The hardness of metals. Hardness Table

In order for the parts and mechanisms to serve for a long time and reliably, the materials from which they are made must meet the necessary working conditions. That is why it is important to control the permissible values ​​of their main mechanical indicators. The mechanical properties include hardness, strength, toughness, ductility. The hardness of metals is the primary structural characteristic.

The concept

The hardness of metals and alloys is the property of a material to create resistance when another body penetrates into its surface layers, which does not deform and does not collapse under associated loads (indenter). Determined with the aim of:

• obtaining information on permissible structural features and operation options;
• analysis of the state under the influence of time;
• control of the results of heat treatment.

The strength and resistance of the surface to aging partially depend on this indicator. Investigate both the source material and the finished parts.

Study Options

An indicator is a quantity called a hardness number. There are various methods for measuring the hardness of metals. The most accurate studies are the use of various types of calculations, indenters and corresponding hardness testers:

1. Brinell: the essence of the apparatus is the indentation of a ball into the metal or alloy under study, the calculation of the diameter of the imprint and the subsequent mathematical calculation of the mechanical parameter.
2. Rockwell: using a ball or diamond cone tip. The value is displayed on the scale or calculated.
3. Vickers: the most accurate measurement of metal hardness using a diamond pyramidal tip.

To determine the parametric correspondence between the indicators of different measurement methods for the same material, there are special formulas and tables.

Factors determining the measurement option

In laboratory conditions, with the necessary range of equipment, the choice of the research method is carried out depending on certain characteristics of the workpiece.

1. The estimated value of the mechanical parameter. For structural steels and materials with low hardness up to 450-650 HB, the Brinell method is used; for tool, alloy steels and other alloys - Rockwell; for carbides - Vickers.
2. The dimensions of the test sample. Particularly small and thin parts are examined using a Vickers hardness tester.
3. The thickness of the metal at the point of measurement, in particular, cemented or nitrided layer.

All requirements and compliance are documented by GOST.

Features of the Brinell technique

Hardness tests of metals and alloys using a Brinell hardness tester are carried out with the following features:

1. Indenter - a ball made of alloy steel or from a tungsten carbide alloy with a diameter of 1, 2, 2.5, 5, or 10 mm (GOST 3722-81).
2. Duration of static indentation: for cast iron and steel - 10-15 s., For non-ferrous alloys - 30, a duration of 60 s. Is also possible , and in some cases - 120 and 180 s.
3. The boundary value of the mechanical parameter: 450 HB when measured with a steel ball; 650 HB when using carbide.
4. Possible loads. Using the weights included in the package, the actual deformation force on the test sample is adjusted. Their minimum permissible values: 153.2, 187.5, 250 N; maximum - 9807, 14710, 29420 N (GOST 23677-79).

Using the formulas, depending on the diameter of the selected ball and on the material being tested, the corresponding allowable indentation force can be calculated.

Designation example:

400HB10 / 1500/20, where 400HB is the Brinell metal hardness; 10 - ball diameter, 10 mm; 1500 - static load, 1500 kgf; 20 - the period of the indentation, 20 s.

To establish the exact numbers, it is rational to examine the same sample in several places, and determine the overall result by finding the average value from the obtained ones.

Brinell hardness test

The research process proceeds in the following sequence:

1. Checking the details for compliance with the requirements (GOST 9012-59, GOST 2789).
2. Checking the health of the device.
3. Selection of the necessary ball, determination of the possible effort, installation of weights for its formation, indentation period.
4. Hardness tester launch and sample deformation.
5. Measuring the diameter of the recess.
6. Empirical calculation.

HB = F / A,

where F is the load, kgf or N; A is the print area, mm ² .

HB = (0.102 * F) / (π * D * h),

where D is the diameter of the ball, mm; h - imprint depth, mm.

The hardness of metals, measured in this way, has an empirical relationship with the calculation of strength parameters. The method is accurate, especially for soft alloys. It is fundamental in systems for determining the values ​​of this mechanical property.

Features of the Rockwell Methodology

This measurement method was invented in the 20s of the XX century, more automated than the previous one. It is used for harder materials. Its main characteristics (GOST 9013-59; GOST 23677-79):

1. The presence of a primary load of 10 kgf.
2. Exposure period: 10-60 s.
3. Boundary values ​​of possible indicators: HRA: 20-88; HRB: 20-100; HRC: 20-70.
4. The number is visualized on the dial of the hardness tester, and can also be calculated arithmetically.
5. Scales and indenters. There are 11 different scales, depending on the type of indenter and the maximum permissible static load. The most common to use: A, B and C.

A: diamond cone tip, apex angle 120˚, total allowable static force - 60 kgf, HRA; thin products are studied, mainly rolled products.

C: also a diamond cone, designed for a maximum force of 150 kgf, HRC, is applicable for hard and hardened materials.

B: 1.588 mm ball made of hardened steel or hard tungsten carbide alloy, load - 100 kgf, HRB, is used to assess the hardness of annealed products.

Ball-shaped tip (1,588 mm) is applicable for Rockwell B, F, G scales. There are also E, H, K scales for which a ball with a diameter of 3.175 mm is used (GOST 9013-59).

The number of samples done with a Rockwell hardness tester in one area is limited by the size of the part. Repeated test is allowed at a distance of 3-4 diameters from the previous deformation point. The thickness of the test product is also regulated. It should be no less than 10 times the depth of penetration of the tip.

Designation example:

50HRC - Rockwell metal hardness, measured with a diamond tip, its number is 50.

Rockwell Study Plan

The measurement of the hardness of the metal is more simplified than for the Brinell method.

1. Assessment of the size and surface characteristics of the part.
2. Checking the health of the device.
3. Determination of tip type and allowable load.
4. Sample installation.
5. The implementation of the primary effort on the material, the value of 10 kgf.
6. The implementation of the full appropriate effort.
7. Reading the resulting number on the dial scale.

A mathematical calculation is also possible in order to accurately determine the mechanical parameter.

Under the condition of using a diamond cone with a load of 60 or 150 kgf:

HR = 100 - ((Hh) / 0.002;

when testing with a ball under a force of 100 kgf:

HR = 130 - ((Hh) / 0.002,

where h is the penetration depth of the indenter with a primary force of 10 kgf; H - indenter penetration depth at full load; 0.002 - coefficient governing the magnitude of the movement of the tip when changing the number of hardness by 1 unit.

The Rockwell method is simple but not accurate enough. At the same time, it allows the measurement of mechanical properties for hard metals and alloys.

Characteristics of the Vickers Methodology

Determination of the hardness of metals by this method is the most simple and accurate. The work of the hardness tester is based on pressing a diamond pyramidal tip into the sample.

Key Features:

1. Indenter: a diamond pyramid with an angle at the apex of 136 °.
2. Maximum permissible load: for alloyed cast iron and steel - 5-100 kgf; for copper alloys - 2.5-50 kgf; for aluminum and alloys based on it - 1-100 kgf.
3. Static load holding period: from 10 to 15 s.
4. Test materials: steel and non-ferrous metals with a hardness of more than 450-500 HB, including products after chemical-thermal treatment.

Designation example:

700HV20 / 15,

where 700HV is the Vickers hardness number; 20 - load, 20 kgf; 15 - period of static effort, 15 s.

Vickers study sequence

The procedure is extremely simplified.

1. Checking the sample and equipment. Particular attention is paid to the surface of the part.
2. Selection of allowable effort.
3. Installation of test material.
4. Launching the hardness tester to work.
5. Reading the result on the dial.

The mathematical calculation by this method is as follows:

HV = 1.8544 * (F / d ² ),

where F is the load, kgf; d is the average value of the lengths of the print diagonals, mm.

It allows you to measure the high hardness of metals, thin and small parts, while providing high accuracy results.

Ways to switch between scales

Having determined the diameter of the print using special equipment, you can use tables to determine hardness. The hardness table of metals is a proven assistant in the calculation of this mechanical parameter. So, if the Brinell value is known, the corresponding Vickers or Rockwell number can be easily determined.

An example of some match values:

The table of hardness of metals is based on experimental data and has high accuracy. There are also graphical dependences of Brinell hardness on the carbon content of the iron-carbon alloy. So, in accordance with such dependences, for steel with a carbon content of 0.2%, it is 130 HB.

Sample Requirements

In accordance with the requirements of GOSTs, the tested parts must meet the following characteristics:

1. The workpiece should be flat, lay firmly on the table of the hardness tester, its edges should be smooth or carefully processed.
2. The surface should have a minimum roughness. It must be sanded and cleaned, including using chemical compounds. At the same time, during machining processes, it is important to prevent the formation of hardening and increase in temperature of the processed layer.
3. The part should correspond to the selected method for determining hardness by parametric properties.

Fulfillment of primary requirements is a prerequisite for the accuracy of measurements.

The hardness of metals is an important fundamental mechanical property that determines some of their other mechanical and technological features, the results of previous processing processes, the influence of time factors, and possible operating conditions. The choice of research methodology depends on the approximate characteristics of the sample, its parameters and chemical composition.