Method and device for determining hardness value of bolt through Leeb hardness of end face of high-temperature bolt

Abstract

The invention discloses a method and device for confirming the hardness value of a bolt through the Leeb hardness of the end face of the high-temperature bolt, and the method comprises the following steps: 1) grinding and polishing the end face of the high-temperature bolt which cannot be disassembled or is disassembled on site to form a detection area; 2) carrying out hardness detection on the detection area prepared on site, recording detection data, and calculating an average value; (3) sampling on-site bolt rod parts with the same material and the same specification, and carrying out a Brinell hardness test; recording detection data, and calculating an average value; and 4) drawing a relation curve of the Leeb hardness of the end surface of the high-temperature bolt and the Brinell hardness of the rod part of the laboratory according to the recorded detection data, and establishing a model formula of the Leeb hardness of the end surface and the Brinell hardness of the rod part through the relation curve. According to the invention, the Leeb hardness test can be carried out on the end surface of the high-temperature bolt during field test, and a bolt end surface Leeb hardness and rod Brinell hardness conversion model is established according to the test results of the portable Leeb hardness tester on the end surface of the bolt and the table Brinell hardness on the rod part of the bolt with the same material and the same specification.

Description

Technical Field

[0001] The present invention belongs to the field of metal material detection, and in particular relates to a method and a device for confirming the hardness value of a bolt by measuring the Leeb hardness of a high-temperature bolt end face. Background Art

[0002] Steam turbine high-temperature bolts are critical components of generator sets. Failure can cause generator failure, threatening the safe operation of the entire unit and even resulting in personal injury or death. High-temperature bolts with sizes M32 or larger must undergo a hardness test before installation, and a specific percentage of hardness checks must be conducted during overhauls. Hardness testing of high-temperature bolts has become the most important supervisory tool for assessing their safety and, in turn, ensuring safe and stable unit operation. Currently, DL / T439-2018, the "Technical Guidelines for High-Temperature Fasteners in Thermal Power Plants," stipulates that Brinell hardness testing should be performed on the polished bolt shaft. However, during unit overhauls, some bolts often cannot be removed or are incompletely removed, making inspection impossible at the shaft. Furthermore, the end faces of some bolts are small or below the bolt hole plane, making it impossible to place a Brinell hardness tester, requiring Leeb hardness testing to be performed on the end faces. GB / T 17394.4-2014, "Metallic Materials - Leeb Hardness Test - Part 4: Hardness Conversion Tables," provides a conversion table for Leeb hardness to Brinell hardness when the elastic modulus of the D-punch material is approximately 210 GPa. However, high-temperature fastener bolts used in large-scale generator sets currently use high-temperature alloys and high-alloy steel forgings, whose elastic moduli differ from these values, making this table incompletely applicable. Automatically converted Brinell hardness values ​​from on-site Leeb hardness testers often exhibit significant errors. More critically, there is a significant discrepancy between the hardness of the bolt end face and the hardness of the shank, which is more pronounced for lighter bolts. These three factors lead to inaccurate on-site end face hardness values ​​for high-temperature bolts, failing to reflect their true hardness. Therefore, it is necessary to develop a method for determining bolt hardness using Leeb hardness on the end face, enabling quick and accurate on-site inspection of high-temperature bolts. Summary of the Invention

[0003] The present invention provides a method and apparatus for determining bolt hardness using the Leeb hardness test on the end face of a high-temperature bolt. This method allows for on-site Leeb hardness testing of the end face of a high-temperature bolt. By comparing the results of a portable Leeb hardness tester on the bolt end face with a benchtop Brinell hardness tester on the shank of bolts of the same material and specification, a conversion model between the Leeb hardness of the bolt end face and the Brinell hardness of the shank is established, thereby enabling the determination of the bolt hardness value using the Leeb hardness test on the end face of the high-temperature bolt.

[0004] In order to achieve the above object, the present invention adopts the following technical solutions: A method for confirming the hardness value of a bolt by measuring the Leeb hardness of a high-temperature bolt end face comprises the following steps: 1) Grind and polish the end faces of high-temperature bolts that cannot be disassembled on site or have been disassembled to form a detection area; 2) Perform hardness testing on the test area prepared on-site in step 1), record the test data, and calculate the average value; 3) Take samples of the bolt shanks of the same material and specifications as in step 1) and conduct Brinell hardness tests; record the test data and calculate the average value; 4) Based on the test data recorded in steps 2) and 3), draw a relationship curve between the high-temperature bolt end face Leeb hardness and the laboratory rod Brinell hardness. Based on the relationship curve, establish a model formula for the end face Leeb hardness and the rod Brinell hardness: Y=a+bx+cx 2 in: Y: laboratory benchtop Brinell hardness value, HB; a, b, c: coefficients, obtained by fitting the curve data; x: Leeb hardness of the bolt end face, HLD.

[0005] A further improvement of the present invention is that, in step 1), grinding and polishing tools are used to grind and polish the end faces of the high-temperature bolts that cannot be disassembled on site or have been disassembled.

[0006] A further improvement of the present invention is that, in step 1), the high-temperature bolts are selected from bolts that cannot be disassembled on site or have been disassembled and are of small specifications, and the bolt materials cover different operating temperatures of the steam turbine and the specifications are all M32 or above.

[0007] A further improvement of the present invention is that in step 2), when the hardness of the test area prepared on-site in step 1) is tested using a portable Leeb hardness tester, the surface roughness of the end face does not exceed 1.6 μm, which is consistent with the Brinell hardness.

[0008] A further improvement of the present invention is that the Leeb hardness test is performed at least at three different positions, the distance between two adjacent indentations is at least three times the average diameter of the indentations, and the indentations cannot overlap.

[0009] A further improvement of the present invention is that the portable Leeb hardness tester is of model HT-2000A.

[0010] A further improvement of the present invention is that, in step 3), the laboratory Brinell hardness test uses a 5 mm diameter ball, a test force of 7355 N, a holding time of 15 s, hardness testing is performed at at least three locations, the distance between the punch impact point and the edge of the specimen is not less than 5 mm, the distance between two adjacent indentations is at least three times the average diameter of the indentations, and the indentations cannot overlap.

[0011] A further improvement of the present invention is that the Brinell hardness tester used in the laboratory Brinell hardness test is a Q3000CS Brinell hardness tester.

[0012] A further improvement of the present invention is that the data used for data analysis in step 4) is the average value of the data detected in steps 2) and 3), and when calculating the average value, the difference between the maximum and minimum values ​​does not exceed 10HLD / 5HBW.

[0013] A device for confirming the hardness value of a bolt by measuring the Leeb hardness of a high-temperature bolt end face, comprising: The inspection bolt processing unit is used to grind and polish the end faces of high-temperature bolts that cannot be disassembled on site or have been disassembled to form an inspection area; Leeb hardness testing unit, used to perform hardness testing on the test area prepared on-site in the bolt processing unit to be tested, record the test data, and calculate the average value; The Brinell hardness testing unit is used to sample the shank of bolts of the same material and specification in the bolt processing unit for Brinell hardness testing; record the test data and calculate the average value; The analysis unit is used to draw a relationship curve between the Leeb hardness of the high-temperature bolt end face and the Brinell hardness of the laboratory rod based on the test data recorded in the Leeb hardness test unit and the Brinell hardness test unit. Through the relationship curve, a model formula for the Leeb hardness of the end face and the Brinell hardness of the rod is established: Y=a+bx+cx 2 in: Y: laboratory benchtop Brinell hardness value, HB; a, b, c: coefficients, obtained by fitting the curve data; x: Leeb hardness of the bolt end face, HLD.

[0014] Compared with the prior art, the present invention has at least the following beneficial technical effects: The present invention provides a method and device for determining the hardness of a high-temperature bolt using the Leeb hardness of its end face. This method is particularly suitable for high-temperature bolts that cannot be disassembled on-site or have already been disassembled. By directly grinding and polishing the bolt end face to form a test area and using a portable Leeb hardness tester for hardness testing, the bolts do not need to be transported back to the laboratory, significantly improving the convenience and efficiency of testing. Compared to traditional destructive hardness testing methods, the present invention utilizes non-destructive Leeb hardness testing, which does not damage the bolt, ensuring the bolt's integrity and subsequent safety. By sampling the shank of a bolt made of the same material and specification for Brinell hardness testing on-site and comparing the data with the Leeb hardness test data from the end face, an accurate relationship curve and model formula between the Leeb hardness of the end face and the Brinell hardness of the shank can be established. This helps eliminate hardness testing errors caused by factors such as material and specification differences, improving the accuracy of test results. By plotting the relationship curve and establishing the model formula, the present invention achieves the conversion between the Leeb hardness of the end face and the Brinell hardness of the shank. This allows for quick and accurate determination of the bolt's hardness on-site simply by measuring the Leeb hardness of the bolt end face, greatly facilitating on-site inspections. The present invention eliminates the need to transport bolts back to the laboratory for destructive testing, thus significantly reducing transportation, testing costs, and time. Furthermore, the simple and efficient testing process improves work efficiency and reduces labor costs. The present invention is suitable for testing the hardness of various high-temperature bolts, enabling accurate hardness testing regardless of whether the bolts are in use or disassembled.

[0015] In summary, the present invention provides a method and device for confirming the hardness value of a bolt by measuring the Leeb hardness of the high-temperature bolt end face. By constructing a conversion model between the Leeb hardness of the high-temperature bolt end face and the Brinell hardness of the rod, and leveraging the convenience of the Leeb hardness test in the high-temperature bolt hardness test environment of a steam turbine unit, the method achieves the goal of confirming the hardness value of a bolt by measuring the Leeb hardness of the high-temperature bolt end face. This method is simple to operate and has low cost. It can quickly perform Leeb hardness testing on site to obtain Brinell hardness information of the bolt, providing a reliable basis for quality assessment and safe use of the bolt. The present invention not only solves the problem of difficult on-site bolt disassembly, but also ensures the reliability of the data, while saving manpower and material costs for related testing. BRIEF DESCRIPTION OF THE DRAWINGS

[0016] In order to more clearly illustrate the specific embodiments of the present invention or the technical solutions in the prior art, the following briefly introduces the drawings required for use in the specific embodiments or the description of the prior art. Obviously, the drawings described below are some embodiments of the present invention. For ordinary technicians in this field, other drawings can be obtained based on these drawings without paying any creative work.

[0017] Figure 1This is the relationship curve between Leeb hardness and Brinell hardness of the present invention.

[0018] Figure 2 This is a structural block diagram of a device for confirming the hardness value of a bolt by measuring the Leeb hardness of the high-temperature bolt end face according to the present invention. DETAILED DESCRIPTION

[0019] Hereinafter, only certain exemplary embodiments are briefly described. As will be appreciated by those skilled in the art, the described embodiments may be modified in various ways without departing from the spirit or scope of the present invention. Therefore, the drawings and description are to be considered as illustrative in nature and not restrictive.

[0020] In the description of the present invention, it is to be understood that when used in this specification and the appended claims, the terms “include” and “comprise” indicate the presence of the described features, wholes, steps, operations, elements and / or components, but do not exclude the presence or addition of one or more other features, wholes, steps, operations, elements, components and / or collections thereof.

[0021] It should also be understood that the terms used in the present specification are only for the purpose of describing particular embodiments and are not intended to limit the present invention. As used in the present specification and the appended claims, the singular forms "a", "an", and "the" are intended to include the plural forms unless the context clearly indicates otherwise.

[0022] It should be further understood that the term "and / or" used in the present description and the appended claims refers to and includes any and all possible combinations of one or more of the associated listed items.

[0023] The accompanying drawings illustrate various schematic diagrams of structures according to embodiments disclosed herein. These figures are not drawn to scale; for clarity, some details are exaggerated and some details may be omitted. The shapes of the various regions and layers shown in the figures, as well as their relative sizes and positional relationships, are merely exemplary and may deviate in practice due to manufacturing tolerances or technical limitations. Those skilled in the art may design regions / layers with different shapes, sizes, and relative positions as needed.

[0024] The embodiments of the present invention are described in detail below with reference to the accompanying drawings.

[0025] Example 1 The present invention provides a method for determining the hardness value of a bolt by measuring the Leeb hardness of a high-temperature bolt end face, comprising the following steps: 1) Grind and polish the end faces of high-temperature bolts that cannot be disassembled on site or have been disassembled to form a detection area; 2) Perform hardness testing on the test area prepared on-site in step 1), record the test data, and calculate the average value; 3) Take samples of the bolt shanks of the same material and specifications as in step 1) and conduct Brinell hardness tests; record the test data and calculate the average value; 4) Based on the test data recorded in steps 2) and 3), draw a relationship curve between the high-temperature bolt end face Leeb hardness and the laboratory rod Brinell hardness. Based on the relationship curve, establish a model formula for the end face Leeb hardness and the rod Brinell hardness: Y=a+bx+cx 2 in: Y: laboratory benchtop Brinell hardness value, HB; a, b, c: coefficients, obtained by fitting the curve data; x: Leeb hardness of the bolt end face, HLD.

[0026] In this embodiment, in step 1), the end faces of high-temperature bolts that cannot be disassembled on site or have been disassembled are ground and polished using grinding and polishing tools.

[0027] In this embodiment, in step 1), the high-temperature bolts selected are bolts that cannot be disassembled on site or have been disassembled and are of small specifications. The bolt materials cover different operating temperatures of the steam turbine and the specifications are all M32 or above.

[0028] In this embodiment, when the hardness of the test area prepared on-site in step 1) is tested using a portable Leeb hardness tester in step 2), the surface roughness of the end face does not exceed 1.6 μm, which is consistent with the Brinell hardness.

[0029] In this embodiment, the Leeb hardness test is performed at least at three different positions, and the distance between two adjacent indentations is at least three times the average diameter of the indentations; the indentations cannot overlap.

[0030] In this embodiment, the portable Leeb hardness tester is model HT-2000A.

[0031] In this embodiment, in step 3) the laboratory Brinell hardness test, a 5 mm diameter ball, a test force of 7355 N, and a holding time of 15 s are used. The hardness test is conducted at at least three locations, and the distance between the punch impact point and the edge of the specimen is not less than 5 mm. The distance between two adjacent indentations is at least three times the average diameter of the indentations; the indentations cannot overlap.

[0032] In this embodiment, the Brinell hardness tester used in the laboratory Brinell hardness test is a Q3000CS Brinell hardness tester.

[0033] In this embodiment, the data used for data analysis in step 4) is the average value of the data detected in steps 2) and 3), and the difference between the maximum and minimum values ​​when calculating the average value does not exceed 10HLD / 5HBW.

[0034] Example 2 The present invention provides a method for confirming the true hardness of a bolt by measuring the Leeb hardness of a high-temperature bolt end face, comprising the following steps: 1) Use grinding and polishing tools to grind and polish the end faces of high-temperature bolts made of different materials, and the surface roughness parameter Ra is not greater than 1.6μm; 2) Use an HT-2000A portable Leeb hardness tester to perform a hardness test on the bolt end face inspection area in 1), with the hardness tester operating perpendicular to the inspection plane. Test three sets of data separately, record the test data, and calculate the average value. 3) Take cross-sectional samples of the bolt shank of the same material and specification on site, grind and polish them using grinding and polishing tools to form a test plane area. The test area is smooth and flat, free of oxide scale and external contaminants, and the surface roughness parameter Ra is not greater than 1.6μm; 4) Use a Q3000CS Brinell hardness tester to perform a hardness test on the test plane formed during the preparation of the test specimen in 3), with the tester operating perpendicular to the test plane. Test three sets of data separately, record the test data, and calculate the average. The distance between the punch impact point and the specimen edge must be no less than 5 mm, and the distance between two adjacent indentations must be at least three times the average indentation diameter. 5) Based on the test data recorded in 3) and 4), use data analysis software to fit the relationship curve between the end face Leeb hardness and the laboratory Brinell hardness (such as Figure 1 As shown). Through this curve, the model formula of the high-temperature bolt end face Leeb hardness and the rod Brinell hardness is obtained; 6) Using another set of bolt samples, obtain the Leeb hardness values ​​of the end faces. Substitute these Leeb hardness values ​​into the hardness conversion model constructed in Step 5 to calculate the predicted Brinell hardness values ​​for the shank. Laboratory testing was then conducted on the Brinell hardness values ​​of the sample bolt shanks, and the error between the predicted and experimental values ​​was calculated. The error was within ±3%, demonstrating the effectiveness of this method.

[0035] The method described in the present invention achieves the goal of confirming the hardness of a bolt by measuring the Leeb hardness of the high-temperature bolt end face. This method achieves this goal by constructing a conversion model between the Leeb hardness of the high-temperature bolt end face and the Brinell hardness of the shank, leveraging the convenience of Leeb hardness testing in the high-temperature bolt hardness testing environment of a steam turbine unit. This method is simple to operate and relatively low in cost. It can quickly perform on-site Leeb hardness testing to obtain Brinell hardness information on the bolt, providing a reliable basis for bolt quality assessment and safe use. This invention not only solves the problem of difficult on-site bolt disassembly, but also ensures data reliability, while saving manpower and material costs for related testing.

[0036] Example 3 Select 2 high-temperature bolts of martensite, austenite and bainite, all with specifications of M32mm or above. The specific implementation steps and results are as follows: 1) After the bolt end is ground and polished, the hardness of the bolt end surface test area is tested using the HT-2000A portable Leeb hardness tester. The hardness tester is applied perpendicular to the test plane. The average test data is shown in Table 1. 2) For the bolt damage sample for end hardness test, the Brinell hardness test was performed on the rod using a Q3000CS Brinell hardness tester. The hardness tester was applied perpendicular to the test plane. The average test data is shown in Table 1. 3) Based on the test data recorded in the first two steps, use data analysis software to fit the relationship curve between the end face Leeb hardness and the laboratory Brinell hardness (such as Figure 1 As shown). Through this curve, the model formula of the high-temperature bolt end face Leeb hardness and the rod Brinell hardness is obtained; Y=-1575+5.77x-0.0043x 2 in: Y: laboratory benchtop Brinell hardness value, HB; x: Leeb hardness of the bolt end face, HLD.

[0037] Table 1 Bolt hardness test results

[0038] Example 4 like Figure 2 As shown, the present invention provides a device for confirming the hardness value of a bolt by measuring the Leeb hardness of the high-temperature bolt end face, comprising: The inspection bolt processing unit is used to grind and polish the end faces of high-temperature bolts that cannot be disassembled on site or have been disassembled to form an inspection area; Leeb hardness testing unit, used to perform hardness testing on the test area prepared on-site in the bolt processing unit to be tested, record the test data, and calculate the average value; The Brinell hardness testing unit is used to sample the shank of bolts of the same material and specification in the bolt processing unit for Brinell hardness testing; record the test data and calculate the average value; The analysis unit is used to draw a relationship curve between the Leeb hardness of the high-temperature bolt end face and the Brinell hardness of the laboratory rod based on the test data recorded in the Leeb hardness test unit and the Brinell hardness test unit. Through the relationship curve, a model formula for the Leeb hardness of the end face and the Brinell hardness of the rod is established: Y=a+bx+cx 2 in: Y: laboratory benchtop Brinell hardness value, HB; a, b, c: coefficients, obtained by fitting the curve data; x: Leeb hardness of the bolt end face, HLD.

[0039] In summary, the present invention can perform Leeb hardness testing on the end face of a high-temperature bolt during on-site inspection. By comparing the test results of a portable Leeb hardness tester on the bolt end face with those of a desktop Brinell hardness tester on the shank of bolts of the same material and specification, a conversion model between the Leeb hardness of the bolt end face and the Brinell hardness of the shank is established, thereby achieving the purpose of confirming the bolt hardness value through the Leeb hardness of the high-temperature bolt end face.

[0040] The above shows and describes the basic principles and main features of the present invention and the advantages of the present invention. It is obvious to those skilled in the art that the present invention is not limited to the details of the above exemplary embodiments, and that the present invention can be implemented in other specific forms without departing from the spirit or essential characteristics of the present invention. Therefore, from all points of view, the embodiments should be regarded as illustrative and non-restrictive. The scope of the present invention is defined by the appended claims rather than the above description, and it is intended that all changes that fall within the meaning and range of equivalents of the claims are included in the present invention. Any reference signs in the claims should not be construed as limiting the claim to which they relate.

[0041] In addition, it should be understood that although this specification describes the embodiments, not every embodiment contains only one independent technical solution. This description is for clarity only. Those skilled in the art should consider the specification as a whole. The technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art. The above content is only for the purpose of illustrating the technical concept of the present invention and cannot be used to limit the scope of protection of the present invention. Any changes made based on the technical solution in accordance with the technical concept proposed by the present invention fall within the scope of protection of the claims of the present invention.

Claims

1. A method for confirming the hardness value of a bolt by measuring the Leeb hardness of the high-temperature bolt end face, characterized in that: The following steps are involved: 1) Grind and polish the end faces of high-temperature bolts that cannot be disassembled on site or have been disassembled to form a detection area; 2) Perform hardness testing on the test area prepared on-site in step 1), record the test data, and calculate the average value; 3) Take samples of the bolt shanks of the same material and specifications as in step 1) and conduct Brinell hardness tests; record the test data and calculate the average value; 4) Based on the test data recorded in steps 2) and 3), draw a relationship curve between the high-temperature bolt end face Leeb hardness and the laboratory rod Brinell hardness. Based on the relationship curve, establish a model formula for the end face Leeb hardness and the rod Brinell hardness: Y=a+bx+cx 2 in: Y: laboratory benchtop Brinell hardness value, HB; a, b, c: coefficients, obtained by fitting the curve data; x: Leeb hardness of the bolt end face, HLD.

2. The method for confirming the bolt hardness value by measuring the Leeb hardness of the high-temperature bolt end surface according to claim 1, characterized in that: In step 1), grinding and polishing tools are used to grind and polish the end faces of high-temperature bolts that cannot be disassembled on site or have been disassembled.

3. The method for confirming the bolt hardness value by measuring the Leeb hardness of the high-temperature bolt end surface according to claim 1, characterized in that: In step 1), the high-temperature bolts selected are the bolts that cannot be disassembled on site or the bolts that have been disassembled and are of small specifications. The bolt materials cover the different operating temperatures of the turbine and the specifications are all M32 or above.

4. The method for confirming the bolt hardness value by measuring the Leeb hardness of the high-temperature bolt end surface according to claim 1, characterized in that: In step 2), when the hardness of the test area prepared on-site in step 1) is tested using a portable Leeb hardness tester, the surface roughness of the end face does not exceed 1.6 μm, which is consistent with the Brinell hardness.

5. The method for confirming the bolt hardness value by measuring the Leeb hardness of the high-temperature bolt end surface according to claim 4, characterized in that: The Leeb hardness test shall be carried out at least at three different positions, with the distance between two adjacent indentations being at least three times the average diameter of the indentations; the indentations shall not overlap.

6. The method for confirming the bolt hardness value by measuring the Leeb hardness of the high-temperature bolt end surface according to claim 4, characterized in that: The model of the portable Leeb hardness tester is HT-2000A.

7. The method for confirming the bolt hardness value by measuring the Leeb hardness of the high-temperature bolt end surface according to claim 1, characterized in that: Step 3) Laboratory Brinell hardness test using a 5mm diameter ball, 7355N test force, and a dwell time of 15s. Hardness testing is performed at at least three locations, with the distance between the punch impact point and the specimen edge being no less than 5mm. The distance between two adjacent indentations is at least three times the average indentation diameter; indentations cannot overlap.

8. The method for confirming the bolt hardness value by measuring the Leeb hardness of the high-temperature bolt end surface according to claim 7, characterized in that: The Brinell hardness tester used in the laboratory Brinell hardness test is the Q3000CS Brinell hardness tester.

9. The method for confirming the bolt hardness value by measuring the Leeb hardness of the high-temperature bolt end surface according to claim 1, characterized in that: The data used for data analysis in step 4) is the average value of the data detected in steps 2) and 3). When calculating the average value, the difference between the maximum and minimum values ​​does not exceed 10HLD / 5HBW.

10. A device for confirming the hardness value of a bolt by measuring the Leeb hardness of the high-temperature bolt end face, characterized in that: include: The inspection bolt processing unit is used to grind and polish the end faces of high-temperature bolts that cannot be disassembled on site or have been disassembled to form an inspection area; Leeb hardness testing unit, used to perform hardness testing on the test area prepared on-site in the bolt processing unit to be tested, record the test data, and calculate the average value; The Brinell hardness testing unit is used to sample the shank of bolts of the same material and specification in the bolt processing unit for Brinell hardness testing; record the test data and calculate the average value; The analysis unit is used to draw a relationship curve between the Leeb hardness of the high-temperature bolt end face and the Brinell hardness of the laboratory rod based on the test data recorded in the Leeb hardness test unit and the Brinell hardness test unit. Through the relationship curve, a model formula for the Leeb hardness of the end face and the Brinell hardness of the rod is established: Y=a+bx+cx 2 in: Y: laboratory benchtop Brinell hardness value, HB; a, b, c: coefficients, obtained by fitting the curve data; x: Leeb hardness of the bolt end face, HLD.

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