ASTM A923-23

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Designation: A923 − 22 A923 − 23
Standard Test Methods for
Detecting Detrimental Intermetallic Phase in Duplex
Austenitic/Ferritic Stainless Steels
This standard is issued under the fixed designation A923; 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 (´) indicates an editorial change since the last revision or reapproval.
1. Scope*
1.1 The purpose of these test methods is to allow detection of the presence of intermetallic phases in certain duplex stainless steels
as listed in Table 1, Table 2, and Table 3 to the extent that toughness or corrosion resistance is affected significantly. These test
methods will not necessarily detect losses of toughness or corrosion resistance attributable to other causes. Similar test methods
for other duplex stainless steels are described in Test Method A1084, but the procedures described in this standard differ
significantly from Test Methods A, B, and C in A1084.
1.2 Duplex (austenitic-ferritic) stainless steels are susceptible to the formation of intermetallic compounds during exposures in the
temperature range from approximately 600 to 1750 °F (320 to 955 °C). The speed of these precipitation reactions is a function of
composition and thermal or thermomechanical history of each individual piece. The presence of these phases is detrimental to
toughness and corrosion resistance.
1.3 Correct heat treatment of duplex stainless steels can eliminate these detrimental phases. Rapid cooling of the product provides
the maximum resistance to formation of detrimental phases by subsequent thermal exposures.
1.4 Compliance with the chemical and mechanical requirements for the applicable product specification does not necessarily
indicate the absence of detrimental phases in the product.
1.5 These test methods include the following:
1.5.1 Test Method A—Sodium Hydroxide Etch Test for Classification of Etch Structures of Duplex Stainless Steels (Sections 3 –
7).
1.5.2 Test Method B—Charpy Impact Test for Classification of Structures of Duplex Stainless Steels (Sections 8 – 13).
1.5.3 Test Method C—Ferric Chloride Corrosion Test for Classification of Structures of Duplex Stainless Steels (Sections 14 – 20).
1.6 The presence of detrimental intermetallic phases is readily detected in all three tests, provided that a sample of appropriate
location and orientation is selected. Because the occurrence of intermetallic phases is a function of temperature and cooling rate,
it is essential that the tests be applied to the region of the material experiencing the conditions most likely to promote the formation
These test methods are under the jurisdiction of ASTM Committee A01 on Steel, Stainless Steel and Related Alloysand are the direct responsibility of Subcommittee
A01.14 on Methods of Corrosion Testing.
Current edition approved June 1, 2022May 15, 2023. Published June 2022June 2023. Originally approved in 1994. Last previous edition approved in 20142022 as
A923 – 14.A923 – 22. DOI: 10.1520/A0923-22.10.1520/A0923-23.
*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
A923 − 23
of an intermetallic phase. In the case of common heat treatment, this region will be that which cooled most slowly. Except for
rapidly cooled material, it may be necessary to sample from a location determined to be the most slowly cooled for the material
piece to be characterized.
1.7 The tests do not determine the precise nature of the detrimental phase but rather the presence or absence of an intermetallic
phase to the extent that it is detrimental to the toughness and corrosion resistance of the material.
1.8 Examples of the correlation of thermal exposures, the occurrence of intermetallic phases, and the degradation of toughness and
corrosion resistance are given in Appendix X1 and Appendix X2.
1.9 The values stated in either inch-pound or SI units are to be regarded as the standard. The values given in parentheses are for
information only.
1.10 This standard does not purport to address all of the safety concerns, if any, associated with its use. It is the responsibility
of the user of this standard to establish appropriate safety, health, and environmental practices and determine the applicability of
regulatory limitations prior to use.
1.11 This international standard was developed in accordance with internationally recognized principles on standardization
established in the Decision on Principles for the Development of International Standards, Guides and Recommendations issued
by the World Trade Organization Technical Barriers to Trade (TBT) Committee.
2. Referenced Documents
2.1 ASTM Standards:
A370 Test Methods and Definitions for Mechanical Testing of Steel Products
A1084 Test Method for Detecting Detrimental Phases in Lean Duplex Austenitic/Ferritic Stainless Steels
G48 Test Methods for Pitting and Crevice Corrosion Resistance of Stainless Steels and Related Alloys by Use of Ferric Chloride
Solution
TEST METHOD A—SODIUM HYDROXIDE ETCH TEST FOR CLASSIFICATION OF ETCH STRUCTURES OF
DUPLEX STAINLESS STEELS
3. Scope
3.1 The sodium hydroxide etch test may be used for the acceptance of material but not for rejection. This test method may be used
with other evaluation tests to provide a rapid method for identifying those specimens that are free of detrimental intermetallic
phases as measured in these other tests.
3.2 The sodium hydroxide etch test may be used to screen specimens intended for testing in Test Method B, Charpy Impact Test
for Classification of Structures of Duplex Stainless Steels, and in Test Method C, Ferric Chloride Corrosion Test for Classification
of Structures of Duplex Stainless Steels.
3.3 Reference photomicrographs are provided to show classifications of etch structures of a particular stainless steel type that are
equivalent to acceptable or to possibly unacceptable performance for each practice. When Test Method A is used as a screening
test for Test Method B or Test Method C, specimens having acceptable etch structures need not be subjected to Test Method B or
Test Method C.
3.4 Table 1 indicates the applicability and acceptance criteria for Test Method A. When Test Method A is specified as an
acceptance test, specimens having other than acceptable etch structures may, at the option of the producer, be tested by Test Method
B or Test Method C.
3.5 The steel shall be tested in the final solution heat treated condition or such other conditions as are agreed upon between the
producer and the user.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at service@astm.org. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
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TABLE 1 Applicability and Acceptance Criteria for
Test Method A
Acceptable Nonacceptable
Grade
Etch Structure Etch Structure
S82441, S31803, unaffected possibly affected
S32205, S32750, structure (Fig. 1, structure (Fig. 3, Fig. 4)
S32760, Fig. 2)
S32550,S32520,
J92205, J93404
affected structure (Fig. 5,
Fig. 6)
centerline structure (Fig.
7)
4. Apparatus
4.1 Source of Direct Current—Battery, generator, or rectifier capable of supplying approximately 15 V and 20 A.
4.2 Ammeter—Range from 0 to 30 A (see Note 1).
4.3 Variable Resistance (see Note 1).
4.4 Cathode—A cylindrical piece of conductive metal.
4.5 Large Electric Clamp, to hold the specimen to be etched.
4.6 Metallurgical Microscope, for examination of etched microstructures to 400 to 500 diameters.
4.7 Electrodes of the Etching Cell—The specimen to be etched is made the anode, and a cylindrical piece of metal as large as the
specimen to be etched is made the cathode.
4.8 Electrolyte, sodium hydroxide (NaOH), reagent grade.
NOTE 1—The variable resistance and 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 For mill products, examination shall be made on a longitudinal or transverse section. For cast products, examination shall be
made on a separately cast test coupon which was heat treated in the same furnace load as the casting it represents. Unless otherwise
specified, selection of the test coupon size shall be at the discretion of the producer. Because high temperature or mechanical
deformation associated with particular cutting processes may alter the structure of the steel, the cutting of the test specimen should
be by a technique that prevents these effects. Alternatively, after the specimens are cut, any material that may have been affected
by high temperature or deformation associated with the cutting should be removed by machining or wet grinding prior to testing.
5.2 For mill products, the specimen should allow for a survey across the full thickness of the section or, in the case of a heavy
section, a survey from one surface through the mid-thickness of the section. The specimen shall include the mid-thickness.
5.3 For cast materials, the specimen shall be taken at approximately 1/4T.
5.4 Polishing—On all materials, cross-sectional surfaces should be polished to a metallographic finish suitable for examination at
400× after etching. Specimens containing welds should include base metal, weld heat–affected zone, and weld metal. The area to
be etched may be prepared by grinding to an 80- or 120-grit finish on a grinding belt or wheel without excessive heating and then
1 1 2 3
by polishing on successively finer emery papers, No. 1, ⁄2, ⁄0, ⁄0, ⁄0, and finer. Other methods of polishing may be acceptable.
5.5 Etching Solution—The solution for etching is prepared by adding 40 g of reagent grade sodium hydroxide (NaOH) to 100 g
of distilled water.
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5.6 Etching Conditions—The polished specimen should be etched at approximately 1 to 3 V dc, for 5 to 60 s (see Note 2).
NOTE 2—When etching is performed at 1 to 3 V dc with a platinum cathode for 5 to 60 s, any intermetallic phase is revealed by yellow, then brown,
staining, followed by staining of the ferrite.
5.7 Rinsing—Following etching, the specimen should be rinsed thoroughly in hot water and in acetone or alcohol, followed by
air drying.
6. Classification of Etch Structures
6.1 The etched surface shall be examined on a metallurgical microscope at 400 to 500×.
6.2 The etched cross-sectional areas should be examined thoroughly by complete traverse of the full sample and across all zones
such as weld metal, weld-affected zones, and base metal on specimens containing welds.
6.3 The etch structures are classified into the following types:
6.3.1 Unaffected Structure (Fig. 1, Fig. 2)—The ferrite has been etched without revelation of intermetallic phase. The interphase
FIG. 1 Unaffected Structure in S31803: (a) Longitudinal Section and (b) Transverse Section (500× Magnification Before Reproduction)
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FIG. 2 (a) J92205: Unaffected Structure-Statically Cast (400× Magnification)
FIG. 2 (b) J92205: Unaffected Structure-Centrifugally Cast (400× Magnification) (continued)
boundaries are smooth.
6.3.2 Possibly Affected Structure (Fig. 3, Fig. 4)—The ferrite has been etched with isolated indications of possible intermetallic
phase. The interphase boundaries may show a fine waviness.
6.3.3 Affected Structure (Fig. 5, Fig. 6)—The indications of an intermetallic phase are readily revealed before or simultaneously
with the staining of the ferrite during etching.
6.3.4 Centerline Structure (Fig. 7)—An intermetallic phase is observed as a continuous or semi-continuous phase in the
mid-thickness region of the product, with or without the affected structure outside of the mid-thickness region, indicative of
segregation.
7. Interpretation and Use of the Etch Structure Classifications
7.1 When Test Method A is used as a screening test, the use of these etch structures depends on the test method for which the
specimens are being screened. Important characteristics of each of the test methods are described as follows.
7.2 Test Method B—The Charpy impact test detects reductions in toughness from that of the optimal composition and processing.
Such reductions may be attributable to intermetallic phases or to other causes not necessarily detectable by Test Method A. A
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FIG. 3 Possibly Affected Structure in S31803: (a) Longitudinal Section and (b) Transverse Section (500× Magnification Before Repro-
duction)
FIG. 4 (a) J92205: Possibly Affected Structure-Statically Cast (400× Magnification)
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FIG. 4 (b) J92205: Possibly Affected Structure-Centrifugally Cast (400× Magnification) (continued)
FIG. 5 Affected Structure in 8a S31803: (a) Longitudinal Section and (b) Transverse Section (500× Magnification Before Reproduction)
Possibly Affected Structure is likely to be associated with a loss of Charpy impact toughness ranging from slight to severe. An
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FIG. 6 J92205: Affected Structure (400× Magnification)
FIG. 7 S31803: Longitudinal Section (500× Magnification Before Reproduction)
Affected Structure is associated with a severe loss of Charpy impact toughness. A Centerline Structure may or may not be detected
by a Charpy test, depending on the orientation of the Charpy specimen.
7.3 Test Method C—The ferric chloride corrosion test is a 24-h test in 10 % ferric chloride. It will detect a loss of corrosion
resistance associated with local depletion of chromium and molybdenum as a result of the precipitation of chromium-rich and
possibly molybdenum-rich phases, not limited to intermetallic phases. An Affected Structure is associated with significant weight
loss in the corrosion test. A Possibly Affected Structure is likely to be associated with significant weight loss in the corrosion test.
TEST METHOD B—CHARPY IMPACT TEST FOR
CLASSIFICATION OF STRUCTURES OF
DUPLEX STAINLESS STEELS
8. Scope
8.1 This test method describes the procedure for conducting the Charpy impact test as a method of detecting the precipitation of
detrimental intermetallic phases in duplex stainless steels. The presence or absence of an indication of intermetallic phase in this
test is not necessarily a measure of performance of the material in service with regard to any property other than that measured
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directly. The Charpy procedure as here applied is different from that as commonly applied for the determination of toughness and
should not be used when characterization of material toughness is the purpose of the testing.
8.2 The Charpy impact test may be used to evaluate mill and cast products, provided that it is possible to obtain a specimen of
relevant location and geometry.
8.3 Table 2 indicates the applicability and acceptance criteria for Test Method B.
9. Rapid Screening Test
9.1 Before testing by the Charpy impact test, specimens of the steel may be given a rapid screening test in accordance with the
procedures of Test Method A, Sodium Hydroxide Etch Test for Classification of Etch Structures of Duplex Stainless Steels.
Preparation, etching, and the classification of etch structures are described therein. Specimens having an etch structure described
as Unaffected Structure in Test Method A will be essentially free of detrimental effect on impact toughness related to the formation
of intermetallic phase. Other mechanisms for loss of toughness may occur independently but are beyond the scope of this test
method. Specimens showing Unaffected Structure in Test Method A are acceptable with respect to the absence of intermetallic
phases, without the necessity of performing the Charpy test as described in Test Method B. All specimens having other than
Unaffected Structure shall be tested by the Charpy impact test.
10. Apparatus
10.1 The test apparatus shall be as described in Test Methods and Definitions A370.
11. Preparation of Test Specimens
11.1 The test specimen shall be as described in Test Methods and Definitions A370.
11.2 An impact test for the purpose of detecting intermetallic phases shall consist of a single specimen taken from the product
piece or lot to be represented.
11.3 For mill products, the test specimen may be aligned in either the longitudinal or transverse direction (see Note 3). For
castings, the test specimen shall be parallel to the side of the test coupon; the notch may be either parallel to or perpendicular to
the coupon surface.
TABLE 2 Applicability and Acceptance Criteria for
Test Method B
Test Minimum Impact
Grade Condition
A
Temperature Energy
A
S82441 base metal –40 °F (–40 °C) 40 ft-lb (54 J)
A
S31803, base metal –40 °F (–40 °C) 40 ft-lb (54 J)
S32205,
...