304, 304L, 316, 316L, 317, 317L AISI: 201, 202, 301, 304, 304L, 304H, 316, 316L, 316H, 317, 317L, 321, 347 ACI: CF-3M, CF-8M, ACIB: CF-3, CF-8 ACI: CF-3, CF-8, CF-3M, CF-8M Nitric Acid TestC (240 h in boiling solution) Ferric Sulfate–Sulfuric Acid Test (120 h in boiling solution) Copper–Copper Sulfate– Sulfuric Acid Test (24 h in boiling solution) Copper–Copper Sulfate– 50 % Sulfuric Acid Testing Boiling Solution Chromium carbide in: 304, 304L, CF-3, CF-8 Chromium carbide and sigma phase in:D 316, 316L, 317, 317L, 321, 347, CF-3M, CF- 8M End-grain in: all grades Chromium carbide in: 304, 304L, 316, 316L, 317, 317L, CF-3, CF-8 Chromium carbide and sigma phase in: 321, CF-3M, CF-8ME Chromium carbide in: 201, 202, 301, 304, 304L, 316, 316L, 317, 317L, 321, 347 Chromium carbide in: CF-3M, CF-8M A AISI: American Iron and Steel Institute designations for austenitic stainless steels. B ACI: Alloy Casting Institute designations. C The nitric acid test may be also applied to AISI 309, 310, 348, and AISI 410, 430, 446, and ACI CN-7M. D Must be tested in nitric acid test when destined for service in nitric acid. E To date, no data have been published on the effect of sigma phase on corrosion of AISI 347 in this test. A 262 – 02a 2 4.5 Large Electric Clamp—To hold specimen to be etched. 4.6 Metallurgical Microscope—For examination of etched microstructures at 250 to 500 diameters. 4.7 Electrodes of the Etching Cell—The specimen to be etched is made the anode, and a stainless steel beaker or a piece of stainless steel as large as the specimen to be etched is made the cathode. 4.8 Electrolyte——Oxalic acid, (H2C2O4·2H2O), reagent grade, 10 weight % solution. NOTE 1—The 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 Specimens 5.1 Cutting—Sawing is preferred to shearing, especially on the extra-low–carbon grades. Shearing cold works adjacent metal and affects the response to subsequent sensitization. Microscopical examination of an etch made on a specimen containing sheared edges, should be made on metal unaffected by shearing. 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 such surface finishing should be performed as is required to remove foreign material and obtain a standard, uniform finish as described in 5.3. For very heavy sections, specimens should be machined to repre- sent the appropriate surface while maintaining reasonable specimen size for convenient testing. Ordinarily, removal of more material than necessary will have little influence on the test results. However, in the special case of surface carburiza- tion (sometimes encountered, for instance, in tubing or castings when lubricants or binders containing carbonaceous materials are employed) it may be possible by heavy grinding or machining to completely remove the carburized surface. Such treatment of test specimens is not permissible, except in tests undertaken to demonstrate such effects. 5.3 Polishing—On all types of materials, cross sectional surfaces should be polished for etching and microscopical examination. Specimens containing welds should include base plate, weld heat-affected zone, and weld metal. Scale should be removed from the area to be etched by grinding to an 80- or 120-grit finish on a grinding belt or wheel without excessive heating and then polishing on successively finer emery papers, No. 1, 1⁄2 , 1⁄0 , 2⁄0 , and 3⁄0 , or finer. This polishing operation can be carried out in a relatively short time since all large scratches need not be removed. Whenever practical, a polished area of 1 cm2 or more is desirable. If any cross-sectional dimension is less than 1 cm, a minimum length of 1 cm should be polished. When the available length is less than 1 cm, a full cross section should be used. 5.4 Etching Solution—The solution used for etching is prepared by adding 100 g of reagent grade oxalic acid crystals (H2C2O4·2H2O) to 900 mL of distilled water and stirring until all crystals are dissolved. 5.5 Etching Conditions—The polished specimen should be etched at 1 A/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, and 5.5.2 The variable resistance should be adjusted until the ammeter reading in amperes is equal to the total immersed area of the specimen in square centimetres. 5.6 Etching Precautions: 5.6.1 Caution—Etching should be carried out under a ventilated hood. Gas, which is rapidly evolved at the electrodes with some entrainment of oxalic acid, is poisonous and irritating to mucous membranes. 5.6.2 A yellow-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 such as 30 % HNO3. 5.6.3 The temperature of the etching solution gradually increases during etching. The temperature should be kept below 50°C by alternating two beakers. One may be cooled in tap water while the other is used for etching. The rate of heating depends on the total current (ammeter reading) passing through the cell. Therefore, the area etched should be kept as small as possible while at the same time meeting the require- ments 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 Rinsing—Following etching, the specimen should be thoroughly rinsed in hot water and in acetone or alcohol to avoid crystallization 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 carbide sensitization, it may be difficult to reveal the presence of step structures by electrolytic etching with oxalic acid. In such cases, an electrolyte of a 10 % solution of ammonium persul- fate, (NH4)2S2O8, may be used in place of oxalic acid. An etch of 5 or 10 min at 1 A/cm2 in a solution at room temperature readily develops step structures on such specimens. TABLE 2 Applicability of Etch Test AISI Grade No. ACI Grade No. Practice B—Ferric Sulfate–Sulfuric Acid Test 304, 304L, 316, 316L, 317, 317L CF-3, CF-8, CF-3M,CF-8M Practice C—Nitric Acid Test 304, 304L CF-8, CF-3 Practice E—Copper–Copper Sulfate–16 % Sulfuric Acid Test 201, 202, 301, 304, 304L, 304H, 316, 316L, 316H, 317, 317L, 321, 347 . . . Practice F—Copper–Copper Sulfate–50 % Sulfuric Acid Test . . . CF-8M, CF-3M A 262 – 02a 3 6. Classification of Etch Structures 6.1 The etched surface is examined on a metallurgical microscope at 2503 to 5003 for wrought steels and at about 2503 for cast steels. 6.2 The etched cross-sectional areas should be thoroughly examined by complete traverse from inside to outside diam- eters of rods and tubes, from face to face on plates, and across all zones such as weld metal, weld-affected zones, and base plates on specimens containing welds. 6.3 The etch structures are classified into the following types (Note 2): 6.3.1 Step Structure (Fig. 1)—Steps only between grains, no ditches at grain boundaries. 6.3.2 Dual Structure (Fig. 2)—Some ditches at grain boundaries in addition to steps, but no single grain completely surrounded by ditches. 6.3.3 Ditch Structure (Fig. 3)—One or more grains com- pletely 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 some shallow etch pits at 5003. (Of importance only when nitric acid test is used.) 6.3.7 End-Grain Pitting II (Fig. 7)—Structure contains numerous, deep end-grain pits at 5003. (Of importance only when nitric acid test is used.) NOTE 2—All photomicrographs were made with specimens that were etched under standard conditions: 10 % oxalic acid, room temperature, 1.5 min at 1 A/cm2. 6.4 The evaluation of etch structures containing steps only and of those showing grains completely surrounded by ditches in every field can be carried out relatively rapidly. In cases that appear to be dual structures, more extensive examination is required to determine if there are any grains completely encircled. If an encircled grain is found, the steel should be evaluated as a ditch structure. Areas near surfaces should be examined for evidence of surface carburization. 6.4.1 On stainless steel castings (also on weld metal), the steps between grains formed by electrolytic oxalic acid etching tend to be less prominent than those on wrought materials or are entirely absent. However, any susceptibility to intergranular attack is readily detected by pronounced ditches. FIG. 1 Step Structure (5003) (Steps between grains, no ditches at grain boundaries) FIG. 2 Dual Structure (2503) (Some ditches at grain boundaries in addition to steps, but no one grain completely surrounded) FIG. 3 Ditch Structure (5003) (One or more grains completely surrounded by ditches) A 262 – 02a 4 6.5 Some wrought specimens, especially from bar stock, may contain a random pattern of pits. If these pits are sharp and so deep that they appear black (Fig. 7) it is possible that the specimen may be susceptible to end grain attack in nitric acid only. Therefore, even though the grain boundaries all have step structures, specimens having as much or more end grain pitting than that shown in Fig. 7 cannot be safely assumed to have low nitric acid rates and should be subjected to the nitric acid test whenever it is specified. Such sharp, deep pits should not be confused with the shallow pits shown in Fig. 1 and Fig. 6. 7. Use of Etch Structure Classifications 7.1 The use of these classifications depends on the hot acid corrosion test for which stainless steel specimens are being screened by etching in oxalic acid and is described in each of the practices. Important characteristics of each of these tests are described below. FIG. 4 Isolated Ferrite Pools (2503) (Observed in castings and welds. Steps between austenite matrix and ferrite pools) FIG. 5 Interdendritic Ditches (2503) (Observed in castings and welds. Deep interconnected ditches) To differentiate between the types of pits, use a magnification of 5003 and focus in the plane of etched surface. The pits which now appear completely black are end grain pits. FIG. 6 End Grain Pitting I (5003) (A few deep end grain pits (see 1 in figure) and shallow etch pits (2)) This or a greater concentration of end grain pits at 5003 (using standard etching conditions) indicates that the specimen must be tested when screening is for nitric acid test. FIG. 7 End Grain Pitting II (5003) A 262 – 02a 5 7.2 Practice B—Ferric Sulfate–Sulfuric Acid Test is a 120-h test in boiling 50 % solution that detects susceptibility to intergranular attack associated primarily with chromium car- bide precipitate. It does not detect susceptibility associated with sigma phase in wrought chrom