User report:
Strength testing during surface hardening

Illustration of the strength curve of a tooth flank

The Department of Forming Manufacturing Processes at the Laboratory for Machine Tools and Production Engineering ( WZL ) at RWTH Aachen University is involved, among other things, in the investigation of edge zone machining processes such as deep rolling and mechanical surface hammering (MOH). In MOH, high-frequency impacts of a mostly spherical ram head produce a surface smoothing or targeted structuring, residual compressive stresses are introduced into the workpiece and work hardening is achieved in the edge zone.

Reason for using the imprint method

Upsetting tests were carried out to generate flow curves for the base material, but this is only feasible to a limited extent for the surface layer, as the production of the upsetting specimens with the material properties of the surface 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 should be noted that a case hardening depth of the upsetting specimens EHT = 3 mm is generated, which is difficult to realize. In comparison, the case hardening depth for the tooth flank analogue specimens is EHT = 1.2 +/- 0.2 mm including a grinding allowance of 0.2 mm.

"With the indentation method, an alternative to the upsetting test was selected. With this method, the flow curves are recorded directly on the tooth flank analogue specimens, so that a complicated manufacturing process of specimens is not necessary. In addition, the method makes it possible to generate a depth profile of the flow curves and thus also to map the transition region between the case-hardened surface layer and the base material." (2020)
Strength profile in the marginal zone of a tooth flank analogy specimen

Test procedure and results

Measurement and result

In order to achieve the most accurate analysis of the surface layer, the component was scanned layer by layer at 100 µm intervals from the surface to the core strength. The average value from five measuring points was evaluated for each layer.

Using the indentation method according to DIN SPEC 4864, a strength curve is obtained from the surface layer into the core of the case-hardened component. The flow curves were provided to the WZL of RWTH Aachen as true and technical flow curves. In the next step, the measured flow curves are transferred to 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.

Further information about the WZL project

As part of the DFG-funded research project KL500/192-1 - Optimization of the Case-Hardening Behavior of Spur Gears through 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 surface integrity in the form of residual stresses, hardness and topography, cause-effect relationships are established between MOH machining and the dimple bearing capacity and friction of tooth flanks in rolling contact. The influence of MOH on the edge zone of cylindrical case-hardened tooth flank analog specimens 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 behavior. The plastic material behavior is stored in the material model using 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 in the interior. Therefore, a distinction is made in the model between the surface layer and the base material. Consequently, material models are deposited. In previous work, upsetting tests were carried out to generate flow curves.

LarsUhlmann, Research Associate in the Department of Forming Manufacturing Processes at the WZL

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