Hardness test

 

Definition of hardness by comparison of several materials, i.e. the resistance to penetration of an object that is always harder than the material under test. The sense or purpose of hardness testing is to easily and quickly determine the hardness (a property that may change along a wide range) attributable for the corresponding state of a material. Often no special sampling is required and the workpiece under test is not rendered unserviceable in many cases. Moreover, to a certain extent the hardness test results allow for conclusions with regard to tensile strength.

Leading manufacturers of hardness testing machinery are, among others EMCO-TEST Prüfmaschinen GmbH, Qness GmbH, Zwick GmbH & Co. KG, Schütz + Licht Prüftechnik GmbH.

In engineering, the hardness of metals is measured by means of impression processes, which involve an indentation body (ball, cone, pyramid) made of steel, hard metal or diamond penetrating the material under test with a specified load.

 

Differentiation is made between static hardness tests, in which the application of load is performed slowly (Brinell, Rockwell and Vickers hardness tests) and dynamic hardness tests (Poldi or Baumann hammer, i.e. scleroscopic hardness or scleroscopic hardness test according to Shore), in which the indentation body either penetrates the material in thrust or by impact.

 

Brinell hardness test:

 

For hardness testing according to the Swedish engineer J. A. Brinell (1849 - 1925) a hard metal ball is pressed into the material under test under a suitable standard load and for a specified duration (10 to 15 seconds for steels and cast iron, 10 to 180 seconds for non-ferrous metals and their alloys). The hardness value is calculated as the ratio between the load and the impression surface produced by the indentor. This hardness test has been standardized in DIN EN ISO 6506-1 to 4. The steel balls previously used are no longer permitted from 2006, as specified in the amended standard. Meanwhile balls made of sintered hard metal are required for all materials. The balls used have diameters of 10 mm, 5 mm, 2.5 mm, 2 mm and 1 mm. The Brinell hardness value is proportional to the quotient of test force F and the impression surface A of the left impression (Figure 1).

 

Eq. 1:

According to the DIN EN ISO 6506-1 standard, the following parameters must always be stated: hardness value, procedure, ball diameter and test force, for example 300 HBW 10/3000.
Hardness value = 300; HBW = procedure (W = Tungsten carbide hard metal, material of the indentor); 10 = ball diameter in mm; 3000 = force F in N

 

Eq. 2:

 

In unalloyed or low-alloy steels there is a direct relation between Brinell hardness and tensile strength (within a certain tolerance range):

 

Rockwell hardness test:

 

Hardness test according to Stanley Rockwell, standardized in ISO 6508-1 (DIN EN 10109). In contrast to the methods by Brinell and Vickers, Rockwell hardness of a material is determined by the depth of indentation of an indenting body upon application of a specified preliminary and test force.

The indentation body is pressed into the surface of the workpiece under test at a specified test force and thus preloaded. The depth of indentation of the indenting body at preliminary load is used as reference level. Subsequently, the indentation body is at least twice subjected to the main load for a period of maximum six seconds; then the load is removed so that only the preliminary load is applied. The difference between indentation depth prior and subsequent to application of the main load is the Rockwell hardness value of the material.

Depending on the standard scale applied, the Rockwell hardness value is calculated from the depth of indentation, which is read off a dial indicator connected to the probe tip. For example, in procedures according to scale C (HRC), a cone-shaped diamond indentor is used.

 

The test sequence may be described as follows: Application of preliminary force - resetting the dial indicator to zero - application of main force - maintaining the force during the period of application (depending on the creep behavior of the material) - releasing the main force - reading the hardness value off the dial indicator - releasing the preliminary force.

Vickers hardness test:

 

Hardness test according to DIN EN ISO 6507-1 to 4 using a diamond indentor that is pressed into the surface of the specimen under specified test load. This test method was developed in 1925 by Smith and Sandland and was named after the British aircraft construction company Vickers; it is very similar to Brinell hardness test. In contrast to Rockwell hardness test an equilateral diamond pyramid with an opening angle of 136° is pressed into the workpiece under a specified test force (Fig. 2)

The diagonals d of the impression produced in the specimen surface are measured and a mean value is calculated from the individual variables; then, Vickers hardness HV is calculated using Eq. 3, taking into account the test force F:

 

Eq. 3:

Eq. 4:

 

According to the standard, the test procedure and the test force used must always be specified, for example 600 HV 10.
610 = hardness value; HV = procedure; 10 = test force F in N

The appropriate test forces according to DIN EN ISO 6507 must be used.

Moreover, the Vickers hardness test method can be divided in three processes:

 

Shore hardness test:

 

A rarely used procedure, in which an indentor with a weight of 20 g and a rounded tip (also referred to as hammer) falls from a height of 112 mm onto the surface under test from which it rebounds. A locking device arrests the indentor in its highest rebound position, which is then used as a dimension for hardness. This is a very simple procedure but its precision depends on both the mass of the material (with small workpieces it is likely for the test set-up to be shifted or displaced) and on the perfectly perpendicular drop axis. The hardness value is expressed in shore points.

 

 

  • Fig. 1: Brinell hardness test (schematic illustration, source: Wikipedia)
  • Fig. 2: Vickers hardness test (schematic illustration, source: Wikipedia)
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