Metrological Characterization of a Primary Vickers Hardness ...

MAPAN Metrological - Journal of Metrology Characterization Society of a India, Primary Vol. Vickers 25, No. Hardness 2, 2010; pp. Standard 1-7 Machine - NIS EgyptORIGINAL ARTICLEMetrological Characterization of a Primary VickersHardness Standard Machine - NIS EgyptG. MOHAMED 1 , M. IBRAHIM1, A.E. ABUELEZZ 1 , M. ADLY 2 and A. KHATAB 21National Institute of Standards, Egypt2Faculty of Engineering, Cairo University, Egypt[Received: 31.08.2009 ; Revised: 02.02.2010 ; Accepted: 03.02.2010]AbstractThe main goal of NIS is to provide traceability to international measurement standards to help theEgyptian industry to penetrate world markets. One of the most important fields of metrology ishardness measurements. A Primary Vickers Hardness Standard Machine (PVHM) was designed toimprove the capabilities to qualify the primary hardness test blocks locally. The machine has beenconstructed to perform Vickers hardness test for scales HV10, HV20, HV30, and HV50 with expandeduncertainty of ±1.4 % .This standard machine has been metrologically characterized by NIS. In thispaper metrological characterization of the machine will be shown. The characterization includes themethods and results of direct verification of influences parameters (test forces, indentation measuringsystem, indenter and test cycle time), and a comparison with PTB standard machine.1. IntroductionThe progress of engineering is greatly dependenton the development and correct use of newer andbetter materials. Thus, considerable attention hasbeen given to the mechanical properties of suchmaterials, which can be properly assessed. Theirbehavior in service could be obtained from the resultsof laboratory tests and experiments. The accuratematerials testing machines is an important andessential matter to the whole engineering andconstruction industries. In Egypt, the Force andMaterials Metrology Department (FMMD) of theNational Institute of Standards (NIS) has beenresponsible for this task. For many years, thedepartment has extended the routine hardness testservices and calibration of hardness testing machineto industrial organizations [1]. Direct contact withindustry together with the expanding reconstruction© Metrology Society of India, All rights reserved 2010.work taking place since the middle seventies showedthe urgent need for updating and extending thecapabilities of the hardness testing section of FMMDat NIS.NIS-PHVM machine is at the top of metrologicalchain where it has the best accuracy and uncertainty,so it is the national level. The primary hardnessreference blocks for the calibration laboratory levelcan be calibrated using the primary hardnessstandard machine. Naturally, direct calibration andthe verification of these machines should be at thehighest possible accuracy [2]. The expanded estimateduncertainty is ±1.4%. This reported expandeduncertainty of measurement is calculated followingthe standards and international documents and theyis stated as the standard uncertainty of measurementmultiplied by the appropriated coverage factorcorresponding to a coverage probability of 95%.1

G. Mohamed, M. Ibrahim, A.E. Abuelezz, M. Adly and A. Khatab2. NIS Primary Vickers Hardness StandardMachineThe PHVM was designed to achieve thedevelopment and modernization of the FMMD of theNIS. Intentions are to achieve two main features. Thefirst feature concerned with constructing primaryVickers standard machine to cover a range from HV10to HV50 through HV (10, 20, 30 and 50) which are themost common in Vickers hardness scale to improvethe calibration capabilities of FMMD for hardness.The second feature is the ability to develop themachine to cover the range of Vickers until HV120and also to improve the function of the machine in thefuture [1].The metrological characterization consists of thedirect verification of the machine and a comparisonwith PTB primary hardness standard machine; in thedirect verification, the main verified parameters arethe forces generated by the PVHM, the geometry ofindenters and indentation measuring system.2.1 The Main Structure of the MachineThe machine consists of dead weights for thegeneration of the test forces which can be selectedmanually, a very flexible control unit that permits toset all the most important parameters involved in thetest cycle (times and velocities) and separated imageprocessing system measurements [1].The main structure of the mechanical system ofthe machine includes;i) Load generation system (E): This system utilizedto generate the required load values to obtain thedesired thought four standard test loads HV (10,20, 30 and 50). This system includes the indenter'sframe and the weights.ii) Load and unload system (G): Testing weightsselection is designed to be manual. The mainparameter which aimed to be controlled is thetest time so it should be automated. The selectionof the weights can be performed manually usinga specified tools fixed on two column.iii) Machine fixed frame (M): It's a frame used tosupport the machine. It should be rigid enough toresist the buckling or deflection due to the appliedcompressive load the main fixed frame includesFig. 1. Photograph for the standardMGER$Fig. 2. Overall construction of the standard machine2

Metrological Characterization of a Primary Vickers Hardness Standard Machine - NIS Egypttwo platens connected by four tie bars and othertwo bars inserted to carry the pins used to selectthe desired weights manually. The fixed frameshould be adjusted by leveling nuts utilized toadjust the machine vertically (Figs. 1 and 2)3. Direct VerificationThe Vickers hardness value obtained by dividingthe test force by the contact area of indentation asreported earlier [3];á2× F × Sin( )FHV = 0.102× 2 = 0.1891 ×2 2ddSo it was noted that the main parameters thataffects the hardness test results could be the testingforce (F), the angle between the opposite faces at thevertex of pyramidal indenter (á), and the indentiondiagonal (d). These parameters can be classified intothree main sources;• Test force• Indenter geometry• Indentation measuring system.• Test cycle time which has been calibrated withstop watch.The relative uncertainty U tof the test cycle time is± 0.1 %.3.1 Force(1)The NIS PVHM generates forces by means of deadweights in the range from HV10 up to HV50. Toperform the verification, the anvil of PHVMs has beenremoved to create room to lodge the load cells.Two different load cells with an adequateuncertainty (class 00 of ISO 376:2005) have been usedfor the direct verification of the forces generated bythe PHVMs (Fig. 3). These load cells have beencalibrated at NIS before the verification [3] .The resultsof test force verification are shown in Fig. 4 andTable 1.whereuFHYTMSFi1= ×F nHere S Fiis the standard deviation of the test-forceindication values in the i-th height position.The combined relative standard uncertainty (u F) ofthe test force calibration is calculated as follows;u u 2 u2F FRS FHTMwhere u FRSis the relative uncertainty of measurementof the force transducer (from calibration certificateIncluding temperature dependence long-termstability, and interpolation deviation (± 0.05%);u FHTMis the relative standard uncertainty of the testforce generated by the hardness testing machine.The relative deviation of force is calculated fromthe following equation;F FFRSrelF(2)(3)(4)Fig. 3. A photograph of the machine calibration using load cell3

G. Mohamed, M. Ibrahim, A.E. Abuelezz, M. Adly and A. KhatabFig. 4. The test force accuracy calibration of the standard machineTable 1Test force calibration resultsNominal Series1 Series 2 Series 3 Mean Relativeu u FHTMload F1 F2 F3 F divisionF RSÄF relN N N N N % %98.07 98.18 98.15 98.01 98.11 -0.044 0.044196.1 196.15 196.19 196.07 196.13 0.002 0.015294.2 294.10 293.70 294.20 294.00 0.071 0.042490.3 490.13 490.10 490.15 490.13 0.045 0.002PVHM force calibration shows that the maximumrelative deviation of force is ± 0.07% and uncertaintyu Fof ± 0.16%.3.2 Vickers Diamond IndenterVickers diamond indenters have been calibratedby a DKD laboratory. All results fulfill the ISOspecifications [4]. The specification of the indentersused can be summarized as follows.• The vertex of the pyramidal indenter is 136 ± 0.1º,• The indenter should be certified and the certificatemust include the value of standard uncertainty(U Ind) not more than ± 0.1 %• The angle between the axis of the diamondpyramid and the axis of the indenter-holder(normal to the seating surface) should be less than0.3°.4

Metrological Characterization of a Primary Vickers Hardness Standard Machine - NIS Egypt3.3 Indentation Measuring System VerificationCamera Control Drive (CCD) image processingsystem was used for indentation measurements. Thissystem is provided with magnification lens and CCDcamera interfaced with image processing software(Fig. 5). The results of Indentation measuring systemverification are shown in Table 2.uLHYTMsLi1= ×L n(5)where s Liis the standard deviation of the lengthindication values for the i-th indication value of theobject micrometer.The combined relative standard uncertainty of thereference instrument for the measuring system iscalculated as follows [3]:u = u + u + u2 2L LRS ms LHTM(6)where u LRSis the relative uncertainty of measurementof the object micrometer (reference standard) from thecalibration certificate (± 0.3 %), u msis the relativeuncertainty of measurement due to the resolution ofthe measuring system, and u LHTMis the relativestandard uncertainty of measurement of the hardnesstesting machine.The relative deviation of indentation measuringsystem is calculated from the following equation;RS-Ä Lrel= L L(7)LThe calibration of image processing system isshowing uncertainty of ± 0.62 %3.4 The Expanded Uncertainty of the Machine byDirect VerificationThe combined uncertainty of the standardmachine u c[5] is then estimated as;2 2 2 2u = u + u + u + u = 0.7% (8)cF Ind L tThe relative expected uncertainty of the primaryVickers hardness testing U expcan be calculated fromthe following equation.U 2expexpu cU 2 0.7 1.4 %The relative expanded uncertainty of the primaryVickers hardness testing machine which had beenevaluated from direct verification was found to be ±1.4 %.(9)Fig. 5. A photograph of the indentation measuring system5

G. Mohamed, M. Ibrahim, A.E. Abuelezz, M. Adly and A. KhatabTable 2Measuring system uncertaintyNominal Series1 Series 2 Series 3 Mean Relative u FHTMu MSLRS F1 F2 F3 L divisionÄL relmm mm mm mm mm % % %0.050 0.0499 0.05000 0.0499 0.049933 -0.133 0.055 0.030.100 0.0999 0.09999 0.0999 0.099900 -0.100 0.000 0.030.150 0.1499 0.14995 0.14995 0.149950 -0.033 0.000 0.030.200 0.1999 0.19995 0.19997 0.199940 -0.030 0.009 0.030.250 0.2499 0.24993 0.24995 0.249943 -0.023 0.0023 0.030.300 0.2999 0.29997 0.29996 0.299970 -0.010 0.0016 0.030.350 0.3499 0.34996 0.34995 0.349953 -0.013 0.001 0.034. ComparisonThe machine was verified to ensure itsperformance using standard hardness test blockswhich have been tested on PTB primary Vickershardness testing machine. Two standard blocks wereused for this purpose. The applied load values were30 kgf and 10 kgf with hardness values of 832 HVand 394 HV and with uncertainty of ± 3.17 HV and ±2.33 HV respectively. Five indentations were done oneach of them and Vickers hardness values werecalculated. Finally, the coefficient E n[5] , whichevaluates the agreement between the measurementdeviations found in the comparison and theuncertainties stated by the participant it can becalculated from the following equation,XlabXref2U 2Uwhere X labis the measurement result of the establishedmachine, X refis the reference value calculated by theweighed mean value, U labis the uncertainty stated bythe established machine and U refis the uncertainty ofX refcalculated by the uncertainty of the weighed meanvalue. For the reference values only the measurementresults with E 1 are considered in Table 5.n5. ConclusionTable 3The results of testing 394 HV hardness test block• A new Primary Vickers hardness standard machinehas been constructed to improve NIS hardnessmeasurements capabilities.Test block 394 HV 10 394 U ref=±2.3HV10R1 R2 R3 R4 R5 AverageLH (mm) 0.218 0.212 0.218 0.214 0.219 0.216LV (mm) 0.217 0.217 0.213 0.218 0.216 0.216Mean 0.218 0.15 0.216 0.216 0.218 0.216F(N) 98,07 98,07 98,07 98,07 98,07 98,07HV 392 403 399 397 392 397Error % -0.53 2.22 1.31 0.85 -0.53 0.66Enlabref(10)6

Metrological Characterization of a Primary Vickers Hardness Standard Machine - NIS EgyptTable 4The results of testing 832 HV hardness test blockTest block 832 HV 10 832 U ref=± 3.2 HV 30R1 R2 R3 R4 R5 AverageLH (mm) 0.258 0.259 0.258 0.259 0.257 0.258LV (mm) 0.256 0.257 0.255 0.258 0.254 0.256Mean 0.257 0.258 0.256 0.259 0.256 0.257F(N) 294.2 294.2 294.2 294.2 294.2 294.2HV 842 836 845 832 852 841Error % 1.20 0.43 1.58 0.04 2.35 1.12Table 5The calculated values of E nTest blocks X labX refU labU refE n(HV) (HV)394 HV10 397 394 5.52 2.3 0.51832 HV 30 841 832 11.7 3.2 0.75• The direct verification of the constructed primaryVickers hardness testing machine shows that theexpanded uncertainty is ± 1.4 %.• the normalized error (E n) value is showing a goodagreement between the measurements carried outon the PTB primary Vickers hardness testingmachine and the measurements carried out onthe NIS primary Vickers hardness testingMachine,AcknowledgementThe authors are thankful to Dr. G. Aggag, Head,Force and Material Metrology Department, NIS, Egypt,for his valuable cooperation and help. Thanks are alsodue to Dr. K. Herrmann, PTB, Germany and Dr .M.Amer, NIS, Egypt for valuable suggestions andremarks.References[1] G. Mohamed, M. Ibrahim, A. Abu El-Ezzand A. Khatab, Proposed Design forPrimary Vickers Hardness StandardMachine. 1st Arab Conference onCalibration and Measurements November,Cairo, Egypt, (2007) .[2] EA-10/16, EA Guidelines on the Estimationof Uncertainty in HardnessMeasure-ments., European Co-operation.[3] ISO 6507-2: Metallic Materials-VickersHardness Test-Part 2: Verification andCalibration of the Testing Machine, (2005).[4] ISO 6507-3: Metallic Materials - VickersHardness Test - Part 3: Calibration ofReference Test Blocks, (2005).[5] K. Herrmann, Guidelines for theEvaluation of the Uncertainty of HardnessMeasurements, Physikalisch-TechnischeBundesanstalt Bundesallee [PTB]Braunschweig, Germany, MAPAN -Journal of Metrology Society of India, 20(2005) 5-13.7