HardnessTesting

Hardness Testing

The hardness of a material can be defined as "the resistance the material exhibits to permanent deformation by penetration of another harder material". Hardness is not a fundamental property of a material and the quantitative value should always be evaluated in relation to:

  • given load on the indenter
  • a specific loading time profile and a specific load duration
  • a specific indenter geometry

The principal purpose of the hardness test is to determine the suitability of a material, or the particular treatment to which the material has been subjected.

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How to Hardness test

The hardness test is typically performed by measuring the depth of indenter penetration (Rockwell, Instrumented Indentation Testing, Ball Indentation Hardness) or by measuring the size of an impression left by an indenter (Vickers, Knoop and Brinell.)

The most suitable indentation hardness test method depends on the materials microstructure, e.g. the homogeneity of the material. It is important that the material, under the indent performed by the hardness tester, is representative of the whole microstructure, unless the task is to study the different constituents in the microstructure. This means that if a microstructure is very coarse and heterogeneous, a larger impression is required than for a homogeneous material.

 

How to select the test method

When selecting a method one should consider:

  • The type of material to be tested
  • Whether compliance with a standard is required
  • The approximate hardness of the material
  • Homogeneity/heterogeneity of material
  • Size of the part
  • Whether mounting is necessary
  • The number of samples to be tested
  • The required accuracy of the result

The four most common indentation hardness tests

Rockwell

Rockwell

Rockwell is a fast method, developed for production control and it has a direct readout. The Rockwell hardness (HR) is calculated by measuring the depth of an indent, after an indenter has been forced into the specimen material at a given load.

Vickers

Vickers

The Vickers Hardness (HV) is calculated by measuring the diagonal lengths of an indent left by introducing a diamond pyramid indenter with a given load into the sample material. The size of the diagonals of indent is read optically in order to determine the hardness using a table or formula.

Knoop

Knoop

Knoop (HK) is an alternative to the Vickers test in the micro hardness range, mainly to overcome cracking in brittle materials (for example ceramics), but also to facilitate testing of thin layers. The indenter is an asymmetrical pyramidal diamond. The size of the indent is based on a measurement of the long diagonal, which is read optically in order to determine the hardness.

Brinell

Brinell

Brinell indentation gives a relatively large impression with a tungsten carbide ball, denotation HBW (W is the chemical symbol for tungsten). The size of the indent is read optically in order to determine the hardness. Typical applications are forgings and castings where the structural elements are large and inhomogeneous or structures are too coarse for other methods (Rockwell/Vickers) to give a representative result.

Influencing factors

A number of factors influence the hardness result. The lower the load used in the hardness test, the more factors need to be controlled, a few of them are mentioned here.

  • External factors such as light, dirt, vibrations, temperature and humidity should be controlled
  • Pre-conditions for testing such as a solid horizontal table for the tester and stage as well as thoroughly clamping or support of the sample with a holder or anvil should be secured. Indenter should be perpendicular to tested surface
  • During testing illumination settings should be held constant when using Vickers, Knoop or Brinell.
  • The tester needs to be recalibrated/verified every time you change indenter or objective lens.
Influencing factors on Hardness Testing

Preparation requirements

The required surface condition depends on the type of test and load used.

Usually a ground surface is sufficient for macro hardness testing, and sometimes no preparation is required. Micro hardness testing requires a polished or electro-polished surface. It is important that the borders/corners of an optically evaluated impression are clearly visible.

In the micro hardness range the lower the loads used during hardness testing, the greater are the requirements for surface preparation. This can be performed mechanically, chemically or electrochemically. It is important that no change of surface properties is induced in the specimen during preparation due to heating or cold working.

Deformations
Deformations introduced during cutting and grinding need to be removed by polishing down to 6, 3 or 1 μm, depending on the test load. For very small loads, less than 300 gf1, the surface needs to be completely free of deformations, and the specimens require oxide polishing or even electrolytic polishing to obtain a completely damage-free surface. One should also take into account that soft and/or ductile materials (i.e. for HV less than 120-150) are more sensitive when it comes to introducing preparation artifacts.

In general, however, it can be said that the variation of the measured hardness result relates directly to the quality of the surface preparation. Thus it is a good idea to consider the trade-off between surface quality and test result variation before deciding on an inferior surface preparation.

Reference: 1 Metallography and practice, George F. Vander Voort

In the table below the preparation requirements for the different hardness tests are described:

HAT

Hardness testing loads

  • The term "micro hardness testing" is often used when indentation loads are below or equal to1 kgf.
  • The term “macro hardness testing” is used when loads are higher than 1 kgf. If standards permit, use the highest possible load/force for largest indent and therefore best accuracy

Officially, the applied loads have to be expressed in Newton (N). However, historically loads were expressed in kilogram-force (kgf), gram-force (gf) or pond (p). The correlation between kgf, kp and N is: 1 kgf = 1000 gf = 1 kp = 9.81 N

The loads used by each of the four methods for metallic materials* comply with the different ISO and ASTM standards:

Hardness testing Method Table
Hardness Testing Indent Spacing

Indent spacing

During hardness testing, the indentation will deform the surrounding material and alter its properties. In order to avoid misinterpretations of the perceived hardness, the standards therefore prescribe a certain distance between multiple indentations.

Example
For steel and copper and copper alloys, Vickers indentations have to be spaced with at least 3 diagonal widths, whereas for lead, zinc, aluminium and tin the spacing between indents has to be at least 6 diagonal widths.

Troubleshooting

Problem

It can be difficult to obtain plane-parallel surfaces during preparation. Also, the indenter should be perpendicular to the test surface. For Vickers the measured diagonals should not deviate more than 5% from each other. For Knoop the two halves of the long diagonals must not differ by more than 10% from each other.

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Solution:

If the deviation is not due to anisotropy in the material, the best solution is to use a fixture to hold the specimen so that the indenter penetrates the surface perpendicularly. If no fixture is available, make sure the mechanical preparation of the specimens results in plane-parallel end surfaces.
If the surface finish of a specimen is too rough, it might be problematic to evaluate the corners of an indent, especially if automatic equipment is used. Scratches from the preparation may cause misreading of the indent size when using automatic hardness testing.
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Solution

Use a polished surface, the requirements of the surface depends of the applied load and hardness of the material, the softer the material, the better polish is required. See Preparation requirements in the section about How to hardness test, and find a suitable preparation method for the material in the e-metalog.
If the specimen is not properly cleaned after mechanical preparation and an optical reading of the hardness test takes place, an automatic reading might result in a misinterpretation of the corners of the indent.
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Solution

Always ensure that the specimens are cleaned properly, otherwise contaminants e.g. dirt or fibres from the polishing cloth might complicate the reading.
For a heavily etched sample, it might be difficult to evaluate the corners of an indent, which may lead to a less accurate hardness value.
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Solution

Etching should be avoided as far as possible, since it results in a less reflective surface. If etching is necessary, a light etch is preferable so that it will be possible to discriminate the corners of the indent. Sometimes, it can be necessary to etch, for example when evaluating a weld.
Hardness appears greater than expected.
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Solution

Check the rules for proper indent spacing for the intended hardness test. If the hardness indentations are too close to each other, strain hardening can appear.

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