Testing principle of Leeb hardness tester
Testing principle of Leeb hardness tester First, the basic principle of the Leeb hardness tester With the development of single-chip technology, in 1978, the Swiss Dr. Leeb first proposed a new method of hardness testing. Its basic principle is that an impact body with a certain quality is tested in a certain The surface of the sample is impacted by force, and the impact velocity and rebound velocity at a distance of 1 mm from the surface of the sample are measured. Using the electromagnetic principle, a voltage proportional to the velocity is induced. The Leeb hardness value is expressed as the ratio of the rebound speed of the impact body to the impact speed. 7. The magnetic properties of the test piece should be less than 300 Gauss
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Calculation formula: HL = 1000 * (VB / VA)
In the formula: HL-Leeb hardness value
VB-rebound speed of impact body
VA——Impact speed of impact body 2. Impact device of Leeb hardness tester (samples are available in the hardness tester accessories)
There are seven kinds of Leeb hardness: D, DC, D = 15, C, G, E, DL:
D: Overall size: f20 * 70mm, weight: 75g. Universal type, used for most hardness measurement.
DC: Dimensions: f20 * 86mm, weight: 50g. The impact device is very short and is mainly used in very cramped places, such as holes or cylinders.
D + 15: Dimensions: f20 * 162mm, weight: 80g. The head is small, used for measuring the hardness of grooves or concave surfaces.
C: Dimensions: f20 * 141mm, weight: 75g. The impact energy is the smallest, used to measure small and light, thin parts and surface hardened layer.
G: Dimensions: f30 * 254mm, weight: 250g. The impact energy is large, and the requirements on the measurement surface are low. Used for large, heavy and rough forging castings.
E: Appearance size: f20 * 162, weight 80g indenter is artificial diamond, used for the determination of extremely hard materials.
DL: Dimensions: f20 * 202mm, weight: 80g, the head is thinner, used for the determination of the hardness of narrow grooves and gear faces.
Third, the use of special-shaped support ring (hardware tester samples are welcome)
In the field work, curved test pieces are often encountered, and various curved surfaces have different effects on the hardness test results. Under the correct operation, the impact position on the surface of the test piece is the same as the flat test piece, so the universal support ring is can. However, when the curvature is small to a certain size, the difference in elastic state due to the deformation of the plane condition will significantly lower the rebound speed of the impact body, thereby lowering the value of the Leeb hardness. Therefore, it is recommended to use a small support ring for the sample. For samples with a smaller radius of curvature, it is recommended to use a special-shaped support ring. 4. The measuring range of the Leeb hardness tester is based on the Leeb principle. As long as the material has a certain rigidity and can form a rebound, the accurate Leeb hardness value can be measured, but many materials have no corresponding conversion relationship with the hardness of other systems. , So the Leeb hardness tester currently only has a conversion table for 9 materials. The specific materials are as follows: steel and cast steel, alloy tool steel, gray cast iron, ductile iron, cast aluminum alloy, copper-zinc alloy, copper-tin alloy, pure copper, stainless copper.
For some samples of special materials, users can use the fitting curve software provided by the company to make a special conversion table. In actual production, there are many kinds of metal materials used. Because the Leeb hardness tester is sensitive to the processing method of the material and the alloy element composition of the material, the hardness conversion table stored in the Leeb hardness tester chip cannot meet the needs of users. In the test, users can use the fitting software to make their own hardness conversion table.
5. Factors affecting the test accuracy of the Leeb hardness tester
1. Errors caused by data conversion Errors when converting the Leeb hardness to other hardness include two aspects: on the one hand, the measurement error of the Leeb hardness itself, which involves the dispersion during the test according to the method and for multiple sets of the same model Leeb The measurement error of the hardness tester. On the other hand, it is to compare the errors produced by the hardness measured by different hardness test methods. This is because there is no clear physical relationship between one of the various hardness test methods, and it is affected by the unreliable measurement in the comparison.
2. The conversion table stored in the hardness tester for errors caused by special materials may produce deviations for the following steel types:
All austenitic steel heat-resistant tool steels and Leystenite chromium steels (tool steels) hard materials will cause an increase in the elastic modulus, resulting in a low L value. This type of steel should be tested on the cross section. Local cooling and hardening will cause a high L value. Magnetic steel will make the L value low due to the influence of the magnetic field.
The surface hardened steel and soft substrate will make the L value low. When the hardened layer is greater than 0.8mm (C type impact device is 0.2mm), it will not affect the L value.
3. Errors in gear detection Under normal circumstances, due to the small tooth surface, the test error is relatively large. For this, users can design corresponding tooling according to the situation, which will help reduce errors.
4. The influence of material elasticity and plasticity The Richter scale value is not only related to hardness and strength, but also related to the elastic modulus. The hardness value is a characteristic parameter of the hardness and plasticity of the material, because the components of the two must be jointly determined.
In the elastic part, first of all, it is obviously affected by the E modulus. In this respect, when the static hardness of the material is the same, and the E value is different, the material with a low E value has a larger L value.
5. Error caused by hot rolling direction When the measured workpiece is formed by hot rolling process, if the test direction is consistent with the rolling direction, the test value will be low due to the large elastic modulus E, so the test direction should be perpendicular to the hot Rolling direction. For example: when measuring the hardness of the cylindrical section, it should be tested in the radial direction. (Generally, the direction of cylindrical hot rolling is axial).
6. Influence of sample weight, roughness and thickness
8. The influence of other factors should be noted when measuring the hardness of the pipe fittings: the pipe fittings should be firmly supported, the test point should be close to the support point and parallel to the support force, the pipe wall is thin, and a suitable core is placed in the pipe.