Model-supported Control of Surface Integrity During Hard Turning

 

The machining of harder steels with geometrically defined cutting edges is becoming increasingly important with the development of super-hard and low-iron tool materials [1]. One of these processes is the hard turning process, in which the high strain rate leads to a localized temperature increase. Approximately 90% of the process power is converted into heat. The distribution of the heat flows has a decisive influence on the process result, which can be influenced by the setting of the process parameters and control variables. A typical problem with hard machining is the material modifications at the component edge zone. In most cases, this refers to the so-called white and dark views (Figure 1).

The white layer is a few micrometers thick, hard and brittle. It also influences the magnitude and position of the residual stresses. In contrast, the dark layer is much thicker, soft and ductile. The presence of a dark layer leads to a crack nucleation and propagation. Thus, these two layers have a decisive influence on the fatigue life of the component. Therefore, they are usually removed before the component is used. The formation of white and dark layers is a Result of microstructural changes, which are mainly caused by the formation of new quenching and tempering zones[2].

  Figure 2: Schematic representation of the relationships between process parameters, process state variables and process target variables Copyright: IEHK Figure 2: Schematic representation of the relationships between process parameters, process state variables and process target variables

A targeted, function-oriented adjustment of the boundary layer condition can only be carried out according to figure 2 by knowing the thermal and mechanical process state variables, if their effect on the boundary zone can be quantitatively described. [4].

 

The aim of this project is the calibration of a micro-magnetic sensor, which shall be used for the determination of residual stresses, existing microstructure types and grain size distributions, and the development of a FE model for the simulation of the load-dependent boundary zone modification, which is based on phenomenological and physical couplings. Within the phenomenological motivated model, the modified Bai-Wierzbicki material model is applied and extended by the parameters relevant to the formation of the white and dark layers, such as hardness. In addition, dynamic recrystallization is modelled by the Zener-Hollomon parameter. While the physical model results from the implementation of a crystal plasticity model within a phase field simulation.

This project is supported by the German Research Foundation (DFG).

[1] A. Barbacki; M. Kawalec; A. Hamrol (2003): Turning and grinding as a source of microstructural changes in the surface layer of hardened steel. In: Journal of Materials Processing Technology 133 (1), S. 21–25. DOI: 10.1016/S0924-0136(02)00211-X.

[2] D. Umbrello; J.C. Outeiro; R. M’Saoubi; A.D. Jayal; I.S. Jawahir (2010): A numerical model incorporating the microstructure alteration for predicting residual stresses in hard machining of AISI 52100 steel. In: CIRP Annals 59 (1), S. 113–116. DOI: 10.1016/j.cirp.2010.03.061.

[3] Manco, G. L.; Caruso, S.; Rotella, G. (2010): FE modeling of microstructural changes in hard turning of AISI 52100 steel. In: Int J Mater Form 3 (S1), S. 447–450. DOI: 10.1007/s12289-010-0803-3.

[4] Barry, J.; Byrne, G. (2002): TEM study on the surface white layer in two turned hardened steels. In: Materials Science and Engineering: A. Jg. 325, Nr. 1-2, S. 356– 364