Hardness, the character of any material, is the most vital factor while ensuring the quality production and manufacture of any heavy item. That’s why it is necessary to determine the hardness value first for structural integrity. For this purpose, many hardness testing methods are introduced and one of them is the Vickers hardness test.
To understand the Vickers Hardness test, it is necessary to have a list of ASTM Hardness guidance related to the test. This list explains the uses, applications and scale of the test. You can select appropriate values and scales from these tables while performing the test. It is recommended to have a copy of tables of the test for accurate outcomes.
I. Significance of Vickers Hardness Standard Test
Vickers hardness test is the nondestructive testing method, which is significant while evaluating materials, a quality check of the manufacturing procedure, and creating new products. Vickers hardness testing method is the only procedure that we can say is universal because of its scaling, as it can be used for testing hard objects such as steel and also for soft objects for example plastic. As the test is a non-destructive method, the sample object is not completely deformed or destroyed, that’s why it can be used for any other purpose also.
II. Principles of Vickers Hardness Test
The general principle of the Vickers hardness test, defined by ASTM E-384, is based on an optical measurement system. The Vickers hardness test can be a micro indentation test and also for the macro hardness test. It is mainly divided into two steps.
- The test force (load) F is applied on the specified square base pyramid shaped diamond indenter for the specific time when it collides with the testing sample object in the direction normal to the test surface.
- Next, after the collision, the lengths of both diagonals are measured and then the mean of these diagonals is calculated. This value is looked up on the scale to find the hardness of the object. For the Knoop hardness test, similar to the Vicker hardness test, the length of the long diagonal is only measured to calculate the hardness value.
Loads in the Vicker hardness test are very light, ranging from 10gm to 30kg.
III. Test Specimen
There is no standard shape or size for a Vickers or Knoop test specimen. The specimen on which the indentation is made should conform to the following:
- Preparation: The surface should be polished in the way that the parameters and the tip of the indentation should provide clear measurements.
- Alignment: The sample object should be placed in a way that the surface is perpendicular to the axis of the indenter.
- Mounted: Spacemen should be mounted when surface gradient tests are performed.
- Thickness: There should be no bulge or other marks in the measuring value of the thickness of the sample object. The thickness of the sample should be more than ten times bigger than the depth caused by indentation.
IV. Test Procedure
The Vickers hardness test works on the same procedure, as any other hardness test. The test force is applied on an indenter to create a penetration on the test specimen. For this surface of the test needs to be straight and polished. Following are the steps to follow while processing the Vickers hardness test.
- First, polish the testing specimen and make it smooth and clear.
- Then, put the material on the testing surface to contact with a diamond shaped inverted pyramid indenter. Keep in mind that every Vickers indenter is of the same size, shape, and features.
- After the preparation of the testing equipment and calibration, drop the load on the intender to create a deep indentation for the specific dwell time.
- After the dwell time, remove the load from the testing surface and measure the value of indentation through an optical device such as a microscope. Where the dimension of the indentation is examined.
- Then manually apply the standard formula to find the hardness value of the material.
Generally, if the indentation is shallow and not well defined then the hardness of the material is high.
V. Verification Testing
There are three types of verification methods to get the accurate and reliable result of the Vickers hardness test.
Direct verification
Direct verification is required for every new testing machine, or any adjustment, modification or repair which could affect the outcomes, test force and measuring system also. It needs to be applied when indirect verification cannot provide the required results.
Indirect verification
It should be done yearly or when the machine is moved, modified or repaired.
Periodic verification
Periodic verification is done a week before the machine is to be used, but you can do periodic verification every day before using the machine also. Also, this verification is needed when the machine is moved or anything like tester or indenter is changed.
VI. ASTM Standards
Vickers Scale (HV) | Knoop Scale^ (HK) | Test force (N) | Approximate Test force (kgf) | Approximate Test force (gf) |
0.001 | 0.001 | 0.009807 | 0.001 | 1 |
0.01 | 0.01 | 0.09807 | 0.01 | 10 |
0.015 | 0.015 | 0.1471 | 0.015 | 15 |
0.02 | 0.02 | 0.1961 | 0.02 | 20 |
0.025 | 0.025 | 0.2451 | 0.025 | 25 |
0.05 | 0.05 | 0.4903 | 0.05 | 50 |
0.1 | 0.1 | 0.9807 | 0.1 | 100 |
0.2 | 0.2 | 1.961 | 0.2 | 200 |
0.3 | 0.3 | 2.942 | 0.3 | 300 |
0.5 | 0.5 | 4.903 | 0.5 | 500 |
1 | 1 | 9.807 | 1 | 1000 |
2 | 2 | 19.61 | 2 | 2000 |
3 |
| 29.41 | 3 |
|
5 |
| 49.03 | 5 |
|
10 |
| 98.07 | 10 |
|
20 |
| 196.1 | 20 |
|
30 |
| 294.1 | 30 |
|
50 |
| 490.3 | 50 |
|
100 |
| 980.7 | 100 |
|
120 |
| 1177 | 120 |
|
VI. ISO Standards
Microindentation Force Ranges (<= 1 kgf)
Hardness | Force gf | Range % | |
Vickers | Knoop |
| Less |
HK > HK < 100 250 < HK > 100 250 < HK > | HV > HV < 100 <= 240 < HV > 100 240 < HV > | 1 <= 100 < 100<= 100<= 100<= 500 500 500 | <12 <12 <12 <4 <3 <7 <3 <2
|
Microindentation Force Ranges (>1 kgf)
Hardness Range of Standardized Test Blocks | Force gf | Range % |
|
| Maximum |
100 to 240 inclusive Over 240 to 600 inclusive Over 600 | >1 >1 >1 | 3 2 1.5 |
Conclusion
It is necessary to proceed with any hardness test such as the Vickers Hardness test according to the ASTM standards. For accurate and reliable results, scales provided by the ASTM should be used while performing any Vickers or Knoop hardness test. To learn more about the hardness tests, their procedures, or ASTM standard, stay updated with our website.