JPH062030A - Quenching method - Google Patents

Abstract

PURPOSE:To prevent the unevenness of strain in the heat stress caused by difference in cooling speed at the time of cooling in high temp. while restraining lowering of the hardness in the inner part, at the time of executing quenching. CONSTITUTION:A material to be quenched is dipped into high temp. coolant of mineral oil at the temp. C just below the starting temp. of martensitic transformation and thereafter, cooled by low temp. coolant of mineral oil at the temp. D lower than the temp. of the high temp. coolant. As the high temp. coolant uses the mineral oil having comparatively small heat capacity and the cooling speed is not so fast, partial temp. difference is small and the unevenness of the strain caused by the heat stress is small. Further, as the temp. of the high temp. coolant is set at the temp. C just below the starting temp. of the martensitic transformation, the cooling speed is not so low and lowering of the hardness can be restrained. Further, as this material is cooled by the low temp. coolant, without receiving the influence from the outer part, the deviation in strain caused by the martensitic transformation is small.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a method for quenching steel.

[0002]

2. Description of the Related Art Conventionally, case hardening steel such as chrome steel or chrome molybdenum steel is subjected to a carburizing treatment as a surface treatment, followed by a quenching treatment. As this quenching method, a predetermined quenching temperature is maintained. The treated material is immersed in a nitrate slightly higher than the martensitic transformation start temperature and subjected to quenching, and when the whole untreated material has a uniform temperature, it is taken out from the nitrate and allowed to cool in the atmosphere, A marquenching process is performed to reduce heat treatment strain or variation in strain between lots.

Since the above-mentioned method is a method of substantially performing martensitic transformation in the atmosphere, for example, the position of the object to be processed in the heat treatment equipment, the thickness of the object to be processed, or the shape of the projections and depressions, etc. Differences occur in the cooling temperature due to differences in air flow and seasonal temperature fluctuations. Due to this difference in cooling temperature, the presence or absence of the volume expansion due to the martensitic transformation or the ratio thereof varies depending on the location of the object to be processed, so that distortion occurs or there is a problem that the distortion varies between lots.

In order to solve this problem, for example, as described in JP-A-02-101113,
There is provided a method in which the temperature of the whole untreated material is made uniform by immersing it in a high temperature coolant which is a nitrate having a martensite transformation start temperature or higher, and immediately thereafter, it is dipped in a coolant having a martensite transformation start temperature or lower.

[0005]

However, in the above-mentioned conventional quenching method, the temperature of each part of the object to be treated is made uniform by the high temperature coolant to be equal to or higher than the martensite transformation start temperature, and then lowered by the low temperature coolant. Since it is rapidly cooled, in the region where the martensitic transformation is performed, the difference in cooling rate is small and the variation in strain due to the martensitic transformation stress is small, but in the region before the martensitic transformation is performed, it is relatively high as a high temperature coolant. Since the nitrate having a large heat capacity is used, the cooling rate in the cooling step with the high temperature coolant is high, and the difference in the partial cooling rate of the object to be processed becomes large.

This will be explained with reference to FIG. 2. Curves a and b in the figure show the temperature change of the object to be treated, a curve a is a portion having a high cooling rate, a curve b is a portion having a low cooling rate,
Generally, the surface layer of the object to be processed has a high cooling rate, and the inside has a low cooling rate. f is the high temperature coolant temperature, g is the low temperature coolant temperature, and e is the martensite transformation start temperature. There is a problem that the temperature difference gradually increases from the start of cooling with the high-temperature coolant, and shows a fairly large temperature difference in the vicinity of the high-temperature coolant temperature f, which causes a large variation in the distortion of thermal stress.

In order to solve the above problems, it is conceivable to use mineral oil having a heat capacity smaller than that of nitrate as a high temperature coolant to cool it at a temperature higher than the martensite transformation start temperature to slow down the cooling rate. The temperature change of the object to be treated in this case is as shown by the curves c and d in FIG. 4, and since the partial difference in the cooling rate is small, the variation in the distortion of the thermal stress is small, but the cooling rate is slow. If too much, the hardness of the object to be treated will be reduced. That is, when nitrate is used as the coolant, high hardness can be obtained because the cooling rate is fast as in the above curves a and b, but when mineral oil is used as the coolant, it is as in curves c and d. The cooling rate slows down,
In particular, there is a problem that the cooling rate is considerably slow in a portion where the cooling rate of the curve d is low, that is, inside the object to be processed, and thus the internal hardness of the object to be processed is lowered.

Therefore, the present invention solves the problems of the above-mentioned conventional quenching method, and suppresses the variation of the thermal stress due to the difference in the cooling rate during high temperature cooling while suppressing the decrease of the internal hardness. The object is to provide a quenching method that can be performed.

[0009]

Therefore, in the quenching method of the present invention, the object to be treated kept at a predetermined quenching temperature is a high temperature coolant which is a mineral oil at a temperature just below the martensite transformation start temperature. It is characterized in that it is immersed therein and then cooled with a low temperature coolant having a temperature lower than the temperature of the high temperature coolant.

[0010]

The operation of the present invention and the effect of the present invention will be described with reference to FIG. 1. Curve A shows the cooling rate at a portion having a high cooling rate, and curve B shows the cooling rate at a portion having a low cooling rate. Show. C is the temperature of the high temperature coolant, D is the temperature of the low temperature coolant, and E is the martensite transformation start temperature.

The object to be treated kept at a predetermined quenching temperature is cooled by a high temperature coolant, but since the high temperature coolant is mineral oil having a relatively small heat capacity, it is cooled as shown by curves A and B. Since the speed is not so fast, the partial temperature difference is small and the variation in strain due to thermal stress is small. Furthermore, since the temperature of the high-temperature coolant is set just below the martensite transformation start temperature, the cooling rate does not become too slow as in the case of cooling with a mineral oil having a martensite transformation start temperature or higher, so that the decrease in hardness is also suppressed. You can

At this time, since the temperature of the high temperature coolant is lower than the martensite transformation start temperature, the martensite transformation starts due to the cooling, but since the temperature is directly below the transformation start temperature, the martensite transformation starts. Is 5 to 1
It is about 0%, and there is almost no influence of strain variation due to martensitic transformation stress.

After that, since it is cooled to a predetermined temperature with a low temperature coolant, the martensite transformation proceeds at a predetermined cooling rate without being affected by the outside, so that the variation in strain due to the martensite transformation is small.

[0014]

EXAMPLES Examples of the present invention will be described in detail below. The examples and comparative examples 1, 2, and 3 are JIS SCr4.
20H material module 2.3, counter gear of automatic transmission for vehicle with outer diameter about 140mm, gas carburizing process with carbon potential 0.8%, temperature 930 °, carburizing and diffusing for 4 hours, up to 850 °. After the temperature is lowered, the quenching treatment is subsequently performed under the conditions shown in Table 1. Table 2 shows the standard deviation of the strain measured under each condition, that is, the variation of the strain. H is the right tooth profile, L. H is the measurement result of the left tooth profile. Table 3 shows the hardness measured under each condition, and the unit of the hardness is Vickers hardness (Hv).

[Table 1]

[Table 2]

[Table 3]

In the quenching method of this embodiment, since the martensite transformation start temperature is about 220 ° when the carbon potential is 0.8%, the temperature of the high temperature coolant is set just below the martensite transformation start temperature. It is set to 210 °. The temperature of this high-temperature coolant will be described later, but is 200 ° to 2
About 10 ° is desirable. The low-temperature coolant is 80 ° mineral oil.

Comparing this example with Comparative Example 1, Comparative Example 1 shows the case where the high temperature coolant is 240 ° mineral oil and the low temperature coolant is 80 ° mineral oil. From the results, it can be seen that the surface hardness is equivalent to that of this example, but the internal hardness is lowered. This is because the cooling rate by the high temperature coolant is too slow, and it is apparent that the hardness inside the object to be processed, which has a slow cooling rate, is lowered. On the other hand, Table 2
Comparing the variations in the strains of 1 and 2, it can be seen that the variations in the strains of the present embodiment and Comparative Example 1 are equal. This indicates that even if the temperature of the high-temperature coolant of this example is set to the martensitic transformation start temperature or lower, the variation in strain is not affected. That is, even if martensitic transformation starts in the high temperature cooling step, the martensitic transformation progresses to about 5 to 10% of the whole, and most martensitic transformation progresses in the low temperature cooling step. It shows that there is almost no effect on the variation of the distortion of.

Comparing this example with Comparative Examples 2 and 3, Comparative Example 2 is a case where the high temperature coolant is 180 ° mineral oil and the low temperature coolant is 80 ° mineral oil. Example 3
Is a high temperature coolant of 140 ° mineral oil and a low temperature coolant of 80
In the case of the mineral oil of °, it can be seen from the results of hardness in Table 3 that the cooling rate is high and the internal hardness is high because the temperature of the high temperature coolant is lower than that in this example. However, looking at the variation in distortion in Table 2, it can be seen that the variation in distortion is greater than in this example. This is because the temperature of the high temperature coolant is too low, so the degree of progress of martensitic transformation is high in the high temperature cooling step, and the low temperature cooling step is performed to reduce the partial temperature difference due to the difference in cooling rate during the martensitic transformation. However, it is clear that there is a large variation in strain due to martensitic transformation in the high temperature cooling step.

From the above results, the temperature of the high temperature coolant in the material of SCr420H is a temperature at which the cooling rate does not become too slow, and the degree of martensite transformation is 5 to 5.
If it is about 10%, the hardness is not lowered so much,
Since the variation in strain due to the martensitic transformation can be reduced, it may be about 200 ° to 210 °. Further, although the temperature of the low-temperature coolant is 80 ° in this example, the low-temperature cooling step is performed to a temperature at which the martensite transformation progresses to about 70 to 80%, and if the partial difference in the cooling rate is reduced, martensite is reduced. The variation in strain due to transformation can be reduced, and thus the temperature is not limited to this.

As described above, according to the quenching method of this embodiment, since the high temperature coolant is the mineral oil having a relatively small heat capacity, the cooling rate is too high as shown by the curves A and B in FIG. Since there is no such difference, the partial temperature difference is small and the variation in strain due to thermal stress is small. Further, since the temperature of the high-temperature coolant is set to 210 ° immediately below the martensite transformation start temperature (220 ° in the case of carbon potential 0.8%), it is 240 ° above the martensite transformation start temperature.
Since the cooling rate does not become too slow as compared with the case of cooling with the mineral oil, it is possible to suppress the decrease in hardness.

At this time, since the temperature of the high temperature coolant is lower than the martensite transformation start temperature, the martensite transformation starts due to the cooling, but since the temperature is 210 ° immediately below the transformation start temperature, The martensitic transformation is about 5 to 10%, and the influence of strain variation due to the martensitic transformation stress is small.

After that, since it is cooled to a predetermined temperature with a low-temperature coolant, the martensite transformation proceeds at a predetermined cooling rate without being affected by the outside, so that the variation in strain due to the martensite transformation is small.

The present invention is not limited to the above embodiments, but various modifications can be made based on the spirit of the present invention, and they are not excluded from the scope of the present invention.

[Brief description of drawings]

FIG. 1 is a cooling curve diagram of the quenching method of the present invention.

FIG. 2 is a cooling curve diagram of a conventional quenching method.

[Explanation of symbols]

 A Cooling curve at a fast cooling rate B Cooling curve at a slow cooling rate C High-temperature coolant temperature D Low-temperature coolant temperature E Martensite transformation start temperature

Claims (2)

[Claims] 1. An object to be treated, which has been maintained at a predetermined quenching temperature, is immersed in a high temperature coolant which is a mineral oil having a temperature just below the martensitic transformation start temperature, and then at a temperature lower than the temperature of the high temperature coolant. A quenching method characterized by cooling with a low-temperature coolant. 2. The quenching method according to claim 1, wherein the low temperature coolant is mineral oil.