In the field of massive forged components the mechanical engineering industry searches for processes with increasing energy and resource efficiency. The new generation bainitic steels are promising for such application because of the high strength, toughness and fatigue properties. In order to achieve the desired mechanical properties, the development of the bainitic microstructure depending on the parameters of the thermomechanical process and on the cooling procedure must be well-known. In the present work diverse experimental techniques were applied for the investigation of the microstructural development during thermomechanical treatment and subsequent continuous cooling through the bainitic transformation range. The thermomechanical processes were simulated using dilatometers and at the same time, the specimens were analyzed using an eddy current sensor or using in-situ X-ray diffraction measurements at synchrotron (DESY). The results show that the eddy current sensor is suitable for the monitoring of the microstructural development during cooling and during deformation. From the investigations suitable process parameters were deduced for achieving a possibly fine bainitic microstructure. The main factors are a relatively low deformation temperature in austenitic range, a fast cooling (> 2 K/s) into the bainitic range, bainitic transformation and/or a short deformation in the lower bainite range, and finally a slower cooling until room temperature.
Carbonitriding serves to increase the strength and wear properties of steel components. The carbon and nitrogen concentration and the depth distribution decisively determine the resulting properties. Optimal profiles create an ideal microstructure of martensite, residual austenite, finely distributed nitrides, carbonitrides and inherent compressive stress in the surface zone. The reliability of carbonitriding heat treatment process is strongly dependent on the possibilities of process control. Previous investigations aimed at measuring the nitrogen potential of the atmosphere by means of an ammonia sensor in the exhaust gas as well as by a wire sensor. Carbon potential is conventionally controlled using an oxygen probe. In order to further increase the process reliability of carbonitriding, simulation of the carbon and nitrogen profiles and of any precipitation of carbides and nitrides are necessary. The first step is the determination of interdependent alloying influences on the carbon and nitrogen contents. The carbon and nitrogen activities in the atmosphere and the alloy surface are near equilibrium after long-time carbonitriding. Depending on the composition of the material, significantly different effects can be described. These influences must be considered. In addition to the reached equilibrium content, the diffusion, the position of phase boundaries in the phase diagrams and the formation of precipitation with influence of carbon and nitrogen as well as interactions with other alloying elements are to be worked out in order to further develop controlled carbonitriding for the reliable adjustment of the heat treatment results.
In industrial case hardening, the temporal and local non-destructive characterization of occurring microstructural constituents creates new possibilities for automating manufacturing processes showing a high level of process reliability. Furthermore, component properties within the scope of quality assurance and product liability can be fully documented. By analyzing the higher harmonics of eddy current testing, the structure-specific magnetic properties can be used to differentiate between the microstructural constituents formed. The eddy current sensor can be integrated into the cooling path. This enables in-situ test signal recording in order to continuously monitor the graded microstructure formation in the peripheral zone as well as deeper in the component for quality assurance. An increasing carbon content leads, among other things, to a higher proportion of residual austenite. This results in a lower test signal amplitude, which, for example, can be correlated with the hardening depth. The results of this testing method, used for the first time for graded components, are presented here.
During forging the tools have to resist high thermal and mechanical loads. Therefore, forging dies are usually nitrided in order to increase the wear resistance of the surface areas. Compared to nitriding carbonitriding increases the hardness depths in shorter treatment times. Due to the (carbo-)nitriding, the surface near region gets a better hot hardness and a better wear resistance than the untreated material. In a second process step by nitriding after carbonitriding, a wear and corrosion resistant compound layer can be created at the surface. In the present work the wear behavior of carbonitrided and nitrided hot working steel ENX38CrMoV5-3 (1.2367) was investigated. The evaluation concerning abrasion and thermal fatigue in the contact of forging die and workpiece was carried out by model wear tests like the two-disc test (Amsler test), the ball-on-disc test and a cyclic induction heating and quenching. The conditions of the heating cycles were chosen close to the forging process. The surface areas were investigated after several cycles in order to gain information about the mechanisms of the surface modification. Finally, the treatment combination carbonitring/nitriding was transferred to forging dies and the wear behavior was investigated in fatigue-life experiments under application conditions.
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