Figure Schematic diagram of hardness test 1 factors affecting the hardness of tinplate
1.1 Effect of sample preparation
In the sampling process of the tinplate hardness test, due to the tinplate There are differences in rolling direction and width direction performance, so the test sampling site should be in accordance with the relevant standards, and the test result is the average value of each test value. During the processing of the specimen, the surface magnetization or cold heat caused by the magnetic field should be avoided Caused by work hardening, try to keep the sample flat, smooth, no scratches and burrs. The processing size of the sample should not be too large, otherwise it is not appropriate to keep the sample flat. In the hardness test, there is a requirement for the thickness of the sample. When testing the Rockwell hardness of the surface, there is no trace on the back of the sample, indicating that the thickness of the sample meets the requirements. However, the thickness of tinplate is mostly between 0.15mm-0.35mm, so when testing HR30T, the back of the sample will show traces to varying degrees. If there are obvious traces on the back surface of the specimen during the test, it means that the tinplate specimen exceeds the thickness specification. For thinner tinplate, the HR15T test is required and then converted into HR30T.
The influence of tinplate surface state, when the surface of the sample is oily, it will make the hardness value of the sample low, the surface roughness of the sample is large, there are defects such as cracks, inclusions or corrosion points, it will make the hardness value fluctuated and dispersed, inaccurate . When the surface of the specimen is curved or convex, deviations will occur between the test surface and the contact surface of the indenter, affecting the accuracy of the test hardness value. In the tinplate hardness test, the coated tinplate will cause the hardness value to decrease. Therefore, the surface coating should be removed during the test and then tested to ensure that the test surface is clean, free of defects, and free of dirt so that accurate measurements can be made. Hardness value.
In the test preparation stage, to ensure the consistency of the test temperature, changes in temperature will affect the Rockwell hardness of the surface, especially the high hardness test specimens. Ensure that the indenter and experimental anvil of the hardness tester meet the requirements for use. When the surface of the indenter has oil contamination, roughness, cracks, or breakage, it will seriously affect the hardness value of the tinplate sample test. For tinplate hardness tests, the use of diamond anvils is generally stipulated. This is mainly due to the thin tinplate, which makes the indentation penetrate the back of the specimen, which can easily damage the center of the steel anvil and cause the test error.
1.2 Effect of test operation
Although the Rockwell hardness test of the surface of tinplate is simple, there are many factors that affect the test result during the test operation. Therefore, the following points should be noted in the test operation to reduce the test error.
(1) The standard test block should be kept updated and valid. The standard test block is generally valid for one year. Before use, the hardness tester must be calibrated with a standard test block.
(2) The results of the first two tests after replacing the indenter and the test bench during the test are not used.
(3) Ensure that the sample is in good contact with the test bench during the test. Therefore, try to select an anvil with a small cross-section and place the test specimen on the test bench before placing it on the test bench.
(4) At least three tests for each sample, the distance between the center of each indentation The distance from the center of the indentation and the distance from the edge of the specimen should not be less than 3mm.
(5) The speed of the test force applied during the test should be even and even to prevent The acceleration is too fast or too slow, causing inaccurate test hardness values.
2 different tinplate hardness and use
Tinplate is widely used in metal packaging cans and top and bottom cover, but Different specifications of tinplate application are not the same. Different regulations The hardness of the tinplate is not the same, and the hardness has a close relationship with the application of tinplate. A traditional cold-rolled tin plate (SR material) is a three-piece food and beverage can production Commonly used raw materials, sheet thickness 0.20mm or more, secondary Cold-rolled tin plate (DR material) sheet thickness decreases, hardness value increases, tensile strength Increased strength. With the technological innovation of can making industry, secondary cold rolling Tin plates are increasingly used in food and beverage cans, and secondary cold rolled tinplates have Helps reduce costs, save steel, reduce carbon emissions, and achieve resource savings For environmental friendly purposes. According to literature review, different specifications of tinplate The hardness target values, characteristics, and applications are shown in the following table.
Table Different hardness of tinplate and its use
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