Influence of Pressure on Vacuum oil Quenching

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.