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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) or Diamond Pyramid Hardness (DPH). The hardness number can be converted into units of pascals, but should not be confused with a pressure, which also has units of pascals. 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:

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

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

Contents

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 HV number is then determined by the ratio F/A where F is the force applied to the diamond in kilograms-force and A is the surface area of the resulting indentation in square millimetres. 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}{1.8544}

where d is the average length of the diagonal left by the indenter. Hence,[3]

HV = \frac{F}{A} \approx \frac{1.8544 F}{d^2}

The corresponding units of HV are then kilograms-force per square millimetre (kgf/mm²). To calculate Vickers hardness number using SI units one needs to convert the force applied from kilograms-force to newtons. To do the calculation directly the following equation can be used:[4]

HV = \frac{F}{A} \approx \frac{0.1891 F}{d^2}

where F is newtons and d is millimetres.

Vickers hardness numbers are reported as xxxHVyy, e.g. 440HV30, or xxxHVyy/zz if duration of force differs from 10 s to 15 s, e.g. 440Hv30/20, where:

  • 440 is the hardness number,
  • HV gives the hardness scale (Vickers),
  • 30 indicates the load used in kg.
  • 20 indicates the loading time if it differs from 10 s to 15 s

Vickers values are generally independent of the test force: they will come out the same for 500 gf and 50 kgf, as long as the force is at least 200 gf.[5]

Examples of HV values for various materials[6]
Material Value
316L stainless steel 140HV30
347L stainless steel 180HV30
Carbon steel 55–120HV5
Iron 30–80HV5

See also

References

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Notes

  1. ^ 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.
  2. ^ http://www.ukcalibrations.co.uk/vickers_htm.html
  3. ^ ASTM E92-82 (2003) e2
  4. ^ ISO 6507-1:2005
  5. ^ Vickers Test. Instron website.
  6. ^ Smithells Metals Reference Book, 8th Edition, ch. 22

Bibliography

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

Further reading

  • ASTM E92: Standard method for Vickers hardness of metallic materials
  • ISO 6507-1: Metallic materials - Vickers hardness test - Part 1: Test method
  • ISO 6507-2: Metallic materials - Vickers hardness test - Part 2: Verification and calibration of testing machines
  • ISO 6507-3: Metallic materials - Vickers hardness test - Part 3: Calibration of reference blocks
  • ISO 6507-4: Metallic materials - Vickers hardness test - Part 4: Tables of hardness values

External links


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