Volume 74, Issue 4

Carbonitriding is used to improve the properties of steel components. The carbon and nitrogen content and the microstructure comprising martensite, retained austenite, and finely distributed nitrides and carbides are significant parameters determining component properties. Current state of the art posits that these multi-phase microstructures determine optimal material conditions. The reliability of a process is strongly dependent on the options for control. Previous studies dealt with the measurement of atmospheric nitrogen potential by means of an ammonia sensor in the exhaust gas. The equilibrium contents in the austenite were determined based on this and the mutual effects of carbon and nitrogen. Further improvement of the reliability of the carbonitriding process requires simulation of the carbon and nitrogen profiles and the precipitation condition of carbides and nitrides. Apart from the equilibrium content, the maximum solubilities determined by phase diagram calculations and precipitation in the presence of interdependent carbon and nitrogen effects, including interactions with other alloying elements, need to be examined towards further development of controlled carbonitriding to ensure reliable heat treatment results.

To achieve maximum hardness and to avoid retained austenite during surface hardening processes, homogeneous austenitization is desired. The necessary temperature to achieve this depends on the heating conditions and the initial microstructure. Its measurement is generally tedious and no distinct models are available to model the process. Therefore, a simulation model of the austenitization process based on a 2-D cellular automaton was developed and analyzed. The model predicts the austenitization kinetics as well as the homogenization temperature for a given microstructure and heating process. The model is able to predict the characteristic dependencies that are known from the experiments at least in a qualitative way.

Beginning in the middle of the last century aqueous polymer solutions like polyvinylpyrrolidone (PVP) and polyalkylene glycol (PAG) have been used for immersion quenching because of their unique features and quenching characteristics. Known problems of quenching in polymer solutions are process reproducibility and work safety due to spontaneous reforming of polymer films and almost explosive vapor layers collapse. The boiling and quenching process within polymer solutions and their mechanisms are investigated here. In quasi-steady experiments a newly developed in-line sensor measures the local concentration in the vicinity of a fixed vapor bubble depending on the distance to the phase boundary. These experiments show that the polymer concentration increases with decreasing distance to the vapor bubble surface. Rheological investigations on polymer solutions show that a higher polymer concentration especially increases the solution viscosity considerably. In a typical immersion quenching process with polymer solutions, in the initial phase of the quenching process individual vapor bubbles are formed on the surface of the immersed hot specimen. The high solution viscosity keeps the vapor bubbles on the surface leading to bubble growth and coagulation. Thus, the formation of a vapor layer is promoted. Along with the stabilization of the vapor phase by a concentrated polymer skin a stable vapor layer may emerge. Within the progress of the quenching process, this vapor layer may repeatedly collapse in an explosive like manner. A major influence of this process is the type of polymer (chain length), the polymer concentration and the liquid solution temperature. These effects are also investigated on a larger application-oriented scale setup. Cooling curves and global electric conductance measurements as well as sound and video recordings during specimen quenching in polymer solutions are used to locate vapor layers and their collapses. The experiments show that these effects can be influenced by suitable flow conditions around the specimen.

In low pressure furnaces with oil quenching it is possible to vary the pressure above the quench tank. By regulating the pressure above the tank the pressure inside the bath will be influenced which can affect the cooling characteristics of the oil compared to atmospheric pressure. It has been reported that the length of the vapour phase as well as the boiling phase will be influenced, thus also influencing the distortions [1–3]. An increased pressure in the oil results in a shorter vapour phase. Depending on component geometry and fixturing of the parts during quenching, the presence and behavior of the vapour film will affect distortions of the parts. The possibility to adjust the pressure above the oil bath during quenching introduces a new parameter for adjustment and control of the cooling process. In order to investigate this phenomenon experiments were conducted in a low pressure furnace with an integrated oil bath. The pressure above the bath could be set between 0.4–1.4 bar (absolute pressure). The following were investigated: Cooling curves measured with thermocouples in Inconel probes and gear wheels Distortion of gear wheels after low pressure carburizing The cooling curves showed no significant difference in cooling characteristics for the investigated pressures above the quench tank. This is contradictory to reported results. One reason can be the influence of agitation as well as type of oil. The influence of agitation on the oil was studied. Depending on analyzed distortion parameter a small impact of the pressure above the oil tank could be noted. The trend was less distortions with a higher pressure in the quench bath. However, when it comes to differences in cooling characteristics between different positions of the gears in the load, there were great variations.

This paper deals with a methodology for a characterisation of inductive heat treatment plants to allow comparison of their practical electromagnetic behaviour with conventional simplifying assumptions used in simulations of the heating process. The impact non-sinusoidal currents and non-linear B-H curves on the simulation are specifically dealt with here. A Rogowski coil and digital oscilloscope are used to read in current signals in various induction plants and compare their total harmonic distortion (THD). In the course of parameter studies, the different current signals were used in simulations of induction to compare heating effects. This yielded positive correlations with the THDs.