The Application of Hardness Tester for Metal Casting

1. Brinell hardness tester

Brinell hardness is the first option for metal casting hardness test, especially for the relatively coarse grain gray iron castings, Brinell hardness tester is the only choice, and we need to try to choose 3000kg force, 10mm ball test conditions. When the metal casting size is small, we can also choose Rockwell hardness tester.

Metal casting hardness test preferred Brinell hardness tester is due to the following two points:

a. Iron castings' organization is usually uneven and the grains are larger, the carbon, silicon and other impurities contained are more than those in steel. The sizes of hardness on different small areas or points are different. The Brinell hardness tester has larger size of head and larger indentation and it can detect the average hardness in a range, so the Brinell hardness tester has a higher accuracy, and smaller dispersion of hardness values, the hardness value measured is more representative of the actual state of the workpiece hardness. So Brinell hardness tester is widely used in the foundry industry.

b. The tensile strength is the first mechanical property of metal castings, and almost all of the metal castings standards have tensile strength requirements. The Brinell hardness and tensile strength values have a very close relationship and the two values can be converted. The tensile strength of gray cast iron can be counted by the following formula: σb = 1.82 (HB) 1.85

This relationship can also be obtained by look up the table. Brinell hardness test can help us get the value of the tensile strength quickly and easily and improve test efficiency and reduce testing costs.

2. Rockwell hardness tester

Rockwell hardness tester is also commonly used in the inspection to cast iron. All the parts with smaller grains, if there is not enough space for the Brinell hardness test, can be tested by Rockwell hardness tester. For pearlitic malleable iron, chilled cast iron and steel castings, HRB or HRC scale can be used, and if the material is heterogeneous, several readings should be measured to get the average value.

Rockwell hardness test is quick, convenient and has small indentation, which can be used to directly test the finished piece. It is suitable for testing mass production of finished or semi-finished parts piece by piece.

3. Shore Hardness Tester

In some cases, for some larger metal castings, cutting the sample or cast test blocks for hardness testing is not available, so hardness testing will be difficult. In this case, the common approach is to use portable Shore hardness tester on the smooth surface after finishing.

As a result of following the dynamic hardness principle, many factors affect the result of Shore hardness test; the test accuracy is much lower than the Brinell hardness tester and Rockwell hardness tester, which work on the static indentation hardness test principle. For this reason, in roll standard, Roll hardness comparison is also recommended, which plays as a standard sample, and the hardness value is to rely on cutting the sample to get accurate test. After using Shore hardness tester, you need to check detection accuracy on the contrast roll.

4. Leeb hardness tester

Nowadays, Leeb hardness tester is widely used in metal castings hardness. Leeb Hardness Tester is improved on the basis of Shore hardness tester. It is also applied to the principle of dynamic hardness testing, using computer technology to achieve the miniaturization, electronization of a hardness tester. It is easy to use, and the test results can be easily converted into Brinell hardness value, and it has been widely welcomed.

However, like Shore hardness, Leeb hardness tester accuracy is not high, many factors can affect test accuracy, such as requiring high surface finish, conversion tables lacking the authority, and conversing errors. Therefore, the Leeb hardness test results have been frequently used as a reference value in our country, and Leeb hardness tester is mainly used for those metal castings that have a wider range of requirements. In addition, as a non-formal method, the Leeb hardness tester is not widely recognized in International Organization for Standardization, and it has not been used in Product Standards of metal castings in foreign countries. In international trade, the hardness test results are not accepted by most foreign investors.

Many metal castings are quite large, some weighing several tons, which can not be moved on the hardness tester desktop. The precise hardness test of metal castings mainly use test rod that cast separately or a test block attached on metal castings. However, neither the test rod or test block can fully replace the part itself, even in the same furnace hot metal, and the casting and heat treatment conditions are identical, because of the huge difference in size, both heating rate, especially the cooling rate are different, the two can hardly get the same hardness, for this reason, many customers are more concerned with and believe in workpiece hardness itself, and this requires a portable hardness tester to test metal casting hardness.

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