1、Designation: B931 14Standard Test Method forMetallographically Estimating the Observed Case Depth ofFerrous Powder Metallurgy (PM) Parts1This standard is issued under the fixed designation B931; the number immediately following the designation indicates the year oforiginal adoption or, in the case o
2、f revision, the year of last revision. A number in parentheses indicates the year of last reapproval. Asuperscript epsilon () indicates an editorial change since the last revision or reapproval.1. Scope*1.1 Ametallographic method is described for estimating theobserved case depth of ferrous powder m
3、etallurgy (PM) parts.This method may be used for all types of hardened cases wherethere is a discernible difference between the microstructure ofthe hardened surface and that of the interior of the part.1.2 With the exception of the values for grit size for whichthe U.S. standard designation is the
4、industry standard, thevalues stated in SI units are to be regarded as standard.1.3 This standard does not purport to address all of thesafety concerns, if any, associated with its use. It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and dete
5、rmine the applica-bility of regulatory limitations prior to use.2. Referenced Documents2.1 ASTM Standards:2B243 Terminology of Powder MetallurgyE177 Practice for Use of the Terms Precision and Bias inASTM Test MethodsE407 Practice for Microetching Metals and Alloys3. Terminology3.1 DefinitionsDefini
6、tions of powder metallurgy (PM)terms can be found in Terminology B243. Additional descrip-tive information is available in the Related Material section ofVol 02.05 of the Annual Book of ASTM Standards.3.2 The metallographically estimated observed case depth isdefined as the distance from the surface
7、 of the part to the pointwhere, at a magnification of 100X, there is a discernibledifference in the microstucture of the material.4. Summary of Test Method4.1 The powder metallurgy part is sectioned and the surfaceprepared for metallographic evaluation. The metallographicspecimen is etched and the d
8、istance is measured from thesurface of the part to the point at which a discernible differencein the microstructure of the material is observed.5. Significance and Use5.1 The engineering function of many PM parts may requirean exterior portion of the part to have a hardened layer. Wherecase hardenin
9、g produces a distinct transition in themicrostructure, metallographic estimation of the observed casedepth may be used to check the depth to which the surface hasbeen hardened.6. Apparatus6.1 Equipment for the metallographic preparation of testspecimenssee Appendix X1.6.2 Metallographic Microscope,
10、permitting observation andmeasurement at a magnification of 100.7. Reagents and Materials7.1 Etchants such as 2 to 5 % nital, nital/picralcombinations, or other suitable etchants. For more informationon suitable etchants refer to Practice E407.8. Test Specimens8.1 Cut a test specimen from the PM par
11、t, perpendicular tothe hardened surface at a specified location, being careful toavoid any cutting or grinding procedure that would affect theoriginal microstructure.8.2 Mounting of the test specimen is recommended forconvenience in surface preparation and edge retention. Edgeretention is important
12、for proper measurement of the observedcase depth.9. Procedure9.1 Grind and polish the test specimen using methods suchas those summarized in Appendix X1.9.2 Etch the specimen with etchants such as 2 to 5 % nitalor nital/picral combinations.1This test method is under the jurisdiction of ASTM Committe
13、e B09 on MetalPowders and Metal Powder Products and is the direct responsibility of Subcom-mittee B09.05 on Structural Parts.Current edition approved Sept. 1, 2014. Published September 2014. Originallyapproved in 2003. Last previous edition approved in 2009 as B93109. DOI:10.1520/B0931-14.2For refer
14、enced ASTM standards, visit the ASTM website, www.astm.org, orcontact ASTM Customer Service at serviceastm.org. For Annual Book of ASTMStandards volume information, refer to the standards Document Summary page onthe ASTM website.*A Summary of Changes section appears at the end of this standardCopyri
15、ght ASTM International, 100 Barr Harbor Drive, PO Box C700, West Conshohocken, PA 19428-2959. United States19.2.1 Observed Case Depth:9.2.1.1 Examine the surface region of the part at a magni-fication of 100.9.2.1.2 Measure the distance from the surface of the part tothe point where there is a disce
16、rnible difference in the micro-structure of the material.NOTE 1The nature and amount of intermediate transformation prod-ucts will depend on the material being heat treated, its density, and thetype of surface hardening treatment being used. The sharpness of thechange in the microstructure at the po
17、int of transition will therefore vary.The microstructure expected at this transition point should be agreedbetween the producer and user of the part. Magnifications higher than100 may be used to check the microstructure of the part in the region ofthe transition zone. However, the metallographic est
18、imate of the observedcase depth shall be made at a magnification of 100.10. Report10.1 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, and10.1.4 The metallographically estimated observed casedepth
19、to the nearest 0.1 mm.11. Precision and Bias11.1 The precision of this test method is based on anintralaboratory study of ASTM B931, Standard Test Methodfor Metallographically Estimating the Observed Case Depth ofFerrous Powder Metallurgy (PM) Parts, conducted in 2013. Asingle laboratory participate
20、d in this study, testing two differentinduction-hardened PM parts. Every “test result” represents anindividual determination. The laboratory reported 40 replicatetest results for each of the materials. Except for the use of onlyone laboratory, Practice E691 was followed for the design andanalysis of
21、 the data; the details are given in ASTM ResearchReport No. B09-10213.11.1.1 Repeatability (r)The difference between repetitiveresults obtained by the same operator in a given laboratoryapplying the same test method with the same apparatus underconstant operating conditions on identical test materia
22、l withinshort intervals of time would in the long run, in the normal andcorrect operation of the test method, exceed the followingvalues only in one case in 20.11.1.1.1 Repeatability can be interpreted as maximum dif-ference between two results, obtained under repeatabilityconditions, which is accep
23、ted as plausible due to randomcauses under normal and correct operation of the test method.11.1.1.2 Repeatability limits are listed in Table.11.1.2 Reproducibility (R)The difference between twosingle and independent results obtained by different operatorsapplying the same test method in different la
24、boratories usingdifferent apparatus on identical test material would, in the longrun, 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 maximumdifference between two results, obtained under reprodu
25、cibilityconditions, which is accepted as plausible due to randomcauses under normal and correct operation of the test method.11.1.2.2 Reproducibility limits cannot be calculated from asingle laboratorys results. The reproducibility of this testmethod is being determined and will be available on or b
26、eforeDecember 2018.11.1.3 The above terms (“repeatability limit” and “repro-ducibility limit”) are used as specified in Practice E177.11.1.4 Any judgment in accordance with statement 11.1.1would normally have an approximate 95% probability of beingcorrect. The precision statistics obtained in this I
27、LS must not,however, be treated as exact mathematical quantities which areapplicable to all circumstances and uses. The limited numberof laboratories reporting replicate results essentially guaranteesthat there will be times when differences greater than predictedby the ILS results will arise, somet
28、imes with considerablygreater or smaller frequency than the 95% probability limitwould imply. Consider the repeatability limit as a generalguide, and the associated probability of 95% as only a roughindicator of what can be expected.11.2 BiasAt the time of the study, there was no acceptedreference m
29、aterial suitable for determining the bias for this testmethod, therefore no statement on bias is being made.11.3 The precision statement was determined through sta-tistical examination of 80 results, from a single laboratory, ontwo different PM parts described below:PM sprocket A: induction-hardened
30、 case depth of approxi-mately 900 mPM sprocket B: induction-hardened case depth of approxi-mately 500 m12. Measurement Uncertainty12.1 The precision of Test Method B931 shall be consideredby those performing the test when reporting metallographicallyestimated case depth results.13. Keywords13.1 case
31、 depth; observed case depth; PM; powder metal-lurgy3Supporting data have been filed at ASTM International Headquarters and maybe obtained by requesting Research Report RR:B09-1021. ContactASTM CustomerService at serviceastm.org.AverageARepeatabilityStandard Devia-tionRepeatabilityLimitxsrrSprocket A
32、 880 39.2 110Sprocket B 560 42.3 120AThe average of the laboratories calculated averages.B931 142APPENDIX(Nonmandatory Information)X1. SAMPLE PREPARATIONX1.1 The methods described in this appendix are provenpractices for metallographic preparation of porous PM materi-als. It is recognized that other
33、 procedures or materials used inpreparation of a sample may be equally as good and can beused on the basis of availability and preference of individuallaboratories.X1.2 Method 1X1.2.1 The porous samples should be free of oil or coolant.Remove any oil using Soxhlet extraction. Mount and vacuumimpregn
34、ate samples with epoxy resin, to fill porosity and toprevent the pickup of etchants. Use a sample cup or holder toform the mount. Pour epoxy resin over the sample in the cup toa total depth of about 19 mm. Evacuate the cup to minus88 kPa and hold at that pressure for 10 min. Then restoreambient air
35、pressure to force the resin into most of the sample.Cure at room temperature or at 50 C.X1.2.2 Grind on 240, 400, and 600 grit wet SiC paper, on arotating wheel, and change the polishing direction 90 aftereach paper. Etch samples for 1 min in their normal etchant, forexample, 2 % nital, to begin to
36、open the porosity. Roughpolishing for 8 to 12 min total on 1 m alumina (Al2O3), longnapped cloth (for example Struers felt cloth), at 250 rpm, and300 gf load, using an automated polisher opens smeared pores.This rough polishing opens and exaggerates the pores. Toreturn the pores to their true area f
37、raction, polish for 4 min at125 rpm on a shorter nap cloth (for example Struers MOLcloth), with 1 m diamond paste. Final polishing is done for 20to 30 s using 0.05 m deagglomerated alumina, and a longnapped cloth (for example, Buehler Microcloth), at 125 rpm,and 75 gf load, on an automated polisher.
38、 Polishing may alsobe done by hand for the times indicated. The first twopolishings require moderate pressure and the final polishrequires light pressure.X1.2.3 The metallographic structure should be free ofsmeared porosity. Generally at 800 to 1000, the edge of asmeared over pore will appear as a t
39、hin gray line outlining oneside of the pore, and occasionally outlining most of the pore.X1.3 Method 2X1.3.1 The specimen should be carefully selected so that itis representative of the region of interest. After selection, thespecimen may require sectioning to provide a workable speci-men. Sectionin
40、g may be made employing an abrasive ordiamond wheel.X1.3.2 Heat should be avoided to prevent occurrence ofpossible 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 toavoid
41、 overheating of the specimen.X1.3.3 If a coolant is employed, it may be retained withinthe pores. The lubricant must be removed prior to the prepa-ration of the specimen for examination. This may be accom-plished by using a Soxhlet extractor or an ultrasonic cleaner.The extraction condenser is the m
42、ost efficient and the leastexpensive.X1.3.4 Generally, specimens to be evaluated for case depthare mounted to provide edge retention.There are many kinds ofmounting compounds available. Most common materials in-clude epoxies (powder or liquid), diallyl phthalate, or Bakelite.Of these, Bakelite is so
43、metimes preferred because it is harderand therefore provides improved edge retention. Bakeliterequires equipment to apply heat and pressure, whereas theepoxies do not.X1.3.5 After mounting, the specimen is ground to provide aflat, stress-free surface. A belt grinder is generally used firstwith care
44、to prevent heating of the specimen. Grit size isdependent on the preference of the metallographer, althoughfiner grits are preferred.X1.3.6 The specimen is then hand ground on four emerypapers, generally of 240, 320, 400, and 600 grit.X1.3.7 Etch samples for 1 min in their normal etchant, forexample
45、, 2 % nital, to begin to open the porosity.X1.3.8 Wet polishing follows hand grinding and etching.Several polishing media are employed including diamondpaste, magnesia, alumina, etc. Grit size varies between 1 and0.3 m and is applied to nap-free cloths such as nylon. Toremove remaining scratches and
46、 stress, a soft cloth with finerpolishing compound is employed. Generally a short nappedcloth is preferred. A fine 0.5 m alumina is recommended. Forbest results, and to ensure complete freedom of pores fromworked metal, repeat the polishing and etching procedure.Final polishing generally requires 3
47、to 5 min.X1.3.9 Automated polishing equipment is also available.Automated polishing is accomplished by moving the specimenacross a polishing cloth in an abrasive solution undergoingvibrating action. Cloths and abrasives available are numerousand are generally selected by experience of the metallogra
48、pher.X1.4 Two additional schemes for the preparation of sinteredferrous materials, one manual and the other automated, arediscussed. The first method, basic manual preparation, hasmost likely been used to prepare more samples for metallo-graphic examination than any other single method. The as-sumpt
49、ion is made that the sample has been mounted andpre-ground to give a planar surface.Vacuum impregnation withan epoxy resin is recommended for samples to be used in casedepth measurement.X1.4.1 Basic Manual Sample PreparationX1.4.1.1 Grind samples using progressively finer abrasivepapers.(a) Routinely, 240, 320, 400, then 600 grit (U.S. Standarddesignation) SiC abrasive paper strips are used.B931 143(b) Lubricate and cool the sample with a continuous flowof water.(c) Rotate the sample 90 before proceeding to the nextpaper.(d) Clean the surface of the sample
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