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.*

„The indentation method was selected as an alternative to the compression test. With this method, the flow curves are recorded directly on the tooth flank analog samples, thus eliminating the need for a more difficult sample production process. In addition, it is possible to generate a depth profile of the flow curves with this method and thus also to map the transition area between case-hardened surface layer and base material.“ (2020)

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.

The figure shows the determined flow curves in the case-hardened surface layer up to the core zone.

More information about the WZL project

Optimization of the insert behavior of cylindrical gears by machine-hammered tooth surfaces (Optigear) the influence of machine-hammered tooth flanks made of case-hardened 16MnCr5 (1.7131) on pitting and friction is investigated. Based on the modification of the surface integrity in the form of residual stresses, hardness and topography, cause-effect relationships between MOH machining and the pitting resistance and friction of tooth flanks in rolling contact are established. The influence of the MOH on the edge zone of cylindrical case-hardened tooth flank analog samples is analyzed both experimentally and simulatively.

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

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