Select quenching medium according to cooling characteristics

 

on how to choose the appropriate quenching liquid for quenching, here chengchi industrial quenching furnace briefly introduces several methods of quenching medium, the first is its cooling performance. Therefore, after determining the type of medium, it is advocated to select the variety of medium according to the cooling characteristics of the medium. For example, when we determine that the rapid quenching oil should be selected, the specific variety should be selected from the cooling rate distribution of the oil according to the characteristics of the workpiece and the heat treatment requirements.

No matter what kind of quenching medium you choose, you can choose according to the following five principles.

1. the carbon content of steel-steel with low carbon content may precipitate proeutectoid ferrite in the high temperature stage of cooling, the temperature at which the pearlite transformation is most likely to occur in its supercooled austenite (the temperature at the so-called "nose tip" location) is higher, and the martensite starting point (Ms) is also higher. Therefore, in order to sufficiently harden a workpiece made of such steel, the quenching medium used should have a short vapor film stage and the temperature at which the highest cooling rate occurs should be high. On the contrary, for steels with higher carbon content, the steam film stage of the quenching medium can be longer, and the temperature at which the highest cooling rate occurs should also be lower.

2. the hardenability of steel & mdash;& mdash; Steel with poor permeability requires quenching medium with fast cooling speed, while steel with good cooling speed can use medium with slow hardenability. Generally, as the hardenability of steel increases, the C curve of the decomposition transformation of supercooled austenite will shift to the lower right. Therefore, for steel with poor hardenability, the temperature of the highest cooling rate of the selected quenching medium should be higher, while the steel with good hardenability should be lower. Some have good hardenability

Bainite transformation is easy to occur in supercooled austenite of steel. To avoid bainite transformation, a sufficiently fast low temperature cooling rate is also required.

3. the effective thickness of the workpiece & mdash;& mdash; As soon as the surface of the workpiece cools to the Ms point, the cooling rate of the medium is greatly slowed down, then the heat inside the workpiece to the quenching medium loss rate will immediately slow down, which will inevitably make the workpiece surface within a certain depth of the supercooled austenite cold less than Ms point on the non-martensite transformation, the result, after quenching the workpiece only a very thin martensite layer. For this reason, when the workpiece is relatively thick and large, in order to obtain a sufficient hardening depth, the quenching medium used should have a rapid low-temperature cooling rate. The thin and small workpiece can choose the quenching medium with slow cooling speed.

4. the shape complexity of the workpiece & mdash;& mdash; For workpieces with complex shapes, especially those with inner holes or deeper concave surfaces, in order to reduce the quenching deformation or the need to harden the inner hole, the quenching medium with shorter steam film stage should be selected. This is because the inner hole or concave internal heat dissipation is slower than other parts. Other parts of the workpiece cold fast into the boiling stage to obtain fast cooling, and the inner hole or concave surface is still covered by steam film, cold very slowly. This difference in cooling rate may cause large quenching deformation and low hardness of the concave surface. The solution to this problem is to choose a quenching medium with a shorter steam film stage and a faster cooling rate. Of course, the appropriate increase in the inner hole and the concave surface of the medium flow speed, also have the same effect. In contrast, a workpiece with a simple shape can use a quenching medium with a slightly longer steam film stage. In addition, the more complex the shape of the workpiece, the greater the internal stress during cooling. Accordingly, a workpiece having a complex shape allows a lower maximum cooling rate, while a workpiece having a simple shape allows a higher maximum cooling rate.

allowable deformation of 5. & mdash;& mdash; It is inferred from the hardness difference method to solve the deformation problem that the cooling speed is small, and the cooling belt must be narrow; while the allowable deformation is larger, there can be a wider cooling speed band. The allowable cooling speed bandwidth,

The deformation requirements are often met by using a medium that can obtain its quenching hardness requirements. Allowable cooling rate band is particularly narrow, it is necessary to use a quenching method that can greatly shorten the cooling rate band of the workpiece. Among the methods that can shorten the cooling rate of the workpiece, the most simple and effective is to do isothermal (or graded) quenching. Isothermal grading quenching medium should have the characteristics, first of all, the steam film stage is short, and the change of liquid temperature has little effect on the cooling characteristics; secondly, the thicker workpiece should choose the quenching medium with fast cooling speed, while the smaller workpiece should choose the medium with slow cooling speed.

There are a variety of artifacts to be processed in production. The requirements of different workpieces on the cooling characteristics of the quenching medium may be compatible, that is, the same quenching medium can be used; but it may also be incompatible, that is, no common applicable quenching medium can be found. Therefore, trying to find & ldquo; 1 an ideal quenching medium that can be applied to all the different requirements of different workpieces at the same time & rdquo; is as unrealistic as trying to find a 1 drug to cure all diseases, at least at present.