Selection of Quenching Cooling Medium

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Quenching is the most critical process in the heat treatment process, which ultimately determines the inherent quality and distortion of the quenched parts. Quenching and cooling technology has been continuously improved with the development of heat treatment technology from beginning to end. However, due to the complexity of the cooling process and the characteristics of instantaneous completion, coupled with the limitations of observation and measurement, quenching cooling is somewhat mysterious. Based on my own experience in long-term heat treatment work, the author talks about the cognition and selection of quenching cooling medium.

1.Quenching cooling medium basic properties

(1) Ideal cooling rate

The so-called ideal cooling rate means that for a certain material and specific quenched parts, it is hoped that the cooling rate of the nose of the C curve is greater than the critical cooling rate (the minimum cooling rate to ensure that the workpiece can complete the martensitic transformation) during cooling. . In other temperature ranges, especially in the martensitic transformation temperature range (Ms→Mf), it should be cooled slowly, which is what people often say “the high temperature should be cooled quickly, and the low temperature should be cooled slowly”. Different steel materials and different workpieces have different demands for the above-mentioned “fast” and “slow”. In theory, there is a concept of “ideal quenching cooling medium”, as shown in Figure 1. The “C” curves of different steels are different. It is impossible to obtain the so-called “ideal quenching cooling medium” that can be suitable for quenching various steels and workpieces of different sizes. We can only choose the appropriate quenching cooling medium according to the specific situation. , so that the quenched parts are hardened, the distortion is small, and the metallographic phase is good.

(2) Good stability

The quenching cooling medium should be relatively stable during use, and not easy to decompose, deteriorate and age. Various quenching oils and organic solvents have different degrees of aging, and should be adjusted, updated and maintained in a timely manner.

(3) Uniformity of cooling

Different parts and surfaces of the workpiece should be cooled as uniformly as possible to avoid quenching soft spots and lumps.

(4) Non-corrosive

After quenching, it should be kept clean, easy to clean, and not corrosive to the workpiece.

(5) Environmentally friendly

During quenching, a large amount of smoke, no toxic and irritating gases are not generated, and the waste liquid brought out by the quenching parts does not pollute the environment.

(6) Safety

The quenching cooling medium should be non-flammable, explosive and safe to use.

(7) Economical

Quenching cooling medium should be both high quality and low price, which is too expensive and not welcome by heat treatment enterprises.

2.Factors affecting the cooling performance of quenching cooling medium

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

(1) Temperature

The temperature of the quenching cooling medium is different, and its cooling capacity is also different. The cooling capacity of water and water-based quenching cooling media decreases with the increase of temperature, while oil and salt baths have the opposite. With the increase of temperature, the fluidity is good, which is conducive to heat dissipation and the cooling capacity is improved, so the cooling temperature of the specific workpiece should be well grasped. Very important.

(2) Surface tension

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

(3) Stirring

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

(4) Thermal conductivity

Thermal conductivity, also known as thermal conductivity, is a physical quantity that characterizes the thermal conductivity of a material. The greater the thermal conductivity, the stronger the cooling capacity.

(5) Specific heat capacity

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

(6) Viscosity

Viscosity describes the internal friction between the molecules of a liquid when it flows. 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 cooling effect is better if the viscosity is small.

(7) Heat of vaporization

Heat of vaporization refers to the amount of heat required to completely change a unit mass of liquid into a gas of the same temperature. The chemical stability of water is very high, the heat capacity is large, and it is 8 times that of steel at room temperature. The boiling point of water is low, and its heat of vaporization decreases with the increase of temperature. After the temperature rises to 80 °C, the cooling capacity has almost no change, and the quenching intensity is maintained at about 0.72.

(8) Additives

The purpose of adding additives is to change the cooling performance, such as adding a small amount of salt or alkali to the water, the cooling capacity will be increased several times. Adding polyvinyl alcohol to the water will form a very thin plastic film on the surface of the quenched part, which has poor thermal conductivity and reduces the cooling rate. Suspension or emulsion formed by oil, soap, etc. in water will accelerate the formation of steam film, increase the stability of steam film, and reduce cooling capacity. In addition, in order to change other properties of the quenching cooling medium, people often consciously add oxidants, brighteners, rust inhibitors, corrosion inhibitors, etc., which will affect the cooling performance to varying degrees. The addition of additives generally has the effect of serving multiple purposes with one stone.

(9) Environment

The influence of the environment on the cooling capacity is often not noticed by people. The same temperature medium has different cooling effects in winter and summer, and different quenching effects during the 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 improper selection or operation errors, quenching parts will be rejected. Therefore, using the right quenching cooling medium is the basic condition to ensure product quality. No matter what kind of quenching cooling medium is used, a uniform quenching effect should be obtained:

① Obtain high and uniform surface hardness and sufficient hardening depth.

②Can not be quenched and cracked.

③ The quenching distortion is small. According to the heat treatment technical requirements, materials, shapes and other specific conditions of the quenched parts, the corresponding quenching cooling medium is selected, and the following five basic principles are summarized:

(1) According to the level of carbon content

Carbon is the most important element in all steels. The level of carbon content not only affects the various properties of the steel, but also affects the quenching effect. For carbon steel, its carbon content is less than or equal to 0.5% (mass fraction, the same below), and choose brine, alkaline water, organic solvent, etc.; for medium and low alloy structural steel, double-liquid quenching or a medium with relatively slow cooling speed is often used; for carbon steel Plain tool steel has poor hardenability due to high heat treatment requirements, so alkaline bath and nitrate bath are often used for graded quenching, and oil cooling is rarely used.

(2) According to the hardenability of steel

From the analysis of the “C” curve of the steel, the steel with poor hardenability requires a faster cooling rate. Conversely, steel with good hardenability cools at a slower rate. It is a wise choice to choose a suitable quenching cooling medium according to the hardenability of the 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 rate of the medium will be greatly slowed down, and the heat inside the workpiece will be dissipated to the quenching cooling medium. It is difficult for supercooled austenite to cool below the Ms point. When the quenched part is thick and large, in order to obtain a sufficient depth of the hardened layer, a faster low-temperature cooling rate should be selected. On the contrary, when the workpiece is thin, the quenching cooling medium with low temperature cooling rate can be used. From the maximum allowable cooling rate distribution curve, thick workpieces are allowed to be cooled at a high speed, and thin workpieces should be cooled at a low speed.

(4) According to the complexity of the quenched parts

First, analyze the minimum allowable cooling rate distribution curve. For workpieces with complex shapes, especially workpieces with inner holes or deeper concave surfaces, in order to reduce quenching distortion and the inner hole needs to be hardened, a shorter steam film stage should be selected. Quenching cooling medium. Conversely, for workpieces with relatively simple shapes, a quenching cooling medium with a slightly longer vapor film stage can be used. From the maximum allowable cooling rate distribution curve, the allowable cooling rate for workpieces with complex shapes is low, while the allowable cooling rates for workpieces with simple shapes are high.

(5) According to the allowable deformation

The quenched part requires small distortion and should have a narrow cooling rate band, and when the allowable distortion is large, it can have a wide cooling rate band. If the allowable cooling rate is wide, the medium that can generally reach the quenching hardness can be used. The cooling rate band of the workpiece can be shortened by isothermal quenching or stage quenching.

Due to the wide variety of workpieces, different heat treatment requirements, and endless quenching and cooling media, the same workpiece can be quenched with different media to obtain the same surface hardness, which brings certain difficulties to the selection of quenching cooling media. Based on the principle of economy and rationality, choose a relatively ideal quenching cooling medium.

The selection and use of heat treatment and quenching cooling medium will not only affect product quality and economic benefits, but also endanger the survival and development of enterprises. We should choose the best quenching cooling medium according to the material and product performance requirements of the company’s quenched workpiece to ensure hardening, less deformation and good performance.

Editor: Frank Lee

Copyright: SIMUWU Vacuum Furnace