ASTM_B_721_-_91_1999.pdf

Designation:B 721 91(Reapproved 1999)Standard Test Method forMicrohardness and Case Depth of Powder Metallurgy(P/M)Parts1This standard is issued under the fixed designation B 721;the number immediately following the designation indicates the year oforiginal adoption or,in the case of revision,the year of last revision.A number in parentheses indicates the year of last reapproval.Asuperscript epsilon(e)indicates an editorial change since the last revision or reapproval.1.Scope1.1 This test method covers determination of the microhard-ness of powder metallurgy(P/M)parts and applications ofmicrohardness test results to methods for determination of thecase depth.Technique for measurement of case depth of P/Mparts by observation is also outlined.1.2 Part A:Microhardness MeasurementThis procedurecovers test methods to determine the microhardness of P/Mparts with the Knoop(HK)or the Vickers(HV)indenters.Procedures for surface preparation of the P/M material prior tomicrohardness measurement are included.1.3 Part B:Case Depth MeasurementProcedures andmethods for determination of both effective case depth andobserved case depth for P/M parts are included.The principlesof Part A on Microhardness Measurement are utilized tomeasure case depth.1.4 The values stated in inch-pound units are to be regardedas the standard.The values in parentheses are for informationonly.1.5 This standard does not purport to address the safetyconcerns,if any,associated with its use.It is the responsibilityof the user of this standard to establish appropriate safety andhealth practices and determine the applicability of regulatorylimitations prior to use.2.Referenced Documents2.1 ASTM Standards:B 243 Terminology of Powder Metallurgy2E 384 Test Method for Microhardness of Materials33.Terminology3.1 Definitions of powder metallurgy(P/M)terms can befound in Terminology B 243.Additional descriptive informa-tion is available in the Related Material section of Vol.02.05 ofthe Annual Book of ASTM Standards.4.Summary of Test Method4.1 Part A:Microhardness is measured by using a calibratedmachine to force a diamond indenter of specific geometry,under a known test load,into the surface of the test material.4.1.1 Ordinarily,the impression of the indenter is measuredoptically and correlated with available tables to obtain a valuein the desired hardness scale;or,the optical measurement canbe used to calculate a hardness number.NOTE1This test method is designed specifically for use on P/M parts.It is intended to be a companion to Test Method E 384.There are specificdifferences that are intentional;otherwise the details on equipment andprocedures in Test Method E 384 shall be adhered to.4.1.2 A new technique,on direct HRC equivalent,is rec-ommended as part of the test method.It involves constructinga plot of microhardness equivalent(HRC)versus the measureddiagonal length of the impression made in the material by theindenter.This master plot can then be used to obtain an HRCequivalent directly from the optical measurement.4.2 Part B:Case Depth is defined as the distance from thesurface of a part to a point below the surface where:4.2.1 There is a drop in hardness below some prescribedlevel,4.2.2 There is a divergence from a linear decrease inhardness as a function of the distance from the surface,or4.2.3 There is an easily seen transition in metallurgicalstructure.5.Significance and Use5.1 Especially for P/M materials,microhardness is usefulfor determination of the actual hardness of the metal matrix.Also,in most metallic materials,microhardness can be used todifferentiate between metallurgical phases and non-metallicinclusions.Of particular interest,microhardness tests can beused to determine the actual hardness of the case in surfacehardened materials and to help define the useful thickness ofsuch cases.5.2 Cases,hardened layers,are used on P/M parts and metalparts produced by other methods to provide required propertieseconomically.Proper engineering function of the case requiresproper hardness and a specified thickness.6.Test Specimens6.1 Specimen Mounting:6.1.1 Mounting is recommended for convenience in surfacepreparation,edge retention,and testing.The specimen shouldbe adequately supported in the mounting medium.1This test method is under the jurisdiction of ASTM Committee B-9 on MetalPowders and Metal Powder Productsand is the direct responsibility of Subcommit-tee B09.05on Structural Parts.Current edition approved Feb.22,1991.Published May 1991.2Annual Book of ASTM Standards,Vol 02.05.3Annual Book of ASTM Standards,Vol 03.01.1Copyright ASTM,100 Barr Harbor Drive,West Conshohocken,PA 19428-2959,United States.6.1.2 Edge retention is important to proper depth measure-ment of the case.The mounting material must be selected toprovide good edge retention and sufficient rigidity so that nomovement of the specimen can occur during the application ofload.6.1.3 At sample densities below 6.6 g/cm3it is advisable tovacuum impregnate the specimen with a suitable