JP2921235B2 - Carburizing and quenching method - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention is used to increase the surface hardness of mechanical parts such as gears to improve wear resistance and to generate compressive residual stress on the surface to improve fatigue strength. It relates to a carburizing and quenching method.

[0002]

2. Description of the Related Art In the quenching process of various mechanical parts, a marquenching method has been widely applied in which the inside and outside portions of a member to be treated are uniformly transformed into martensite to thereby prevent cracking and distortion.

That is, FIG. 5 is a diagram for explaining the principle of a general quench process in connection with a CCT diagram. In the quench process, a member to be processed is heated to 800 to 950 ° C.
Immersed in a salt bath maintained at a temperature just above the Ms point, which is the martensitic transformation start temperature, from the austenitizing temperature of about Ms until the inside and outside of the member to be treated have the same temperature.
After maintaining a constant temperature at a temperature just above the point, the supercooled austenite is pulled up and cooled and quenched before the bainite transformation occurs. It is intended to prevent deformation due to volume expansion due to transformation by performing site transformation.

[0004] In the quenching step of carburizing and quenching of gears and the like, the above-described quenching method is used to uniformly cool the surface carburized portion having a large volume expansion rate due to martensitic transformation in order to reduce heat treatment deformation. The law has been applied.

FIG. 6 is an explanatory view showing, as an example, a marquenching process in a carburizing and quenching process of a gear, and a gear that has been carburized and diffused under a predetermined condition is 90%.
From a high temperature of about 0 ° C., it is immersed in a salt bath maintained at 220 ° C. immediately above the Ms point of about 200 ° C. of the surface carburized portion containing about 0.8% of carbon, and kept at this temperature for several minutes. After the temperature of the tip and root surfaces is made uniform, the material is pulled up and air-cooled for quenching, or in order to further improve the uniformity of cooling during air cooling, as shown in FIG. Subsequent to maintaining the constant temperature, the gear is immersed in a coolant at 100 to 150 ° C. and gradually cooled, whereby the surface carburized portion of the gear is uniformly cooled, and the surface carburized portion is simultaneously transformed.

[0006]

However, the Marquench method is intended to eliminate the time difference of the transformation by keeping the inside and outside of the member to be treated at the same temperature, thereby preventing the member from being deformed. It is premised that the martensitic transformation start temperature Ms point is the same, and the martensitic transformation start temperature Ms point is the same, and when this marquenching method is applied to carburized members having different carbon contents inside and outside,
Even when the member to be processed is kept at a temperature just above the Ms point and cooled after eliminating the temperature difference between the inside and the outside, the transformation temperature differs due to the difference in the carbon content between the inside and the outside, so that the transformation timing does not match. In addition, there is a problem that deformation due to heat treatment cannot be eliminated.

That is, taking the above-described gear as an example, the carbon content of the inside of the gear excluding the carburized layer which is the very surface layer of the gear is about 0.2%, and its Ms point is about 40%.
Since the temperature is 0 ° C., the carburized gear is maintained at 220 ° C. immediately above the 200 ° C. which is the Ms point of the surface carburized portion,
At the stage where the inside and outside of the gear are at the same temperature, the inside of the gear has already begun martensitic transformation when it reaches 400 ° C. without uniform temperature difference between the central part and the part immediately below the carburized layer. . Therefore, even if the surface layer portion of the gear is uniformly transformed by the aforementioned marquenching method, uniform transformation of the internal non-carburized portion cannot be obtained,
Although the amount of heat treatment distortion is small, there is a problem that the variation of distortion is not much different from normal oil quenching,
Resolving these problems has been a problem in improving dimensional accuracy and improving the quality of the carburized and quenched member.

[0008]

SUMMARY OF THE INVENTION The present invention has been made in view of the above-mentioned conventional problems, and can reduce the amount of heat treatment distortion and its variation, and improve the dimensional accuracy of a carburized and quenched member. It is intended to provide a method of carburizing and quenching.

[0009]

As shown in FIG. 3, the rate of volume increase due to martensitic transformation increases as the carbon content increases, and uniform cooling of the surface carburized portion having a high carbon content during carburizing and quenching. Needless to say, it is desirable to simultaneously perform martensitic transformation in order to reduce heat treatment distortion.

However, in general, the carbon concentration distribution of the carburized member is as shown in FIG. 2, and the carburized depth is only about 1 mm at most, and it is considered that almost all of the carburized portion is a non-carburized portion. Then, in order to reduce the heat treatment distortion and its variation of the carburized and quenched member, it is important that not only the surface carburized part but also the internal carburized part is uniformly cooled and transformed at the same time. it is conceivable that.

The carburizing and quenching method according to the present invention focuses on the cooling of the internal carburized portion and the uniform transformation of the internal carburized portion together with the surface carburized portion. After immersion in the first heat bath held just above the martensitic transformation start temperature of the internal non-carburized portion of the to-be-processed member from the first, further kept at just above the martensite transformation start temperature of the carburized portion surface of the above-mentioned member 2
Immersed in a heat bath and then cooled, characterized in that such a structure in the carburizing and quenching method was used as means for solving the above-mentioned conventional problems.

That is, the member to be treated which has been carburized under predetermined conditions is subjected to the martensitic transformation start temperature I of the internal non-carburized portion of the member to be treated in advance from the austenite temperature.
It is immersed in a first heat bath maintained at a temperature immediately above the Ms point and maintained for a predetermined time (t1). During this time, after the temperature inside and outside of the member to be treated is made uniform, the inside of the member to be treated starts partially isothermal transformation to bainite.

Next, the member to be processed is drawn out of the first heat bath maintained at a temperature just above the IMs point, and is maintained at a temperature just above the martensitic transformation start temperature OMs point on the surface of the carburized portion of the member to be processed. It is immersed in the second heat bath and held for a predetermined time (t2). As a result, the remaining portion of the internal non-carburized portion undergoes martensite transformation at the same time, and the temperature of each portion of the surface of the carburized portion of the member to be processed is made uniform to the temperature just above the OMs point.

Then, the member to be treated is drawn out of the second heat bath and cooled, whereby the surface carburized portion of the member to be treated is simultaneously and uniformly transformed into martensite, whereby the surface of the member to be treated is changed. A hardened quenched structure and a softer bainite + martensite structure are formed inside, so that abrasion resistance and high fatigue strength and toughness can be secured. In addition, during the transformation of martensite on the surface of the member and the transformation inside the member (bainite + martensite), the members are cooled after eliminating the temperature difference between the respective parts, so that the members are simultaneously and uniformly transformed. The amount of heat treatment distortion and its variation can be significantly reduced.

As for the immersion times t1 and t2 in the first and second heat baths, since the surface of the carburized portion of the member must be surely martensitized, the material of the member to be treated and the carburized portion are not required. Considering the CCT diagram of
In the heat bath, the inside and outside temperatures of the member to be treated are made uniform,
In the heat bath, the temperature of at least each part of the surface of the member to be treated is sufficient to be uniform, and the surface carburized portion does not start isothermal transformation to bainite in the second heat bath. Must be set to time.

Next, the carburizing and quenching method according to the present invention will be described more specifically with reference to the above-described gear example.
Gears made of case hardened steel with a carbon content of 0.2%
By performing carburizing and diffusion treatment at a temperature of about 0 to 950 ° C. for a predetermined time, for example, a surface carburized layer having a carbon content of 0.8% is obtained.

As shown in FIG. 1, the above-mentioned carburized gear is heated from an austenite temperature of about 800 to 900 ° C. to a martensitic transformation start temperature IMs of an internal non-carburized portion (0.2% C) of the gear. (About 400 ° C.), that is, quenched in a first heat bath maintained at 400 to 450 ° C., and immersed for a certain time (t1). Thereby, the temperature inside and outside the gear is made uniform during this time,
The non-carburized part inside the gear begins to transform into bainite at a constant temperature.

The immersion time t1 in the first heat bath depends on the size of the gear, but as described above, in consideration of the elimination of the temperature difference between the inside and outside and the reliable martensitic transformation of the carburized surface, it is about It is considered that about 30 seconds is generally appropriate.

After about 30 seconds, the gear is withdrawn from the first heat bath and the carburized surface of the gear (0.8% C)
OMs point of martensitic transformation start temperature of about 200 ℃
, Ie, immersed in a second heat bath maintained at 200 to 250 ° C. and maintained for a predetermined time (t2), during which the remaining part of the non-carburized portion inside the gear is simultaneously transformed into martensite and The temperature of each part of the carburized part of the gear surface is made uniform. About 30 seconds is considered to be appropriate for the immersion time t2 in the second heat bath for the above-mentioned reason.

The transfer from the first heat bath to the second heat bath needs to be performed within a short time, and is preferably performed within 10 seconds. In addition, the refrigerant materials of the first and second heat baths are changed to the first and second heat baths in order to avoid the change of the refrigerant material properties due to the mixing of the gears, that is, the members to be processed.
It is desirable that both heat baths have the same composition, for example, nitrate.

When about 30 seconds have elapsed, the gear is drawn out of the second heat bath and cooled, whereby the surface of the carburized portion of the gear is simultaneously transformed into martensite, and has a Rockwell hardness of about C60. A quenched structure is obtained. The non-carburized portion inside the gear is hardened to Rockwell hardness C30 to C40 by the bainite and martensite transformation described above.

[0022]

In the method for carburizing and quenching according to the present invention, after the carburized member is once maintained at a temperature just above the martensitic transformation starting temperature of the internal non-carburized portion of the member from the austenitic temperature. Since it is cooled and held at a temperature just above the martensitic transformation start temperature on the surface of the carburized portion of the member to be treated, and then cooled,
The temperature difference of each part of the member during transformation of the internal non-carburized part and the surface carburized part of the treated member disappears, and the carburized part on the surface of the treated member is not only martensitically transformed at the same time,
Since the bainite or martensite transformation is uniformly performed even in the internal non-carburized portion, the occurrence of heat treatment distortion is minimized.

[0023]

EXAMPLE According to JIS G 4106, SMnC 420 was used.
Manganese chrome steel for machine structures (0.2
0% C), and the pitch circle diameter: 2
8 mm, module: 1.0, number of teeth: 28, twist angle:
The 32 ° involute was machined into a helical gear.

Next, the carbon content of the surface of the carburized layer is reduced to about 0.5.
Carburizing at 920 ° C. for 3 hours, diffusion treatment for 2.5 hours, and holding at 900 ° C. for 30 minutes to obtain 8%, and then in a first heat bath (potassium nitrate-lime nitrate bath) held at 420 ° C. To 30
Soaked for seconds.

Immediately after the lapse of 30 seconds, the gear pulled out of the first heat bath was immediately immersed in a second heat bath (potassium nitrate-lime nitrate bath) kept at a temperature of 220 ° C.
After being kept in the heat bath for seconds, it was pulled out and air-cooled to obtain a carburized and quenched gear.

Then, 20 obtained by the same process is obtained.
The torsion angle error was measured for each of the gears, and the carburized gear obtained under the same material and the same carburizing condition was subjected to 220 ° C.
Of the carburized and quenched gears obtained by the conventional marquenching method in which the steel was immersed in a heat bath held at a temperature and then air-cooled and quenched.

The result is shown in FIG. 4, where the average value of the torsion angle error by the conventional Marquench method is -0.13.
μm, and the variation (3σ) is ± 0.8 μm, whereas in the gear obtained by the carburizing and quenching method according to the present invention, the average value of the torsion angle error is −0.11 μm.
m, the variation (3σ) was ± 0.5 μm, and it was confirmed that both the amount of heat treatment distortion and its variation were improved.

[0028]

As described above, in the carburizing and quenching method according to the present invention, the carburized member to be treated is maintained from the austenite temperature to just above the martensitic transformation start temperature of the non-carburized portion inside the member to be treated. After being immersed in the first heat bath, the member is further immersed in the second heat bath held just above the martensitic transformation start temperature on the surface of the carburized portion of the member to be treated, and then cooled. By maintaining the non-processed member at a temperature just above the martensitic transformation start temperature of each of the internal non-carburized part and the surface carburized part, the temperature of each part of the member at the time of transformation can be made uniform, and the surface of the member to be treated can be made uniform. Since the transformation of not only the carburized part but also the internal non-carburized part is controlled to be uniform, heat treatment distortion and its variation are reduced, and the dimensional accuracy of the carburized and quenched member is improved. It leads to excellent effect that it becomes possible.

[Brief description of the drawings]

FIG. 1 shows the principle of the carburizing and quenching method according to the present invention by CCT.
It is explanatory drawing shown in connection with a diagram.

FIG. 2 is a graph showing an example of a carbon concentration distribution of a carburized layer.

FIG. 3 is a graph showing a relationship between a volume increase rate due to martensitic transformation and a carbon content.

FIG. 4 is a graph showing measurement results of strain amounts in Examples of the carburizing and quenching method according to the present invention, together with Comparative Examples.

FIG. 5 is an explanatory diagram showing the principle of a general quench method in association with a CCT diagram.

FIG. 6 is an explanatory view showing a conventional carburizing and quenching method of a gear by a marquenching method.

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Claims (2)

(57) [Claims] After immersing a carburized member in a first heat bath maintained at a temperature just above a martensitic transformation start temperature of an internal non-carburized portion of the member from an austenite temperature, the member is further treated. Immersed in a second heat bath maintained just above the martensitic transformation start temperature on the carburized part surface of
A method of carburizing and quenching, which is followed by cooling. 2. The holding temperature of the first heat bath is 400 to 450.
The carburizing and quenching method according to claim 1, wherein the holding temperature of the second heat bath is 200 to 250 ° C.