The degree of tempering of tin-plated steel sheet (referred to as tinplate, commonly known as tinplate) represents an indicator of the overall mechanical properties of the tinplate, usually expressed in terms of its surface Rockwell hardness. This value plays an important role in the use of tinplate processing. Therefore, in the tinplate inspection, the measurement of the Rockwell hardness value of its surface is often involved. This article summarizes and analyzes several factors affecting the accuracy of the hardness test during the actual inspection process for your reference.
1, the test principle of the surface Rockwell hardness
The Rockwell hardness of the surface is determined by using a diamond cone or steel ball as the indenter. The initial pressure of 29.42N and the force of 147.1N, or 294.2N, or 441.3N force are applied to the sample. The difference is obtained by the difference in indentation depth before and after the test. of. The ASTM (American-Chinese Experimental Materials Association) defines thirty different Rockwell hardness scales, Rockwell hardness scales are divided into two categories: regular Rockwell scales and surface Rockwell scales. Tinplate generally specifies the surface Rockwell hardness HR30T, that is, the use of φ1.588mm (1/16``) steel ball indenter and 29.42N initial pressure and 294.2N force.
2. Several factors that affect the measured value of Rockwell hardness on the surface of tinplate
2.1 Effect of test operation
Surface Rockwell hardness tester is relatively simple to operate, but there are many factors in the operation process that affect the test results, so the following points should be noted to reduce the error.
1 Before using the hardness tester, the hardness tester must be calibrated with a standard test block.
2 The results of the first two tests of changing the indenter and the test bench are not used.
3 It is necessary to ensure that the tinplate sample is in good contact with the test bench, and smaller cross-section anvils should be selected as far as possible.
4 Each sample shall be measured at least three times. The distance between the center of each indentation and the distance between the center of the indentation and the edge of the sample shall not be less than 3mm.
5 When the initial pressure is applied, the movement should be completed once, there should be no pause in the middle, and the large pointer should be within 5 grids of the vertical direction of the lead. If it exceeds this range, it should be re-operated.
2.2 Effect of sampling and sample preparation
1 Effect of sampling site
Since the tinplate original plate has different performances in the direction of the bar and the width direction, the different parts of the sample have a direct influence on the test results. The sampling site should be in accordance with the relevant standards. The test result shall be the arithmetic mean of the measured values ​​of each sample at the specified location.
2 Influence of sample processing quality
The quality of the test sample has a great influence on the accuracy of the test data. Although the tinplate sample processing is very simple, due to the small thickness of the tinplate (usually 0.15-0.35mm), it is easy to cause deformation and scratches, resulting in inaccurate results. Therefore, the prepared tinplate sample must be kept flat, smooth, free of scratches and large cut burrs. Therefore, the sample should be processed with proper stamping or shearing methods. The sample processing size should not be too large (generally 3-4cm2 is appropriate). Otherwise, it is not easy to keep the sample flat.
In addition, the surface of tinplate must not be corroded or carry grease.
2.3 Influence of test machine anvil
Test the surface Rockwell hardness of general materials and use steel anvils. The tinplate surface Rockwell hardness test generally specifies the use of diamond anvils. This is mainly due to the fact that the tinplate is very thin, which will cause the indentation to penetrate the backside and cause the center of the steel anvil to be damaged and deformed, resulting in a test error, ie an “anvil effectâ€. Practice has proved that the anvil has a greater impact on the test results.
2.4 Effect of sample thickness
In the hardness test, the thickness of the sample is required. When testing the surface Rockwell hardness, if the back of the sample does not show any traces, it can be proved that the sample thickness meets the requirements. Otherwise, the test results will be affected. However, the thickness of tinplate is mostly between 0.15-0.35mm. Therefore, when testing the HR30T, the back of the specimen shows different degrees of marks. This shows that most tinplate specimens have exceeded thickness specifications. Therefore, for thin tinplate samples, it is often necessary to test HR15T and then change HR30T. The thicknesses specified in the standards of this country are slightly different and should be determined according to specific standards.
2.5 Effect of tinplate surface coating.
In actual inspection work, tinplates with painted or printed surfaces are encountered. A hardness test was performed on a group of tinplate samples with a print coating, and the coatings were then removed for hardness testing. The results of the two groups were compared and it was found that the hardness value of the coated test was lower than the test value of the removed coating 1 - 2. Of course, due to the different coating materials and thicknesses, the impact may be different. Therefore, the coated tinplate should be treated according to different circumstances.
When it comes to claim certification, the coating should be removed for testing in order to measure the true hardness value. In addition, according to the standard, if there is doubt about the test results, the tin layer should be removed and re-inspected.