Vickers hardness test
The Vickers hardness test was developed in 1924 by Smith and Sandland at Vickers Ltd. as an alternative to Brinell Hardness test designed to measure the hardness of very hard materials. The Vickers hardness test is often easier to use than other hardness testing methods because the calculation of the hardness value required is independent of the size of the indenter and can be applied regardless of the hardness of the material.
As with all common hardness testing methods, the basic principle is based on the resistance that the material offers to a harder indenter, or on the permanent deformation that this causes. The Vickers test can be used for all metals and has the widest range of applications of all hardness testing methods (from extremely soft lead to extremely hard steels or even ceramics). The unit of hardness determined by this test is called Vickers hardness. The abbreviation HV stands for Vickers hardness followed by the test force in kgf, e.g. HV10.
The degree of hardness is not a material characteristic, but an empirical value that must be viewed in connection with the hardness testing method selected and the measuring range or test force used. During the hardness test, the distance between the respective Vickers indentations must be greater than 2,5 times the indentation diagonal in order to avoid the hardness test being influenced by material compaction/work hardening caused by adjacent indentations.
The Vickers hardness test is the only reliable method, particularly for hardness testing on thin layers or films, in the edge areas of a workpiece and for surface hardening. Due to the relatively small indentations and the very good positioning of the indentations, this method is ideal for determining the hardening depth CHD (EHT - hardening depth), SD (RHT - surface hardening depth) and NHT (nitriding hardening depth). In addition, this is a non-destructive testing method (due to the small size of the indentations). If a Vickers indentation is too wide for a thin layer (testing from the side of a prepared sample), a Knoop indenter can be used as an alternative. The Knoop indenter or the test indentation created with it is essentially an elongated Vickers indentation (diamond or rhombus). The hardness is calculated in the same way as Vickers.
Vickers hardness test
After the test load is applied, the Vickers pyramid leaves a square impression in, for example, case-hardened steel. When developing the method, it was determined that the shape of the indenter should be designed in such a way that the impressions created have comparable geometric shapes regardless of the size of the test load. The impression should have precisely defined points that can be easily measured. In addition, the indenter should have a high resistance to self-deformation. A (synthetic) diamond in the shape of a square pyramid met these requirements best. From experience with the Brinell test, it was known that the ideal size of a Brinell ball impression is 3/8 of the ball diameter (indentation depth). Since the ends of the second tangents to a circle meet at an angle of 3° with a chord of 8d/136, it was decided to define this angle as the geometric shape of the pyramid. Further tests have shown that the results of the hardness test for homogeneous material are constant regardless of the level of the applied test load.
The Vickers hardness is determined from the ratio F / A. F is the test load (force) applied to the diamond in kg and A is the area of the impression produced in square millimeters. Since the test force was still given in kp at the time this hardness testing method was developed, the factor 0,102 (reciprocal of 9,81 = factor for converting Newton to kgf) has been included in the metrological calculation of the Vickers hardness that is valid today.
Video Animation Functional Principle
Test loads (typical test loads in BOLD)
|
Methods Vickers – Hardness Test |
Methoden Knoop |
||||
|
test load |
test load |
test load |
test load |
test load |
test load mN milli Newton |
|
0,005 |
49,03 |
1 |
9,806 |
||
|
0,010 |
98,1 |
2 |
19,61 |
0,010 |
98,1 |
|
0,015 |
147,1 |
3 |
29,41 |
||
|
0,020 |
196,1 |
4 |
39,22 |
||
|
0,025 |
245,2 |
5 |
49,03 |
||
|
0,030 |
294,2 |
10 |
98,06 |
||
|
0,050 |
490,3 |
20 |
196,1 |
0,050 |
490,3 |
|
0,100 |
980,6 |
30 |
294,2 |
0,100 |
980,6 |
|
0,200 |
1961 |
40 |
392,2 |
||
|
0,300 |
2942 |
50 |
490,3 |
||
|
0,400 |
3923 |
100 |
980,6 |
||
|
0,500 |
4903 |
120 |
1176 |
0,500 |
4903 |
|
1 |
9806 |
||||
According to DIN EN ISO 6507-1 to 4 (Vickers hardness test), the hardness test method and the test force must always be specified when presenting the results.
Example: 440 HV30
This means:
440 = hardness value
HV = Vickers hardness with diamond pyramid 136°
30 = test load (force F) in kg or kgf
If the test load duration is adhered to in accordance with the standard (10 - 15 seconds), nothing is stated after the hardness value. Only if the test load duration is not adhered to does the time also have to be documented.
Example: 440 HV30/20
Metallographic preparation for hardness tests according to Vickers | Brinell | Knoop
In addition to the microstructure analysis Metallographic preparation is used to prepare for a hardness test (typical for Vickers) on the sample surface or for hardness profiles. For an exact determination of the hardness depth, metallographic preparation down to the polished surface is required. This applies in particular to an automated hardness test using a fully automatic machine for the following evaluations:
- CHD (EHT) case hardening depth-hardening depth
- SHD (Rht) surface hardening depth
- NHD (Nht) nitriding hardness depth
- CD (At) carburization depth
- CLT (VS) bonding layer thickness
- hardness gradients on components
Hardness testing machines according to test force | Vickers
Hardness testing machines belong to the group of material testing machines or material testing machines. The standards listed below consistently use the term hardness testing machine. Since there is no definition for the term hardness testing machine, the terms hardness tester, hardness tester, hardness measuring device, hardness meter or other variations.
According to DIN EN ISO 6507-1 for the Vickers method (only here), the group of hardness testing machines is divided into the following groups depending on the test force:
|
Description |
common name |
test force in Newton |
test procedure |
|
Vickers microhardness test |
micro hardness testing machine |
≤ 0,009.807 to < 1,961 |
< HV0,001 to < HV0,2 |
|
Vickers hardness test in the low-force range |
low-force hardness testing machine |
1,961 ≤ to < 49,03 |
HV0,2 to < HV5 |
|
Hardness test according to Vickers |
Vickers hardness testing machine |
F ≥ 49,03 |
> HV5 |
Since the indenter creates an impression in the material, this test is a destructive test. However, the metal hardness test method according to the Vickers standard DIN EN ISO 6507-1 is an exception to this. The hardness test impression created here is relatively small (at least with low test forces). This can usually be removed with simple means (sandpaper). The Vickers hardness test method is therefore classified as a non-destructive hardness test method.
A hardness tester is a mechanical device in which a defined force is applied to an indenter. Since the indenter is harder than the material to be tested, the tip of the end ring body penetrates the material. If the indenter can penetrate deeper, a larger or deeper impression is created. The indentation size is measured optically in accordance with the standards DIN EN ISO 6507-1 (Vickers) or DIN EN ISO 6506-1 (Brinell). In the measuring methods defined in DIN EN ISO 6508-1 (unlike the two methods mentioned above), the indentation size is not measured (optically) but the depth difference of the indentation is used for qualification. An optical measuring device is required for the Brinell and Vickers methods. For the methods according to Rockwell a depth-distance measuring system is required.
The first commercially usable hardness testing machine was developed by the Swede Ernst August Brinell in 1900. In this hardness testing machine, a very hard ball was pressed into the material to be tested using a weight loading system. The test force to be applied was applied using weights. Later developments also used devices in which the test force was applied using a hydraulic pump or spring force. The weight loading system was the most common until the end of the 20th century. Currently (in 2019), hardness testing machines with a weight loading system (from CHINA) or hardness testing machines with spring loading systems (Italy) are still offered. However, these are increasingly being replaced by more precise, electronic load cells with an associated motorized load system in a closed control loop. The state of the art is also that the optical measuring systems (magnifying glass, ground glass) are being replaced by camera measuring systems and image analysis systems.
Modern Rockwell hardness testing machines do not use pointer instruments (dial gauges) but rather high-resolution electronic sensors.
The world market leader INNOVATEST supplies suitable equipment for every application and every budget:
- Rockwell hardness testing machines with weight loading system (low budget, no power requirement)
- Rockwell hardness testing machines with load cell and pointer instrument
- Rockwell hardness testing machines with electronics and test software integrated into the electronics
- Rockwell hardness testing machines with electronics and WINDOWS hardness testing software
- Vickers / Brinell hardness testing machines with electronics and WINDOWS hardness testing software
- up to fully automatic machines with motorized XY table for automatic test sequences / integrated into production systems
