There are several methods for evaluating the cooling capacity of a quenching medium, including the quenching intensity method, hot wire method, hardness U-curve method, magnetic test method, and others. The cooling curve method is considered the best laboratory measurement method and is widely used.
However, it is important to note that the actual cooling curve of a part during quenching may differ from that obtained in testing. This occurs because the heat transfer process from the part to the environment during quenching is influenced not only by the material of the part, but also by its size and shape.
For example, testing a general quick-quench oil using a standard probe will normally show the vapor film stage, but when the same oil is used as a quenching medium for small fasteners, the vapor film stage may not be visible.
Despite these differences, testing the cooling characteristic curve of a cooling medium using standard methods is still valuable for comparing and selecting different media and monitoring media performance over time.
1. Cooling characteristic curve and three quenching stages
The cooling characteristic curve is widely used nowadays to evaluate the cooling properties of quench cooling media, determine the aging degree of the media, and guide the heat treatment process.
The most commonly used testing methods are:
Heat a probe of a specific size and material to more than 800°C and then immerse it in the quench cooling medium of a specific temperature.
Using a thermocouple at the center of the probe to directly record the temperature change at the center of the probe over time and deriving the curve to determine the cooling rate at different temperatures.
Based on the measured cooling characteristic curve, the cooling process is typically divided into three stages (see Fig. 1):
Vapor film stage (when the workpiece is first immersed in the medium, its temperature is high and the medium around the workpiece vaporizes quickly to form a stable vapor film that surrounds the surface of the workpiece. At this time, cooling is slow due to poor heat conduction of the vapor film);
Boiling stage (as the temperature of the part decreases, the vapor film becomes unstable and quickly leaves the surface of the part in the form of small bubbles, removing heat. This stage has the fastest cooling rate);
Convection stage (as the surface temperature of the part decreases further, boiling stops when it drops below the boiling point of the medium, and the convection stage begins, relying on convective heat transfer).
By superimposing the cooling temperature rate curve with the heating rate temperature curve of the heating process (as shown in Fig. 7), it can be seen that the three stages of the quenching process are closely related to the temperature range of the three stages. of the heating process.
However, compared to the heating process, the transition temperature between the stages of the cooling process is slightly higher for the following reasons:
In the heating process, induction heating starts at the surface and then is transferred to the center of the probe, causing the measured core temperature to lag behind the surface temperature to a certain extent, resulting in a lower test temperature compared to the actual surface temperature.
In the cooling process, the core temperature also lags behind the surface temperature, resulting in a higher test temperature compared to the actual surface temperature.
At the same time, according to equation (4), the heat flux density in the cooling process is proportional to the average cooling speed:
Since the cooling rate of isothermal oil when the film breaks is lower than that of low-viscosity base oil, it can be calculated that its critical heat flux qcr2 is also higher than that of low-viscosity base oil. This means that the film can break at higher temperatures, which is in line with observations made in engineering applications.
4. Conclusion
By analyzing the heat transfer process in the quenching process, it became clear that the stages of vapor film formation and the transition from vapor film to nucleate boiling are much more complex than previously thought. The concept of critical heat flux, used in boiling heat transfer theory, was introduced to explain the vapor film phenomenon in the quenching process.
Experiments were conducted using induction heating to observe and record the boiling and vapor film phenomena during heating and cooling. This combined approach aimed to gain a deeper understanding of the common vapor film phenomenon in quenching processes, which could provide further guidance for the design and development of new quench cooling media with shorter vapor film duration and speed. faster cooling.
Theoretical discussions and experimental verifications have shown that in order to reduce the vapor film in the quenching process and improve the quenching uniformity of the workpiece, the following three aspects should be considered:
(1) Increase the critical heat flux qcr1 and qcr2 for the formation and rupture of the vapor film of the medium itself.
For example, increasing the surface tension of the medium and the difference in density between the gas and liquid phases, making the vapor film more difficult to form and easier to rupture.
(2) An additive that can form a film on the surface of the workpiece is introduced to attach to the surface of the workpiece to form a thermal insulation layer of moderate thickness, so as to reduce the heat transfer coefficient of the surface of the workpiece. workpiece, thereby reducing the qin heat flux density of the workpiece surface, reducing or even eliminating the vapor film.
(3) The electrolyte is introduced to increase the critical heat flux qcr of the medium, and at the same time, a double electrical layer is formed on the surface of the workpiece to reduce the heat flow qin of the workpiece surface, so as to reduce or even eliminate the vapor film.
