The test method as well as the tolerances required should be defined in the suitability test. The product should have a sufficiently large test surface, which is later prepared for the surface hardness test.
For the surface hardness test, you need a flat and scratch-free test surface. Unlike the core hardness test, the sample is not embedded. This complicates the sample preparation. Since most of the products have a coating, it has to be removed beforehand. For this purpose, the screw head is carefully grinded with a fine sandpaper. It is important to ensure that the coating is removed completely on the test spot and, at the same time, not too much of the base material is removed from the screw. It is ideal, if only the coating is removed. Afterwards, the sample is polished with a diamond suspension. You can recognise a good sample preparation by the fact that the residues of the coating are visible next to the test surface.
The test device is reviewed, as it is during the core hardness test. A corresponding hardness test block is measured and values are documented. Then, it is decided, whether the machine can be used. The laboratory of Würth Industrie Service carries out these checks every day for each test method implemented and the corresponding hardness range.
The test is performed mostly in HV0.3. The diamond pyramid is pressed into the surface of the sample with a force of 300g. Thus, the hardness indentation is relatively small. This has the advantage that a possible carburisation is not penetrated. It should also be ensured that the test surface lies flat for the indenter. During this test method, even a minor unevenness has a major impact on the test result. The hardness indentation is distorted and does not give the actual result. The hardness indentations are spread over the test spot and subsequently analysed.
For evaluating the result, a report similar to core hardness is also created here. It should be noted that only an upper tolerance of 390 and 435HV0.3 for the property classes 10.9 and 12.9 is defined for the surface hardness test as per DIN EN ISO 898-1. In case the reading appears too low, a microstructure analysis should be conducted to display and evaluate a possible decarburisation. If the checked surface hardness is too high, there is the risk of carburisation, which can lead to brittle fracture.