Hardness of metals. Table of hardness of metals

In order to ensure that the parts and mechanisms are long and reliable, the materials from which they are made must comply with the necessary working conditions. That is why it is important to control the permissible values of their main mechanical indicators. Mechanical properties include hardness, strength, toughness, ductility. The hardness of metals is the primary design characteristic.

The concept of

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 accompanying loads (indenter). Determine with the purpose:

• Obtaining information on permissible design features and on the possibilities of operation;
• Analysis of the state under the action of time;
• Control of the results of temperature treatment.

The strength and stability of the surface to aging partly depend on this index. Explore both the raw material and finished parts.

Research Options

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

1. Brinell: the essence of the apparatus - ball pressing into the metal or alloy being studied, calculation of the diameter of the print and subsequent mathematical calculation of the mechanical parameter.
2. Rockwell: a ball or diamond cone tip is used. The value is displayed on the scale or determined by calculation.
3. Vickers: The most accurate measurement of the hardness of a metal using a diamond pyramidal tip.

To determine the parametric correspondences 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, in the presence of the necessary assortment of equipment, the choice of the method of investigation is carried out depending on certain characteristics of the workpiece.

1. The approximate value of the mechanical parameter. For structural steels and materials with a small hardness of up to 450-650 HB, Brinell's method is used; For instrumental, alloyed steels and other alloys - Rockwell; For carbides - Vickers.
2. The dimensions of the test sample. Particularly small and fine details are examined using a Vickers hardness tester.
3. The thickness of the metal in the place of measurement, in particular, of a cemented or nitrided layer.

All requirements and conformities are documented by GOST.

Features of the Brinell method

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

1. The indenter is a ball of alloy steel or a carbide-tungsten alloy with a diameter of 1, 2, 2.5, 5 or 10 mm (guest 3722-81).
2. Duration of static indentation: 10-15 seconds for cast iron and steel, 30 for non-ferrous alloys, 60 seconds, and in some cases 120 and 180 seconds.
3. Limit value of the mechanical parameter: 450 HB when measured with a steel ball; 650 NW when using a hard alloy.
4. Possible loads. With the help of the included cargoes, the actual strain force on the test sample is corrected. Their minimum permissible values are: 153.2, 187.5, 250 N; The maximum - 9807, 14710, 29420 N (guest 23677-79).

With the help of formulas, depending on the diameter of the selected ball and on the test material, it is possible to calculate the appropriate allowable punching force.

Type of alloy	Mathematical calculation of the load
Steel, nickel and titanium alloys	30D 2
Cast iron	10D 2 , 30D 2
Copper and copper alloys	5D 2 , 10D 2 , 30D 2
Light metals and alloys	2.5D 2 , 5D 2 , 10D 2 , 15D 2
Lead, tin	1D 2

Example of a designation:

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

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

Determination of Brinell Hardness

The research process proceeds in the following sequence:

1. Checking the details for compliance with requirements (GOST 9012-59, guest 2789).
2. Check the serviceability of the device.
3. Selection of the necessary ball, determination of the possible effort, installation of cargo for its formation, the period of indentation.
4. Running of the hardness tester and deformation of the sample.
5. Measuring the diameter of the recess.
6. An empirical calculation.

HB = F / A,

Where F is the load, kgf or H; A is the area of the print, mm ² .

HB = (0.102 * F) / (π * D * h),

Where D is the diameter of the ball, mm; H - depth of print, mm.

The hardness of metals measured by this method has an empirical relationship with the calculation of the 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 technique

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

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

A: diamond cone tip, angle at the apex 120˚, total permissible force of static influence - 60 kgs, HRA; Thin products, mainly rolling, are investigated.

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

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

A ball-shaped tip (1.588 mm) is applicable for Rockwell scales B, F, G. Also there are scales E, H, K, for which a ball is used with a diameter of 3.175 mm (GOST 9013-59).

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

Example of a designation:

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

The Rockwell Research Plan

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

1. Estimation of the dimensions and characteristics of the surface of a part.
2. Check the serviceability of the device.
3. Determination of tip type and permissible load.
4. Sample installation.
5. Implementation of the primary effort on the material, the value of 10 kgf.
6. Implementing the full appropriate effort.
7. Reading the received number on the dial dial.

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

When using a diamond cone with a load of 60 or 150 kgf:

HR = 100 - ((Hh) / 0.002;

When testing with a ball under the force of 100 kgf:

HR = 130 - ((Hh) / 0.002,

Where h is the penetration depth of the indenter at a primary force of 10 kgf; H is the penetration depth of the indenter at full load; 0.002 - coefficient regulating the amount of displacement of the tip with a change in the number of hardness by 1 unit.

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

Characteristics of the Vickers Technique

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

Key Features:

1. Indentor: diamond pyramid with a vertex angle of 136 °.
2. Maximum permissible load: for alloyed iron and steel - 5-100 kg; For copper alloys - 2,5-50 kgf; For aluminum and alloys based on it - 1-100 kgf.
3. Static load holding period: 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.

Example of a designation:

700HV20 / 15,

Where 700HV is the number of Vickers hardness; 20 - load, 20 kgf; 15 - period of static force, 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. The choice of permissible effort.
3. Installation of the test material.
4. Run the hardness tester into operation.
5. Reading the result on the dial.

The mathematical calculation for this method is as follows:

HV = 1.8544 * (F / d ² ),

Where F is the load, kgf; D is the average value of the diagonal lengths of the print, mm.

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

Ways to switch between scales

Determining the diameter of the print with the help of special equipment, you can use the tables to determine the hardness. The hardness table of metals is a verified assistant in the calculation of this mechanical parameter. So, if you know the value of Brinell, you can easily determine the appropriate number of Vickers or Rockwell.

An example of some match values:

The diameter of the print, Mm	Method of investigation
	Brinell	Rockwell			Vickers
		A	C	B
3.90	241	62.8	24.0	99.8	242
4.09	218	60.8	20.3	96.7	218
4.20	206	59.6	17.9	94.6	206
4.99	143	49.8	-	77.6	143

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

Requirements for the sample

In accordance with the requirements of GOST, the test parts must meet the following characteristics:

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

The fulfillment of primary requirements is an obligatory condition for the accuracy of measurements.

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