Vickers hardness test.html

A Vickers hardness tester

The Vickers hardness test was developed in 1924 by Smith and Sandland at Vickers Ltd as an alternative to the Brinell method to measure the hardness of materials.¹ The Vickers test is often easier to use than other hardness tests since the required calculations are independent of the size of the indenter, and 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 material's 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 surface 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.

The yield strength of the material can be approximated as

$H_V = c {\sigma_y}\approx {3}{\sigma_y}$ .

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

1 Implementation
2 See also
3 References
- 3.1 Notes
- 3.2 Bibliography
4 External links

Implementation

Vicker's test scheme

An indentation left in case hardened steel after a Vickers hardness test.

It was decided that the indenter shape should be capable of producing geometrically similar impressions, irrespective of size, the impression should have well defined points of measurement and the indenter should have high resistance to self deformation. A diamond in the form of a square based pyramid satisfied these conditions. It had been established that the ideal size of a Brinell impression was 3/8 of the ball diameter. As two tangents to the circle at the ends of a chord 3d/8 long, intersect at 136° It was decided to use this as the included angle of the indenter. The angle was varied experimentally and it was found that the hardness value obtained on a homogeneous piece of material remained constant, irrespective of load.² 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 surface area of the resulting indentation. A can be determined by the formula

$A = \frac{d^2}{2 \sin(136^{\circ}/2)}$ ,

which can be approximated by evaluating the sine term to give

$A \approx \frac{d^2}{1.854}$ ,

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

$H_V = \frac{F}{A}\approx \frac{1.854 F}{d^2}$ .

The corresponding units of HV are then kilogram-force per square millimetre (kgf/mm²). 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 (1 kgf/mm² = 9.80665×10⁶ Pa).

Vickers hardness numbers are reported as xxxHVyy, e.g. 440HV30, where:

440 is the hardness number,
HV gives the hardness scale (Vickers),
30 indicates the load used in kg.

Examples of HV values for various materials³
Material	Value
316L stainless steel	140HV30
347L stainless steel	180HV30
Carbon steel	55–120HV5
Iron	30–80HV5

References

Notes

^ R.L. Smith & G.E. Sandland, "An Accurate Method of Determining the Hardness of Metals, with Particular Reference to Those of a High Degree of Hardness," Proceedings of the Institution of Mechanical Engineers, Vol. I, 1922, p 623–641.
^ http://www.ukcalibrations.co.uk/vickers_htm.html
^ Smithells Metals Reference Book, 8th Edition, ch. 22

Bibliography

Meyers and Chawla (1999). "Section 3.8", Mechanical Behavior of Materials. Prentice Hall, Inc.

External links

Vickers hardness test
Conversion table - Vickers, Brinell, and Rockwell scales