Vickers hardness test

A Vickers hardness tester

The Vickers hardness test was developed in 1921 by Robert L. Smith and George E. 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 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 pressure.

Implementation

Vickers test scheme

The pyramidal diamond indenter of a Vickers hardness tester.

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.

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 ⅜ 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, giving an angle to the horizontal plane 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.^[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 millimeters. 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 in millimeters. Hence,^[3]

HV={\frac {F}{A}}\approx {\frac {1.8544F}{d^{2}}}\quad [{\textrm {kgf/mm}}^{2}]

where F is in kgf and d is in millimeters.

The corresponding units of HV are then kilograms-force per square millimeter (kgf/mm²). To calculate Vickers hardness number using SI units one needs to convert the force applied from newtons to kilogram-force by dividing by 9.806 65 (standard gravity). This leads to the following equation:^[4]

HV\approx {0.1891}{\frac {F}{d^{2}}}\quad [{\textrm {kgf/mm}}^{2}],

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/mm2), but results directly in the Vickers hardness number (usually given without units), which is in fact kilograms-force per square millimeter (kgf/mm²).

To convert the Vickers hardness number to SI units the hardness number in kilograms-force per square millimeter (kgf/mm²) has to be multiplied with the standard gravity (9.806 65) to get the hardness in MPa (N/mm²) and furthermore divided by 1000 to get the hardness in GPa.

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 kgf.
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]

For thin samples indentation depth can be an issue due to substrate effects. As a rule of thumb the sample thickness should be kept greater than 2.5 times the indent diameter. Alternatively indent depth can be calculated according to:

h = \frac{d}{2\sqrt{2}\tan{\frac{\theta}{2}}} \approx \frac{d}{7.0006},

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
Martensite	1000HV
Diamond	10000HV

Precautions

When doing the hardness tests the minimum distance between indentations and the distance from the indentation to the edge of the specimen must be taken into account to avoid interaction between the work-hardened regions and effects of the edge. These minimum distances are different for ISO 6507-1 and ASTM E384 standards.

Standard	Distance between indentations	Distance from the center of the indentation to the edge of the specimen
ISO 6507-1	> 3·d for steel and copper alloys and > 6·d for light metals	2.5·d for steel and copper alloys and > 3·d for light metals
ASTM E384	2.5·d	2.5·d

Estimating tensile strength

If HV is expressed in kg/mm² then the yield strength (in MPa) of the material can be approximated as $σ u$ ≈ HV× $c$ ≈ HV/0.3, where $c$ is a constant determined by geometrical factors usually ranging between 2 and 4.^[7]

Application

The fin attachment pins and sleeves in the Convair 580 airliner were specified by the aircraft manufacturer to be hardened to a Vickers Hardness specification of 390HV5, the '5' meaning five kiloponds. However, on the aircraft flying Partnair Flight 394 the pins were later found to have been replaced with sub-standard parts, leading to rapid wear and finally loss of the aircraft. On examination, accident investigators found that the sub-standard pins had a hardness value of only some 200-230HV5.^[8]

Currently some watch companies are improving the crystal watch glass by testing the strength using the Vickers hardness test. While they are progressively making their watches better, they are also using it as a marketing information for the consumer. The everyday consumer is now being made aware of crystal strength in their watches due to the Vickers hardness test. Some manufactures are promoting that they have watch glass at 6000 Vickers compared to other watches rating at 700.^[9]

References

↑ 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.
↑ The Vickers Hardness Testing Machine. UKcalibrations.co.uk. Retrieved on 2016-06-03.
↑ ASTM E384-10e2
↑ ISO 6507-1:2005(E)
↑ Vickers Test. Instron website.
↑ Smithells Metals Reference Book, 8th Edition, ch. 22
↑ "Hardness". matter.org.uk.
↑ Report on the Convair 340/580 LN-PAA aircraft accident North of Hirtshals, Denmark on September 8, 1989 | aibn. Aibn.no. Retrieved on 2016-06-03.
↑ "51 of the Best Military and Tactical Watches for Men-Great gift idea!". 2016-08-18. Retrieved 2016-08-26.

External links

Video on the Vickers hardness test
Vickers hardness test
Conversion table – Vickers, Brinell, and Rockwell scales

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