Volume 73, Issue 6

Carburising followed by martensitic hardening is state of the art in the gear industry today. Carburisation followed by isothermal transformation into a partial or fully bainitic microstructure has not been systematically examined and evaluated yet. Especially isothermal transformation of carburised parts into lower bainite is promising in terms of the characteristic mechanical values of treated parts, due to the increased toughness of the bainite compared to a carbon rich martensite and finely distributed retained austenite. Using this procedure for case hardening requires a quenching technology for isothermal bainitic transformation and additional changes to the process. Examinations of the transformation characteristics of carburised samples in the dilatometer were discussed in the first part of the publication already. This second part examines the microstructures and residual stresses caused by “bainitic case hardening”. The 3 rd part will follow in HTM 1/2019.

Although metallic materials processed by laser beam melting have significantly different microstructures compared to their conventional counterparts no adjusted heat treatment parameters have been published to date. Conventional heat treatment will therefore not always result in the desired mechanical properties. This project examines the effect of heat treatment on the properties of laser beam melted components produced with AlSi10Mg (EN AC-43000) precipitation hardening cast aluminium alloy and X5CrNiCuNb16-4 (1.4542 or 17-4 PH) martensitic precipitation hardening steel. Comparisons to conventionally manufactured material are made in parallel. The kinetics of phase transformation during heat treatment is analysed in-situ by means of differential scanning calorimetry (AlSi10Mg and X5CrNiCuNb16-4) and dilatometry (X5CrNiCuNb16-4). The results show considerable differences in phase transformation kinetics between laser beam melted and conventionally processed materials.

Nitriding is a widely used treatment to increase the performance of unalloyed and alloyed steels. Because nitriding majorly affects the hardness of the surface region of the material it is common to perform a quenching and tempering treatment before nitriding in order to get a suitable core hardness and microstructure for application. The hardness from pre-heat treatment is nearly not affected by the following nitriding process if the temperature of tempering exceeds the nitriding temperature more than ca. 30 °C. Nevertheless in some cases where a high nitriding depth, i. e. 0.8 mm and more, is necessary, the nitriding parameters temperature and time can affect the material's state, which was achieved by tempering, even though the nitriding temperature is lower than the temperature of the tempering treatment. The current investigations concern the tempering behaviour of typical nitriding and heat treatable steels. In nitriding experiments with different treatment temperature and time the stability of the pre-heat treatment state was investigated and the Hollomon-Jaffe parameter was used to describe the effect of temperature and time on the material's properties. Furthermore it was shown, that the Hollomon-Jaffe parameter is also suitable to predict the tempering effect of a long time nitriding process.

Material properties like yield stress and hardness change during forming. Metastable austenitic steels are characterized by an elevated work hardening potential due to strain-induced transformation of austenite into martensite especially by multi-stage processing. An application of a new rotary swaging process design with Eccentric Flat Shaped Dies (EFSD) enables a severe shear strain in the workpiece. Eccentric rotary swaging typically provides a helical material flow. According to the process parameters, the microstructure features a typical eddy pattern with a spiral shaped grain orientation. The forming process can be carried out in one or more processing steps. The properties of the formed workpieces can be adjusted both by a variation of the process parameters values and by a specific subsequent heat treatment. This work is focused on microscopic investigations of the microstructure of in one and two steps eccentrically swaged parts with and without subsequent heat treatment.

In many metal forming applications, especially in micro cold forming, the tools are loaded differently in various functional regions. Tailored material properties in the specific regions are therefore advantageous. To meet this requirement, different tool steels can be applied in the corresponding regions of the tools, with a good bonding in between. This kind of composite materials can be produced by means of a newly developed successive spray forming process. In addition, the different steels in the composite tools may require quite different heat treatment conditions in order to achieve the expected material properties and performance. For this purpose, a selective heat treatment based on middle frequency induction heating has been developed to austenitize the different steels at different temperatures. In this study, these two newly developed processes have been applied to produce composite tool steels for micro cold forming tools. The investigation is focused on the correlation between the processing conditions and the microstructure and properties of the composite tool steels. It shows that fine and homogeneous microstructure can be achieved in the composite tool steels, with a gradual transition of chemical and physical properties in between. The microstructure and properties of the composite tool steels are clearly correlated with their austenitization conditions during the selective induction heating.