Vickers hardness test

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The Vickers hardness test was developed in the early 1920s as an alternative method to measure the hardness of materials. Reasons why it is easier to use than other hardness tests is that calculations are not dependent on the size of the indenter, and also that the indenter can be used for all materials irrespective of hardness. The basic principle, as with all common measures of hardness, is to observe the questioned materials' ability to resist plastic deformation from a standard source. The Vickers test can be used for all metals and has one of the widest scales among hardness tests. The unit of hardness given by the test is known as the Vickers Pyramid Number (HV). The hardness number can be converted into units of Pa, but should not be confused with a pressure, which also has units of Pa. The hardness number is determined by the load over the projected area of the indentation and not the area normal to the force, and is therefore not a pressure.

The hardness number is not really a true property of the material and is an empirical value that should be seen in conjunction with the experimental methods and hardness scale used. When doing the hardness tests the distance between indentations must be more than 2.5 indentation diameters apart to avoid interaction between the work-hardened regions.

Image:Vickerhard.JPG

The yield strength of the material can be approximated as

<math>H_V = c {\sigma}\approx {3}{\sigma}.</math>

where c is a constant determined by geometrical factors usually ranging between 2 and 4.

The Vickers hardness test uses a diamond, with the shape of square-based pyramid with an angle of 136° between opposite faces as an indenter. It is based on the principle that impressions made by this indenter are geometrically similar regardless of load. Accordingly, loads of various magnitudes are applied to a flat surface, depending on the hardness of the material to be measured. The Vickers Pyramid Number (HV) is then determined by the ratio F/A where F is the force applied to the diamond and A is the projected surface area of the resulting indentation, as seen from above. A can be determined by the formula

which can be approximated by evaluating the sine term to give

<math>A \approx \frac{l^2}{1.854},</math>

where l is the average length of the diagonal left by the indenter. Hence,

<math>H_V = \frac{F}{A}\approx \frac{1.854 F}{l^2}.</math>

The corresponding units of HV are then kgf per mm squared. To convert a Vickers hardness number in SI units (MPa or GPa) one needs to convert the force applied from kgf to newtons and the area from mm² to m² to give results in pascals.

A practical method to convert HV to SI units:

To convert HV to MPa multiply by 9.807

To convert HV to GPa multiply by 0.009807

Vickers hardness numbers are reported as xxxHVyy, e.g. 440HV30, where 440 is the hardness number, HV gives the hardness scale (Vickers), and 30 indicates the load used in kgf.

[edit] Examples

Some HV values<ref>Smithells Metals Reference Book, 8th Edition, chptr. 23</ref>

Stainless Steels 140-180HV30 (316L & 347L stainless respectively)
Carbon Steel 55-120HV5 (Note: load is different to that of stainless)
Iron 30-80HV5

[edit] References

<references/> Meyers and Chawla (1999): Section 3.8 of Mechanical Behavior of Materials, Prentice Hall, Inc. www.gordonengland.co.uk/hardness/vickers.htm