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I. Basic principle of the Leeb hardness tester: With the development of microchip technology, in 1978, Dr. Leeb from Switzerland proposed a completely new method for measuring hardness. The basic principle of this method is that an impact body with a certain mass strikes the surface of the test specimen under a specified testing force; the impact velocity and rebound velocity of this impact body at a distance of 1 mm from the specimen surface are measured, and electromagnetic principles are utilized to generate a voltage proportional to the velocity. The Rockwell hardness value is expressed as the ratio of the rebound velocity of the impact body to the impact velocity. Calculation formula: HL = 1000 * (VB/VA). Where: HL – Rockwell hardness value; VB – Rebound velocity of the impact body; VA – Impact velocity of the impact body. II. Impact device of the Rockwell hardness tester. There are seven types of Rockwell hardness scales: D, DC, D=15, C, G, E, DL. D: Dimensions: f20*70mm; weight: 75g. It is a general-purpose type suitable for most hardness measurements. DC: External dimensions: f20*86mm, weight: 50g. The impact device is very short and is mainly used in very confined spaces, such as holes or cylinders. D+15: External dimensions: f20*162mm, weight: 80g. The head is small, used for measuring surface hardness on grooved or recessed surfaces. C: External dimensions: f20*141mm, weight: 75g. It has the lowest impact energy and is used to measure small, lightweight, thin components as well as surface-hardened layers. G: External dimensions: f30*254mm, weight: 250g. It has high impact energy and low requirements for the measurement surface. Used for large, heavy, and rough-surfaced forged and cast parts. E: External dimensions: f20*162, weight 80g. The indenter is made of synthetic diamond and is used for testing materials with extremely high hardness. DL: Dimensions: f20*202mm, weight: 80g. The head is more slender, designed for measuring the hardness of narrow grooves and gear surfaces. III. Use of special-shaped support rings: In field work, curved test pieces are often encountered, and different types of curves have varying effects on the hardness test results. With proper operation, the location where the impact occurs on the surface of the test piece is the same as that in the case of flat test pieces; therefore, standard support rings are sufficient. However, when the curvature reaches a certain level, the significant differences in the elastic behavior resulting from the planar condition cause the rebound velocity of the impact body to be low, thereby leading to a lower value for the Rockwell hardness. Therefore, for the specimen, it is recommended to use a small support ring during measurement. For specimens with a smaller radius of curvature, it is recommended to use special-shaped support rings. IV. Measurement range of the Rockwell hardness tester: According to the Rockwell principle, as long as a material has sufficient rigidity to allow for rebound, an accurate Rockwell hardness value can be determined. However, for many materials, there is no conversion formula between Rockwell hardness and other types of hardness measures; therefore, the Rockwell hardness tester currently comes equipped with conversion tables for only 9 types of 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, and stainless copper. For test samples of some special materials, users can use the fitting curve software provided by the company to create custom conversion tables. In actual production, a wide variety of metal materials are used. Since the Rockwell hardness tester is sensitive to the processing method of the material as well as its alloy composition, the hardness conversion tables stored in the tester’s chips may not meet all of the users’ needs. Therefore, during testing, users can use fitting software to create their own custom hardness conversion tables. V. Factors Affecting the Testing Accuracy of the Rockwell Hardness Tester 1. Errors arising from data conversion The errors that occur when converting Rockwell hardness to other types of hardness include two aspects: one is the measurement error of Rockwell hardness itself, which relates to the variability in tests conducted according to the specified methods, as well as the measurement errors associated with multiple Rockwell hardness testers of the same model. On the other hand, it involves comparing the errors in hardness values obtained using different hardness testing methods; this is due to the lack of a clear physical relationship between these various testing methods, as well as the impact of unreliable measurements in such comparisons. 2. Errors caused by special materials: The conversion tables stored in the hardness tester may yield deviations for the following types of steel: all austenitic steels, heat-resistant tool steels, and ledeburite chromium steels (tool steels). Hard materials cause an increase in the elastic modulus, resulting in a lower L value. Such steels should be tested across their cross-section; local cooling and hardening can result in a higher L value, while magnetic steels, due to the influence of magnetic fields, yield a lower L value. Surface-hardened steel with a soft matrix results in a lower L value; however, when the hardened layer is greater than 0.8 mm (0.2 mm for the Type C impact device), it has no effect on the L value. 3. Errors in gear inspection: Generally, due to the small size of the tooth surfaces, the testing errors are relatively large. In this regard, users can design appropriate fixtures based on the specific situation, which can help reduce these errors. 4. Influence of material elasticity and plasticity: The Rockwell value is related not only to hardness and strength but also to the elastic modulus. Hardness values are characteristic parameters of a material’s hardness and plasticity, as the properties of these two aspects must be determined together. In the elastic region, it is primarily influenced by the E-modulus; in this regard, when the static hardness of the material is the same but the values of E differ, the material with a lower E-value will have a larger L-value. 5. Errors caused by the hot rolling direction: When the specimen in question was formed through hot rolling, if the testing direction is aligned with the rolling direction, the higher value of the elastic modulus E will result in lower test values. Therefore, the testing direction should be perpendicular to the hot rolling direction. For example, when measuring the hardness of a cylindrical cross-section, it is better to conduct the test radially. (The general hot-rolling direction for cylinders is axial.) 6. Effects of sample weight, roughness, and thickness. 7. The magnetism of the specimen should be less than 300 gauss. 8. Effects of other factors. When measuring the hardness of pipe fittings, it is necessary to ensure that the fittings are securely supported; the testing points should be located near the support points and parallel to the force of support. For pipes with thin walls, an appropriate core should be placed inside them.