Cooling Characteristic Curve of Quenching Medium: What Does It Really Show?

Abstract

At the same time of testing the cooling characteristics of quenching medium with a standard tester, the conditions around the probe were recorded with a camera.

By comparison, it is found that the cooling stages divided according to the shape of the measured cooling characteristic curve are quite different from the actual cooling conditions on the probe surface, which explains the reasons for this difference.

Through analysis and reasoning, it is concluded that the cooling stage of the probe can not be divided from the cooling characteristic curve of the quenching medium;

It is impossible to accurately calculate the possible cooling conditions of the actual workpiece based on the measured cooling characteristic curve;

The cooling characteristic curve of quenching medium should only be used in the comparison of cooling characteristics of medium.

In the past twenty years, the application of the cooling characteristic curve of quenching medium has brought considerable technical progress to the heat treatment industry.

Now, the development and research of quenching medium, the comparison and selection of medium, the product quality control in heat treatment production, and even the analysis and solution of heat treatment quality and technical problems encountered in production have all been inseparable from the cooling characteristic curve of quenching medium.

But what can these cooling characteristic curves tell us?

There are basically consistent answers to this question in the industry.

The explanations provided in the highly authoritative American Metal Manual and the monographs of well-known experts in the industry, G.E. Totten, are very representative.

As shown in the figure, stage A in Fig. 1 is generally referred to as the cooling steam film stage (also known as the film boiling stage), stage B is generally referred to as the boiling stage (also known as the bubble boiling stage), and stage C is referred to as the convection stage.

In the steam film stage, the whole test block is surrounded by the steam film.

In the boiling cooling stage, the whole surface of the test block is boiling.

In the convection cooling stage, the test block is cooled by convection heat transfer.

The points on any curve in the figure can be found by time or temperature coordinates.

In other books and periodicals, the cooling characteristic curve of liquid quenching medium, no matter what testing standard is used, divides the cooling stages and explains the cooling mechanism of each stage in the way shown in Fig. 1.

Cooling Characteristic Curve of Quenching Medium: What Does It Really Show? 1

Fig. 1 Classification of quenching cooling stages in liquid medium and heat dissipation mechanism of each stage

In the research and evaluation of quenching medium, two curves shown in Fig. 1 are usually used to express and compare the cooling characteristics of the medium.

From the cooling rate curve, the characteristic temperature of the quenching medium, the temperature at which the highest cooling rate occurs, the value of the highest cooling rate, and the starting temperature of convection are pointed out.

From the cooling process curve, the time required for cooling from 800 ℃ to 400 ℃ (or 300 ℃) is usually indicated.

Some people also use the cooling speed value corresponding to each temperature on the cooling speed curve directly or indirectly as the cooling speed value obtained by the workpiece at the same temperature in actual production.

It is well known that under the same cooling conditions, small workpieces cool quickly, while large workpieces cool slowly.

According to this general principle, people will naturally associate it with the cooling rate curve of the quenching medium.

This leads to the understanding that under the same cooling conditions, the parts with the same effective thickness on the workpiece should have the same cooling process and cooling effect, and the corresponding relationship between their temperature, cooling speed and cooling time can be found on the cooling characteristic curve of the quenching medium.

Considering that the hot end of the thermocouple for temperature measurement is at the geometric center of the probe and the influence of the probe shape factor, we have always held some doubts about the accuracy of the above understanding and practice.

In order to clarify various doubts in this regard, after completing the research of “four stage theory of cooling in liquid quenching medium”, the subject was studied through experiments and observations.

There are three purposes of the study:

1. Review the rationality of current understanding and use.

2. If there is a problem, find out the cause of the problem.

3. Determine the reasonable application occasion and reasonable application limit of the quenching medium cooling characteristic curve.

Test Methods and Results

Test methods and instruments

Cooling Characteristic Curve of Quenching Medium: What Does It Really Show? 2

Fig. 2 Comparison Chart of Test and Observation Results of 50 ℃ Base Oil

In the process of detecting the cooling characteristics of the quenching medium, the camera is used to synchronously observe and record the phenomenon on the surface of the probe.

In order to obtain clearer images, colorless or light colored quenching media with good transparency are used.

Such as clean water, salt water, highly refined base oil, rapid quenching oil, PAG quenching liquid and other media.

Sweden IVF cooling characteristic tester is used to test the cooling characteristics of quenching medium.

The camera is a Panasonic NV-GS11 camera.

Take 25 pictures per second at a shutter speed of 1/100 second.

The heating temperature of 850 ℃ is usually used in the test.

The liquid temperature of water-based medium shall be 10 ℃~ 70 ℃, and the liquid temperature of oily medium shall be 30 ℃~ 100 ℃.

2. Test results

The commonly seen cooling characteristic curve of the quenching medium was obtained, and the camera data of the cooling near the surface of the probe during the cooling process were also obtained.

In the following, water, base oil and rapid quenching oil are taken as representatives to introduce the test results in this paper.

Among them, the camera observation results corresponding to some selected points on the cooling speed curve are drawn on the same diagram in schematic form.

The cooling characteristic curve of base oil and the cooling conditions of several selected points are shown in Fig. 2.

The cooling characteristic curve of rapid quenching oil and the cooling conditions of several selected points are shown in Fig. 3.

The cooling characteristic curve of 60 ℃ clean water and the cooling conditions of several selected points are shown in Fig. 4.

Cooling Characteristic Curve of Quenching Medium: What Does It Really Show? 3

Fig. 3 Comparison Chart of Test and Observation Results of 50 ℃ Rapid Quenching Oil

Cooling Characteristic Curve of Quenching Medium: What Does It Really Show? 4

Fig. 4 Comparison Chart of Test and Observation Results of 60 ℃ Clean Water

Analysis of test results

1. Relationship between cooling characteristic curve and cooling medium heat dissipation stage

The research and analysis focus on three aspects:

First, the relationship between the cooling characteristic curve of the medium and the cooling condition observed by the camera;

Second, the common law between the cooling characteristics of different quenching media and the camera results;

The third is the relationship between the cooling characteristic curve and the actual cooling condition of the workpiece.

With a little attention, it will be found that the cooling stage of the selected point on the cooling characteristic curve is quite different from the actual cooling stage of the probe at the same time. It is mainly shown in:

a) Except at the beginning of the steam film stage, as shown in point 1 in Fig. 2, at all other selected points, the actual cooling condition is different from the phase composition indicated on the medium cooling characteristic curve.

b) On the cooling characteristic curve of the medium, a certain probe temperature corresponds to a single cooling stage, except for the dividing point of the cooling stage.

However, the camera results show that in most cooling processes, there are two or three cooling stages in different parts of the probe.

For example, even at the characteristic temperature point of special concern, when testing in base oil, the upper and lower ends of the probe have already entered the boiling cooling stage.

This shows that the probe at that time indicated that there were two cooling stages at the same time.

At the characteristic temperature point of fast quenching oil and 60 ℃ clean water, there are three cooling stages on the probe at the same time.

At the time of the highest cooling rate, there are three cooling stages on the probe in three mediums at the same time, but the proportion of each stage in different mediums is different.

At the beginning temperature of convection, there are also three cooling stages in the base oil and rapid quenching oil.

During the test in clean water, the upper and middle sections of the probe are still in the boiling cooling stage at the starting point of convection, indicating that there are two cooling stages at the same time.

c) For different media types, the characteristic temperature on the cooling characteristic curve, the temperature at which the highest cooling rate occurs, and the number of cooling stages and the proportion of each stage on the camera picture when the convection starts are compared.

It turns out that there is no common ground between different media.

d) All these results show that there is no simple correspondence between the current quenching medium cooling characteristic curve and the cooling stages observed by camera.

Therefore, the cooling stage of the probe cannot be divided from the cooling characteristic curve of the quenching medium.

2. How the cooling characteristic curve of quenching medium is formed

Based on the three-stage theory of cooling in quenching medium (as shown in the division method in Fig. 1) and the understanding that the effective thickness can determine the cooling process, it is impossible to explain the formation reason of the cooling characteristic curve shown in Fig. 1.

For example, according to the stage division shown in Fig. 1, once the probe reaches the so-called characteristic temperature point, the whole probe will enter the boiling cooling stage.

Because the temperature of the probe was very high at that time, the maximum cooling speed value of the whole cooling process should appear on the corresponding cooling speed curve, but the maximum cooling speed value in the graph line appears at a lower temperature.

In fact, there are two problems involved here: one is that the hot end of the thermocouple for temperature measurement is located at the geometric center of the probe.

It measures the temperature change of internal points;

The other is the factors that determine the cooling characteristics of a certain point of the probe. In addition to the heat transfer characteristics of the probe itself, the cooling mechanism (stage) of the cooling medium at different temperatures plays a very important role.

The newly proposed “four stage theory of cooling in liquid quenching medium” can easily explain this problem.

According to the four stage theory, the cooling mechanism in liquid quenching medium can be divided into steam film stage, intermediate stage, boiling stage and convection stage according to the workpiece temperature.

Here, only the temperature change of internal points is analyzed to explain why there are often two or three cooling stages in the observation results corresponding to Figures 2 to 4.

Cooling Characteristic Curve of Quenching Medium: What Does It Really Show? 5

Fig. 5 During cooling, the internal point is radiating towards the outer part

During quenching and cooling, the temperature of a point P inside is reduced by radiating heat to the outer part, as shown in Fig. 5.

The final reason for this heat dissipation is the cooling effect of the liquid medium on the workpiece surface.

The far and near different surface parts are cooled, and then the P point is cooled by heat conduction.

Whether the cooling surface is in the vapor film stage, boiling stage, or convection stage, the closer it is to the P point, the earlier the cooling condition will affect the P point;

The farther away from the P point, the later the temperature drop will affect the P point.

Therefore, the actual cooling condition of point P at any time is the result of the combined influence of the cooling conditions on different surfaces far and near within a certain time range before that time.

The cooling characteristic curve of the internal point represents the change of this effect with time and P point temperature.

The graph lines usually used to describe the cooling characteristics of quenching medium are also such curves.

They are neither the cooling process curve of the workpiece (probe) surface nor the curve of the cooling rate obtained on the workpiece (probe) surface varying with the surface temperature.

It is no doubt inappropriate to divide the three stages of cooling in liquid quenching medium with such a curve.

Relation between cooling characteristic curve and actual workpiece cooling

The cooling characteristics of quenching medium are mostly measured by thermocouple method.

Different measurement standards are formed due to different materials, shapes and sizes of probe rods and positions of thermocouples.

The measured cooling characteristic curves are different with different standards.

Out of the habit of converting inches and centimeters, Fahrenheit and Celsius, people have tried to establish conversion relations between cooling characteristics measured by different standards.

However, all efforts in this regard ended in failure.

Up to now, the heat treatment industry has to face the fact that there is no fixed relationship between the cooling characteristic curves of the same quenching medium measured by different standards.

Here, “no comparability” means that there is no universal, i.e. regular conversion relationship between cooling characteristics detected by different standards.

Here, I just want to use this fact to help us analyze the problems raised in this section.

Based on the above fact that there is no comparability between the cooling characteristic curves detected by different standards, if the actual workpiece is regarded as another probe with different materials, shapes and sizes, and thermocouple positions.

Then, there is no comparability between the cooling characteristics of the workpiece quenched in a quenching medium and the cooling characteristics of the same medium detected by a standard cooling characteristic instrument.

In other words, the cooling characteristic curve of the quenching medium cannot be used (accurately) to calculate the cooling process of the actual workpiece.

Furthermore, based on the same reasoning, the next conclusion can be drawn: there is no comparability between the cooling characteristic curves detected by all standard methods and the cooling characteristics of the actual workpiece.

Finally, according to the same principle, it can also be concluded that when cooling in the same quenching medium, there is no comparability between the cooling characteristics of workpieces with different shapes, sizes and materials.

Cooling Characteristic Curve of Quenching Medium: What Does It Really Show? 6

Reasonable use of cooling characteristic curve of quenching medium

The previous discussion has shown that although the cooling characteristic curve of quenching medium is very helpful to heat treatment workers, their role should not be expanded.

In brief, the reasonable application range of the cooling characteristic curve of quenching medium can be summarized as follows:

1. Test the cooling characteristics of quenching medium products.

Compare the cooling characteristics of different products, it can be both qualitative and quantitative.

It is mainly suitable for research and development of quenching medium, inspection and selection of products, etc.

2. Understand the stability and change degree of cooling characteristics of quenching medium in use.

It can be both qualitative and quantitative. It is mainly suitable for the quality management of heat treatment production units, as well as the analysis and solution of workpiece heat treatment technology and quality problems.

3. Qualitatively predict the quenching hardness of different workpieces and the depth of hardened layer.

It is mainly used to select quenching medium for different workpieces and introduce the applicable range of different quenching medium.

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