Measurement of hardness of metals

The hardness of steel is one of the most important mechanical properties of steel. Generally speaking, the hardness of a material is not its intrinsic property. Therefore, it cannot be given an exact and unambiguous definition based on the fundamental units of measurement - mass, length and time. therefore hardness measurement is the result of a specific measurement procedure - a hardness measurement method. How many methods hardness measurements - so many definitions of the concept of "hardness".

The oldest method of measuring hardness

The hardness of materials has long been measured by their resistance to scratching or cutting.. for instance, material B scratches material C, but does not scratch material A. AND, vice versa, material A scratches material B only slightly, and material B is very strong. Thus, the relative hardness of minerals is still estimated according to the Mohs mineralogical scale, which was introduced, German Moos back in 1811 year. Mohs scale maximum hardness, 10 has a diamond, and the minimum hardness, 1 - talc.

A practical method for measuring hardness

In practice, a similar method of measuring hardness is often used using a special set of files., each of which is thermally hardened to a certain hardness - from low to high. Such a set of six files is shown in the figure below..

nabor-nahilnikov-dlya-opredeleniya-tverdostiPicture – Set of files for measuring the hardness of metals

If the file slides over the material without leaving any marks, then this material is considered more solid, than a file, if the trace remains - less hard. This relative method of measuring hardness is limited to practical applications and does not provide accurate numerical data or scales., especially for modern metals and materials.

What is hardness?

Hardness measurement applies to most materials, especially for metals. The very concept of "hardness" has many definitions - from physicists, metallurgists and mechanical engineers. In mineralogy, hardness – is the resistance to scratching by another substance, and for metallurgists - the ability of the material to resist plastic deformation.

The most commonly used determination of the hardness of metals is: "Resistance of metal to local plastic deformation, arising from the introduction of a more rigid body into it, eg, tip (indentora) test instrument". but, hardness can have other manifestations such as, eg, rigidity, scratch resistance, abrasion or cutting. The hardness of a metal gives it the ability to resist irreversible deformation. – flexible, fracture or change in shape. The greater the hardness of the metal, the more his resistance he renders irreversible, eg, plastic deformation. This variety of manifestations of hardness gives a variety of methods for measuring it..

Hardness measurement by static indentation methods

To measure the hardness, the most commonly used methods are static indentation into the material of the ball., diamond cone or diamond pyramid. These methods include:

  • Brinell method according to GOST 9012-59) - steel ball;
  • Rockwell method according to GOST 9013-59) - diamond cone;
  • Super-Rockwell method according to GOST 22975-78 – steel ball or diamond cone;
  • Vickers method according to GOST 2999-75 - diamond pyramid;
  • microhardness test method according to GOST 9450-75 - diamond pyramid.

Scratch hardness measurement

This is the oldest method for measuring hardness.. Mohs scratch testing of materials is based on this principle., which are used for minerals. Besides, GUEST 21318-75 defines a method for measuring the microhardness of a material by scratching it - drawing a groove – special diamond pyramids.

Dynamic methods for measuring hardness

Impact imprint method according to GOST 18661-73

Used for massive parts and structures, when other methods cannot be applied. Hardness is measured using a special device. A reference bar with a known hardness is placed in the device.. The instrument is placed on the surface of the part to be examined.. When struck with a hammer on the upper part of the striker, a ball with a diameter 10 mm is pressed simultaneously into the part and into the standard. By comparing the holes on the part and the sample, the hardness of the material of the part is judged..

Method of elastic rebound of the striker (Shor method) according to GOST 23273-78

Use a special device – scleroscope, inside which a diamond-tipped striker falls freely. The measurement of hardness is carried out by the height of the rebound of the striker.

Electromagnetic method for measuring the hardness of steel

This method is based on the dependence of the magnetic characteristics of steel on its structure.. Since each steel structure corresponds to a certain hardness, then between the magnetic characteristics and hardness, you can establish some relationship. These are less accurate., than mechanical methods and require a lot of experience for their application. They are used in mass testing of the hardness of the same type of parts..

Ultrasonic Method for Measuring the Hardness of Steel

This method of measuring hardness is called the ultrasonic contact impedance method.. The diamond pyramid is pressed against the test sample at a constant force and elastic vibrations are excited. The lower the sample hardness, the more the indenter pushes through its surface. Used to measure hardness in hard to reach places.