Volume 76, Issue 4

Heat treatment simulation of inductive surface hardening of large bearing rings is a challenging multi-physical task. Besides the determination of material and process parameters of induction heating, the quenching process must be modelled to obtain realistic results concerning surface hardening depth as well as information about residual stresses and distortions of the bearing rings. A common method to model quenching processes is to determine heat transfer coefficients for the specific process depending on component surface temperature. This method was used to characterize the shower cooling process using an aqueous polymer solution of a modified polyalkylene glycol (PAG) type. A specifically designed test set-up allowed to determine the heat transfer coefficients for different distances between shower and hot specimen as well as for different impingement angles of the fluid relative to gravitation. Additionally, the calculated heat transfer coefficients were checked and corrected by FEM simulations.

Nitriding is used to achieve a high hardness in the surface layer through the precipitation of nitrides. However, to realize high nitriding hardness depths, treatment times of many hours are necessary, which usually also result in a decrease in strength within the nitrided layer and base material. With induction heat treatment, on the other hand, high hardness depths can be achieved in a very short time. However, the maximum hardness increase is limited by the alloy content of the material. By combining nitriding and induction hardening, high hardness depths can be achieved in short treatment times as an alternative to deep nitriding. In addition to a significant saving in process energy surface layer properties that cannot be achieved with the individual processes are expected. In order to fully exploit the potential of the combination treatment, at first suitable conditions must be set during nitriding for the subsequent induction hardening. In the present work, nitriding layers with low-porosity compound layers as well as only diffusion layers were produced and analyzed on typical nitriding and tempering steels for this purpose.

Case hardening processes such as carbonitriding can be used to improve the performance of powder metallurgical (PM) structural components. The partially open porosity of these components leads to a significant increase in diffusion, which in turn leads to a change in the element gradient in the surface layer (and consequently the surface layer state) compared to melting metallurgic materials. Within the scope of a two-part work, the surface layer states in common densities are investigated after the case hardening process. The present part 2 comprises the characterisation of the resulting surface layer states depending on the carbon and nitrogen profile and the tempering heat treatment. Through the deep rolling and the subsequent carbonitriding treatment, hardness of up to 850 HV0.1, retained austenite contents of up to 25 vol-% and residual stresses of up to –300 MPa are determined across densities. The extensions to the numerical models described in this paper enable the FE model to predict the surface layer states in a wide range of process combinations and densities with an error tolerance of ±20 %.

The graphite inclusions typical of grey solidified cast iron materials reduce the load-bearing capacity under locally concentrated pressure and simultaneous sliding stress. Surface treatment processes such as nitriding and electron beam remelting are known to improve the local stress behaviour. In this paper, the effects of the above-mentioned individual processes and their combination on the tribological stress behaviour of ferritic and pearlitic cast irons with different graphite morphologies are discussed. The results obtained in the model wear test ball-plate show that the specific wear coefficient of the investigated cast irons with different graphite morphology can already be reduced by at least one order of magnitude by an approx. 0.5–0.9 mm thick remelted surface layer with a surface hardness of 650–750 HV1. This treatment eliminates the graphite and produces ledeburitic carbides instead. The potential of an additional nitriding treatment depends on the parameters used, i. e. the nitrided layer thickness produced as well as the phase composition and the pore fraction of the compound layer. Based on stress calculations, the experimentally determined main influences such as the coefficient of friction, the pore fraction in the compound layer and the magnitude of the Hertzian pressure on the contact stress could essentially be confirmed.