How to Select Quenching Cooling Medium? (40 Year Experience Summary)

Quenching is the most critical process in heat treatment process, which ultimately determines the internal quality and distortion of quenched parts.

Quenching cooling technology has been continuously improved with the development of heat treatment technology.

However, due to the complexity of the cooling process and the characteristics of instantaneous completion, as well as the limitations of observation and measurement, the quenching cooling is somewhat mysterious.

For a long time, the machinery industry generally has the malpractice of “emphasizing cold and neglecting heat” (that is, attaching importance to cold processing and neglecting hot processing), while the heat treatment industry generally has the phenomenon of “emphasizing heat and neglecting cold” (that is, attaching importance to heating and neglecting cooling), which is very abnormal.

Based on my long-term experience in heat treatment, I would like to talk about the cognition and selection of quenching cooling medium today.

1. Basic properties of quenching cooling medium

(1) Ideal cooling rate

The so-called ideal cooling speed means that for a certain material and a specific quenched part, the cooling speed at the nose of the C curve is expected to be greater than the critical cooling speed (the minimum cooling speed to ensure that the workpiece can complete the martensite transformation).

In other temperature ranges, especially in the martensite transformation temperature range (Ms → Mf), it should be cooled slowly, which is often said that “high temperature should be cooled quickly and low temperature should be cooled slowly”.

Different steels and different workpieces have different demands for “fast” and “slow”. Theoretically, there is a concept of “ideal quenching cooling medium”, as shown in Fig. 1.

The “C” curve of different steels is different.

It is impossible to obtain the so-called “ideal quenching cooling medium” suitable for quenching of various steels and workpieces of different sizes.

We can only select the appropriate quenching cooling medium according to the specific conditions to make the quenched parts hard, with small distortion and good metallography.

Fig. 1 ideal quenching cooling medium curve

(2) Good stability

The quenching cooling medium shall be relatively stable during use and not easy to decompose, deteriorate and age.

Various quenching oils and organic solvents have different degrees of aging, which should be adjusted, updated and maintained in a timely manner.

(3) Uniformity of cooling

Different parts and surfaces of the workpiece shall be cooled as evenly as possible to avoid quenching soft spots and soft blocks.

(4) Non corrosive

After quenching, it shall be kept clean and easy to clean, and shall not corrode the workpiece.

(5) Environmentally friendly

During quenching, no large amount of smoke, toxic and irritating gas will be produced, and the waste liquid from quenching parts will not cause pollution to the environment.

(6) Safety

The quenching cooling medium is not flammable and explosive, and it is safe to use.

(7) Economy

Quenching cooling medium should be both good quality and good price.

It is too expensive and is not welcomed by heat treatment enterprises.

2. Factors affecting the cooling performance of quenching cooling medium

There are many factors affecting the cooling performance of quenching cooling medium, which can be summarized as following 9 kinds.

(1) Temperature

The cooling capacity of quenching cooling medium varies with the temperature.

The cooling capacity of water and water-based quenching cooling medium decreases with the increase of temperature, while that of oil and salt bath is opposite.

With the increase of temperature, the fluidity is good, which is conducive to heat dissipation and the cooling capacity is improved.

Therefore, it is important to grasp the cooling temperature of specific workpiece.

(2) Surface tension

The surface tension directly affects the cooling speed.

Generally speaking, the quenching cooling medium with small surface tension is in close contact with the surface of the quenched parts, so the heat dissipation is fast and the cooling capacity is improved.

(3) Stir

Stirring will increase the heat transfer coefficient of quenching cooling medium, destroy the vapor film as soon as possible, improve the cooling speed, and make the quenched parts cool evenly.

(4) Thermal conductivity

Thermal conductivity, also known as thermal conductivity, is a physical quantity that represents the thermal conductivity of a substance.

The greater the thermal conductivity, the stronger the cooling capacity.

(5) Specific heat capacity

The greater the specific heat capacity of quenching cooling medium, the greater the cooling speed.

(6) Viscosity

Viscosity represents the internal friction force between liquid molecules during liquid flow.

The quenching cooling medium with high viscosity has poor fluidity, which is not conducive to convection heat dissipation, and its cooling capacity is poor.

On the contrary, the quenching cooling medium with low viscosity has good cooling effect.

(7) Heat of vaporization

Vaporization heat refers to the heat required for a unit mass of liquid to completely change into a gas at the same temperature.

The chemical stability of water is very high and the heat capacity is large.

It is 8 times of steel at room temperature.

The boiling point of water is low.

Its vaporization heat decreases with the increase of temperature.

The cooling capacity of water drops sharply with the increase of temperature.

However, after the water temperature rises to 80 ℃, its cooling capacity hardly changes, and the quenching intensity is maintained at about 0.72.

(8) Additive

The purpose of adding additives is to change the cooling performance.

If a small amount of salt or alkali is added to the water, the cooling capacity will be increased several times.

When polyvinyl alcohol is added to the water, a very thin plastic film will be formed on the surface of the quenched parts, which has poor thermal conductivity and reduces the cooling speed.

The suspension or emulsion formed by oil and soap in the water will accelerate the formation of steam film, increase the stability of steam film and reduce the cooling capacity.

In addition, in order to change other properties of quenching cooling medium, people often consciously add oxidant, brightener, rust inhibitor and antiseptic, which will affect the cooling performance to varying degrees.

The addition of additives will generally achieve multiple effects with one stone.

(9) Environment

The influence of the environment on the cooling capacity is often ignored by people.

The cooling effect of the same temperature medium is different in winter and summer, and the quenching effect is different in day and night.

3. Basic principles for selecting quenching cooling medium

A large number of facts tell people that many heat treatment quality accidents are related to quenching cooling medium.

If the quenching medium is improperly selected or misoperated, it will lead to the waste of quenching parts.

Therefore, the use of quenching cooling medium is the basic condition to ensure product quality.

No matter what quenching cooling medium is selected, uniform quenching effect should be obtained:

① High and uniform surface hardness and sufficient hardening depth are obtained.

② It cannot be quenched.

③ Quenching distortion is small.

According to the heat treatment technical requirements, materials, shapes and other specific conditions of quenched parts, the corresponding quenching cooling medium is selected, and the following five basic principles are summarized:

(1) According to the carbon content

Carbon is the most important element in all steels.

The carbon content not only affects the properties of steels, but also affects the quenching effect. For carbon steel, the carbon content ≤ 0.5% (mass fraction, the same below) can be salt water, alkaline water, organic solvent, etc;

For medium and low alloy structural steel, double liquid quenching or medium with relatively slow cooling rate is often used;

As for carbon tool steel, due to the high heat treatment requirements and poor hardenability, it is often quenched by alkali bath and nitrate bath, and rarely by oil cooling.

(2) According to the hardenability of steel

According to the “C” curve of the steel, the steel with poor hardenability requires faster cooling speed.

On the contrary, the cooling rate of steel with good hardenability is slower.

It is wise to choose proper quenching cooling medium according to the hardenability of steel.

(3) According to the effective diameter of the workpiece

Each steel has a critical quenching diameter.

When the surface of the quenched part is cooled to the Ms point, the cooling speed of the medium will be greatly reduced immediately, and the heat inside the workpiece will also be greatly reduced to the quenching cooling medium.

The undercooled austenite within a certain depth on the surface of the workpiece will be difficult to cool below the Ms point.

When the quenched parts are relatively thick, in order to obtain sufficient quenching layer depth, a faster low-temperature cooling speed should be selected.

On the contrary, when the workpiece is thin, the quenching cooling medium with low temperature and low cooling rate can be used.

From the distribution curve of the maximum allowable cooling speed, the thick workpieces are allowed to be cooled at high speed, and the thin workpieces should be cooled at low speed.

(4) According to the complexity of quenched parts

Based on the analysis of the allowable minimum cooling speed distribution curve, the quenching cooling medium with short steam film stage shall be selected for the workpieces with complex shapes, especially the workpieces with inner holes or deep concave surfaces, in order to reduce the quenching distortion and the need to harden the inner holes.

On the contrary, for workpieces with relatively simple shape, quenching cooling medium with a slightly longer steam film stage can be used.

From the distribution curve of the maximum allowable cooling speed, it can be seen that the allowable cooling speed of workpieces with complex shapes is low, while the allowable cooling speed of workpieces with simple shapes is high.

(5) According to the allowable deformation

The quenched parts shall have small distortion and a narrow cooling speed band.

When the allowable distortion is large, there can be a wide cooling speed band.

For the allowable cooling speed bandwidth, the medium that can generally achieve quenching hardness can be used.

The cooling speed band of the workpiece can be shortened by isothermal quenching or step quenching.

As there are many kinds of workpieces, different heat treatment requirements, quenching and cooling media emerge in endlessly, and the same kind of workpieces quenched with different media also get the same surface hardness, so it is difficult to select quenching and cooling media.

Based on the principle of economy and rationality, relatively ideal quenching cooling medium is selected.

4. Application examples of quenching cooling medium

(1) Neutral salt quenchant for quenching of high speed steel

The so-called neutral salt quenchant generally refers to two formulations:

The first type: 50% BaCl2 + 30% KCl + 20% NaCl (mass fraction), melting point 560 ℃, service temperature 580 ~ 620 ℃, suitable for effective diameter ≤ 20mm, can ensure the cooling speed within the temperature range of 1000 ~ 800 ℃ of the workpiece ≥ 7 ℃ / s, and prevent the precipitation of eutectic carbide from affecting the performance of the tool.

The second type: 48% CaCl2 + 31% BaCl2 + 21% NaCl, melting point 435 ℃, service temperature 460 ~ 550 ℃.

The phase diagram is shown in Fig. 2. For the convenience of batching, the factory changed it to 50% CaC12 + 30% bac12 + 20% NaC1, with a melting point of 440 ° C and a service temperature of 460-550 ° C.

Applicable to high-speed steel workpiece with effective diameter < 40mm.

The neutral salts of the two formulations are used differently.

It is recommended to use Ca based salts when the furnace is continuously opened for more than 5 days a week, because the Ca based salts have strong temperature absorption and are easily deliquescent in the air;

If the size of quenched parts is small and the furnace is not opened every day, barium based salt is suitable.

Fig. 2 ternary phase diagram of CaCl2, BaCl2 and NaCl

(2) Nitrate bath quenchant

Nitrate refers to the four salts of NaNO3, KNO3, NaNO2 and KNO2.

As the quenchant, few single components are used, and two or three mixed salts are commonly used.

The formula and melting point are shown in Fig. 3.

The most used formula is: 55% KNO3 + 45NaNO2, melting point 137 ℃, use temperature 160-550 ℃;

50% KNO3 + 50% NaNO2, melting point 140 ℃, service temperature 160-550 ℃.

Used for quenching of low alloy steel products and isothermal quenching of high speed steel and high alloy steel workpieces.

Some enterprises use nitrate salt bath as quenching liquid after carburization of large gears, which solves the problem of heat treatment of large gears.

Fig. 3 melting curve of nitrate system

(3) Quenchant of nitrate aqueous solution

There are mainly two nitrate water and three nitrate water.

① The two nitrate water is 25% NaNO3 + 25% NaNO2 + 50% water, and the service temperature is less than 60 ℃.

Due to the quenching of 45 steel small-size taps, dies and small workpieces, not only the quenching crack problem is solved, but also the quenching hardness is relatively uniform.

In actual production, carbon tool steel molds with sharp corners, grooves and cross-sectional size changes are often encountered.

The hardness requirements are 59-63HRC, water quenching is easy to crack, oil quenching is not hard, and water quenching and oil cooling are difficult to ensure the quality.

However, the problem is solved by using nitrate water nitrate salt bath isothermal quenching. That is to say, it can be estimated by 1s / 5mm in the nitrate water that there are white nitrate bubbles on the surface of the workpiece.

At this time, the temperature is about 200 ℃, and the workpiece is immediately put into the 180 ℃ nitrate bath for isothermal 30-60min.

In this way, the workpiece will not crack, the deformation is small, and the hardness meets the requirements.

(4) Boiling water quenching

Some people use boiling water quenching of 45 steel to replace quenching and tempering and achieved good results.

φ 40~ φ 80mm45 steel is heated at 840 ℃ and quenched into boiling water to obtain a hardness of about 250HBW, which is very uniform.

Boiling water quenching can replace the normalizing treatment of 45 steel as the final heat treatment.

High speed steel is quenched in boiling water at 850-870 ℃ instead of the original quenching and tempering treatment.

Double refining treatment of bearing steel – boiling water quenching.

Since the cooling speed of boiling water is slower than that of oil, there is no need to worry about cracking caused by boiling water quenching.

The specific operation is as follows: the bearing steel is quenched in boiling water immediately after final forging, the workpiece is cooled to 500-400 ℃, and the water is air cooled, and then (730-740 ℃) × (3 ~ 4h) annealing and air cooling after furnace discharge can obtain ultrafine grains and fine carbides.

There are many other applications of boiling water quenching, which are not listed one by one. As long as it is properly applied, it can save energy and increase efficiency.

(5) Bluing quenchant

It is a kind of quenching cooling medium prepared by the factory, and the workpiece after quenching is beautiful and corrosion-resistant.

There are two formulations with different colors.

① 70% NaNO3 + 20% KNO3 + 10% NaNO2, the workpiece is black after quenching.

② 70% NaNO2 + 20% KNO3 + 10% NaNO3, the workpiece is blue after quenching.

After mixing the three nitrates evenly in proportion, add appropriate amount of water to make them supersaturated solution, and heat them to 40-60 ℃ for use.

(6) Water soluble polymer quenchant — polyalkylene glycol (PAG)

PAG has a unique reverse solubility, that is, the solubility in water decreases with the increase of temperature.

The cooling speed can be adjusted by changing the concentration, temperature and stirring.

PAG series coolant quenching ability covers all fields between water and oil.

Since the production of PAG in the United States in the 1960s, it has been widely used in the heat treatment industry.

At present, it has successfully replaced alkaline water and oil for quenching and cooling of carbon steel and low alloy steel.

(7) Quenching oil

Quenching oil has been serialized, which includes ordinary quenching oil, bright quenching oil, fast quenching oil, fast bright quenching oil, ultra fast quenching oil, vacuum quenching oil, graded quenching oil and isothermal quenching oil.

Although quenching oil has many advantages as quenching cooling medium, its disadvantages are also very prominent.

For example, oil smoke pollutes the environment and endangers human health, it is easy to age and cause fire, and the treatment of waste oil is also a headache.

In the tool and die industry, the author suggests that quenching oil should be eliminated as soon as possible and new quenching cooling medium with energy saving and environmental protection should be developed and applied.

(8) Gas quenching

The steel with high hardenability and small size can be gas quenched.

The cooling capacity of the gas is related to the type, pressure and flow rate of the gas.

The high-speed steel mechanical blade (thickness < 20mm) is directly air cooled after induction heating, which can achieve high hardness above 63HRC.

For Cr12 type high alloy steel, it can be quenched in the air.

In order to improve the cooling speed, it can be cooled by blowing.

The effective size of the template > 50mm can even be cooled on the water-cooled copper plate.

In recent years, vacuum high-pressure gas quenching has developed rapidly. The commonly used cooling gases include N2, He, H2 and Ar.

H2 has the best thermal conductivity, but it is easy to explode when mixed with air, with poor safety.

It will also cause decarburization of steel above 1000 ℃, which limits its application.

Although the cooling effect of N2 is poor, it is cheap and safe, so it is widely used.

5. Conclusion

Selecting and using the quenching cooling medium for heat treatment not only affects the product quality and economic benefits, but also endangers the survival and development of enterprises.

We should select the best quenching cooling medium according to the material and product performance requirements for quenching workpieces of the enterprise, so as to ensure hardening, less deformation and good performance.

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