The indentation method according to DIN SPEC 4864
for determination of strength during surface hardening
Testing after surface hardening - Application report of the WZL of the RWTH Aachen
WZL of the RWTH Aachen University is concerned with the investigation of edge zone machining processes such as deep rolling and mechanical surface hammering (MOH). In the MOH, high-frequency blows of a mostly spherical ram head are used to smooth or selectively structure the surface, to introduce residual compressive stresses into the workpiece and to achieve work hardening in the edge zone.*
Base for the use of the indentation method
To generate flow curves for the base material, upsetting tests have been carried out, but this is only conditionally feasible for the edge layer, as the production of upsetting samples with the material properties of the edge layer is only possible to a limited extent. It is necessary that the entire diameter of the upsetting specimens is hardened according to the case hardness of the tooth flank analogue specimens. It is important to note that a case hardness depth of the upsetting samples EHT = 3 mm is produced, which is difficult to realize. In comparison, the case depth of the tooth flank analogy samples EHT = 1.2 +/- 0.2 mm including a grinding allowance of 0.2 mm.*
Test procedure and results
Measurement and result
To achieve the most accurate analysis of the surface layer, the component was scanned layer by layer at 100 µm intervals from surface to core strength. The mean value of five measuring points was evaluated for each layer.
The indentation method according to DIN SPEC 4864 provides a strength curve from the surface layer to the core of the case-hardened component. The flow curves were made available to the WZL of the RWTH Aachen as true and technical flow curves. In the next step, the measured flow curves are transferred into a material model that describes the flow behavior as a function of the distance to the surface.
More information about the WZL project
The simulative approach is implemented with the finite element method (FEM). A decisive factor in FE simulations is the description of the material behaviour. The plastic material behaviour is stored in the material model on the basis of recorded flow curves. Due to case hardening, the flow behavior of the surface layer differs from that of the base material, which is still present inside. Therefore, the model differentiates between the surface layer and the base material. Consequently, material models are stored. In previous work, upsetting tests were carried out to generate flow curves.*
*Lars Uhlmann, research associate in the department of Forming Technologies at the WZL