Heat Treatment Quenching Oil: Selection Principles and Methods

1. Heat treatment cooling curve

The heat treatment process generally includes three processes: heating, heat preservation and cooling.

The heating is to transform pearlite into austenite, and the heat preservation is to completely austenitize.

The cooling method varies according to the process, mainly controlling the cooling rate, which changes into pearlite, bainite, martensite or mixed structure due to different cooling rates.

Generally, martensite is expected to be obtained during quenching, and tempered martensite (low temperature), troostite (medium temperature) and sorbite (high temperature) are obtained according to different tempering temperatures after quenching.

The isothermal transformation curve of eutectoid steel is shown in Fig. 1, which basically reflects the inoculation time, transformation completion time and transformation products required for the transformation of eutectoid steel at different temperatures.

Heat Treatment Quenching Oil: Selection Principles and Methods 1

Fig. 1 Isothermal transformation curve of eutectoid steel

In actual heat treatment production, there are many cases of continuous cooling except for step isothermal quenching process.

Quenching requires that the martensite structure speed must be greater than the critical cooling speed, and the surface cooling speed of the part is generally greater than the core cooling speed.

One of the selection principles of quenching oil: the cooling rate must be greater than the critical cooling rate in order to obtain martensite during quenching, and it is also necessary to consider reducing deformation and preventing cracks.

The cooling rate must be moderate and not excessive. As shown in Fig. 2.

Heat Treatment Quenching Oil: Selection Principles and Methods 2

Fig. 2 TTT curve and CCT curve of eutectoid carbon steel

2. Ideal quenching method

According to the cooling transformation law of steel, it is expected that the cooling speed will be high at the critical area temperature, and the steel will pass through the nose area of the C curve as soon as possible to avoid the transformation into pearlite or bainite structure.

In the dangerous area where the martensite transformation starts, the cooling speed must be slow down to reduce the deformation and even cracks caused by the structural stress generated by the structural transformation.

This is the case for double liquid quenching, which is first quenched in water and then cooled in oil.

Single liquid quenching requires that the cooling medium has such cooling characteristics: the cooling speed is fast at the critical area temperature, and slow down at the dangerous area temperature.

The second selection principle of quenching oil: fast cooling at high temperature and slow cooling at low temperature, taking into account the requirements of hardness and deformation. As shown in Fig. 3.

Heat Treatment Quenching Oil: Selection Principles and Methods 3

Fig. 3 Schematic Diagram of Ideal Quenching Method

3. Cooling curve of quenching oil

The heat treatment quenching oil needs to have the above cooling properties.

The cooling speed of parts is fast at the critical area temperature and slow at the dangerous area temperature.

Fig. 4 shows the cooling characteristic curve of Houghton MT355 quenching oil at different oil temperatures, showing the relationship between sample cooling time, cooling rate and temperature, and the cooling capacity of quenching oil at different temperatures.

Heat Treatment Quenching Oil: Selection Principles and Methods 4

Fig. 4

4. Hardness requirements and deformation effects

The critical cooling speed of different materials is different, the hardness requirements of parts are also different when they are used, and the precision and deformation control requirements of parts are also different, so it is necessary to select the quenching oil with appropriate cooling speed according to the actual situation of the hardness requirements and the process and technical requirements of deformation control.

After the selection of quenching oil is determined, various control measures such as quenching temperature, oil temperature, mixing cooling speed and time, oil feeding time, etc. shall be adjusted by controlling the quality of raw materials (part materials and oil materials), combining the tempering process after quenching, tempering temperature, tempering time parameter test to meet the hardness requirements and metallographic structure requirements of parts, and through the adjustment and control of heat treatment process parameters and the coordination of cold and hot with machining, Control the heat treatment deformation within a reasonable range.

5. Selection method of heat treatment quenching oil

Heat Treatment Quenching Oil: Selection Principles and Methods 5

From the heat treatment quenching process, the selection principle of quenching oil was mentioned: the cooling performance of quenching oil must reach the required hardness (the cooling speed is required to be fast) and no cracks are allowed, and the heat treatment deformation must be controlled within a reasonable minimum range (the cooling speed is required to be slow) to find an optimal combination.

This contradictory cooling speed requirement is a requirement for different temperature ranges.

This provides a direction for the development of heat treatment quenching oil.

In view of the limitation of detection conditions, we can only make a rough selection of quenching medium based on general knowledge and experience in the production practice of heat treatment in the factory before and then verify it with tests.

The wide application of the cooling characteristic instrument just provides a kind of detection tool analysis means, which helps us to optimize the scientific selection of quenching oil, analysis and comparison.

We can conditionally test or outsource testing or require oil companies to provide test data reports on the cooling characteristics of quenching oil to provide scientific basis for the selection of quenching oil.

According to the above method, there are many kinds of quenching oils that meet the use requirements.

Heat Treatment Quenching Oil: Selection Principles and Methods 6

According to the actual production situation and the company’s quality policy, the quenching oil that meets the cooling performance requirements should also be compared with other technical indicators and comprehensively compared to select the most suitable one.

The stability and oxidation resistance of quenching oil, the repeatability and reproducibility of cooling performance, brightness, safety and cost are all factors to be considered.

QB/T0001-2007, JB/T7951, SH/T0220 put forward the technical requirements that must be met for the kinematic viscosity, flash point, ignition point, moisture, pour point, corrosion, brightness, saturated vapor pressure (vacuum quenching oil), thermal oxidation stability, cooling performance, maximum cooling speed and cooling time of heat treatment cooling oil.

6. Conclusion

The selection principle and method of quenching oil are introduced above.

Quenching oil base oil, additives, many factors affecting the cooling performance, involving a wide range.

Quenching is an extremely important link in the heat treatment process, which must be carefully selected.

Through analysis and comparison, the recommended quenching oil can be preliminarily determined, and then supplemented by test verification, the same industry use effect, market reflection survey, comprehensive consideration of technical requirements, safety, quality, cost and other factors, finally determine the selection of quenching oil.

Leave a Comment

Your email address will not be published. Required fields are marked *