Effects of differences in hardness measurement

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Effects of differences in hardness measurement procedures on the traceability chain and calibration process

This article has been downloaded from IOPscience. Please scroll down to see the full text article. 2013 Metrologia 50 249 (http://iopscience.iop.org/0026-1394/50/3/249) View the table of contents for this issue, or go to the journal homepage for more

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IOP PUBLISHING

METROLOGIA

Metrologia 50 (2013) 249–256

doi:10.1088/0026-1394/50/3/249

Effects of differences in hardness measurement procedures on the traceability chain and calibration process Nae Hyung Tak, Hae Moo Lee and Gunwoong Bahng Center for Energy Materials Metrology, Division of Industrial Metrology, KRISS, Daejeon, 305-600, Korea

Received 4 February 2013, in final form 10 April 2013 Published 13 May 2013 Online at stacks.iop.org/Met/50/249 Abstract Establishment of traceability in hardness measurement is still under discussion at the ISO TC 164 SC 3 subcommittee on hardness testing. Two paths for establishing traceability are proposed for the Rockwell hardness measurement in ISO 6508-1. One way is taking a route to SI units through direct calibration of machine components, and the other way is taking a route to a hardness measurement standard through indirect calibration of machine performance. In addition to this, the difference in hardness measurement procedures between Rockwell and Brinell/Vickers hardness causes further confusion in establishing a traceability chain and an uncertainty evaluation process. This confusion is partly caused by the characteristics of hardness value, i.e. a procedure-dependent property. In this paper, methods used for the establishment of traceability for Rockwell, Brinell and Vickers hardness measurements are discussed based on the concept defined in the VIM.

1. Introduction Hardness is one of the most widely used material properties for quality control of heat-treated machine components and structural materials. Because of its simple and quick test process, its application is currently expanding with the development of new methods such as Leeb hardness and instrumented indentation test. However, still there is confusion over the way of establishing traceability as well as uncertainty evaluation in hardness measurement. More precisely, it was initiated with the publication of the EA guideline about the method of uncertainty evaluation in hardness measurement [1]. In this approach, the uncertainty of a hardness machine is evaluated by combining all of the uncertainties of the machine components. This approach is known as direct calibration. Since the machine performance could not be guaranteed even after the calibration of machine components, hardness standard blocks are used to check the overall machine performance after the calibration of machine components. This process is called indirect calibration or indirect verification. With the introduction of the concept of traceability and uncertainty in the 1990s, there was a discussion about which way is the right path for the establishment of traceability 0026-1394/13/030249+08$33.00

in hardness measurement and uncertainty evaluation [2]. Unfortunately, there has been no definitive conclusion until now and both traceability paths are introduced in ISO 6508-1 for the Rockwell hardness test method [3]. In addition to this, differences in hardness measurement procedures cause further confusion. For the Rockwell hardness measurement, force application and measurement of indentation depth are performed at the same time. However, for the Brinell and Vickers hardness measurement, force application to make an indentation on the surface of the specimen and the measurement of the diameter or diagonal length of indentation are accomplished separately. Therefore, the traceability chain and the uncertainty evaluation process should be different. However, this difference in the measurement procedure is not considered in the calibration process described in those relevant ISO standards [4, 5]. In this paper, it will be suggested that the EA guideline for the uncertainty evaluation of a hardness testing machine, the so-called direct calibration, is applicable for the NMI level only for the realization of hardness measurement standards and the so-called indirect calibration is the right path for traceability regarding hardness values when the measurement standard of hardness is available from NMIs. The EA guideline may be adopted by the calibration laboratory when

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Table 1. Examples of intrinsic properties and procedural properties of materials. Intrinsic properties Procedural properties

Young’s modulus, shear modulus, Poisson’s ratio, sound velocity, thermal conductivity, thermal diffusivity, emissivity, electrical resistivity, electrical conductivity, optical transmissivity, etc Ductility, yield strength, tensile strength, hardness, friction coefficient, fatigue, creep rate, corrosion rate, diffusion rate, fracture toughness, etc

the measurement standard is not available from NMIs. Also for the uncertainty evaluation, the difference in the measurement procedure among hardness measurement methods, whether the force application and indent depth or diameter/diagonal measurement are separated or not, should be considered in the evaluation of uncertainty and calibration process.

2. Material property and traceability Material properties can be grouped into two areas [6]. One is intrinsic properties and the other is procedural properties. Intrinsic properties, such as the elastic constant of materials, heat conductivity, etc, are procedure-independent. For example, to measure Young’s modulus two methods are available. One method is calculating Young’s modulus from the slope of the stress–strain curve. The other is obtaining Young’s modulus from the ratio of ultrasonic sound velocities in materials. Even if the principles of the methods are totally different, the results are comparable. In contrast to this, procedural properties such as strength and hardness are procedure-dependent. In other words, if the measurement procedure changes, then the result also changes and there is no compatibility between the data obtained with different procedures. Most of the important engineering properties are procedural properties. Hardness is also a procedural property as can be known from its unique units such as HRC, HRA, HBW, HV, etc. This is the main reason why the hardness values obtained with different procedures are not comparable to each other. Table 1 shows the list of material properties of both groups. According to the VIM, a quantity is defined as the ‘property of a phenomenon, body, or substance, where the property has a magnitude that can be expressed as a number and a reference’ [7]. In this explanation, the definition of reference is worth noting. In note 2 of this clause, ‘a reference can be a measurement unit, a measurement procedure, a reference material, or a combination of such’. Therefore, procedure-dependent properties of materials can be grouped into the second category of quantity, which is expressed by a combination of number and reference procedure. According to this definition of quantity, the so-called hardness units, such as HRC, HRA, HV, HRB, etc, are not a unit but a symbol of procedure, if we strictly apply the definition. To avoid any mistake of comparing hardness data obtained with different procedures, a unique unit and name symbolizing hardness measurement procedure was introduced, e.g. HRC, HBW, HV, etc. In this sense, the unit which is used for the expression of material strength, MPa, may wrongly suggest 250

that it is an intrinsic property even though it is also a proceduredependent property [8]. The next question is then how to establish traceability through an unbroken calibration chain regarding the procedure-dependent properties. According to clause 2.41 of the VIM, metrological traceability is defined as a ‘property of a measurement result whereby the result can be related to a reference through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty’ [7]. Note that the measurement result should be related to a ‘reference’. Further explanation can be found in note 1 of this clause saying that ‘reference’ can be a definition of a unit or a measurement procedure. As a matter of fact, the reference should be a reference procedure for proceduredependent material properties. To produce a measurement result traceable to the measurement standard, a calibration hierarchy should be established. Considering this fact, it can be understood that a primary reference procedure with lower uncertainty is necessary to set up a calibration hierarchy for proceduredependent properties. Specifically for hardness, the Working Group on Hardness under the Consultative Committee for Mass and Related Quantities (CCM) is now working to develop a primary reference procedure, and it has been proposed only for the Rockwell hardness measurement [9]. According to 5.1 of the VIM, a measurement standard is defined as ‘realization of the definition of a given quantity, with stated quantity value and associated measurement uncertainty, used as a reference’ [7]. Therefore, the hardness testing machine which can realize the primary reference procedure with lower uncertainty becomes the measurement standard of hardness to be used as a reference and can be called a primary hardness standard machine. For the evaluation of uncertainty of the primary hardness standard machine, which lies at the top of the traceability system in hardness as a measurement standard, a process described in the EA guideline should be applied. Uncertainty of each machine component in SI units is evaluated and converted into the hardness value by multiplying by the sensitivity coefficient. All of the uncertainties of machine components are combined and the uncertainty of the primary hardness machine in hardness value can be figured out. The EA guideline is applicable to NMIs only who are in charge of establishing the hardness measurement standard. This guideline may be adopted by a calibration laboratory for the uncertainty evaluation of the hardness calibration machine when the measurement standard is not available from NMIs. Because the EA guideline was published earlier, even before the concept of traceability in hardness was clearly established, it was misunderstood that this process is applicable to any hardness machine without regarding whether the measurement standard of hardness is available or not. In addition to this, since the uncertainty evaluation process in this EA guideline is based on the combination of uncertainties of machine components by converting the SI units into the hardness value using the sensitivity coefficient, it was misunderstood that the hardness value is traceable to SI units. Metrologia, 50 (2013) 249–256


Figure 1. Structure of the metrological chain for the dissemination of Rockwell hardness scales appeared in ISO 6508-1.

As has been discussed above, the hardness value is traceable to the hardness measurement standard realized by NMIs.

3. Calibration hierarchy in hardness value To establish traceability in hardness value, an unbroken chain of calibration including uncertainty evaluation in hardness units is necessary. This means that the calibration of a hardness machine in hardness units is the key element in a calibration hierarchy. In this sense, the so-called indirect calibration with reference block in hardness units should be regarded as a calibration process. The so-called direct calibration of force and length is actually a verification of machine components, which is necessary before the calibration is performed. Although this verification process is a requisite for the calibration, it cannot be called calibration since the result of verification is not included in the evaluation of uncertainty of the hardness machine. Specifically, verification of machine components is checking about the performance of load cell, depth measuring device, timer, etc, after their calibration in SI units and these results do not contribute to the uncertainty evaluation. They are used only for making a decision whether they satisfy the requirements or not, which are specified in the relevant ISO standards. According to note 1 affiliated to the definition of verification in the VIM, measurement uncertainty should be taken into consideration for the verification. This is the reason why calibration of machine components is necessary for verification but it is only for the evaluation of uncertainty of those components to make a decision whether the machine components satisfy the requirements or not. After verification, the overall performance of the machine is evaluated using reference blocks and this is the process of calibration in hardness units. The unbroken chain of Metrologia, 50 (2013) 249–256

calibration in hardness value is connected via hardness reference blocks with a specified value and uncertainty in hardness units. This is called an indirect calibration or indirect verification, as is shown in figure 1 without a clear definition of it even though it is the calibration process itself. Figure 2 shows the revised structure of the metrological chain for the dissemination of the Rockwell hardness scale based on the calibration hierarchy proposed in this paper. The measurement standard of the Rockwell hardness is realized by the primary hardness standard machine with the primary reference procedure developed by CCM-WGH. Using this primary hardness standard machine, a primary hardness reference block with hardness reference value and uncertainty can be produced. This primary hardness reference block is applied for the calibration of the hardness measurement machine of a calibration laboratory. The machine of the calibration laboratory is used for the production of secondary reference blocks to be used for the calibration of the working level hardness machine. When the hardness calibration machine of the calibration laboratory is calibrated with the primary hardness reference blocks, the uncertainty of the hardness calibration machine, UHCM , is evaluated as is shown in equation (1) by combining uncertainties from the measurements and the blocks. UHCM = k · u2crm + u2x¯ + u2r (1) where ucrm , ux¯ and ur are the uncertainties from the primary hardness reference block, measurement results on the primary reference block and the resolution of the measuring machine, respectively. Equation (G.2) that appeared in ISO 6508-1, for the evaluation of uncertainty of the Rockwell hardness testing machine, is basically the same as that described here. Once the uncertainty of the hardness calibration machine is evaluated through calibration with primary reference blocks, 251

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Figure 2. Revised structure of the metrological chain for the dissemination of Rockwell hardness scales. The box for the primary hardness reference block is moved one step higher since it is an action being carried out at the NMI level. Arrows with thick lines indicate the traceability chain in hardness value. Note that the description in figure 1, indirect verification and direct verification, is removed.

the uncertainty of the secondary reference block, USCRM , can be calculated by equation (2) as shown below: USCRM = k · u2HCM + u2x¯ + u2r (2) where uHCM , ux¯ and ur are the uncertainties from the hardness calibration machine, measurement results on the blank secondary reference block and the resolution of the measuring machine, respectively. USCRM becomes the uncertainty of the secondary hardness reference block and will be recorded in the certification of the secondary reference block produced by the calibration laboratory. Equation (G.1) that appeared in ISO 6508-1 for the evaluation of uncertainty of single hardness measurement was derived based on the same concept. Even though the components of a force application machine, such as force, alignment, ball diameter, ball sphericity, etc, are verified and confirmed that they satisfy the requirements, there is a possibility that the measurement result may show biased data when it is measured with the reference block for verification of the overall performance of the force application machine. In this case, this bias may be included in the uncertainty evaluation or the data may be adjusted, as is recommended in the Rockwell hardness measurement. The same process is applicable for the evaluation of uncertainty of hardness measurement in the field. Equation (1) is applied for the uncertainty evaluation of the hardness testing machine using a secondary hardness reference block supplied by the calibration laboratory. Equation (2) is applied for the uncertainty evaluation of hardness measurement results obtained by the working level hardness machine. Many of the quantities measured by clinical laboratories are also procedure-dependent. The traceability system is quite similar with the hardness value as can be found in ISO 15189 and ISO 17511 [10, 11]. For the Rockwell hardness machine, the traceability can be understood straightforwardly since the force application and hardness value reading are performed simultaneously. 252

However, for the Brinell and Vickers hardness testing, indentation by force application is followed by indent measurement separately. This may cause confusion in the traceability path compared with the case of the Rockwell hardness. To establish traceability in the Brinell hardness case, the first step will be the development of a primary reference procedure with lower uncertainty, as is the case for the Rockwell hardness. This is the role of CCM-WGH and it is under discussion to develop a primary reference procedure for the Brinell and Vickers hardness. After the primary reference procedure becomes available, NMIs should develop a primary reference machine that can realize the primary reference procedure. Using this primary hardness machine, including the force application machine and the diameter measuring device, the measurement standard of the Brinell hardness is realized and NMIs can assign a hardness value and uncertainty to the primary hardness reference block in hardness units, as is the case for the Rockwell hardness. However, the force application machine and the diameter measuring device are independently operated and hence two kinds of reference blocks are necessary. The first one is a block having a reference indent, which will be used for the calibration and uncertainty evaluation of the diameter measuring device in the hardness unit, HBW or HV. The second one is a block with a reference value in the hardness unit and this will be used for the verification of the total performance of the force application machine. Uncertainty of the secondary reference blocks, produced by the calibration laboratory, should be evaluated by combining the uncertainty of the diameter measuring device and the uncertainty of measurements made on the blank secondary reference blocks. The uncertainty of the diameter measuring device is evaluated using the primary reference indent supplied by NMIs. The equations for uncertainty evaluation are the same as in the case for the Rockwell hardness measurement except for the fact that the uncertainty of the diameter measuring device is the main concern rather than Metrologia, 50 (2013) 249–256


Figure 3. Revised traceability chain for the Brinell hardness value based on the path for the dissemination of hardness standard by a reference indent. The reference indent is used for the calibration of the diameter measuring device.

Figure 4. Flow chart of activities required for the national metrology institute level.

the force application machine. As a result, the traceability path, which is in ISO 6506-1, should be modified. Basically, the traceability path in ISO 6506-1 is the same as shown in figure 1 for the Rockwell hardness measurement; however, the traceability path for the Brinell and Vickers hardness value should be defined carefully considering the fact that the force application machine and the indent diameter measuring device are independently operated one after the other. Figure 3 shows the new proposed traceability path for the Brinell hardness value focused on diameter measurement considering the difference in the measurement procedure between Rockwell and Brinell hardness. Figures 4, 5 and 6 are diagrams showing the activities of a national metrology laboratory, a calibration laboratory and a working level laboratory, respectively, for the dissemination of Brinell hardness standards. Table 2 shows the process of detailed verification and calibration process for the calibration laboratory based on the Metrologia, 50 (2013) 249–256

new proposal for the establishment of traceability in the Brinell hardness measurement. The calibration laboratory should calibrate the diameter measuring device using the primary reference indent supplied by NMIs in hardness units, not in SI units. Note that the diameter measurement device should be verified using the conventional standard scale or object micrometer before calibration. Through this process, the uncertainty of the diameter measuring device in hardness units can be evaluated. The uncertainty evaluation in hardness units is performed only for the diameter measuring device using the primary reference indent made on the primary reference block. In other words, the traceability chain of hardness values is connected via the reference indent on the reference blocks. The second step of the process for the calibration laboratory is verification of the force application machine components followed by performance verification of the 253

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Figure 5. Flow chart of activities required for the calibration laboratory.

Figure 6. Flow chart of activities required for the working level laboratory.

machine using reference blocks supplied by NMIs in hardness units. The verification of the force application machine is completed when the diameter of the indent made by that machine is measured with the calibrated diameter measuring device. According to the calibration process that appeared in ISO 6506-2 for verification and calibration of a Brinell hardness testing machine, the diameter measuring device is calibrated 254

using a length standard such as an object micrometer or standard scale [12]. Since the calibration hierarchy should be established based on the hardness value, this process should be regarded as a verification of the measuring device before calibration. If we follow the procedure in ISO 65062, the actual traceability based on hardness units cannot be established since calibration of the diameter measuring device in hardness units is missing. In addition to this, duplication of Metrologia, 50 (2013) 249–256


Table 2. Verification and calibration process for the calibration laboratory to produce secondary hardness reference blocks to be used at the working level. Order of action

Object

Tool

① Verification using the standard scale

Diameter measurement Standard scale device ② Calibration using primary reference indent and Diameter measurement Primary reference indent supplied by NMI uncertainty evaluation device Calibration of diameter measurement device is completed and it is ready for the measurement of indent diameter ③ Verification of machine components and testing Force application Force measurement tool, ball diameter measurement cycles machine equipment, testing cycle measurement system ④ Indentation on primary reference blocks for the Force application Primary reference blocks with values and verification of the total performance of force machine uncertainties in hardness units supplied by NMI application machine ⑤ Verification of the performance of force Force application Diameter measuring device calibrated by step ② is application machine machine used for this process Force application machine is ready for indentation of candidate secondary reference blocks to be calibrated by diameter measuring device ⑥ Indentation of secondary reference blocks

Force application machine verified by step ⑤ is used. One of the indents should be defined as a reference indent to be used for the calibration of the diameter measuring device at the working level ⑦ Calibration of the blocks indented by the force Indented secondary Diameter measuring device calibrated by step ② is application machine reference blocks used for this process. Calibration on the hardness block as well as reference indent should be carried out Secondary reference blocks with hardness value and uncertainty as well as reference indent are produced for application at the working level Candidate secondary reference blocks

uncertainty from the force application machine is unavoidable in the final uncertainty evaluation. Currently, reference blocks with a reference indent are commercially available, but they are used just for the confirmation of the diameter measuring process of the operator. Considering the fact that most of the uncertainties in the Brinell hardness measurement come from the diameter measurement, as was revealed by the key comparison on the Brinell hardness measurement, introducing the reference indent into the right position in the traceability chain is very important to reduce the scattering of data [13]. Considering the importance of the reference indent, it is recommended in ISO 6506 to identify a reference indent among the five indents at least which are made for the evaluation of the hardness of blocks with specified information on the location and direction of measurement [12]. However, this reference indent is not for the establishment of traceability in hardness units but just for the correction of the operator’s skill in diameter measurement. For the blocks with a reference indent, the traceability can be established through the calibration of the diameter measuring device using the reference indent and the rest of the surface area of the blocks is used for the verification of the overall performance of the force application machine. Commercially, a Brinell hardness machine with an automatic measuring system is available. Even in this case, it is desirable to use reference indentation supplied by NMIs with an uncertainty value; however, this is not available at this time as far as we know. For the time being until the measurement standards of Brinell and Vickers hardness are established by NMIs participating in CCM-WGH, we think that either direct calibration on the measuring device or calibration using a Metrologia, 50 (2013) 249–256

reference indent supplied with the automatic measuring system may be adopted. In either case, it is recommended to indicate how the measuring system was verified or calibrated. The calibration and verification process for the Vickers hardness is the same as that for the Brinell hardness. A reference block with a reference indent is necessary for the calibration of diagonal measuring equipment. Currently, it is calibrated using the standard scale, as is specified in ISO 6507. In this case, the calibration hierarchy as well as traceability cannot be established. It is recommended that the ISO standards of Brinell and Vickers hardness be amended about the traceability, uncertainty evaluation and calibration hierarchy according to the definition in the VIM. The same verification and calibration process is applicable for the working level hardness machine as is applied for the calibration laboratory level hardness machine.

4. Conclusion Procedure-dependent properties of materials should be treated consistently under the international metrological structure. Since these are quantities expressed by number and procedure, proper description of the calibration chain and uncertainty evaluation is important to guarantee the comparability of the measured data. The measurement standard of hardness is realized by the NMIs based on a primary reference procedure and a primary reference machine with lower uncertainty. This measurement standard is located at the top level of the hardness calibration hierarchy and the traceability of the measured hardness value obtained by industry ends at this point. However, it should be noticed that this does not mean 255

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that hardness is fully independent of SI units. To realize the measurement standard of hardness, calibration of machine components traceable to SI units is essential. In other words, traceability in hardness units is only a part of the overall scheme of the metrological structure of hardness measurement. Basically, the metrological structure including the traceability chain is the same irrespective of the hardness measurement procedure as long as it is a procedure-dependent property. However, careful description of the traceability and calibration hierarchy is necessary to obtain reliable and comparable data, especially for the case where the force application process and the indent measurement process are not carried out simultaneously, as is the case for the Brinell and Vickers hardness measurement. To avoid any possible misunderstanding of the calibration and verification, the terminology of calibration should be applied for the process related to uncertainty evaluation in hardness units only even though the verification process is essential and a part of the overall calibration activity. In this way, the reliability and comparability of hardness data can be guaranteed based on firm metrological grounds. If the measurement standard is not established and a primary reference block is not available from the NMIs, the calibration laboratory may evaluate the uncertainty of their hardness calibration machine according to the EA guideline. The only problem is that comparability of data obtained by the end users may not be guaranteed in this case since several reference block producers may supply with different levels of hardness value and the uncertainty depends on the performance of the hardness calibration machine they use. To overcome this possible problem, we think that it should be recommended in the ISO standard to indicate the reference blocks which were used for the calibration of the hardness testing machine in the final test report. Traceability on the impact property, which is one of the many procedure-dependent material properties, can be established following the same process discussed in this paper. International cooperation is recommended to establish

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a primary reference procedure for the impact test as well as production of primary reference materials. It is also recommended that those testing methods for proceduredependent material properties defined in the related ISO standards should be amended according to the concept of traceability and calibration described in this paper and the VIM.

References [1] European Co-operation for Accreditation 2001 EA guidelines on the estimation of uncertainty in hardness measurements EA-10/16rev.00 [2] Germak A, Herrmann K and Low S 2010 Traceability in hardness measurements: from the definition to industry Metrologia 47 S59–66 [3] ISO 6508-1 2005 Metallic Materials—Rockwell Hardness Test—Part 1: Test Method [4] ISO 6506-1 2005 Metallic Materials—Brinell Hardness Test—Part 1: Test Method [5] ISO 6507-1 2005 Metallic Materials—Vickers Hardness Test—Part 1: Test Method [6] Munro R G 2003 Data Evaluation Theory and Practice for Materials Properties NIST Special Publication 960-11, (Washington, DC: NIST) [7] JCGM 2012 International Vocabulary of Metrology—Basic and General Concepts and Associated Terms JCGM 200:2012 [8] Bahng G W et al 2012 Establishment of traceability in measurement of mechanical properties of materials Metrologia 47 S32–40 [9] CCM-WGH/11-01, Rockwell C hardness scale (HRC) definition, BIPM website, www.bipm.org/wg/ AllowedDocuments.jsp?wg=CCM-WGH [10] ISO 15189 2003 Medical Laboratories—Particular Requirements For Quality And Competence [11] ISO 17511 2003 In Vitro Diagnostic Medical Devices—Measurement of Quantities in Biological Samples—Metrological Traceability of Values Assigned to Calibrators and Control Materials [12] ISO 6506-2 2005 Metallic Materials—Brinell Hardness Test—Part 2: Verification and Calibration of Testing Machines [13] 2012 Brinell hardness key comparison Preliminary Report

Metrologia, 50 (2013) 249–256