Steam Film Phenomenon in Quenching and Cooling Process Explained

There are many methods to judge the cooling capacity of quenching cooling medium, such as quenching intensity method, hot wire method, hardness U curve method and magnetic test method, among which the cooling curve method is recognized as the best laboratory measurement method, so it is also the most widely used.

However, the cooling curve of the workpiece in the actual quenching process is often different from the cooling characteristic curve obtained from the test, because the process of heat transfer from the workpiece to the medium in the quenching process is not only related to the workpiece material, but also closely related to the size and shape of the workpiece.

For example, the cooling curve obtained by testing the general rapid quenching oil with a standard probe can usually observe the vapor film stage, while when it is used as the quenching cooling medium for small fasteners, the vapor film stage may not be observed.

However, using standard methods to test the cooling characteristic curve of quenching cooling medium is still very useful for comparison and selection of cooling performance of different mediums and monitoring the change of medium performance during use.

1. Cooling characteristic curve and three stages of quenching

At present, the cooling characteristic curve has been widely used to evaluate the cooling characteristics of quenching cooling medium, judge the aging degree of the medium, and guide the heat treatment process.

At present, the most widely used test methods are:

Heat the probe with a certain size and material to more than 800 ℃, and then immerse it into the quenching cooling medium with a certain temperature as a whole.

Use the thermocouple at the center of the probe to directly record the curve of the temperature change at the center of the probe with time, and obtain the cooling speed at different temperatures after derivation of the curve.

According to the measured cooling characteristic curve, the whole cooling process is generally divided into three stages (see Fig. 1):

Steam film stage (when the workpiece is just immersed in the medium, the temperature of the workpiece is high, and the medium around the workpiece is rapidly vaporized to form a stable steam film wrapped on the surface of the workpiece.

Because the heat conduction of the steam film is poor, the cooling speed is slow at this time);

Boiling stage (as the temperature of the workpiece decreases, it is difficult for the steam film to exist stably, but it quickly leaves the surface of the workpiece in the form of small bubbles to take away heat, and this stage has the fastest cooling speed);

Convection stage (the temperature of the workpiece surface decreases further, and when it is lower than the boiling point of the medium, the boiling stops, and then it enters the convection stage, relying on convection heat transfer).

Steam Film Phenomenon in Quenching and Cooling Process Explained 1

Fig. 1 Cooling characteristic curve and three stages of quenching

However, because the cooling curve usually tests the change of the probe core temperature with time, it cannot really reflect the temperature change of the probe surface.

Based on this, Dr. Zhang Kejian put forward the “four stages” theory, which believes that there should be an “intermediate stage” between the steam film stage and the boiling stage, to describe the process of coexistence of boiling and steam film phenomenon on the workpiece surface after the “advanced expansion point” appears on the workpiece surface (see Fig. 2).

This theory well explains that the cooling process of workpiece in quenching cooling medium is a very complex phenomenon, and can not be described comprehensively only by the cooling curve measured by the thermocouple at the center of the probe.

However, just knowing the complexity of the steam film breaking process is not enough to help us understand the essential causes of steam film formation and breaking, nor can it provide more reference for heat treatment practitioners.

Kobasko proposed to use the concept of critical heat flux from boiling heat transfer process to evaluate the cooling performance of quenching cooling medium, which can provide more help for medium development.

Steam Film Phenomenon in Quenching and Cooling Process Explained 2
Phase nameTheory divisionActual division
Vapor blanket stageAbove T0Above T1
Intermediate stageT0~T*T1~T2
Boiling stageT*~TT2~Tb
Convection stageTb~liquid temperatureTb~liquid temperature

Fig. 2 “Four Stage” Theory of Quenching

2. Critical heat flux and three stages of quenching

Fig. 3 shows the change of surface heat flow density and three successive stages in the continuous heating process, as the metal surface temperature rises and the wall superheat (surface temperature minus the boiling point of the medium) rises, as well as the change of thermal conductivity α of the medium in this process.

However, quenching and cooling is a cooling process, and the corresponding changes in heat flow density and thermal conductivity should be from the top right to the left of Fig. 3.

At the same time, there is a very short time transient boiling process (see Fig. 4).

Steam Film Phenomenon in Quenching and Cooling Process Explained 3

Fig. 3 Boiling phenomenon and changes of heat flux and thermal conductivity during heating

Steam Film Phenomenon in Quenching and Cooling Process Explained 4

Fig. 4 Instantaneous Boiling Phenomenon at the Initial Stage of Quenching Process

French designed a large number of experiments in 1926-1930 to study the instantaneous boiling process.

It was found that the duration of the instantaneous boiling process at the beginning of the quenching process was less than 1s for workpieces of all shapes and sizes.

qcr1 is the critical heat flux when vapor film appears after a short transient boiling process, and qcr2 is the critical heat flux when vapor film boiling changes to nucleate boiling.

According to the theoretical calculation model of qcr proposed by S. Kutateladze based on the theory of fluid mechanics, qcr1 (unit: W/m2) can be calculated according to formula (1):

Steam Film Phenomenon in Quenching and Cooling Process Explained 5

Where k ≈ 0.14;

  • r* – Heat of steam formation, J/kg;
  • ρ’ – Liquid density, kg/m3
  • ρ” – Steam density, kg/m3;
  • g – Gravitational acceleration, m/s2
  • σ – Surface tension of medium, N/m.

At the same time, qcr1 and qcr2 satisfy the following relationship:

Steam Film Phenomenon in Quenching and Cooling Process Explained 6

It is worth pointing out that both qcr1 and qcr2 here represent the nature of the cooling medium itself, independent of the quenched workpiece.

qcr2 can be used to measure the cylinder with large aspect ratio (to avoid the test error caused by the “leading expansion point” – the leading expansion point of the sphere has great randomness, while the lower edge corner of the short cylinder is always easier to break the film as the leading expansion point), Silver material (the thermal conductivity changes little with temperature, and the thermal conductivity is high, and the core temperature is more consistent with the surface temperature).

The cooling rate when the steam film breaks after the probe is immersed in the cooling medium is converted, because the instantaneous temperature change at a certain time and the heat flow density meet the following relationship:

Steam Film Phenomenon in Quenching and Cooling Process Explained 7

Where

c – medium heat capacity;

dT – d τ Average temperature change in time;
V – volume;
S – surface area;
Steam Film Phenomenon in Quenching and Cooling Process Explained 8 – Surface temperature gradient.

Thus, the heat flux q:

Steam Film Phenomenon in Quenching and Cooling Process Explained 9

Where Steam Film Phenomenon in Quenching and Cooling Process Explained 10 is the average cooling speed.

Measure the change of cooling rate during the whole cooling process, and then calculate the minimum heat flux q, which is the minimum critical heat flux qcr2 when the film breaks, and then calculate qcr1.

Combined with the initial (maximum) heat flux qin of heat transfer from the workpiece surface to the outside after the high-temperature workpiece is immersed in the medium, it can be inferred that two different cooling processes may occur in the actual quenching cooling process.

When qin < qcr1, the heat flow density of the workpiece heat transfer cannot reach the critical heat flow density qcr1 required for the formation of the cooling medium vapor film, so the stable vapor film cannot be formed.

At this time, the vapor film stage cannot be observed.

After the workpiece is immersed in the medium, it will directly enter the boiling stage and then transition to the convection stage;

When qin ≥ qcr1, after the workpiece is immersed in the medium, a vapor film can be formed on the surface, so that a three-stage cooling process of complete vapor film stage boiling stage convection stage can be observed.

This theory can also explain different steam film phenomena in engineering application practice. For example, increasing the surface roughness of the workpiece and reducing the size of the workpiece are actually equivalent to increasing the specific surface area of the workpiece and reducing the heat flow density of the workpiece, qin, so as to eliminate (qin < qcr1) or shorten the steam film time (it can be reduced to qcr2 faster to cause the steam film to break);

Adding a certain amount of inorganic salts to the water can increase the surface tension σ of the aqueous solution, and at the same time increase the density difference picture between the medium and the steam, thus increasing qcr1.

At the same time, in the salt solution, the double electric layer formed on the surface of the workpiece will reduce the heat flow density qin of the workpiece, which makes it more difficult to form or accelerate the rupture of the vapor film under the double action.

At the same time, this theory also helps us to explore many quenching and cooling problems that are difficult to explain by the “three-stage theory”.

For example, the medium low hardenability high carbon chromium steel is prone to reverse quenching, and the carburizing layer surface is prone to non martensite structure (ignoring alloy element depletion).

3. Effect of refrigerant on steam film during cooling of different quenching oils

Based on the above theory, we designed an experiment to investigate the influence of the addition of refrigerant in base oil on the steam film during quenching and cooling.

It is well known that adding refrigerant to the base oil can significantly shorten the duration of its vapor film, thereby improving the cooling capacity of the base oil and improving the uniformity of the workpiece quenching process.

In this paper, we respectively dissolve the same concentration of refrigerant in low viscosity and high viscosity base oil to simulate the cooling performance of fast quenching oil and isothermal quenching oil, and observe the steam film during heating and cooling through experiments.

The test uses a 8mm diameter nickel chromium alloy probe, places a thermocouple temperature probe in its geometric center, uses an induction coil with a constant heating power of 2.7kW for heating, the inner diameter of the coil is 12.5mm, and uses a temperature recorder to record the time temperature curve of the temperature rise and drop process.

The test device is shown in Fig. 5.

Steam Film Phenomenon in Quenching and Cooling Process Explained 11

Fig. 5 Induction heating and temperature recording device

(1) Generation of steam film during temperature rise

Fig. 6 shows the time temperature curve of low viscosity base oil, fast oil and isothermal oil during the heating process.

The heating rate temperature curve can be obtained by differentiating the curve.

Steam Film Phenomenon in Quenching and Cooling Process Explained 12

Fig. 6 Time temperature curve and heating rate temperature curve of heating process

It can be seen from the figure that except for fast oil, the probe has two obvious inflection points on the temperature rise curves of base oil and isothermal oil.

The increase of probe temperature reflects the increase of net heat after the heat applied to probe by induction heating minus the heat taken away by quenching oil from probe surface.

Among them, since the material, size and distance between the probe and the coil remain unchanged, and the power of the heating coil remains unchanged, it can be considered that the heat increase rate caused by induction heating also remains unchanged.

In the first stage of the low temperature zone, the medium has poor cooling capacity through convection heat transfer, so the temperature rise rate of the probe is fast;

After that, the medium begins to boil violently.

At this stage, the cooling capacity of the medium is greatly enhanced, and the temperature rise rate of the probe is significantly reduced;

As the temperature continues to rise, a vapor film appears. At this time, the cooling capacity of the medium becomes worse again, so the temperature rise rate of the probe becomes faster again.

At the same time, compared with fast oil and base oil, the duration of convection stage and the temperature of transition to boiling stage are basically the same.

It can be seen that the introduction of refrigerant will not significantly change the cooling capacity of the medium in the convection stage, nor will it significantly change the boiling point of base oil.

However, under the heating power of 2.7 kW, the probe surface can no longer produce a stable vapor film.

This is because the addition of refrigerant increases the critical heat flux qcr2 of the medium, and the vapor film is easier to crack;

At the same time, the polymer film formed by the refrigerant on the probe surface reduces the thermal conductivity of the workpiece surface, thereby reducing the heat flow density of the workpiece, making it qin < qcr1, resulting in the failure of the formation of the vapor film.

(2) Disappearance of vapor film in cooling process

Through induction heating, the probe temperature is raised to 1000 ℃ and then stopped heating.

The cooling process of the probe over time is recorded to obtain the well-known cooling time temperature curve.

After differentiation, the cooling rate temperature curve during quenching can be obtained, as shown in Fig. 7.

Steam Film Phenomenon in Quenching and Cooling Process Explained 13

Fig. 7 Time temperature curve and cooling rate temperature curve in the cooling process (the solid line is the heating process, and the dotted line is the cooling process)

Superimposing it with the heating rate temperature curve of the heating process, it can be seen that the three stages of the quenching process have a good correlation with the temperature range of the three stages of the heating process.

Only compared with the heating process, the transition temperature between stages in the cooling process is slightly higher than that in the heating process. as a result of:

For the heating process, the induction heating is surface heating and then transferred to the center of the probe, so the measured center temperature lags behind the surface temperature to a certain extent, and the test temperature should be lower than the actual surface temperature;

For the cooling process, the core temperature also lags behind the surface temperature, and the test temperature should be higher than the actual surface temperature.

At the same time, according to equation (4), the heat flow density in the cooling process is proportional to the average cooling speed:

Steam Film Phenomenon in Quenching and Cooling Process Explained 14
(5)

As the cooling rate of isothermal oil when breaking the film is lower than that of low viscosity base oil, it can be calculated that its critical heat flux qcr2 is also greater than that of low viscosity base oil, so the film can be broken at higher temperatures, which is basically consistent with the phenomenon observed in practical engineering applications.

4. Conclusion

So far, by analyzing the heat transfer process in the quenching process, it can be seen that the steam film stage and the transition stage from steam film to nucleate boiling in the quenching process are much more complex than we usually know.

At the same time, the concept of critical heat exchange density proposed by heat exchange theory in boiling heat transfer process is introduced to explain the phenomenon of steam film in quenching process.

By means of induction heating, the phenomena of boiling and steam film in the process of heating and cooling are recorded and observed, and the two processes are combined to try to have a deeper understanding of the phenomenon of steam film that commonly exists in the quenching process, which can provide more help for the design and development of new quenching cooling media with shorter steam film time and faster cooling speed.

Based on theoretical discussion and experimental verification, in order to reduce the steam film in the quenching process and improve the quenching uniformity of the workpiece, we think the following three aspects can be started:

(1) Increase the critical heat flux qcr1 and qcr2 for the formation and rupture of the vapor film of the medium itself.

For example, increase the surface tension of the medium and the density difference 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, thereby reducing the heat flow density qin of the surface of the workpiece, thereby reducing or even eliminating the steam film.

(3) Electrolyte is introduced to increase the critical heat flux qcr of the medium, and at the same time, a double electric layer is formed on the workpiece surface to reduce the heat flux qin of the workpiece surface, so as to reduce or even eliminate the vapor film.

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