JPS6056014A - Method and device for cooling in heat treatment of metal - Google Patents

Abstract

PURPOSE:To enable cooling which has the initial cooling rate satisfying a desired heat treatment and is thereafter slow with less quenching strain and crack by using a gas-liquid mixture formed by dispersing fine foam of gas into liquid as a cooling medium. CONSTITUTION:An air-water mixture is put into an inside vessel 2 of a cooling vessel 1 having a double construction and liquid such as water or oil is put into an outside vessel 3. The temp. of said liquid is controlled by a heater 4 to maintain the temp. of the above-mentioned mixture or to control the temp. thereof. The above-described mixture is subjected to flow rate control by a throttle valve 7 from a device 5 for forming said mixture and is admitted into the vessel 2 in the circumferential tangential direction along the inside wall of the lower part thereof so as to be distributed uniformly in the tank 2. The mixture is then discharged from an overflow port 8. The air foam is thus uniformly dispersed at the horizontal section of the vessel 2 and at the same time the floating of the air foam is limited. The quasi-static state of the cooling liquid in the vessel is obtd. with the decreased amt. of the mixture to be introduced. The inflow rate is conversely increased by the valve 7 or turning flow is formed in the vessel 2 by a stirrer 9 to make the cooling liquid more positively convectional.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a cooling method in metal heat treatment and an apparatus used in the method.

Conventionally, the selection of cooling capacity in quenching, especially in immersion quenching, is usually done by changing the coolant; factors such as liquid temperature and stirring are also taken into account as secondary effects. Therefore, the selection range of cooling capacity is determined by the ability to select cooling liquids with different cooling capacities, and currently single-phase systems such as air cooling, hot cooling, oil cooling, and water cooling are used.

However, in cases such as steel, where rapid cooling to 500-600°C is required for hardening, or slow cooling to around 200°C to prevent thermal distortion and cracking, there is no suitable single-phase cooling liquid. method is known.

■ Use water with a softening agent (vinyl compound water-soluble resin, salts, alcohol compounds, etc.) dissolved in water as the coolant.

■ Spray water as fine particles through a nozzle.

(2) When the temperature of the material during immersion quenching reaches around 200°C, the material is pulled up using a stepwise cooling method such as air cooling and slow immersion cooling.

Each of these methods had the following drawbacks.

■ Changes in cooling capacity are small, making it difficult to maintain and manage the set cooling capacity (concentration). Also, the coolant is expensive. (Depending on the compound, it adheres to the material to be cooled and requires cleaning, and the concentration also changes).

■ Since it is not an immersion quenching method, many nozzles are arranged concentrically and the material to be cooled is placed on the center line and sprayed.
In this case, even if the nozzle jet amount and jet pressure are adjusted, the jetting conditions cannot be made the same.Furthermore, uniform cooling is difficult when cooling not only an axially symmetrical single item but also a complicated shape or a large amount of material at the same time. Therefore, it is easy to cause uneven cooling of the material, and furthermore, the cooling capacity can be set and controlled closer to the air side than the water side.In other words, the cooling capacity (degree of cooling) is salt>water>oil>air, but spraying In the system, the amount of air is greater than the amount of water, so the degree of cooling is lower than that of immersion in water, and the range of cooling capacity options is narrow.

■ It is difficult to accurately grasp the stage of material withdrawal, and it is difficult to heat treat a large number of materials at the same time.

Moreover, the heat treatment process becomes complicated.

In particular, when heat treating steel (carbon steel, alloy steel), the material must be heated to an appropriate temperature and cooled at an appropriate rate. Generally, it is necessary to quickly take out the material from the heating furnace and cool it extremely quickly at around 500 to 600° C. as a heat treatment (quenching). But 180
At around ~200°C, it is desirable to cool gradually to prevent quenching distortion and cracking. In the case of copper with poor hardenability or materials with large mass (dimensions), the cooling rate is too slow even if oil or, in some cases, water is used, and the above-mentioned hardening distortion and cracking may occur. In order to prevent this, we changed the cooling medium from water to salt, and from salt to oil.
Alternatively, in the case of oil cooling, changing from a low cooling capacity to a low cooling capacity (in this case, the selection range of cooling capacity is small), or removing it from the cooling tank at around 200℃ after water cooling and cooling in the air. Two-stage cooling is performed. The former involves replacing with one with a smaller cooling capacity, so the cooling rate in the high-temperature range is also slow, which may result in insufficient hardening, while the latter makes it difficult to take out the material at the same time. It is difficult to maintain and manage heat treatment conditions.

In addition, in the cooling method that uses a single phase liquid as a cooling medium, a non-uniform vapor film is formed on the metal surface due to contact between the high temperature metal to be processed and the low temperature liquid, but the vapor film is adiabatic. This makes stable and uniform cooling difficult.

Therefore, the object of the present invention is to provide a uniform and stable method for metal heat treatment that overcomes the drawbacks of the conventional methods (1) to (3) above, is inexpensive, simple, and can be easily controlled to a desired cooling capacity. The objective is to provide a cooling method. In particular, it is an object of the present invention to provide a cooling method that has an initial cooling rate that satisfies the desired heat treatment, and that allows for subsequent gradual surface cooling with less distortion and cracking, and an apparatus therefor. .

Other objects of the invention will become apparent from the description below.

The present inventors have discovered that the above object can be achieved by using a liquid-gas mixture in which gas microbubbles are dispersed in a liquid as a cooling medium, and have completed the present invention.

That is, as mentioned above, when a single phase liquid is used as a coolant, a vapor film consisting of air bubbles is formed on the surface of the metal material to be treated due to the contact between the liquid and the high temperature metal material to be treated, and the film is The aim is to uniformly and stably cool the metal to be processed. Using a liquid-gas mixture in which microbubbles are dispersed as a cooling medium, cooling is surprisingly performed in a state in which the microbubbles are evenly dispersed. The material to be processed can be cooled uniformly and stably.

Furthermore, the present inventor found that the cooling ability of a gas-liquid mixture changes significantly depending on the gas content, and therefore, the gas content (
It was discovered that the cooling ability of the gas-liquid mixture can be changed at any stage of the cooling process by changing the void ratio.

Therefore, the metal heat treatment cooling method of the present invention uses a liquid-gas mixture liquid in which gas microbubbles are dispersed in a liquid as a cooling medium, and maintains a uniformly dispersed state of the microbubbles to cool the material to be cooled. It is characterized in that it prevents the formation of a vapor film and enables uniform and stable cooling.

The cooling capacity of the liquid-gas mixture can be controlled by controlling the gas content and/or the liquid temperature.

The relationship between the gas content (void ratio) and the cooling capacity of the liquid-gas mixture is shown in FIGS. 1 to 3.

Figure 1 shows a water-air mixture having a constant liquid temperature of 20°C and a constant air content rate (void rate) of 0 to 10-10°C as a material to be treated in an N2 gas atmosphere. A cooling curve is shown when a copper cylinder with a diameter of 30 mm and a length of 120 mm heated to ℃ is immersed and cooled. The horizontal axis shows the cooling time, and the vertical axis shows the dimensionless surface temperature. On the vertical axis 0 jar 1 issue F), Tt:
Temperature of the material to be treated, TO: Initial temperature of the material to be treated, Tinf
: Indicates liquid temperature. The temperature of the material to be treated is measured using the diameter α5IIl.
2 below the surface of the treated material using a CA sheathed thermocouple (grounded).
The temperature of III was measured as follows.

For reference, cooling curves when cold (Co/d) and hot (HOT) marquench oils are used as coolants are also shown.

FIG. 2 is a cooling curve showing the influence of liquid temperature on the cooling capacity of a water-air mixture. The material to be treated was J30x12 heated to 800°C in a N2 atmosphere, as in the case in Fig. 1.
0m copper cylinder, temperature measurement method, Tt, To, and Tin
f is as described above.

Figure 3 shows the air content (void fraction) of the water-air mixture and the g50x120mm copper cylinder at 800'0 to 400°C.
It is a graph which shows the relationship with the time required for cooling to.

As can be seen from FIGS. 1 to 3, the cooling curve of the material to be treated changes depending on the void ratio and liquid temperature. Therefore, by setting the void ratio and liquid temperature so as to obtain a cooling curve suitable for a specific material to be processed, the material to be processed can be cooled through a desired cooling process. In the cooling process, the void ratio and, in some cases, the liquid temperature may be changed to change the cooling ability of the coolant.

For example, in immersion hardening of steel (carbon steel or alloy steel), rapid cooling at 500 to 600°C and slow cooling at around 200°C are required, so liquid-gas (e.g. water-air) is required. The liquid temperature and void ratio may be set or controlled so that the cooling curve becomes gentle around 200°C.

For example, cooling is performed using a water-air mixture whose water temperature is set to 20° C. and the void ratio is set to 0 to 10 degrees.

Or about 20 in a water-air mixture with a void ratio of 2 to 10
The mixture is cooled to around 0°C, and then the void ratio of the mixture is reduced to around 1% to lower the cooling ability, and the mixture is slowly cooled to around 200°C (see Figure 3). Alternatively, the cooling capacity may be lowered by raising the liquid temperature around 200°C. However, in either case, it is necessary to maintain a uniformly dispersed state of microbubbles.

Next, a case where the cooling is immersion cooling and the cooling liquid is a water-air mixture will be specifically described.

A water-air mixture liquid whose air bubble mixing rate (void rate) and liquid temperature are set in advance is introduced into the cooling tank. Since the density of water in the mixed liquid in the tank is approximately 1,000 times that of air, separation will inevitably occur after a certain period of time. Therefore, in order to maintain a constant void ratio, the water-air mixture is introduced into the cooling tank from the bottom of the tank at a flow rate that causes a rise in water level that is equal to or slightly greater than the floating speed of the air bubbles. . whether the mixed liquid is discharged from the lower part of the tank along the inner wall of the tank in the tangential direction of the cypress, or into the tank via a diffuser;
Alternatively, the bubble distribution within the agitated cypress can be maintained uniformly.

It is preferable that the bubble diameter is smaller than about 1M.

If the diameter is larger than 1B, coalescence of bubbles will occur.

Repeat 7 times to uniformly disperse the bubbles. [When the diameter of L is about 111 m, the underwater ascent speed is several z seconds, which is sufficient for practical use.

A fourth embodiment of the apparatus that can be used for immersion cooling according to the present invention
- Shown in Figure 5.

The cooling tank 1 has a double structure including an inner tank 2 for storing an air-water mixture and an outer tank Ia3 for keeping the mixture warm or controlling the temperature of the mixture. The outer tank 3 is filled with a suitable liquid such as water, oil, ethylene glycol, etc., and the temperature of the liquid in the outer tank 3 is controlled by a heater 4, which is also installed inside the outer tank 3. The temperature of the liquid is maintained or the temperature of the liquid is controlled. The mixed liquid is passed from the water-air mixed liquid preparation device 5 through an inlet 6 provided at the lower part of the inner tank 2, the flow rate of which is controlled by a throttle valve 7 provided at the inlet 6, and is applied to the inner wall of the lower part of the inner tank 2. The mixed liquid flows in a tangential direction along the circumference, so that the mixed liquid is uniformly distributed in the tank 2, and then flows out from the overflow port 8 at the upper part of the inner tank 2. A diffuser installed at the inlet 6 (
The mixed liquid may be introduced through a tube (not shown). As a result, the air bubbles are uniformly dispersed within the horizontal cross section of the inner tank, and at the same time, the floating of air bubbles is suppressed, and with a small amount of mixed liquid introduced, the coolant in the tank is kept in a quasi-quiet state (immersion cooling in static coolant). ) can be obtained. On the other hand, in order to actively effect the convection of the cooling liquid (mixed liquid), the flow rate of the cooling liquid (mixed liquid) can be actively increased using the throttle valve 7, or the agitator 9 in the inner chamber driven by the motor 10 can be used to Circulation can be formed within. The broken coolant 11 is placed approximately in the center of the inner tank 2.

In order to reduce the void ratio of the mixed liquid, it is sufficient to introduce the mixed liquid into the inner tank 2 at a flow rate that results in a water level rising speed that is slower than the floating speed of air bubbles, while keeping the void ratio constant. In order to achieve this, the water level rising speed should be set to be equal to or higher than the air bubble floating speed, preferably slightly higher than the air bubble floating speed. In order to increase the void ratio, a water-air mixture having a large air content ratio is introduced into the inner tank at a flow rate that causes the water level to rise faster than the floating speed of air bubbles. The composition of the mixed liquid used as the cooling liquid is not limited to water-air, but may be replaced with any liquid-gas such as water-nitrogen, oil-air, oil-nitrogen, etc.

In spray quenching of metals, the spray cooling liquid is, for example, a water-air mixture containing micro air bubbles, and the cooling capacity of the cooling liquid is determined not only by the normal discharge water storage (nozzle pressure) and water temperature, but also by the void ratio of the mixture. control, and the nozzles are placed around the material to ensure uniform cooling of the material.

FIG. 6 shows one embodiment of an apparatus that can be used for blast a hardening.

That is, a water-air mixture in which micro air is dispersed in water is formed with a preset bubble inclusion rate (volumetric void ratio) and liquid temperature using an arbitrary water-air mixture preparation device, or, for example, Water in which air is dissolved is created by a reduced pressure method, and the mixed liquid or water is supplied to a plurality of nozzles 12 arranged concentrically around the material and ejected. Cool down.

Next, a case where carbon steel (845c) is used as the material to be treated and is immersed and hardened will be illustrated by way of example.

Example Two solid cylindrical carbon steel specimens with a diameter of 30 cm and a length of 1 B (ha+) were heated to 800°C in a nitrogen gas atmosphere, and then heated at 20°C in the apparatus shown in Figure 4. The liquid temperature (void ratio: 1x and 5x) was immersed in two volumes of water-air mixture (void ratio of 1x and 5x) held at
20°C).

As a result, all carbon steel specimens had a surface hardness of 700HV.
It was about 400 HV at the radial center. This was almost the same hardness as conventional cooling using only water.

Next, two annular V grooves with an interval of 90 grooves were provided in the axial center of each of the two carbon steel specimens, and the dimensional difference between the grooves before and after the heat treatment was measured. As a result, the length change as the sum of strain changes due to expansion and thermal deformation corresponding to an increase in precursor hardness is 0.351 relative strain in the case of conventional water-only cooling; case is also 0
．． The number decreased to about 2 inches, and a good effect was brought about in practical use.

According to the cooling method of the present invention, the cooling capacity can be easily changed, and the formation of a vapor film and therefore unstable cooling in conventional cooling methods using liquid can be avoided.

Since the use of a relaxation agent that suppresses the cooling ability is omitted, there is no need to worry about adhesion of the relaxation agent to the material to be treated and maintenance of the concentration. Furthermore, when the gas-liquid mixture is water-air,
It's cheap.

[Brief explanation of drawings]

Figure 1 is a graph showing the cooling curve of a steel sample when an air-water mixture (liquid temperature: approximately 20°C) having various void ratios (volume ratio) is used as a cooling medium. , is a graph showing cooling curves of steel samples when air-water mixtures having various liquid temperatures are used as coolants. 4 is a graph showing the change in cooling time of the sample from 800°C to 400°C; FIG. 4 is a vertical cross-sectional view of an apparatus that can be used in the cooling method of immersion quenching; It is a partial sectional view taken along A-A of
FIG. 6 is a partial plan view of an apparatus that can be used in the cooling method of the spray hardening method. 1...Cooling tank 2...Inner tank 3...Outer tank 5...
Air-water mixed liquid preparation device - 7... Throttle valve 12... Nozzle Patent applicant Toyota Central Research Institute Co., Ltd. Representative Patent attorney Yumi Kaede (and 1 other person) 3 years old Fig. M) ヰ4 Figure 0 Y5 Enya

Claims (8)

[Claims] (1) Gold I! 4. In the cooling process of heat treatment, a liquid-gas mixture in which gas microbubbles are dispersed in a liquid is used as a cooling medium, and by maintaining a uniformly dispersed state of the microbubbles, the formation of a vapor film on the material to be cooled is prevented. 1. A cooling method for metal heat treatment, which is characterized in that it enables uniform and stable cooling. (2) The method according to claim 1, characterized in that the cooling capacity of the liquid-gas mixture is controlled by controlling the gas content and/or temperature of the liquid-gas mixture. (3) The method according to claim 1 or 2, wherein the cooling medium is an air-water mixture containing air bubbles in an amount of up to 10 volumes. (4) The method according to claim 1, 2 or 3, wherein the metal heat treatment is immersion hardening of copper. (5) The air-water mixture is caused to flow from the lower part of the cooling tank in the melting direction along the tank inner wall at a flow rate that is equal to or slightly higher than the floating speed of the air bubbles and the water level rise speed in the cooling tank. 5. A method as claimed in claim 4, characterized in that the air bubble content is kept constant and the air bubbles are uniformly distributed. (6) The method according to any one of claims 3 to 5, wherein the air bubble diameter is in the range of α1 to α1. (7) The metal heat treatment is PJT mist quenching of steel, the coolant is an air-water mixture containing up to 10 (by volume) micro air bubbles, and the cooling capacity is controlled by the amount of ejected liquid, liquid temperature and air A feature characterized in that the material to be cooled is uniformly cooled by controlling the bubble content and injecting the coolant from a plurality of nozzles around the circumference of the material to be cooled. !
I- The method according to item 1. (8) Cooling tank with a double N structure consisting of an inner tank for storing the liquid-gas mixture and an outer tank for keeping the liquid-gas mixture warm or controlling the liquid temperature: A cooling tank provided at the bottom of the inner tank. It is characterized by comprising an inlet configured to allow the mixed liquid to flow in a circumferential tangential direction along the inner wall of the lower part of the inner tank, a throttle valve for controlling the flow rate, and an overflow port provided at the upper part of the inner tank. Cooling equipment for metal immersion quenching.