Implementation
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, 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. 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
which can be approximated by evaluating the sine term to give
where d is the average length of the diagonal left by the indenter in millimeters. Hence,
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 kilogram-force to newtons by multiplying by 9.806 65 (standard gravity) and convert mm to m. To do the calculation directly, the following equation can be used:
where F is newtons and d is millimeters.
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.
Material | Value |
---|---|
316L stainless steel | 140HV30 |
347L stainless steel | 180HV30 |
Carbon steel | 55–120HV5 |
Iron | 30–80HV5 |
Martensite | 1000HV |
Diamond | 10000HV |
Read more about this topic: Vickers Hardness Test