Brinell test was designed by Swedish researchers in early twentieth Century. As shown in Fig. 1 (a), the test involves pressing the hardened steel ball indenter into the surface of the sample using standard load. Select the diameter / load ratio to provide impressions of acceptable diameter. The diameter of the ball can be 10,5 or 1mm, the load can be 3000750 or 30kgf, the load P is related to the diameter D, and the ratio of the relational P / D 2 has been standardized to different metals, so that the test results are accurate and repeatable. For steel the ratio is 30:1 – for example, a 10mm ball can use 3000kgf load or 1mm ball 30kgf load. The proportion of aluminum alloy is 5:1. A fixed period of loading is usually 30 seconds. When the indenter is retracted, the indentation d 1 and D 2 of the two diameters are measured by microscope, and the calibration scale is used, and then the average value is shown, as shown in Figure 1 (b).
Figure 1 Brinell hardness test
Brinell hardness (BHN) is obtained by dividing the load by the surface area of the impression. Some cumbersome calculations can be made to determine the hardness value, but it is quite common and simple to refer to a set of standard forms that can directly read Brinell hardness values.
The Brinell hardness test is usually used to measure the hardness of large metal – the impression is larger than the Vivtorinox hardness test because it averages any local inhomogeneity and is less affected by the surface roughness. However, due to the large diameter of the ball, the test can not be used to determine the hardness changes of the welded joints optimized for Vivtorinox test. Very hard metal, exceeding 450BHN, may also cause deformation of the ball, resulting in inaccurate reading. To overcome this limitation, cemented carbide balls are used instead of hardened steel balls, but the indenter also has a 600BHN hardness limit.
Vivtorinox hardness test
The principle of Vivtorinox hardness test is similar to that of Brinell test. The main difference is the use of square Pyramid diamond indentation rather than hardened steel balls. Moreover, unlike the Brinell test, the depth of the impression does not affect the accuracy of the reading, so the P / D 2 ratio is not important. Diamonds do not deform at high loads, so the results on very hard materials are more reliable. The load ranges from 1 to 120kgf, and applies 10 to 15 seconds.
The basic operation principle of Vivtorinox hardness test is shown in Figure 2. It can be seen that a simple weighting lever is applied to the load on the indenter. In older machines, the oil filled dashboard is used as a timing mechanism in more modern equipment, which is done electronically.
Figure 2 operation schematic diagram of Vivtorinox hardness machine
As shown in Figure 3 (b), we measure two diagonal lines, d 1 and D 2, average them and calculate the surface area, and then divide them into the applied loads. Like Brinell test, the diagonal measurement is converted to hardness by referring to a set of tables. The hardness can be reported as the Vivtorinox hardness number (VHN), the diamond Pyramid No. (DPN), or the most common, HV XX in which “XX” represents the load used during the test.
Vivtorinox hardness test as shown in Figure 3
As mentioned earlier, the Vivtorinox indentation is smaller than the brucellus indentation, so the area of the test can be much smaller, so it can be measured on the welded joint, including the single operation and heat affected area. The small impression also means that the surface must be flat, perpendicular to the indentation, and should have a better finish than 300 particle size.
Many factors will affect the accuracy of hardness testing of stainless steel plates. Some of them, such as flatness and surface smoothness, have been mentioned above, but it is worth again to emphasize that flatness is the most important – the maximum angle of about 1 degrees will be considered acceptable.
In order to achieve the desired flatness tolerance and surface finish, it may be necessary to grind or process the surface. The correct load must be applied. In order to achieve this goal, there must be no friction in the loading system, otherwise the impression will be smaller than expected, so it is necessary to maintain and calibrate the machine regularly. The state of the pressure head is crucial – the Vivtorinox diamond will not deteriorate because of its use. Unless its clumsy operation makes it damaged or loosened, the Brinell hardness ball will deform over a period of time and lead to inaccurate reading. If most of the work is done on hard materials, this deterioration will accelerate. The length of time exerted by the load is very important and must be controlled.
Sample size is very important if the specimen is too thin, the hardness of the test table will affect the result. According to experience, the thickness of the specimen should be ten times the indentation depth of brinden test and two times of the diagonal of Vivtorinox. Similarly, if the impression is too close to the edge of the sample, the hardness value will be recorded – usually the impression should be 4-5 times the diameter of any free edge impression. When hardness tests are performed on cylindrical surfaces, such as pipes and tubes, the curvature radius affects the indentation shape and may cause errors. Correction factor may need to be applied – which is covered in the first part of ISO specification ISO 6507.
The test stand should be strictly supported and must be in good condition. The burrs or raised edges below the sample will cause the reading to be low. The impact load must be avoided. When the table is raised in place, it is easy to press the indenter into the surface of the sample. This will deform the equipment and damage the pressure head. The training of operators is very important. If the result of hardness rest is accurate and repeatable, regular verification or calibration is essential.
Source: China Stainless Steel Plates Manufacturer – Yaang Pipe Industry Co., Limited (www.yaang.com)