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Innovative Methods for Rapid Control of Material Hardness

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On the scientific portal IOPscience, which collects information about new technologies in various fields of human activity, in the section «Materials Science and Engineering», the article «Innovative Methods for Rapid Control of Material Hardness» is published.

The article describes briefly the methods of measuring the hardness of structural steels using  hardness testers applying indirect measurement method.

Technical parameters and common description of Leeb hardness tester TKM-359CE manufactured by NPP Mashproject LLC are provided as an example.

The article is coauthored by А. А. Fayustov (Ph.D. of Economic Sciences, Associate Professor at the Department of Innovation Management - State University of Management, Moscow)  and employee of NPP Mashproject LLC O. D. Novokshonova

We thank our colleague for the time he dedicated to investigate this theme.

 

Innovative Methods for Rapid Control of Material Hardness

A.A. Fayustov1, O.D. Novokshonova2

1State University of Management, Moscow
2LLC "NPP" Mashroekt ", St. Petersburg

1. Introduction

When developing and using structural materials, it is necessary to evaluate their physical and mechanical characteristics [1-4]. One of the characteristics that can determine the quality of products and materials is hardness.

The hardness of metals is important to consider in most types of works performed with them. For example, when manufacturing bulky structures with considerable weight, where several types of metals are used, it is important to know that they will interact in the best possible way and successfully withstand a heavy load.

It is particularly important to consider the metal hardness index in the following areas of activity: shipbuilding; automobile production; aircraft construction; production of building materials based on metal and in combination with other materials.

In any of these areas, resistance to mechanical stress determines human safety, the ability to perform a task and the service life of products.

Hardness refers to the property of a material that allows it to resist the introduction of a harder and more elastic-deformed body, called an indenter, and made of a hard alloy or diamond, which has the best indicator of resistance to mechanical influences. Indentation is one of the methods of mechanical testing [5]. Indentation allows one to:

  • test the product in a short time using only a small area of the product;
  • use this type of test when other methods cannot be used for one reason or another;
  • test the material almost without damage, which makes it possible to attribute this type of measurement to non-destructive testing;
  • use small-sized devices, measure the hardness of products during their operation using automation and integration with digital computing technology;
  • establish an analytical or empirical relationship between the results obtained and data from other tests [6].

1.1. Relevance

Depending on the various parameters of the workpiece, such as size, design, and material properties, two main types of hardness testers can be used: stationary (classic) and portable (portable, electronic)
hardness testers.

A stationary hardness tester is a classical hardness tester that measures hardness using a standard method: the Brinell [7], Rockwell [8], Vickers [9], or Shore method, etc. As a rule, the name of such a hardness tester includes the name of the hardness scale.

There are stationary hardness meters that allow you to measure the hardness not only on one scale,
but also on several hardness scales. For example, there are hardness meters that allow you to measure Rockwell, Brinell, and Vickers hardness. A stationary hardness tester of any type is a large, heavy (from 50 kg and above) table or floor device. The simplest models are fully mechanical devices. The main advantage of such devices is the classical method of measuring hardness, as it was conceived by the author, for example, the Brinell method, in which indentation occurs under constant load with a spherical indenter. All previously existing stationary hardness testers use measurement methods standardized in regulatory and technical documents of the state (national) standard level [7-11].

Unlike stationary ones, portable hardness meters are small, light and compact devices that measure hardness using several hardness scales at once. As a rule, a modern portable hardness tester allows you to measure the hardness on all the main existing scales. With a portable hardness tester, the measurement is carried out on the part itself, which is a clear advantage of such devices. A portable hardness tester is an electronic device, so it usually has a whole set of useful functions: the ability to save measurement results to memory, generate and edit control protocols, transfer the results to a computer for subsequent processing, print the results on a printer directly from the hardness tester, and so on. Portable hardness meters allow you to measure hardness with an accuracy that is not inferior to stationary ones, while they have a number of undeniable advantages. However, there are some specific problems of hardness measurement that can only be solved with a classical hardness tester. But such tasks are very few, and one can say that in 99 cases out of 100, you can use a portable device.

Portable models are used in cases where it is impossible to use stationary versions, for example, if the workpiece parts are too large, or because of their heavy weight, when the object of research cannot be transported to the laboratory.

1.2. Problem statement

At this stage, modern stationary hardness testers are already optionally equipped with an interface for connecting to a computer, a microscope, and a printer for printing the results of the study. They have a keyboard for entering measurement parameters, and the results are displayed on the built-in displays.

At the same time, portable hardness testers have a small weight and dimensions, most of them fit in a pocket. Despite their size, some small-sized hardness testers have impressive functionality: a graphic display, detailed adjustment of measurement parameters, a camera, and a removable memory card for storing calibrations and research results.

Methods for measuring the hardness of materials that underlie the operation of hardness meters can be classified as follows:

  • Static methods are a group of methods that demonstrate resistance to plastic deformation. The indenter is a diamond tip, or a steel ball, which is gradually pressed into the surface of the material, after which the print is analyzed.
  • Dynamic methods are a group of methods that demonstrate both the deformation resistance of the tested materials and the elasticity. The result of the impact of the indenter on the surface of the material is analyzed.
  • Indirect methods are a group of methods that allow one to evaluate related properties of a material, such as changes in the frequency of a missed sound wave.

1.3. Theory

Take a closer look at the group of portable hardness testers as the most promising in their further development. Portable devices are classified into the following types according to the principle of their operation:

2. Dynamic hardness testers

Dynamic hardness testers operate according to the method of the rebound. The principle of operation of the hardness tester is based on a dynamic method of hardness control. The method consists in determining the rate of rebound of the carbide indenter from the surface of the controlled product. The device sensor is installed on the product whose hardness is to be measured. The main parts of the sensor are the indenter and the electromagnetic coil. When the indenter bounces off the test product, an electromotive force is produced in the coil, which is proportional to the speed of the rebound from the product surface.

Since the bounce rate of the indenter is an indicator of hardness, there is a functional relationship
between the bounce rate V and the hardness of the material H:  H=f (V)

The electronic unit of the hardness tester receives the signal from the sensor of the device, converts it into units of hardness, displays the measurement results, statistical processing and other functions of this hardness tester.

These hardness testers are very convenient and easy to use, but they have one significant drawback. Dynamic hardness testers are not recommended to measure the hardness of products weighing less than 5 kg or with a wall thickness of less than 10 mm at the measuring point. In some cases, the hardness of such parts can be measured by grinding them to a solid plate through a layer of grease, but this is only if the shape of the part allows it.

3. Ultrasonic hardness testers

They function through the introduction of a sensor into the surface of the material and subsequent
measurement of the frequency of indenter vibrations. The hardness is calculated based on the degree
of variation in the frequency of vibrations (Fig. 1).

Ultrasonic hardness testers rely on the method of ultrasonic contact impedance. To measure, the sensor is mounted on the product and simply pressed against it. At the end of the sensor there is a diamond pyramid, which is embedded in the product to a very small depth (about 50 microns) and thus determines the hardness. The measurement method is very similar to the classic Vickers method, but it uses microloads: 1 kg, 5 kg or 10 kg. Unlike a dynamic hardness tester, such devices have almost no restrictions on the mass and wall thickness of the product. These devices also have their drawback: it is not always possible to measure the hardness of products with a coarse-grained structure (some cast iron, stainless steel), and there may also be difficulties with measuring very soft metals and nonferrous metal alloys (for example, aluminum).

UCI Hardness tester TKM‑459CE

Figure 1. Scheme for measuring the hardness of the gear tooth with an ultrasonic hardness tester TKM-459C

Combined hardness testers are able to perform measurements using the methods described above simultaneously. A combined hardness tester is a device that can measure hardness using both dynamic and ultrasonic methods by simply replacing the sensor that is used. This is the most functional option if we consider portable devices. The hardness tester has almost no restrictions on use. It is the best method of express control, as it allows getting more accurate data.

4. The main criteria for the selection of hardness testers for express hardness control

If measurement accuracy is an important requirement, and the device will be used primarily in the company's laboratory, preference is given to stationary options (ideally, to the universal ones).

When monitoring the hardness of large-sized parts, in hard-to-reach places and in cases where it is impossible to completely disassemble the components (pipelines, rails, ship repairs, etc.), the only correct solution is to use portable hardness meters [12].

Combined devices have an advantage in the case of a large range of controlled products, since the dynamic method is well suited for geometrically simple massive parts, and the ultrasonic one for small samples of complex material.

If the measuring instrument is used in a complex environment in the presence of external factors (high humidity, the presence of liquids and aggressive media), since modern portable devices have complex electronics that are sensitive to external influences, it must be protected by a shock-resistant, moisture-resistant (or even waterproof) case with rubber pads for easy retention.

There are various options for connecting the electronic unit to the indenter. Manufacturers offer 3 options for connecting the sensor with an indenter to the recording equipment (main module): built-in, wired or wireless (Bluetooth communication). Usability depends on this selection.

The following options may be important for users: the availability of RAM, the type of sensor and the ability to complete additional types of sensors, the ability to connect to a PC and an external printer directly, as well as the ability to work with multiple scales and convert results between scales.

Practical implementation of express control methods. As an example of practical application of innovative measuring instruments, the authors present the technical characteristics of the TKM-359C ultrasonic hardness tester, as a typical tool of the TKM family of hardness testers developed by Research and Production Enterprise Mashproekt, LLC (Saint Petersburg), widely used in production processes and in training sessions in materials science at the State University of Management [13, 14].

The ultra-modern high-precision hardness tester TKM-359C is designed for rapid measurement of the hardness of metal and alloy products, including quality control of heat treatment, hardening of high frequency current and evaluation of mechanical strength.

The hardness tester is intended for use by enterprises that pay special attention to control measures. It is made in a shock-resistant, dust-and moisture-proof casing that allows it to be used in the most
severe operating conditions.

Hardness tester TKM‑359C-Mashproject LCC

Figure 2. Appearance of the TKM-359C hardness tester with a set of sensors for measurements Legend, left to right: sensor type G, sensor type D with a connector, sensor type E with a connector, head piece Z-359

The hardness tester is equipped with a color graphic OLED display that provides easy reading in low light conditions and at subzero temperatures.

TKM-359C implements measurements in the main HB, HRC, and HV hardness scales standardized in Russia. The control is also carried out according to the HRA, HRB, HL, HSD scales and the ultimate strength (MPa, recalculated according to government standards GOST 22791-77). The user can program their own scales.

The device is equipped with additional scales for monitoring the hardness of various types of metals and alloys without introducing additional characteristics (calibrations).

The unique statistical processing system allows you to analyze the measurement results directly on the display of the hardness tester.

Dynamic hardness tester TKM-359 (modifications TKM-359C, TKM-359M) has passed metrological certification, entered in the State Register of measuring instruments of the Russian Federation (number in the register no. 48898-12) and in the State Register of measuring instruments of the Republic of Belarus (number in the register no. РБ 03 03 3797 17).

Registered in the State system for ensuring the uniformity of measurements of the Republic of Kazakhstan (number in the register no. KZ.02.03.05068-2013/48898-12).

Approved for use in Russia, Ukraine, Belarus and Kazakhstan. An award-winning product.

The hardness tester has an interactive, intuitive graphical interface that meets the global standards of computer technology relying on the principle of “TURN TO WORK”.

Basic technical characteristics of the hardness tester are given in Table 1 [13].

Table 1. General technical characteristics of TKM-359 hardness testers

Parameters

TKM-359C (TKM-359M)

Range of hardness measurements on the main scales:

Brinell

90 - 450 НВ

Rockwell С

20 - 70 HRC

Vickers

240 - 940 HV

Limits of absolute error in the measurement of hardness on the main scales on measures of hardness of the 2nd category

Brinell

Within (90...150) НВ

±10 НВ

Within (150...300) НВ

±15 НВ

Within (300...450) НВ

±20 НВ

Rockwell С

±2 HRC

Vickers

Within (240...500)HV

±15 HV

Within (500...800)HV

±20 HV

Within (800...940)HV

±25 HV

Range of hardness control by reference scales:

Ultimate tensile strength σв

350…1,500 MPa

Rockwell А

70,5 - 85,5 HRА

Rockwell В

51 - 100 HRВ

Shore D

35 - 202 HSD

Leeb D

150-900 HL

Diameter of the sensor installation area:

in the plane

1 mm (for ultrasonic hardness tester)

in the slot (blind hole)

5 mm (for ultrasonic hardness tester)

Number of measurements to calculate the average value

1 – 99

Number of algorithms for discarding the results of incorrectly performed measurements

3

Memory capacity of measurement results

12 400

Number of additional individual calibrations

50 (5 for each scale of the instrument)

Number of user programmable scales

3

Overall dimensions of the electronic unit of the hardness tester no more than:

Modification of TKM-359C 121 х 69 х 41 mm
Modification of TKM-359M 150 х 81 х 31 mm
Mass of the electronic unit of the hardness tester, NMT 0,3 kg

Mass sensors, NMT

0,3 kg
Operating temperature range from minus 15 to plus 35° C
Service life of the hardness tester 5 years old

Display, statistical processing, and plotting graphs of measurement results blocks

Display output - all results in the block,
- results in the block greater than / less than the value set by the user,
- results that are outside the range specified by the user.
Statistical processing - maximum, minimum, average, standard deviation from the average,
the average deviation from the value set by the user, the number of results greater than or less than - the value, and the maximum deviation up or down from the value,
- the number of results that fall outside the user-defined range (upper / lower bound), and the maximum deviation from the upper/lower bound.
Plotting - relative to the average value,
- relative to the value set by the user,
- relative to the range set by the user.
Power supply of the hardness tester
Self-contained battery. Non-rechargeable equivalent batteries may be used.

5. Requirements for the controlled product

  • Products weighing more than 5 kg and thickness more than 6 mm do not require additional measures.
  • Products of rigid construction (for example, pipes, rails, ships, etc.) with an expected hardness of 90 to 250 NV, and a thickness of more than 4 mm, do not require additional measures.
  • The remaining products must be fixed (“grinded”) on the base plate with a fixing paste (for example, TSIATIM grease).
  • The roughness of the controlled surface that provides the highest measurement accuracy (without increasing the number of averaging) for the sensor type “D”: 3.2 Ra
  • for the “G” type sensor: 7.2 Ra
  • for the sensor type “E”: 1.6 Ra.

An example of a fragment of reporting data on measurement results generated using the program included in the delivery package is shown in Table 2 and in Figures 3 and 4.

Table 2. Control Report Form (fragment)

Operator: Fayustov A.
Part name:
Reference measure of hardness МТБ 174 HB
Material: GOST 9031-75
Serial number of the measure, date verification:
No. 5529; 17.04.2019;
Vostok-7, LLC
The name of the device:
TKM-359C hardness tester
Instrument type: Portable Leeb hardness tester
The serial number of the device:
Serial No. 19010
Number of approval certificate for measuring instruments
RU.C.28.002.A № 45288
Date of verification certificate:
04.03.2020

REPORT ON HARDNESS TESTING

Average: 168.2

RMS: 168.3

Min: 161.8, Max: 177.9

Measurement number

HB block (Brinell)
Deviation from the average
1 168.80

 

0.6

 

2

163.50 -4.7
3 167.00 -1.2
4 163.00 -5.2
5 170.30 2.1
6 164.60 -3.6
7 171.10 3
8 161.80 -6.4
9 177.80 9.6
10 174.80 9.6

scatter_plot_of_values_in_a_sample_of_10_dimensions_article.jpg

Figure 3. Scatter plot of values in a sample of 10 dimensions

diagram_constructed.jpg

Figure 4. Diagram constructed by a computer program based on the measured values of the model measure 174 HB on the Brinell scale (example)

6. Conclusion

When measuring hardness using the classical static method, it is necessary to consider many factors that affect the measurement results, the main ones are: the load value, the location of the print, the roughness of the sample surface, other mechanical influences, the error in measuring the size of the print, etc.

Along with this, the most promising, from the point of view of control efficiency, are new modern methods of hardness measurement implemented in dynamic and ultrasonic hardness meters that rely on the method of dynamic rebound from the sample surface and the degree of change in the frequency of indenter vibrations when it is embedded in the material surface and provide the specified values of measurement error on the main scales.

All the considered methods of hardness measurement, of course, do not exclude each other, and can and should be used together to determine the objective characteristics of materials used in various structures and technologies.

7. References

[1] Kablov E N 2015 Innovative developments of FSUE "VIAM" SSC RF for the implementation of "Strategic directions for the development of materials and technologies for their processing for the period up to 2030" Aviation materials and technologies 1(34) pp 3 - 33
[2] Kablov E N 2015 Composites: Today and Tomorrow Metals of Eurasia 1 pp 36 - 39

[3] Buznik V M, Kablov E N, Koshurina A A 2015 Materials for complex technical devices for arctic applications Scientific and technical problems of Arctic development (Moscow: Nauka) pp 275 - 285
[4] Oreshko E I, Utkin D A, Erasov V S, Lyakhov A A 2020 Methods for measuring the hardness of materials (review) Proceedings of VIAM 1(20) pp 101 - 117
[5] Markovets M P 1979 Determination of the mechanical properties of metals by hardness (Moscow: Mashinostroenie) 191 p
[6] Kolmakov A G, Terent'ev V F, Bagirov M B 2005 Methods for measuring hardness 2nd ed. Rev. and add. (Moscow: Intermet Engineering) 150 p
[7] GOST 9012-59 Metals and alloys Brinell hardness measurement method
[8] GOST 9013-59 Metals and alloys Rockwell hardness measurement method
[9] GOST 2999-75 Metals and alloys Method for measuring Vickers hardness
[10] GOST 22975-78 Metals and alloys Method for measuring Rockwell hardness at low loads (Super-Rockwell)
[11] GOST 23677-79 Hardness testers for metals General technical requirements
[12] Strutynsky A V, Khudyakov S A 2020 Comparison of the characteristics of small-sized hardness testers URL: http://www.armada-ndt.ru/articles/8444/ (date of treatment 08/05/2020)
[13] Portable dynamic hardness testers TKM-359 (modifications TKM-359C, TKM-359M) Operating manual for TKM359CM RE [Electronic resource]
[14] Fayustov A A 2019 The use of innovative tools for measuring the hardness of materials in the educational process Young scientist 10(248) pp 30-35 URL: https://moluch.ru/archive/248/57068/ (date of access: 06.08.2020)