ASTM G98-23

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Designation: G98 − 17 (Reapproved 2022) G98 − 23
Standard Test Method for
Galling Resistance of Materials
This standard is issued under the fixed designation G98; 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 This test method covers a laboratory test which ranks the galling resistance of material couples. Most galling studies have been
conducted on bare metals and alloys; however, non-metallics, coatings, and surface modified alloys may also be evaluated by this
test method.
1.2 This test method is not designed for evaluating the galling resistance of material couples sliding under lubricated conditions
because galling usually will not occur under lubricated sliding conditions using this test method.
1.3 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.
1.4 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:
E177 Practice for Use of the Terms Precision and Bias in ASTM Test Methods
E691 Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method
G40 Terminology Relating to Wear and Erosion
G83 Test Method for Wear Testing with a Crossed-Cylinder Apparatus (Withdrawn 2005)
3. Terminology
3.1 Definitions: Terminology Relating to Wear and Erosion G40:
3.1.1 galling—a form of surface damage arising between sliding solids, distinguished by macroscopic, usually localized,
roughening and creation of protrusions above the original surface; it often includes plastic flow or material transfer, or both.
3.1.1.1 Discussion—
The onset of galling usually requires that the contact pressure exceeds some threshold value. Galling can be a precursor to seizing
or loss of function. The identification of galling is somewhat subjective, and complete agreement does not exist, even among
experts.
3.2 Definitions of Terms Specific to This Standard:
This test method is under the jurisdiction of ASTM Committee G02 on Wear and Erosion and is the direct responsibility of Subcommittee G02.40 on Non-Abrasive Wear.
Current edition approved Nov. 1, 2022Nov. 1, 2023. Published November 2022November 2023. Originally approved in 1989. Last previous edition approved in 20172022
as G98 – 17.G98 – 17 (2022). DOI: 10.1520/G0098-17R22.10.1520/G0098-23.
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volume information, refer to the standard’s Document Summary page on the ASTM website.
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G98 − 23
3.2.1 threshold galling stress—the stress midway between the highest non-galled stress and the lowest galled stress as determined
by this test method.
4. Summary of Test Method
4.1 This test method uses available laboratory equipment capable of maintaining a constant, compressive load between two flat
specimens, such as hydraulic or screw feed compression testing machines. One specimen is slowly rotated one revolution 360°
relative to the other specimen. The surfaces are examined for galling after sliding. The criterion for whether galling occurs is the
appearance of the specimens based on unassisted visual examination. If the specimens have not galled, a new set of specimens is
tested at increased load. This process is continued until galling occurs.
4.2 Appropriate load intervals are chosen to determine the threshold galling stress within an acceptable range.
4.3 The higher the threshold galling stressing, the more galling resistant is the test couple.
5. Significance and Use
5.1 This test method is designed to rank material couples in their resistance to the failure mode caused by galling and not merely
to classify the surface appearance of sliding surfaces.
5.2 This test method should be considered when damaged (galled) surfaces render components non-serviceable. Experience has
shown that galling is most prevalent in sliding systems that are slow moving and operate intermittently. The galling and seizure
of threaded components is a classic example which this test method most closely simulates.
5.3 Other galling-prone examples include: sealing surfaces of value trim which may leak excessively due to galling; and pump
wear rings that may function ineffectively due to galling.
5.4 If the equipment continues to operate satisfactorily and loses dimension gradually, then mechanical wear should be evaluated
by a different test such as the crossed cylinder Test Method (see Test Method G83). Chain belt pins and bushings are examples
of this type of problem.
5.5 This test method should not be used for quantitative or final design purposes since many environmental factors influence the
galling performance of materials in service. Lubrication, alignment, stiffness and geometry are only some of the factors that can
affect how materials perform. This test method has proven valuable in screening materials for prototypical testing that more closely
simulates actual service conditions.
6. Apparatus
6.1 Commonly available laboratory equipment has been used to conduct galling tests. Both Brinell hardness testers and
servo-hydraulic testing machines have proven to be satisfactory as loading devices. Any apparatus that can apply and maintain a
constant compressive load should be acceptable.
7. Test Specimens
7.1 This test method uses a cylindrical flat-on-flat geometry. One specimen is called the button (or pin) and is generally (but not
necessarily) rotated about its axis on the flat specimen called the block.
7.2 Some typical button geometries are shown in Fig. 1.
7.3 The only critical dimension is diameter “B’’ of the contact area. The 6.4 mm (0.25 in.) diameter hole accommodates a ball
bearing for alignment purposes during the test. All other dimensions may be varied to the user’s convenience.
7.4 The block specimen must have sufficient area to accommodate at least one test; however, most users have found that blocks
of length 76 mm (3 in.) to 152 mm (6 in.) are ideal for multiple tests. A reasonable width is 19 mm (0.75 in.). Thickness is not
critical. Tests have been successfully run on blocks with thicknesses from 1.5 mm (0.06 in.) to 25.4 mm (1 in.).
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Metric Equivalents
in. mm in. mm
1 5
⁄4 6.4 ⁄8 16
3 3
⁄8 10 ⁄4 19
1 7
⁄2 13 ⁄8 22
0.500 ± 0.003 12.7 ± 0.08 1 ⁄8 29
FIG. 1 Typical Button Geometries
7.5 Maintain block flatness at 0.33 mm/m (0.004 in./ft).
7.6 The arithmetic average surface finish of both test surfaces should be between 0.25 μm and 1.1 μm (10 μin. and 45 μin.). Leave
specimens as-ground or polished with abrasive paper to achieve the finish.
8. Procedure
8.1 An overall view of the galling test set-up is shown in Fig. 2.
8.2 Cleaning—Immediately prior to testing, clean the test surfaces of the specimens using a procedure that will remove any scale,
oil film, or foreign matter. The following cleaning technique is suggested for metallic specimens: clean the button and block in
trichloroethane, ultrasonically, if possible; a methanol rinse may be used to remove any traces of trichloroethane residue (see Note
1). Materials with open grains (some powder metals) must be dried to remove all traces of the cleaning solvent which may be
entrapped in the material. Demagnetize steel specimens having residual magnetism.
NOTE 1—This cleaning procedure is not appropriate for polymers. If a polymer is being tested, a cleaning procedure that does not alter the chemistry or
surface should be determined.
8.3 Mount specimens in the loading device and degrease again if possible. Lightly load the specimens. Twist the button by hand
to make sure it is seated flat on the block.
FIG. 2 Schematic Diagram of Galling Test Set-Up
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8.4 Apply the selected load. If there is no estimate of the galling resistance of the test couple, it is advisable to start with 890 N
(200 lb) and increase the stress
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