During the standard testing of the cooling characteristics of quenching medium using a tester, the surrounding conditions of the probe were recorded with a camera. However, upon comparison, it was discovered that the cooling stages, which were divided based on the shape of the cooling characteristic curve, were quite different from the actual cooling conditions on the probe surface, providing an explanation for the difference observed.
After thorough analysis and reasoning, it was concluded that it is impossible to divide the cooling stage of the probe from the cooling characteristic curve of the quenching medium. Additionally, accurately calculating the possible cooling conditions of the actual workpiece based on the measured cooling characteristic curve is impossible. Therefore, the cooling characteristic curve of the quenching medium should only be used to compare the cooling characteristics of the medium.
Over the past twenty years, the cooling characteristic curve of quenching medium has significantly advanced the heat treatment industry, allowing for the development and research of quenching medium, the comparison and selection of medium, product quality control in heat treatment production, and even the analysis and solution of heat treatment quality and technical problems encountered in production.
The cooling characteristic curves of quenching medium can tell us about the different cooling stages, which have been divided based on the shape of the cooling characteristic curve as shown in Figure 1. Stage A is commonly known as the cooling steam film stage (also referred to as the film boiling stage), Stage B is the boiling stage (also referred to as the bubble boiling stage), and Stage C is the convection stage. The points on any curve in the figure can be found using time or temperature coordinates.
Other books and periodicals that discuss the cooling characteristic curve of liquid quenching medium, regardless of the testing standard used, typically divide the cooling stages and explain the cooling mechanism of each stage as illustrated in Figure 1.
Fig. 1 Classification of quenching cooling stages in liquid medium and heat dissipation mechanism of each stage
When researching and evaluating quenching mediums, two curves displayed in Fig. 1 are commonly used to express and compare the cooling characteristics of the medium.
The cooling rate curve highlights the quenching medium’s characteristic temperature, the temperature at which the highest cooling rate occurs, the value of the highest cooling rate, and the starting temperature of convection.
On the other hand, the cooling process curve usually indicates the time required to cool from 800℃ to 400℃ (or 300℃).
Some individuals also use the cooling speed value corresponding to each temperature on the cooling speed curve, either directly or indirectly, as the cooling speed value obtained by the workpiece at the same temperature in actual production.
It’s widely acknowledged that small workpieces cool quicker than large workpieces under the same cooling conditions.
Based on this general principle, people tend to connect it with the cooling rate curve of the quenching medium.
This leads to the assumption that under the same cooling conditions, workpiece parts with the same effective thickness should have the same cooling process and effect. The corresponding relationship between their temperature, cooling speed, and cooling time can be found on the cooling characteristic curve of the quenching medium.
However, taking into account the hot end of the thermocouple for temperature measurement at the geometric center of the probe and the probe shape factor’s influence, we have always had doubts about the accuracy of the above understanding and practice.
To clarify these doubts, after completing the research on the “four-stage theory of cooling in liquid quenching medium,” the subject was studied through experiments and observations.
There are three purposes of the study:
- Review the rationality of our current understanding and usage.
- Identify the root cause of any problems that arise.
- Determine the appropriate circumstances and reasonable limits for applying the quenching medium cooling characteristic curve.
Test Methods and Results
Test methods and instruments
Fig. 2 Comparison Chart of Test and Observation Results of 50 ℃ Base Oil
To detect the cooling characteristics of the quenching medium, a camera is used to synchronously observe and record the surface phenomenon of the probe.
To obtain clearer images, colorless or lightly colored quenching media with good transparency are preferred. Examples include clean water, salt water, highly refined base oil, rapid quenching oil, PAG quenching liquid, and other similar media.
The Sweden IVF cooling characteristic tester is used to test the cooling characteristics of the quenching medium, while a Panasonic NV-GS11 camera captures the images.
The camera takes 25 pictures per second at a shutter speed of 1/100 second. Typically, the heating temperature of 850℃ is used during the test.
For water-based medium, the liquid temperature should be between 10℃ to 70℃, and for oily medium, it should be between 30℃ to 100℃.
2. Test results
The cooling characteristic curve of the quenching medium and camera data of surface cooling during the process were obtained and analyzed in this study. Specifically, the test results of water, base oil, and rapid quenching oil are presented below.
Schematic diagrams in Fig. 2, Fig. 3, and Fig. 4 illustrate camera observation results for selected points on the cooling speed curve of base oil, rapid quenching oil, and 60 ℃ clean water, respectively.
Fig. 2 displays the cooling characteristic curve of base oil, including the cooling conditions of several selected points. Similarly, Fig. 3 shows the cooling characteristic curve of rapid quenching oil and the cooling conditions of several selected points. Finally, Fig. 4 presents the cooling characteristic curve of 60 ℃ clean water and the cooling conditions of several selected points.
Fig. 3 Comparison Chart of Test and Observation Results of 50 ℃ Rapid Quenching Oil
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:
There are three main points to consider:
First, the correlation between the cooling characteristic curve of the medium and the cooling conditions observed by the camera.
Second, the similarities in the cooling characteristics of different quenching media and camera results.
Third, the relationship between the cooling characteristic curve and the actual cooling conditions of the workpiece.
Upon closer inspection, it becomes apparent that the cooling stage of the chosen point on the cooling characteristic curve differs significantly from the actual cooling stage of the probe at the same time. This discrepancy is primarily demonstrated by:
a) With the exception of the initial steam film stage as depicted in point 1 in Fig. 2, the actual cooling condition differs from the phase composition indicated on the medium cooling characteristic curve at all other selected points.
b) The cooling characteristic curve of the medium shows that a particular probe temperature corresponds to a single cooling stage, except for the dividing point of the cooling stage.
However, the camera results demonstrate that in most cooling processes, there are two or three cooling stages in different parts of the probe. For instance, even at the characteristic temperature point of special concern, when tested in base oil, the upper and lower ends of the probe have already entered the boiling cooling stage. This suggests that the probe indicated two cooling stages simultaneously at that 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 point of highest cooling rate, the probe experiences three cooling stages simultaneously in three different mediums, although the proportion of each stage varies across mediums.
At the initial temperature of convection, the base oil and rapid quenching oil also display three cooling stages.
During the clean water test, the upper and middle sections of the probe remain in the boiling cooling stage at the beginning of convection, indicating the presence of two cooling stages concurrently.
When comparing different types of media, including the characteristic temperature on the cooling characteristic curve, the temperature at which the highest cooling rate occurs, and the number and proportion of cooling stages in the camera picture at the onset of convection, there appears to be no commonalities.
These results demonstrate that there is no direct correlation between the cooling stages observed by the camera and the cooling characteristic curve of the quenching medium.
Therefore, it is not possible to distinguish the cooling stage of the probe based on the cooling characteristic curve of the quenching medium.
2. How the cooling characteristic curve of quenching medium is formed
The cooling characteristic curve shown in Fig. 1 cannot be explained solely based on the three-stage theory of cooling in quenching medium (as depicted in the division method in Fig. 1) and the understanding that the effective thickness determines the cooling process.
For instance, when the probe reaches the so-called characteristic temperature point, as per the stage division shown in Fig. 1, the whole probe enters the boiling cooling stage.
As the temperature of the probe is very high at this point, the corresponding cooling speed curve should display the maximum cooling speed value of the entire cooling process. However, the graph line indicates that the maximum cooling speed value appears at a lower temperature.
In fact, two problems arise here: firstly, the hot end of the thermocouple used for temperature measurement is located at the geometric center of the probe and measures the temperature change of internal points.
Secondly, factors beyond the heat transfer characteristics of the probe itself, such as the cooling mechanism (stage) of the cooling medium at different temperatures, play a crucial role in determining the cooling characteristics of a specific point of the probe.
The newly proposed “four-stage theory of cooling in liquid quenching medium” provides a more comprehensive explanation for this problem. According to this theory, the cooling mechanism in liquid quenching medium can be divided into steam film stage, intermediate stage, boiling stage, and convection stage based on the workpiece temperature.
Therefore, the observation results corresponding to Figures 2 to 4 often exhibit two or three cooling stages when only the temperature change of internal points is analyzed.
Fig. 5 During cooling, the internal point is radiating towards the outer part
During the quenching and cooling process, the temperature of a point P inside the object decreases as heat is radiated to the outer part, as depicted in Fig. 5.
The primary reason for this heat dissipation is the cooling effect of the liquid medium on the workpiece surface. The near and far surfaces are cooled first, and then the point P is cooled through heat conduction.
The closer the cooling surface is to the point P, whether in the vapor film stage, boiling stage, or convection stage, the earlier the cooling condition will affect the point P. Conversely, the farther away from the point P, the later the temperature drop will affect it.
Therefore, the actual cooling condition of the point P at any given time is the result of the combined influence of the cooling conditions on different surfaces near and far 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 curves usually used to describe the cooling characteristics of quenching medium are also such curves. They neither represent the cooling process curve of the workpiece surface nor the curve of the cooling rate obtained on the workpiece surface as it varies with the surface temperature.
It is inappropriate to divide the cooling process in liquid quenching medium into three stages using such curves.
Relation between cooling characteristic curve and actual workpiece cooling
The cooling characteristics of quenching mediums are typically measured using the thermocouple method.
Different measurement standards are established based on various factors such as the materials, shapes, and sizes of probe rods, as well as the positions of thermocouples.
As a result, the cooling characteristic curves obtained using different standards can vary.
Despite attempts to establish conversion relations between cooling characteristics measured by different standards, all such efforts have failed.
Thus, the heat treatment industry must acknowledge the fact that there is no fixed relationship between the cooling characteristic curves of the same quenching medium measured by different standards.
In other words, there is no universal conversion relationship between cooling characteristics detected by different standards, which renders them incomparable.
I would like to use the following fact to help analyze the issues raised in this section: there is no comparability between the cooling characteristic curves detected by different standards. If we consider the actual workpiece as another probe with different materials, shapes, sizes, and thermocouple positions, this fact applies.
Therefore, there is no comparability between the cooling characteristics of a 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 accurately calculate the cooling process of the actual workpiece.
Furthermore, based on the same reasoning, we can draw the conclusion that there is no comparability between the cooling characteristic curves detected by all standard methods and the cooling characteristics of the actual workpiece.
Lastly, following the same principle, we can conclude that there is no comparability between the cooling characteristics of workpieces with different shapes, sizes, and materials, even when they are cooled in the same quenching medium.
Reasonable use of cooling characteristic curve of quenching medium
The preceding discussion has demonstrated that while the cooling characteristic curve of a quenching medium is beneficial to heat treatment workers, its role should not be overextended.
In summary, the appropriate range of application for the cooling characteristic curve of a quenching medium can be summarized as follows:
1. Test the cooling characteristics of quenching medium products.
Comparing the cooling characteristics of various products can be done both qualitatively and quantitatively. This is particularly useful for researching and developing quenching mediums, inspecting and selecting products, and other related applications.
2. Understand the stability and change degree of cooling characteristics of quenching medium in use.
The approach can encompass both qualitative and quantitative methods, making it versatile in its application. It is particularly well-suited for managing the quality of heat treatment production units, as well as analyzing and addressing issues related to workpiece heat treatment technology and quality.
3. Qualitatively predict the quenching hardness of different workpieces and the depth of hardened layer.
This content is mainly intended to guide the selection of quenching mediums for different types of workpieces, as well as to provide an overview of the applicable range of various quenching mediums.