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Hardness Testing

The theory of hardness testing

What is hardness?

Hardness is the ability of a material to resist permanent deformation when put under load.

A simple method of testing for comparative hardness is a scratch test. A harder material          will scratch a softer material. Hardness is closely related to tensile properties, in particular tensile strength. However, the relationship is not the same for different metals nor in many cases for hardness values measured using the same test type but under different test conditions.

Figure 1: Relationship Between Tensile Strength and Vickers Hardness for Steel

Why do we Need Hardness Testing?

 Hardness testing provides a relatively quick method of getting an indication of the strength properties of materials. It has several advantages over a full tensile test for this purpose.

  •  It provides a simple, quick quality check
  • Generally only limited sample preparation is required
  • Hardness measurements can be made on small samples

The hardness test can provide information on local property variations within a sample.

Eg across a heat treated bar;

Types of Hardness Test

The commonly used hardness tests press a hard indentor into the metal and then either:

  • Measure the size of the resulting impression – Brinell and Vickers tests
  • Measure the depth of the resulting impression – Rockwell test

Specimen Preparation and Testing

Some general principles apply to the preparation of samples for all hardness tests if reliable test values are to be obtained.

  • The surface of the sample should be prepared by milling, grinding or polishing so that the subsequent indent is clearly defined, and accurate measurement obtained. This is particularly important for Brinell and Vickers tests where the diameter of the impression must be measured.
  •  All unrepresentative material such as oxide scale or decarburised layers must be removed. This is particularly important for Vickers testing where the depth of the impression is small.
  • Care must be taken to avoid altering the properties of the surface during preparation for instance by overheating or cold working. Overheating is a particular problem for harder samples and cold working for softer materials.
  •  Further considerations apply with respect to the minimum specimen thickness which can be used. There must be no evidence of the damage on the face opposite the impression after testing.

There must be sufficient distance between the impression and the edge of the sample otherwise asymmetric impressions may be obtained;

Indentations must be spaced such that the deformed regions around adjacent impressions do not overlap;

By staggering impressions it may be possible to place them more closely together without influencing the hardness value obtained;

Brinell Hardness Test

The Brinell test is widely used for release test purposes as it provides a quick macro-scale indication of hardness, which can be performed on a ground surface.

The test uses a 5 or 10mm diameter spherical indenter made from tungsten carbide, Figure 1.

(N.B. Until recently steel ball indenters were also used, but these are not permitted by the current British Standard.)

A load of 500-3000kgf is applied for 10-30s and the diameter of the resulting impression is measured. Two measurements of the diameter of the impression in mm are made at right angles and the average, d, taken. Measurements are generally made using a microscope, but automatic measuring systems may be found on more modern testing equipment.

The hardness is defined as the average force per unit area of the impression. So for a softer material a deeper impression with a larger surface area will be produced, so the hardness number will be lower.

For a ball diameter of D mm, and an applied load of P kgf the hardness is given by:

HBW =  P /π D (D √ (D² -d²) )

Minimum Specimen Thickness 

The minimum specimen thickness is 10 times the impression depth which is given by:

                               Impression Depth  = P / π D x HBW

 The minimum specimen thickness for a 3,000kgf load and a 10mm diameter indenter is shown in Figure 2.

Spacing of Indents

 The distance of the centre of an indent to the specimen edge or the spacing between indents should be at least 3 times the specimen diameter.

Reporting of Results

 The test report shall include:

  • Reference to the testing standard  – BS EN ISO 6506-1, ASTM E10
  • All details necessary for the complete identification of the test piece
  • The test temperature if not within the range 10 – 35°C
  • The result
  • Additional requirements outside the scope of the standard
  • Details of any occurrence which may have affected the result

 Hardness values are reported as HBW Ball diameter (mm) / Force (kgf)

                           Eg. 350 HBW 10/3000

Vickers Hardness Test

 The Vickers hardness test was introduced in 1925 by Vickers- Armstrong Ltd of Crayford in Kent. It can be used both as a research tool, or for release test purposes although the high standard of specimen preparation required means that Brinell or Rockwell tests are more often used for the latter.

It can be used to measure properties on a range of scales from:

  •  Micro – using loads from 0.01 – 2 kgf
  • Intermediate (low force testing) – using loads from 0.2 – 5kgf
  • Macro – using loads from 5-100 kgf

The principle is similar to that used for Brinell testing, but in this case a diamond pyramidal indenter is used. The size of the impression is measured and the hardness is again defined as the average force per unit area of impression.

The indenter is a square based pyramidal indenter with a 136°C angle between adjacent faces. Details are shown in figure 3;

The arithmetic mean of the two diameters of the impression are used to calculate the hardness

HV = 2Psin(θ/2) / d²

 Where θ is the inter-face angle of the indenter and P is the applied load.

For homogenous materials the Vickers hardness is essentially independent of the load used. Up to 350HV Vickers hardness values are similar to those from Brinell tests, but at higher hardnesses Brinell tests give lower values.

The minimum specimen thickness must be 10x the impression depth which is approximately 0.15 x the specimen diagonal. Hence the minimum specimen depth is 1.5 x the specimen diagonal.

Asymmetrical Indents

Some material may give non-standard shaped impressions. This is due to the elastic properties on unloading

For convex impressions found in cold worked and high hardness materials, for a given impression diameter the true area of the impression is greater than would be expected. Hence the indicated hardness is higher than the true value.

The test report shall include:

 Reference to the testing standard  – BS EN ISO 6507-1, ASTM E92

  • All details necessary for the complete identification of the test piece
  • The result
  • Additional requirements outside the scope of the standard
  • Details of any occurrence which may have affected the result

       Hardness values are reported as HV force (kgf)/ Load application time  (if

      not in the range 10-15s)

e.g. 350HV 30/20

In practice values are often reported as HV or HV30

Rockwell Hardness Test

 The Rockwell test is similar to the Brinell and Vickers tests in that it uses an indenter to test the sample however it does not calculate the hardness of the material from the size of the impression but from its depth.

A preload is applied to establish a datum then the main load is applied and the additional penetration measured.

Indenters are tungsten carbide balls with diameters of 1.59 to 12.5mm. for very hard materials diamond indenters are used e.g hardened steels and cemented carbides.

A large number of Rockwell scales exist and details are shown in the table below.

Where h = difference between the two loads applied

Minimum Specimen Thickness

 The minimum specimen thickness must be 10x the permanent indentation depth for cone indenters and 15 x for ball indenters.

Values for the Rockwell C scale which uses a cone indenter are shown below

No deformation must be visible on the face opposite the test indentation.

Spacing of Indents

 The distance between the centres of the adjacent indents must be a minimum of 4X the diameter of the indentation and no less than 2mm.

The distance of the centre of the indentation from the edge of the sample shall be a minimum of 2.5 x the diameter of the impression and no less than 1mm.

Specimen Preparation

 Specimen must not be heated or cold worked so as not to change the local hardness of the material. Unrepresentative material e.g. decarburised or oxidised layers must be removed.

Reporting Results

 The test report shall include:

 Reference to the testing standard  – BS EN ISO 6508-1, ASTM E18

  • All details necessary for the complete identification of the test piece
  • The test temperature if not within the range 10-35°C
  • The result
  • Additional requirements outside the scope of the standard
  • Details of any occurrence which may have affected the result
  • The actual time for which the load was applied if greater than 6s

 

      Hardness values are reported as HR Rockwell scale Type of Ball

                                  Eg 35 HRCS

  In practice values are often reported as HRC

Hardness Conversions

 Numerous hardness conversions exist which vary greatly in their reliability and applicability. They are at best a guide and the results should be treated with caution.

Different materials produce different correlations between hardnesses measured using different tests and between hardness values and tensile strength.

Even with the same material different correlations exist, for example for austenitic stainless and conventional low alloy steels or for low alloy steels in the quenched or quenched and tempered conditions.

For the best results individual correlations should now be developed for individual classes of material with similar heat treatments over a narrow range of hardness.