ASTM B931-14(2018)e1

Standard Test Method for Metallographically Estimating the Observed Case Depth of Ferrous Powder Metallurgy (PM) Parts

SIGNIFICANCE AND USE
5.1 The engineering function of many PM 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 (PM) 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 With the exception of the values for grit size for which the U.S. standard designation is the industry standard, the values stated in SI units are to be regarded as standard.  
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.

General Information

• Status: Historical
• Publication Date: 31-Aug-2014
• ICS: 77.160 - Powder metallurgy
• Technical Committee: B09 - Metal Powders and Metal Powder Products
• Drafting Committee: B09.05 - Structural Parts

Standards Content (Sample)

NOTICE: This standard has either been superseded and replaced by a new version or withdrawn.
Contact ASTM International (www.astm.org) for the latest information
´1
Designation: B931 − 14 (Reapproved 2018)
Standard Test Method for
Metallographically Estimating the Observed Case Depth of
Ferrous Powder Metallurgy (PM) Parts
This standard is issued under the fixed designation B931; 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.
ε NOTE—Editorially corrected Table 1 in October 2018.
1. Scope tive information is available in the Related Material section of
Vol 02.05 of the Annual Book of ASTM Standards.
1.1 A metallographic method is described for estimating the
observed case depth of ferrous powder metallurgy (PM) parts.
3.2 The metallographically estimated observed case depth is
This method may be used for all types of hardened cases where
defined as the distance from the surface of the part to the point
there is a discernible difference between the microstructure of
where, at a magnification of 100×, there is a discernible
the hardened surface and that of the interior of the part.
difference in the microstucture of the material.
1.2 With the exception of the values for grit size for which
4. Summary of Test Method
the U.S. standard designation is the industry standard, the
values stated in SI units are to be regarded as standard.
4.1 The powder metallurgy part is sectioned and the surface
prepared for metallographic evaluation. The metallographic
1.3 This standard does not purport to address all of the
specimen is etched and the distance is measured from the
safety concerns, if any, associated with its use. It is the
surface of the part to the point at which a discernible difference
responsibility of the user of this standard to establish appro-
in the microstructure of the material is observed.
priate safety, health, and environmental practices and deter-
mine the applicability of regulatory limitations prior to use.
1.4 This international standard was developed in accor- 5. Significance and Use
dance with internationally recognized principles on standard-
5.1 The engineering function of many PM parts may require
ization established in the Decision on Principles for the
an exterior portion of the part to have a hardened layer. Where
Development of International Standards, Guides and Recom-
case hardening produces a distinct transition in the
mendations issued by the World Trade Organization Technical
microstructure, metallographic estimation of the observed case
Barriers to Trade (TBT) Committee.
depth may be used to check the depth to which the surface has
been hardened.
2. Referenced Documents
2.1 ASTM Standards:
6. Apparatus
B243 Terminology of Powder Metallurgy
6.1 Equipment for the metallographic preparation of test
E177 Practice for Use of the Terms Precision and Bias in
specimens—see Appendix X1.
ASTM Test Methods
E407 Practice for Microetching Metals and Alloys
6.2 Metallographic Microscope, permitting observation and
measurement at a magnification of 100×.
3. Terminology
3.1 Definitions—Definitions of powder metallurgy (PM)
7. Reagents and Materials
terms can be found in Terminology B243. Additional descrip-
7.1 Etchants such as 2 to 5 % nital, nital/picral
combinations, or other suitable etchants. For more information
This test method is under the jurisdiction of ASTM Committee B09 on Metal
on suitable etchants refer to Practice E407.
Powders and Metal Powder Products and is the direct responsibility of Subcom-
mittee B09.05 on Structural Parts.
Current edition approved Sept. 1, 2014. Published October 2018. Originally
8. Test Specimens
approved in 2003. Last previous edition approved in 2014 as B931 – 14. DOI:
10.1520/B0931-14R18E01.
8.1 Cut a test specimen from the PM part, perpendicular to
For referenced ASTM standards, visit the ASTM website, www.astm.org, or
the hardened surface at a specified location, being careful to
contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM
avoid any cutting or grinding procedure that would affect the
Standards volume information, refer to the standard’s Document Summary page on
the ASTM website. original microstructure.
Copyright © ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States
´1
B931 − 14 (2018)
8.2 Mounting of the test specimen is recommended for conditions, which is accepted as plausible due to random
convenience in surface preparation and edge retention. Edge causes under normal and correct operation of the test method.
retention is important for proper measurement of the observed 11.1.1.2 Repeatability limits are listed in Table 1.
case depth. 11.1.2 Reproducibility (R)—The difference between two
single and independent results obtained by different operators
9. Procedure
applying the same test method in different laboratories using
different apparatus on identical test material would, in the long
9.1 Grind and polish the test specimen using methods such
run, in the normal and correct operation of the test method,
as those summarized in Appendix X1.
exceed the following values only in one case in 20.
9.2 Etch the specimen with etchants such as 2 to 5 % nital
11.1.2.1 Reproducibility can be interpreted as maximum
or nital/picral combinations.
difference between two results, obtained under reproducibility
9.2.1 Observed Case Depth:
conditions, which is accepted as plausible due to random
9.2.1.1 Examine the surface region of the part at a magni-
causes under normal and correct operation of the test method.
fication of 100×.
11.1.2.2 Reproducibility limits cannot be calculated from a
9.2.1.2 Measure the distance from the surface of the part to
single laboratory’s results. The reproducibility of this test
the point where there is a discernible difference in the micro-
method is being determined and will be available on or before
structure of the material.
December 2018.
NOTE 1—The nature and amount of intermediate transformation prod-
11.1.3 The above terms (“repeatability limit” and “repro-
ucts will depend on the material being heat treated, its density, and the
ducibility limit”) are used as specified in Practice E177.
type of surface hardening treatment being used. The sharpness of the
11.1.4 Any judgment in accordance with statement 11.1.1
change in the microstructure at the point of transition will therefore vary.
would normally have an approximate 95 % probability of
The microstructure expected at this transition point should be agreed
between the producer and user of the part. Magnifications higher than being correct. The precision statistics obtained in this ILS must
100× may be used to check the microstructure of the part in the region of
not, however, be treated as exact mathematical quantities
the transition zone. However, the metallographic estimate of the observed
which are applicable to all circumstances and uses. The limited
case depth shall be made at a magnification of 100×.
number of laboratories reporting replicate results essentially
guarantees that there will be times when differences greater
10. Report
than predicted by the ILS results will arise, sometimes with
10.1 Report the following information:
considerably greater or smaller frequency than the 95 %
10.1.1 The type of material and case measured,
probability limit would imply. Consider the repeatability limit
10.1.2 The type of etchant used,
as a general guide, and the associated probability of 95 % as
10.1.3 The location of the measurement, and
only a rough indicator of what can be expected.
10.1.4 The metallographically estimated observed case
11.2 Bias—At the time of the study, there was no accepted
depth to the nearest 0.1 mm.
reference material suitable for determining the bias for this test
11. Precision and Bias method, therefore no statement on bias is being made.
11.1 The precision of this test method is based on an 11.3 The precision statement was determined through sta-
intralaboratory study of ASTM B931, Standard Test Method tistical examination of 80 results, from a single laboratory, on
for Metallographically Estimating the Observed Case Depth of two different PM parts described below:
Ferrous Powder Metallurgy (PM) Parts, conducted in 2013. A PM sprocket A: induction-hardened case depth of approxi-
single laboratory participated in this study, testing two different mately 900 µm
induction-hardened PM parts. Every “test result” represents an PM sprocket B: induction-hardened case depth of approxi-
individual determination. The laboratory reported 40 replicate
mately 500 µm
test results for each of the materials. Except for the use of only
12. Measurement Uncertainty
one laboratory, Practice E691 was followed for the design and
12.1 The precision of Test Method B931 shall be considered
analysis of the data; the details are given in ASTM Research
Report No. B09-1021. by those performing the test when reporting metallographically
estimated case depth results.
11.1.1 Repeatability (r)—The difference between repetitive
results obtained by the same operator in a given laboratory
13. Keywords
applying the same test method with the same apparatus under
13.1 case depth; observed case depth; PM; powder metal-
constant operating conditions on identical test material within
lurgy
short intervals of time would in the long run, in the normal and
correct operation of the test method, exceed the following
TABLE 1 Observed Case Depth (µm)
values only in one case in 20.
Repeatability
11.1.1.1 Repeatability can be interpreted as maximum dif- Repeatability
A
Average Standard
Limit
ference between two results, obtained under repeatability
Deviation
x¯ s r
r
Sprocket A 880 39.2 110
Sprocket B 560 42.3 120
Supporting data have been filed at ASTM International Headquarters and may
A
be obtained by requesting Research Report RR:B09-1021. Contact ASTM Customer The average of the laboratories’ calculated averages.
Service at [e-mail].
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B931 − 14 (2018)
APPENDIX
(Nonmandatory Information)
X1. SAMPLE PREPARATION
X1.1 The methods described in this appendix are proven X1.3.3 If a coolant is employed, it may be retained within
practices for metallographic preparation of porous PM materi- the pores. The lubricant must be removed prior to the prepa-
als. It is recognized that other procedures or materials used in ration of the specimen for examination. This may be accom-
preparation of a sample may be equally as good and can be plished by using a Soxhlet extractor or an
...

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 previous version. Because
it 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 version
of the standard as published by ASTM is to be considered the official document.
´1
Designation: B931 − 14 B931 − 14 (Reapproved 2018)
Standard Test Method for
Metallographically Estimating the Observed Case Depth of
Ferrous Powder Metallurgy (PM) Parts
This standard is issued under the fixed designation B931; 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.
ε NOTE—Editorially corrected Table 1 in October 2018.
1. Scope*Scope
1.1 A metallographic method is described for estimating the observed case depth of ferrous powder metallurgy (PM) 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 With the exception of the values for grit size for which the U.S. standard designation is the industry standard, the values
stated in SI units are to be regarded as standard.
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 safety, health, and healthenvironmental 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:
B243 Terminology of Powder Metallurgy
E177 Practice for Use of the Terms Precision and Bias in ASTM Test Methods
E407 Practice for Microetching Metals and Alloys
3. Terminology
3.1 Definitions—Definitions of powder metallurgy (PM) terms can be found in Terminology B243. Additional descriptive
information is available in the Related Material section of Vol 02.05 of the Annual Book of ASTM Standards.
3.2 The metallographically estimated observed case depth is defined as the distance from the surface of the part to the point
where, at a magnification of 100X,100×, there is a discernible difference in the microstucture of the material.
4. Summary of Test Method
4.1 The powder metallurgy part is sectioned and the surface prepared for metallographic evaluation. The metallographic
specimen is etched and the distance is measured from the surface of the part to the point at which a discernible difference in the
microstructure of the material is observed.
5. Significance and Use
5.1 The engineering function of many PM 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.
This test method is under the jurisdiction of ASTM Committee B09 on Metal Powders and Metal Powder Products and is the direct responsibility of Subcommittee B09.05
on Structural Parts.
Current edition approved Sept. 1, 2014. Published September 2014October 2018. Originally approved in 2003. Last previous edition approved in 20092014 as
B931B931 – 14.–09. DOI: 10.1520/B0931-14.10.1520/B0931-14R18E01.
For referenced ASTM standards, visit the ASTM website, www.astm.org, or contact ASTM Customer Service at [e-mail]. For Annual Book of ASTM Standards
volume information, refer to the standard’s Document Summary page on the ASTM website.
*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
´1
B931 − 14 (2018)
6. Apparatus
6.1 Equipment for the metallographic preparation of test specimens—see Appendix X1.
6.2 Metallographic Microscope, permitting observation and measurement at a magnification of 100×.
7. Reagents and Materials
7.1 Etchants such as 2 to 5 % nital, nital/picral combinations, or other suitable etchants. For more information on suitable
etchants refer to Practice E407.
8. Test Specimens
8.1 Cut a test specimen from the PM part, perpendicular to the hardened surface at a specified location, being careful to avoid
any cutting or grinding procedure that would affect the original microstructure.
8.2 Mounting of the test specimen is recommended for convenience in surface preparation and edge retention. Edge retention
is important for proper measurement of the observed case depth.
9. Procedure
9.1 Grind and polish the test specimen using methods such as those summarized in Appendix X1.
9.2 Etch the specimen with etchants such as 2 to 5 % nital or nital/picral combinations.
9.2.1 Observed Case Depth:
9.2.1.1 Examine the surface region of the part at a magnification of 100×.
9.2.1.2 Measure the distance from the surface of the part to the point where there is a discernible difference in the microstructure
of the material.
NOTE 1—The nature and amount of intermediate transformation products will depend on the material being heat treated, its density, and the type of
surface hardening treatment being used. The sharpness of the change in the microstructure at the point of transition will therefore vary. The microstructure
expected at this transition point should be agreed between the producer and user of the part. Magnifications higher than 100× may be used to check the
microstructure of the part in the region of the transition zone. However, the metallographic estimate of the observed case depth shall be made at a
magnification of 100×.
10. Report
10.1 Report the following information:
10.1.1 The type of material and case measured,
10.1.2 The type of etchant used,
10.1.3 The location of the measurement, and
10.1.4 The metallographically estimated observed case depth to the nearest 0.1 mm.
11. Precision and Bias
11.1 The precision of this test method is based on an intralaboratory study of ASTM B931, Standard Test Method for
Metallographically Estimating the Observed Case Depth of Ferrous Powder Metallurgy (PM) Parts, conducted in 2013. A single
laboratory participated in this study, testing two different induction-hardened PM parts. Every “test result” represents an individual
determination. The laboratory reported 40 replicate test results for each of the materials. Except for the use of only one laboratory,
Practice E691 was followed for the design and analysis of the data; the details are given in ASTM Research Report No.
B09-1021B09-1021. .
11.1.1 Repeatability (r)—The difference between repetitive results obtained by the same operator in a given laboratory applying
the same test method with the same apparatus under constant operating conditions on identical test material within short intervals
of time would in the long run, in the normal and correct operation of the test method, exceed the following values only in one case
in 20.
11.1.1.1 Repeatability can be interpreted as maximum difference between two results, obtained under repeatability conditions,
which is accepted as plausible due to random causes under normal and correct operation of the test method.
11.1.1.2 Repeatability limits are listed in Table 1.
11.1.2 Reproducibility (R)—The difference between two single and independent results obtained by different operators applying
the same test method in different laboratories using different apparatus on identical test material would, in the long run, in the
normal and correct operation of the test method, exceed the following values only in one case in 20.
11.1.2.1 Reproducibility can be interpreted as maximum difference between two results, obtained under reproducibility
conditions, which is accepted as plausible due to random causes under normal and correct operation of the test method.
Supporting data have been filed at ASTM International Headquarters and may be obtained by requesting Research Report RR:B09-1021. Contact ASTM Customer
Service at [e-mail].
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B931 − 14 (2018)
TABLE 1 Observed Case Depth (μm)
Repeatability
Repeatability
A
Average Standard
Limit
Deviation
x¯ s r
r
Sprocket A 880 39.2 110
Sprocket B 560 42.3 120
A
The average of the laboratories’ calculated averages.
11.1.2.2 Reproducibility limits cannot be calculated from a single laboratory’s results. The reproducibility of this test method
is being determined and will be available on or before December 2018.
11.1.3 The above terms (“repeatability limit” and “reproducibility limit”) are used as specified in Practice E177.
11.1.4 Any judgment in accordance with statement 11.1.1 would normally have an approximate 95%95 % probability of being
correct. The precision statistics obtained in this ILS must not, however, be treated as exact mathematical quantities which are
applicable to all circumstances and uses. The limited number of laboratories reporting replicate results essentially guarantees that
there will be times when differences greater than predicted by the ILS results will arise, sometimes with considerably greater or
smaller frequency than the 95%95 % probability limit would imply. Consider the repeatability limit as a general guide, and the
associated probability of 95%95 % as only a rough indicator of what can be expected.
11.2 Bias—At the time of the study, there was no accepted reference material suitable for determining the bias for this test
method, therefore no statement on bias is being made.
11.3 The precision statement was determined through statistical examination of 80 results, from a single laboratory, on two
different PM parts described below:
PM sprocket A: induction-hardened case depth of approximately 900 μm
PM sprocket B: induction-hardened case depth of approximately 500 μm
12. Measurement Uncertainty
12.1 The precision of Test Method B931 shall be considered by those performing the test when reporting metallographically
estimated case depth results.
13. Keywords
13.1 case depth; observed case depth; PM; powder metallurgy
APPENDIX
(Nonmandatory Information)
X1. SAMPLE PREPARATION
X1.1 The methods described in this appendix are proven practices for metallographic preparation of porous PM materials. It is
recognized that other procedures or materials used in preparation of a sample may be equally as good and can be used on the basis
of availability and preference of individual laboratories.
X1.2 Method 1
X1.2.1 The porous samples should be free of oil or coolant. Remove any oil using Soxhlet extraction. Mount and vacuum
impregnate samples with epoxy resin, to fill porosity and to prevent the pickup of etchants. Use a sample cup or holder to form
the mount. Pour epoxy resin over the sample in the cup to a total depth of about 19 mm. Evacuate the cup to minus 88 kPa and
hold at that pressure for 10 min. Then restore ambient air pressure to force the resin into most of the sample. Cure at room
temperature or at 50 °C. 50°C.
X1.2.2 Grind on 240, 400, and 600 grit wet SiC paper, on a rotating wheel, and change the polishing direction 90° after each paper.
Etch samples for 1 min in their normal etchant, for example, 2 % nital, to begin to open the porosity. Rough polishing for 8 to 12
min total on 1 μm alumina (Al O ), long napped cloth (for example Struers felt cloth), at 250 rpm, and 300 gf load, using an
2 3
automated polisher opens smeared pores. This rough polishing opens and exaggerates the pores. To return the pores to their true
area fraction, polish for 4 min at 125 rpm on a shorter nap cloth (for example Struers MOL cloth), with 1 μm diamond paste. Final
polishing is done for 20 to 30 s using 0.05 μm deagglomerated alumina, and a long napped cloth (for example, Buehler
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B931 − 14 (2018)
Microcloth), at 125 rpm, and 75 gf load, on an automated polisher. Polishing may also be done by hand for the times indicated.
The first two polishings require moderate pressure and the final polish requires light pressure.
X1.2.3 The metallographic structure should be free of smeared porosity. Generally at 800 to 1000×, the edge of a smeared over
pore will appear as a thin gray line outlining one side of the pore, and occasionally outlining most of the pore.
X1.3 Method 2
X1.3.1 The specimen should be carefully selected so that it is representative of the region of interest. After selection, the specimen
may require sectioning to provide a workable specimen. Sectioning may be made employing an abrasive or diamond wheel.
X1.3.2 Heat should be avoided to prevent occurrence of possible changes in microstructure. If slow feeds are employed, a coolant
may not be necessary to avoid temperature buildups. If abrasive wheels are used, then a coolant is often necessary to avoid
overheating of the specimen.
X1.3.3 If a coolant is employed, it may be retained within the pores. The lubricant must be removed prior to the preparation of
the specimen for examination. This may be accomplished by using a Soxhlet extractor or an ultrasonic cleaner. The extraction
condenser is the most efficient and the least expensive.
X1.3.4 Generally, specimens to be evaluated for case depth are mounted to provide edge retention. There are many kinds of
mounting compounds available. Most common materials include epoxies (powder or liquid), diallyl phthalate, or Bakelite. Of
these, Bakelite is sometimes preferred because it is harder and therefore provides improved edge retention. Bakelite requires
equipment to apply heat and pressure, whereas the epoxies do not.
X1.3.5 After mounting, the specimen is ground to provide a flat, stress-free surface. A belt grinder is generally used first with care
to prevent heating of the specimen. Grit size is dependent on the preference of the metallographer, although finer grits are preferred.
X1.3.6 The specimen is then hand ground on four emery papers, generally of 240, 320, 400, and 600 grit.
X1.3.7 Etch samples for 1 min in their normal etchant, for example, 2 % nital, to begin to open the porosity.
X1.3.8 Wet polishing follows hand grinding and etching. Several polishing media are employed including diamond paste,
magnesia, alumina, etc. Grit size varies between 1 and 0.3 μm and is applied to nap-free cloths such as nylon. To remove remaining
scratches and stress,
...