Metrological Characterization of a Primary Vickers Hardness Standard Machine - NIS Egypt

Transcription

1 MAPAN Metrological - Journal of Metrology Characterization Society of a India, Primary Vol. Vickers 5, No. Hardness, 00; pp. Standard -7 Machine - NIS Egypt ORIGINAL ARTICLE Metrological Characterization of a Primary Vickers Hardness Standard Machine - NIS Egypt G. MOHAMED, M. IBRAHIM, A.E. ABUELEZZ, M. ADLY and A. KHATAB National Institute of Standards, Egypt aculty of Engineering, Cairo University, Egypt [Received: ; Revised: ; Accepted: ] Abstract The main goal of NIS is to provide traceability to international measurement standards to help the Egyptian industry to penetrate world markets. One of the most important fields of metrology is hardness measurements. A Primary Vickers Hardness Standard Machine (PVHM) was designed to improve the capabilities to qualify the primary hardness test blocks locally. The machine has been constructed to perform Vickers hardness test for scales HV0, HV0, HV30, and HV50 with expanded uncertainty of ±.4 %.This standard machine has been metrologically characterized by NIS. In this paper metrological characterization of the machine will be shown. The characterization includes the methods and results of direct verification of influences parameters (test forces, indentation measuring system, indenter and test cycle time), and a comparison with PTB standard machine.. Introduction The progress of engineering is greatly dependent on the development and correct use of newer and better materials. Thus, considerable attention has been given to the mechanical properties of such materials, which can be properly assessed. Their behavior in service could be obtained from the results of laboratory tests and experiments. The accurate materials testing machines is an important and essential matter to the whole engineering and construction industries. In Egypt, the orce and Materials Metrology Department (MMD) of the National Institute of Standards (NIS) has been responsible for this task. or many years, the department has extended the routine hardness test services and calibration of hardness testing machine to industrial organizations []. Direct contact with industry together with the expanding reconstruction Metrology Society of India, All rights reserved 00. work taking place since the middle seventies showed the urgent need for updating and extending the capabilities of the hardness testing section of MMD at NIS. NIS-PHVM machine is at the top of metrological chain where it has the best accuracy and uncertainty, so it is the national level. The primary hardness reference blocks for the calibration laboratory level can be calibrated using the primary hardness standard machine. Naturally, direct calibration and the verification of these machines should be at the highest possible accuracy []. The expanded estimated uncertainty is ±.4%. This reported expanded uncertainty of measurement is calculated following the standards and international documents and they is stated as the standard uncertainty of measurement multiplied by the appropriated coverage factor corresponding to a coverage probability of 95%.

2 G. Mohamed, M. Ibrahim, A.E. Abuelezz, M. Adly and A. Khatab. NIS Primary Vickers Hardness Standard Machine The PHVM was designed to achieve the development and modernization of the MMD of the NIS. Intentions are to achieve two main features. The first feature concerned with constructing primary Vickers standard machine to cover a range from HV0 to HV50 through HV (0, 0, 30 and 50) which are the most common in Vickers hardness scale to improve the calibration capabilities of MMD for hardness. The second feature is the ability to develop the machine to cover the range of Vickers until HV0 and also to improve the function of the machine in the future []. The metrological characterization consists of the direct verification of the machine and a comparison with PTB primary hardness standard machine; in the direct verification, the main verified parameters are the forces generated by the PVHM, the geometry of indenters and indentation measuring system.. The Main Structure of the Machine The machine consists of dead weights for the generation of the test forces which can be selected manually, a very flexible control unit that permits to set all the most important parameters involved in the test cycle (times and velocities) and separated image processing system measurements []. The main structure of the mechanical system of the machine includes; i) Load generation system (E): This system utilized to generate the required load values to obtain the desired thought four standard test loads HV (0, 0, 30 and 50). This system includes the indenter's frame and the weights. ii) Load and unload system (G): Testing weights selection is designed to be manual. The main parameter which aimed to be controlled is the test time so it should be automated. The selection of the weights can be performed manually using a specified tools fixed on two column. iii) Machine fixed frame (M): It's a frame used to support the machine. It should be rigid enough to resist the buckling or deflection due to the applied compressive load the main fixed frame includes ig.. Photograph for the standard M G E R $ ig.. Overall construction of the standard machine

3 Metrological Characterization of a Primary Vickers Hardness Standard Machine - NIS Egypt two platens connected by four tie bars and other two bars inserted to carry the pins used to select the desired weights manually. The fixed frame should be adjusted by leveling nuts utilized to adjust the machine vertically (igs. and ) 3. Direct Verification The Vickers hardness value obtained by dividing the test force by the contact area of indentation as reported earlier [3]; á Sin( ) HV = 0.0 = 0.89 d d So it was noted that the main parameters that affects the hardness test results could be the testing force (), the angle between the opposite faces at the vertex of pyramidal indenter (á), and the indention diagonal (d). These parameters can be classified into three main sources; Test force Indenter geometry Indentation measuring system. Test cycle time which has been calibrated with stop watch. The relative uncertainty U t of the test cycle time is ± 0. %. 3. orce () The NIS PVHM generates forces by means of dead weights in the range from HV0 up to HV50. To perform the verification, the anvil of PHVMs has been removed to create room to lodge the load cells. Two different load cells with an adequate uncertainty (class 00 of ISO 376:005) have been used for the direct verification of the forces generated by the PHVMs (ig. 3). These load cells have been calibrated at NIS before the verification [3].The results of test force verification are shown in ig. 4 and Table. where u HYTM Si = n Here S i is the standard deviation of the test-force indication values in the i-th height position. The combined relative standard uncertainty (u ) of the test force calibration is calculated as follows; u u u RS HTM where u RS is the relative uncertainty of measurement of the force transducer (from calibration certificate Including temperature dependence long-term stability, and interpolation deviation (± 0.05%); u HTM is the relative standard uncertainty of the test force generated by the hardness testing machine. The relative deviation of force is calculated from the following equation; RS rel () (3) (4) ig. 3. A photograph of the machine calibration using load cell 3

4 G. Mohamed, M. Ibrahim, A.E. Abuelezz, M. Adly and A. Khatab ig. 4. The test force accuracy calibration of the standard machine Table Test force calibration results Nominal Series Series Series 3 Mean Relativeu u HTM load 3 division RS Ä rel N N N N N % % PVHM force calibration shows that the maximum relative deviation of force is ± 0.07% and uncertainty u of ± 0.6%. 3. Vickers Diamond Indenter Vickers diamond indenters have been calibrated by a DKD laboratory. All results fulfill the ISO specifications [4]. The specification of the indenters used can be summarized as follows. The vertex of the pyramidal indenter is 36 ± 0.º, The indenter should be certified and the certificate must include the value of standard uncertainty (U Ind ) not more than ± 0. % The angle between the axis of the diamond pyramid and the axis of the indenter-holder (normal to the seating surface) should be less than

5 Metrological Characterization of a Primary Vickers Hardness Standard Machine - NIS Egypt 3.3 Indentation Measuring System Verification Camera Control Drive (CCD) image processing system was used for indentation measurements. This system is provided with magnification lens and CCD camera interfaced with image processing software (ig. 5). The results of Indentation measuring system verification are shown in Table. u LHYTM sli = L n (5) where s Li is the standard deviation of the length indication values for the i-th indication value of the object micrometer. The combined relative standard uncertainty of the reference instrument for the measuring system is calculated as follows [3]: u = u + u + u L LRS ms LHTM (6) where u LRS is the relative uncertainty of measurement of the object micrometer (reference standard) from the calibration certificate (± 0.3 %), u ms is the relative uncertainty of measurement due to the resolution of the measuring system, and u LHTM is the relative standard uncertainty of measurement of the hardness testing machine. The relative deviation of indentation measuring system is calculated from the following equation; RS - Ä Lrel = L L (7) L The calibration of image processing system is showing uncertainty of ± 0.6 % 3.4 The Expanded Uncertainty of the Machine by Direct Verification The combined uncertainty of the standard machine u c [5] is then estimated as; u = u + u + u + u = 0.7% (8) c Ind L t The relative expected uncertainty of the primary Vickers hardness testing U exp can be calculated from the following equation. U exp exp u c U % The relative expanded uncertainty of the primary Vickers hardness testing machine which had been evaluated from direct verification was found to be ±.4 %. (9) ig. 5. A photograph of the indentation measuring system 5

6 G. Mohamed, M. Ibrahim, A.E. Abuelezz, M. Adly and A. Khatab Table Measuring system uncertainty Nominal Series Series Series 3 Mean Relative u HTM u MS LRS 3 L division ÄL rel mm mm mm mm mm % % % Comparison The machine was verified to ensure its performance using standard hardness test blocks which have been tested on PTB primary Vickers hardness testing machine. Two standard blocks were used for this purpose. The applied load values were 30 kgf and 0 kgf with hardness values of 83 HV and 394 HV and with uncertainty of ± 3.7 HV and ±.33 HV respectively. ive indentations were done on each of them and Vickers hardness values were calculated. inally, the coefficient E n [5], which evaluates the agreement between the measurement deviations found in the comparison and the uncertainties stated by the participant it can be calculated from the following equation, Xlab X ref U U where X lab is the measurement result of the established machine, X ref is the reference value calculated by the weighed mean value, U lab is the uncertainty stated by the established machine and U ref is the uncertainty of X ref calculated by the uncertainty of the weighed mean value. or the reference values only the measurement results with E are considered in Table 5. n 5. Conclusion Table 3 The results of testing 394 HV hardness test block A new Primary Vickers hardness standard machine has been constructed to improve NIS hardness measurements capabilities. Test block 394 HV U ref =±.3HV0 R R R3 R4 R5 Average LH (mm) LV (mm) Mean (N) 98,07 98,07 98,07 98,07 98,07 98,07 HV Error % E n lab ref (0) 6

7 Metrological Characterization of a Primary Vickers Hardness Standard Machine - NIS Egypt Table 4 The results of testing 83 HV hardness test block Test block 83 HV 0 83 U ref =± 3. HV 30 R R R3 R4 R5 Average LH (mm) LV (mm) Mean (N) HV Error % Table 5 The calculated values of E n Test blocks X lab X ref U lab U ref E n (HV) (HV) 394 HV HV The direct verification of the constructed primary Vickers hardness testing machine shows that the expanded uncertainty is ±.4 %. the normalized error (E n ) value is showing a good agreement between the measurements carried out on the PTB primary Vickers hardness testing machine and the measurements carried out on the NIS primary Vickers hardness testing Machine, Acknowledgement The authors are thankful to Dr. G. Aggag, Head, orce and Material Metrology Department, NIS, Egypt, for his valuable cooperation and help. Thanks are also due to Dr. K. Herrmann, PTB, Germany and Dr.M. Amer, NIS, Egypt for valuable suggestions and remarks. References [] G. Mohamed, M. Ibrahim, A. Abu El-Ezz and A. Khatab, Proposed Design for Primary Vickers Hardness Standard Machine. st Arab Conference on Calibration and Measurements November, Cairo, Egypt, (007). [] EA-0/6, EA Guidelines on the Estimation of Uncertainty in Hardness Measure-ments., European Co-operation. [3] ISO 6507-: Metallic Materials-Vickers Hardness Test-Part : Verification and Calibration of the Testing Machine, (005). [4] ISO : Metallic Materials - Vickers Hardness Test - Part 3: Calibration of Reference Test Blocks, (005). [5] K. Herrmann, Guidelines for the Evaluation of the Uncertainty of Hardness Measurements, Physikalisch-Technische Bundesanstalt Bundesallee [PTB] Braunschweig, Germany, MAPAN - Journal of Metrology Society of India, 0 (005)