Hardness test at incoming goods and the indentation method as an alternative test
Quality assurance in incoming goods
Hardness testing on receipt of goods often serves to identify materials and is of great importance. Hardness testing is successfully used for this purpose. There are, however, exceptions where a risk of material substitution arises due to the hardness test.
The risk of confusion of materials in incoming and outgoing goods increases with the number of similar alloys. For example, it is not possible to distinguish a work-hardened steel and a heat-treated steel with the same tensile strength (see Table 1) by means of classical hardness testing. In this case the same hardness is measured. This creates the risk of material mix-ups during operation..
Risk of confusion in material characterisation by hardness testing - indentation method as solution
The risk of material mix-ups is minimized in practice by assigning unique identifiers at the time of goods issue. These identifiers (e.g. colour markings, QR Code, etc.) provide a remedy. If errors occur in the identification, unintentionally wrong materials are processed further. Hardness testing at goods receipt is often carried out as a random sample to identify materials. The table shows the hardness values, tensile strength RIm and the RIp0.2yield strength of two steels with similar hardness. The Rockwell C values according to DIN EN ISO 6508 show the same hardness and do not indicate a difference in strength. In order to avoid this danger and to clearly minimize the risk of confusion, the indentation method according to DIN SPEC 4864 is used..
The indentation method according to DIN SPEC 4864 provides precise flow curves and comparative characteristic values for the tensile test
The indentation method is a new testing method. It is based on highly accurate 3D measurements of indentations and finite element simulations. The material uplift of the indentation considered in these simulations reflects the plastic deformation behaviour of the material. Within a few seconds, the testing can be carried out fully automatically with the strength testing machine i3D from Imprintec GmbH.
Fig. 1 shows the stress-strain curves determined for quenched and tempered steel (measuring points 1-3) and cold extrusion steel (measuring points 3-6). The results confirm that the tensile strength RIm is at the same level. However, the RIp0.2yield strength is clearly different. The risk of confusion is significantly reduced with the indentation method in comparison to hardness testing.
This article can also be read in the journal “DIALOG – Materialwissenschaften und Werkstofftechnik“.