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The quenching operation is a critical part of the Abstract: heat treatment process. The cooling characteristics of a quenchant will change during its working life. Improper quenching parameters and the drifting of the cooling characteristics can cause unforeseen costs if a malfunctioning component necessitates additional operations, e. g. straightening, machining, rework, or even rejection. For a polymer quenchant the cooling characteristics are influenced by, inter alia, concentration, agitation, temperature, thermal degradation, contamination and drag-out. In this study tests have been performed in order to study how the ageing and contamination of polymer quenchants influence the cooling characteristics. Polymers used for induction hardening in showers as well as in quench baths have been evaluated with regard to e.g cooling curves, refractive index and kinematic viscosity after ageing. Quenchants used in industrial applications have been evaluated. Cooling curves have been evaluated by using the ivf SmartQuench™ system. This is a tool for testing quenching media, primarily against the current ISO and ASTM standards for hardening oils and polymers. A large number of characteristics are recorded or calculated, such as cooling times to certain temperatures and cooling rates at these temperatures. Einfluss von Alterung und Kontamination auf die Kühleigenschaften von PolymerabschreckmedienDas Abschrecken ist ein kritischer VorZusammenfassung: gang während der Wärmebehandlung. Die Kühleigenschaf-ten von Abschreckmedien verändern sich im Laufe ihrer Einsatzdauer. Ungeeignete Härteparameter und die Verän-derung der Kühleigenschaften können unvorhergesehene Kosten verursachen, wenn eine nicht funktionierende Komponente zusätzliche Arbeitsgänge wie z. B. Richten, Zusatzbehandlung, Nachbesserung oder sogar Ausmusterung zur Folge hat. Die Kühleigenschaften von Polymerabschreckmedien werden u. a. von deren Konzentration, Bewegung, Temperatur und Kontamination sowie vom thermischen Abbau und dem Austrag beeinflusst. Durch die Tests dieser Studie sollte festgestellt werden, wie die Alterung und Kontamination von Polymerabschreckmedien die Kühleigen-schaften beeinflussen. Die Auswertung der Polymere für die Induktionshärtung in Duschen oder Abschreckbädern erfolgte z. B. gemäß Kühlkurven, Brechungsindex und kinematischer Viskosität nach der Alterung. Es wurden Abschreckmedien ausgewertet, die in industriellen Prozessen eingesetzt werden. Die Auswertung der Kühlkurven erfolgte mit ivf SmartQuench™. Mit diesem Programm werden Kühlmedien getestet, in der Hauptsache in Bezug auf aktuelle ISO-und ASTM-Standards für das Härten von Ölen und Polymeren. Eine sehr hohe Zahl an Eigenschaften wird aufgezeichnet oder berechnet, wie z. B. Abkühlzeiten auf bestimmte Temperaturen und Kühlgeschwindigkeiten bei diesen Temperaturen.

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 wheelsDistortion of gear wheels after low pressure carburizingThe 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.

One of the most critical parts of the heat treatment process, and usually the least controllable one, is the quenching operation. Improper selection or application of a quenching medium, or a drift in its cooling characteristics during its lifetime, may result in products that do not meet specifications and therefore give rise to substantial additional costs for e.g. straightening, rework, rejection, delayed deliveries and, sometimes, lost goodwill for the heat treater. A greater awareness of the importance of the quenching process came with the introduction in the last couple of decades of ISO and ASTM standards for testing cooling media (hardening oils and polymers), and commercial instruments for testing compliance with these standards. In 2003, a new, advanced system – ivf SmartQuench – was introduced which has three main features: a small, handheld data acquisition unit, advanced computer software and wireless data transmission between the data acquisition unit and the computer. The design facilitates in-situ testing in quench tanks and testing in laboratories. Recently, this system has been extended with a unique software module that allows calculation of heat transfer coefficients (e.g. for the ISO 9950 probe) by the inverse method, and calculation of hardness and microstructure in a cross-section of steel samples.

To establish the process window for the spray quenching step of the induction hardening process is essential for quality control and optimized use of the quenching capacity supplied by the quenching unit. In general, the process window is established by an empirical approach, where the processing is related to the mechanical properties. On the other hand, there has been a rapid development of computational tools that may facilitate and accelerate process optimization. In the present work it is demonstrated how such tools, e.g., FE-simulation and multivariate analysis, can be applied to couple quenching characteristics to mechanical properties. The methodology is applied to induction hardened steel cylinders that were quenched with different flow rates, temperatures and composition of the quenchant. The results show how mechanical properties can be related to characteristics of the quenching, e.g., heat transfer coefficients and characteristics of the cooling curve. Moreover, the work discusses and exemplifies how the process window can be established and how computational tools allow the user to virtually alter the processing and estimate the impact it may have on the mechanical properties.