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What Are The Benefits Of Oil Quenching?

Hardening of steel is one of the main vacuum heat treatment processes. In this process, which was originally carried out with aqueous solutions, the quenching medium affects the final properties of the steel and can be controlled to optimize the hardness, microstructure and deformation of the treated component.
Depending on the type of steel and the desired result, the industry uses oil, salt bath or pressurized inert gas for quenching. Vacuum furnaces are increasingly used in this process.

Problems with oil quenching
Like any hardening process, the goal of oil quenching is to achieve the desired hardness index by rapidly cooling the austenite to martensite phase transformation.

Advantages of vacuum oil quenching
Vacuum heat treatment is becoming more and more popular. However, it is usually cooled with pressurized inert gas. Although it is not widely used, vacuum oil quenching has shown attractive advantages to the industry.

Advantages when heating
When heating in a vacuum furnace, the surface of the component is reliably protected and oxidation or decarburization is completely avoided. A further advantage is that the partial pressure of the gas can be easily controlled. The partial pressure of the inert gas (nitrogen, argon) suppresses the sublimation of alloying elements. In addition, the partial pressure of the reactive gas allows carburizing or carbonitriding at low pressure and high temperature, which helps to shorten the process cycle time.

Control of cooling
When the furnace is vacuum purged, the surface of the component is always protected until it is completely immersed in the quenching oil because the charge transfer is carried out under vacuum or inert gas conditions. Whether oil or gas quenching, the surface protection conditions are basically the same.
The main advantage compared to the traditional atmosphere oil quenching scheme is the ability to accurately control the cooling parameters. When using a vacuum furnace, the standard quenching parameters – temperature and stirring intensity – can be adjusted, as well as the pressure above the quench tank.
Adjusting the pressure above the quench tank can create a pressure difference inside the oil bath, which changes the oil cooling efficiency curve obtained at atmospheric pressure. The highest cooling efficiency is of course the boiling stage. Changes in the quench oil pressure will change the evaporation caused by the heat of the charge.
Increasing the quench oil pressure can suppress evaporation and reduce the generation of vapor. The component still has insulation problems and cools more evenly but at a slower rate. Therefore, vacuum oil quenching is more uniform and produces less deformation.
The control of the quenching oil pressure, together with the choice of initial cooling parameters, temperature and stirring mode, gives the user more opportunities to optimize the hardening process conditions and find the best balance between cooling rate and uniformity (i.e. hardness and deformation).
Compared with high pressure gas quenching (HPGQ) treatment, which is usually carried out in a vacuum or low pressure furnace, oil quenching provides more efficient cooling and a certain safety margin, especially for large cross-section parts or parts made of low hardenability steel. In these applications, the use of quenching oil makes vacuum no longer an obstacle.