ASTM B931-03
(Test Method)Standard Test Method for Metallographically Estimating the Observed Case Depth of Ferrous Powder Metallurgy (P/M) Parts
Standard Test Method for Metallographically Estimating the Observed Case Depth of Ferrous Powder Metallurgy (P/M) Parts
SIGNIFICANCE AND USE
The engineering function of many P/M parts may require an exterior portion of the part to have a hardened layer. Where case hardening produces a distinct transition in the microstructure, metallographic estimation of the observed case depth may be used to check the depth to which the surface has been hardened.
SCOPE
1.1 A metallographic method is described for estimating the observed case depth of ferrous powder metallurgy (P/M) parts. This method may be used for all types of hardened cases where there is a discernible difference between the microstructure of the hardened surface and that of the interior of the part.
1.2 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 and health practices and determine the applicability of regulatory limitations prior to use.
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Designation:B931–03
Standard Test Method for
Metallographically Estimating the Observed Case Depth of
Ferrous Powder Metallurgy (P/M) Parts
This standard is issued under the fixed designation B 931; 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.
1. Scope Where case hardening produces a distinct transition in the
microstructure, metallographic estimation of the observed case
1.1 Ametallographic method is described for estimating the
depth may be used to check the depth to which the surface has
observed case depth of ferrous powder metallurgy (P/M) parts.
been hardened.
Thismethodmaybeusedforalltypesofhardenedcaseswhere
there is a discernible difference between the microstructure of
6. Apparatus
the hardened surface and that of the interior of the part.
6.1 Equipment for the metallographic preparation of test
1.2 This standard does not purport to address all of the
specimens—see Appendix X1.
safety concerns, if any, associated with its use. It is the
6.2 Metallographic Microscope, permitting observation and
responsibility of the user of this standard to establish appro-
measurement at a magnification of 1003.
priate safety and health practices and determine the applica-
bility of regulatory limitations prior to use.
7. Reagents and Materials
7.1 Etchants such as 2 to 5 % nital, nital/picral combina-
2. Referenced Documents
tions, or other suitable etchants. For more information on
2.1 ASTM Standards:
2 suitable etchants refer to Practice E 407.
B 243 Terminology of Powder Metallurgy
E 407 Practice for Microetching Metals and Alloys
8. Test Specimens
3. Terminology 8.1 Cut a test specimen from the P/M part, perpendicular to
the hardened surface at a specified location, being careful to
3.1 Definitions—Definitions of powder metallurgy (P/M)
avoid any cutting or grinding procedure that would affect the
terms can be found in Terminology B 243.Additional descrip-
original microstructure.
tive information is available in the Related Material section of
8.2 Mounting of the test specimen is recommended for
Vol 02.05 of the Annual Book of ASTM Standards.
convenience in surface preparation and edge retention. Edge
3.2 Themetallographicallyestimatedobservedcasedepthis
retention is important for proper measurement of the observed
defined as the distance from the surface of the part to the point
case depth.
where, at a magnification of 100X, there is a discernible
difference in the microstucture of the material.
9. Procedure
9.1 Grind and polish the test specimen using methods such
4. Summary of Test Method
as those summarized in Appendix X1.
4.1 The powder metallurgy part is sectioned and the surface
9.2 Etch the specimen with etchants such as 2 to 5 % nital
prepared for metallographic evaluation. The metallographic
or nital/picral combinations.
specimen is etched and the distance is measured from the
9.2.1 Observed Case Depth:
surface of the part to the point at which a discernible difference
9.2.1.1 Examine the surface region of the part at a magni-
in the microstructure of the material is observed.
fication of 1003.
5. Significance and Use 9.2.1.2 Measure the distance from the surface of the part to
the point where there is a discernible difference in the micro-
5.1 The engineering function of many P/M parts may
structure of the material.
require an exterior portion of the part to have a hardened layer.
NOTE 1—The nature and amount of intermediate transformation prod-
ucts will depend on the material being heat treated, its density, and the
This test method is under the jurisdiction of ASTM Committee B09 on Metal
type of surface hardening treatment being used. The sharpness of the
Powders and Metal Powder Products and is the direct responsibility of Subcom-
change in the microstructure at the point of transition will therefore vary.
mittee B09.05 on Structural Parts.
The microstructure expected at this transition point should be agreed
Current edition approved Oct. 1, 2003. Published October 2003.
between the producer and user of the part. Magnifications higher than
Annual Book of ASTM Standards, Vol 02.05.
Annual Book of ASTM Standards, Vol 03.01. 1003 may be used to check the microstructure of the part in the region of
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959, United States.
B931–03
the transition zone. However, the metallographic estimate of the observed
11. Precision and Bias
case depth shall be made at a magnification of 1003.
11.1 The precision that can be expected through the use of
10. Report
this test method is currently under review by Subcommittee
10.1 Report the following information: B09.05 on Structural Parts.
10.1.1 The type of material and case measured,
10.1.2 The type of etchant used,
12. Keywords
10.1.3 The location of the measurement, and
12.1 case depth; observed case depth; P/M; powder
10.1.4 The metallographically estimated observed case
metallurgy
depth to the nearest 0.1 mm.
APPENDIX
(Nonmandatory Information)
X1. SAMPLE PREPARATION
X1.1 The methods described in this appendix are proven X1.3 Method 2:
practices for metallographic preparation of porous P/M mate-
X1.3.1 The specimen should be carefully selected so that it
rials. It is recognized that other procedures or materials used in
is representative of the region of interest. After selection, the
preparation of a sample may be equally as good and can be
specimen may require sectioning to provide a workable speci-
used on the basis of availability and preference of individual
men. Sectioning may be made employing an abrasive or
laboratories.
diamond wheel.
X1.3.2 Heat should be avoided to prevent occurrence of
X1.2 Method 1:
possiblechangesinmicrostructure.Ifslowfeedsareemployed,
a coolant may not be necessary to avoid temperature buildups.
X1.2.1 The porous samples should be free of oil or coolant.
If abrasive wheels are used, then a coolant is often necessary to
Remove any oil using Soxhlet extraction. Mount and vacuum
avoid overheating of the specimen.
impregnate samples with epoxy resin, to fill porosity and to
X1.3.3 If a coolant is employed, it may be retained within
prevent the pickup of etchants. Use a sample cup or holder to
the pores. The lubricant must be removed prior to the prepa-
form the mount. Pour epoxy resin over the sample in the cup to
ration of the specimen for examination. This may be accom-
a total depth of about 0.75 in (19 mm). Evacuate the cup to
plished by using a Soxhlet extractor or an ultrasonic cleaner.
minus 26 in. of mercury (88 kPa) and hold at that pressure for
The extraction condenser is the most efficient and the least
10min.Thenrestoreambientairpressuretoforcetheresininto
expensive.
most of the sample. Cure at room temperature or at 122°F
X1.3.4 Generally, specimens to be evaluated for case depth
(50°C).
aremountedtoprovideedgeretention.Therearemanykindsof
X1.2.2 Grind on 240, 400, and 600 grit wet SiC paper, on a
mounting compounds available. Most common materials in-
rotating wheel, and change the polishing direction 90° after
clude epoxies (powder or liquid), diallyl phthalate, or Bakelite.
each paper. Etch samples for 1 min in their normal etchant, for
Of these, Bakelite is sometimes preferred because it is harder
example, 2 % nital, to begin to open the porosity. Rough
and therefore provides improved edge retention. Bakelite
polishing for 8 to 12 min total on 1 µm alumina (Al O ), long
2 3
requires equipment to apply heat and pressure, whereas the
napped cloth (for example Struers felt cloth), at 250 rpm, and
epoxies do not.
300 gf load, using an automated polisher opens smeared pores.
X1.3.5 After mounting, the specimen is ground to provide a
This rough polishing opens and exaggerates the pores. To
flat, stress-free surface. A belt grinder is generally used first
return the
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