Editing Vickers hardness test (section)

==Implementation==
[[File:Vickers-path-2.svg|thumb|Vickers test scheme]]
[[File:Vickers anvil diamons.jpg|thumb|The pyramidal [[diamond]] indenter of a Vickers hardness tester]]
[[File:Case hardened steel-vickers hardness test.png|thumb|An indentation left in case-hardened steel after a Vickers hardness test. The difference in length of both diagonals and the illumination gradient, are both classic indications of an out-of-level sample. This is not a good indentation.]]
[[File:Vicker Hardness - Diamond Indentation.jpg|thumb|This is a good indentation.]]

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 hardness test|Brinell]] impression was {{frac|3|8}} of the ball diameter. As two tangents to the circle at the ends of a chord 3''d''/8 long intersect at 136°, it was decided to use this as the included angle between plane faces of the indenter tip. This gives an angle from each face [[Normal (geometry)|normal]] to the horizontal plane normal of 22° on each side. 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.<ref>[http://ukcalibrations.co.uk/vickers-htm The Vickers Hardness Testing Machine]. UKcalibrations.co.uk. Retrieved on 2016-06-03.</ref> 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 millimeters.{{Citation needed|date=September 2024}} 

: <math>A = \frac{d^2}{2 \sin(136^\circ/2)},</math>

which can be approximated by evaluating the sine term to give,

: <math>A \approx \frac{d^2}{1.8544},</math>

where ''d'' is the average length of the diagonal left by the indenter in millimeters. Hence,<ref>ASTM E384-10e2</ref>

: <math>\mathrm{HV} = \frac{F}{A} \approx \frac{1.8544 F}{d^2} \quad [\textrm{kgf/mm}^2]</math>,

where ''F'' is in [[kilogram-force|kgf]] and ''d'' is in millimeters.

The corresponding unit of HV is then the kilogram-force per square millimeter (kgf/mm<sup>2</sup>) or HV number. In the above equation, ''F'' could be in N and ''d'' in mm, giving HV in the SI unit of MPa. To calculate Vickers hardness number (VHN) using SI units one needs to convert the force applied from [[kilogram-force]] to N then [[kilogram-force]] is divided by 9.806 65 ([[standard gravity]]). This leads to the following equation:<ref>ISO 6507-1:2005(E)</ref>

: <math>\mathrm{HV} \approx {0.1891}\frac{F}{d^2} \quad [\textrm{N/mm}^2],</math>

where ''F'' is in N and ''d'' is in millimeters. A common error is that the above formula to calculate the HV number does not result in a number with the unit newton per square millimeter (N/mm<sup>2</sup>), but results directly in the Vickers hardness number (usually given without units), which is in fact  one [[kilogram-force]] per square millimeter (1&nbsp;kgf/mm<sup>2</sup>).

Vickers hardness numbers are reported as '''xxxHVyy''', e.g. '''440HV30''', or '''{{not a typo|xxxHVyy/zz}}''' if duration of force differs from 10&nbsp;s to 15&nbsp;s, e.g. 440HV30/20, where:

* '''440''' is the hardness number,
* '''HV''' names the hardness scale (Vickers),
* '''30''' indicates the load used in kgf.
* '''20''' indicates the loading time if it differs from 10&nbsp;s to 15&nbsp;s
{| class="wikitable"
|+ Examples of HV values for various materials<ref>Smithells Metals Reference Book, 8th Edition, ch. 22</ref>
|-
! Material !! Value
|-
| 316L stainless steel || 140HV30
|-
| 347L [[stainless steel]] || 180HV30
|-
| [[Carbon steel]] || 55–120HV5
|-
| [[Iron]] || 30–80HV5
|-
| [[Martensite]] || 1000HV
|-
| [[Diamond]] || 10000HV
|}