KR20210125159A - Heat treatment method for improving the torsional strength of tubular shafts of automobile constant speed joints - Google Patents

Abstract

The present invention relates to a heat treatment method for improving the torsional strength of a tubular shaft of a vehicle constant velocity joint, which can prevent an accident. The heat treatment method of the present invention comprises: a temperature elevating step (S100); a cementation heat treating step (S200); a slow cooling preparation step (S300); a salt bath treatment step (S400); and a washing completion step (S500) of washing and removing foreign substances on a surface.

Description

Heat treatment method for improving the torsional strength of tubular shafts of automobile constant speed joints of automobile constant speed joints

The present invention relates to a heat treatment method for improving the torsional strength of a tubular shaft of an automobile constant velocity joint, and more particularly, to improve the torsional strength of the tubular shaft by improving the existing heat treatment method, thereby increasing service life and safety accidents of automobiles It relates to a heat treatment method for improving the torsional strength of a tubular shaft of a constant velocity joint of an automobile that can be prevented.

The drive shaft, which is a component of the constant velocity joint of the vehicle, transmits the rotational driving force of the transmission to the drive wheels. It has a structure in which two constant velocity joints (CVJoints) are connected between the hubs.

The constant velocity joint is a component that allows the power transmission to be made evenly without change in the rotational angular velocity. The so-called plunging type constant velocity joint and the fixed constant velocity joint are based on the degree of cleavage (angle adjustment) and plunging (lengthwise contraction and expansion) functions. divided into fixed joints.

The plunging joint is also called a slide-type ball joint, and is intended to absorb the vertical, horizontal, and vertical movements of the vehicle.

In general, it can be seen that a tripod type constant velocity joint or a plunging joint is used for the engine side (inboard side) around the drive shaft, and a fixed joint is used for the drive wheel side (outboard side) around the drive shaft.

Here, as shown in FIG. 1, the shaft unit of the plunging type constant velocity joint includes, as an example, a hollow external shaft (hereinafter, tubular shaft) which is rotated by receiving rotational power from the engine side, and within the tubular shaft. It consists of a plurality of traction balls for transmitting the rotational power of the inserted inner shaft and the tubular shaft to the inner shaft, and a cage for supporting the traction balls.

Meanwhile, as shown in FIG. 2, a plurality of inner grooves are arranged at equal intervals on the inner circumferential surface of the tubular shaft, and as shown in FIG. 1, a plurality of outer circumferential grooves ( outer groove) is formed, and the traction ball is interposed in the space formed by the combination of the inner and outer grooves.

The cage is a tubular structure in which a plurality of holes through which the traction ball passes, and by restraining the traction ball between the outer shaft and the inner shaft, the movable position of the traction ball is limited within a certain range.

According to the shaft unit of the plunging type constant velocity joint, the tubular shaft and the inner shaft are connected in the rotational direction by the traction ball. The so-called rolling operation in which the inner shaft is rotated is performed.

In addition, a so-called plunging operation is made to absorb the displacement by sliding the inner shaft within the tubular shaft with respect to the displacement occurring during vehicle driving. The axial friction of the shaft is reduced.

Meanwhile, as described above, for the rolling operation of the tubular shaft and the inner shaft, a plurality of inner circumferential grooves must be formed on the inner circumferential surface of the tubular shaft, and corresponding outer circumferential grooves must be formed on the outer circumferential surface of the inner shaft. In addition, for the operational stability of the tubular shaft and the inner shaft connected by the traction ball, the specifications of the inner and outer grooves must precisely correspond to the traction ball.

Furthermore, the tubular shaft transmits the torque of the final reduction device to the wheels while always receiving a torsional force due to the length change of the wheels and the final reduction device by the steering device and the suspension device.

Therefore, fatigue builds up on the tubular shaft, which affects the strength and vibration, and eventually results in poor vehicle safety and steering performance. Heat treatment is carried out to obtain the strength for

The heat treatment performed to obtain strength against the torsional force of the tubular shaft is usually heat treated to obtain torsional strength through carburizing heat treatment and austempering. Carburizing at 940°C for 8 hours and diffusion at 940°C for 6 hours, when the temperature is raised to 940°C after 2 hours of preheating and heating, carburizing heat treatment for about 14 hours. Thereafter, while the temperature is gradually reduced for 90 minutes, the temperature is maintained at 800-860°C, and then cooled again in a 240°C salt bath. After cooling the salt bath at 240° C. for 150 minutes, it is cooled in the air for about 1 hour, and washed with water to complete. Here, the heat treatment proceeds continuously from preheating to salt bath cooling, and then the washing process is performed separately.

When the heat treatment of the tubular shaft is completed through the conventional carburizing heat treatment method as described above, the strength against the torsion of the tubular shaft is provided. Usually, the required strength is achieved, but it is often less than the required strength. In other words, in order to maintain the required strength, despite the fact that the minimum value has to correspond to the required strength, the minimum strength is lower than the required hardness, so there are many product defects. There are problems that can lead to

1. Republic of Korea Patent Publication No. 10-2005-0061476, published on June 22, 2005. 2. Republic of Korea Patent Publication No. 10-2006-0030854, published on April 11, 2006.

Therefore, the present invention was created to solve the above-mentioned problems, and the part for the conventional continuous heat treatment method for imparting torsional strength of the tubular shaft of the constant velocity joint of an automobile is a discontinuous heat treatment furnace through preparation for slow cooling in air. An object of the present invention is to provide a heat treatment method for improving the torsional strength of the tubular shaft of an automobile constant velocity joint, which improves the torsional strength of the tubular shaft by improving it, thereby increasing the service life of the tubular shaft and preventing safety accidents of the automobile.

In addition, the present invention improves the defect rate of the tubular shaft due to heat treatment below the torsional strength required by the conventional heat treatment method to more than the torsional strength required for the tubular shaft through the improved heat treatment method, thereby minimizing the defect rate of the product, Another object of the present invention is to provide a heat treatment method for improving the torsional strength of a tubular shaft of an automobile constant velocity joint capable of shortening production time and improving productivity.

In order to achieve the above object, in the heat treatment method for improving torsional strength of the tubular shaft of a constant velocity joint of an automobile according to the present invention, the tubular shaft is charged into a heat treatment furnace to which a reducing gas is supplied, and then the temperature of the furnace is 900 A temperature raising step (S100) to reach ~950 ℃ and; After the temperature raising step, while supplying propane, LNG, and benzene, which are carburized or carburized gases, the internal temperature is maintained in a temperature range of 900 to 950 ° C. a carburizing heat treatment step (S200) of infiltrating and processing; After the carburizing heat treatment step, the slow cooling preparation step (S300) of slowly cooling to 500° C. or less in an anti-oxidation protective gas atmosphere within 30 minutes to 10 hours, and after finishing, preparing for preheating and temperature rise again; After the slow cooling preparation step, the temperature of the heat treatment furnace is again preheated and heated to a temperature of 850 ~ 920 ℃, and the temperature is maintained by adjusting the heating to be maintained at the elevated temperature of 850 ~ 920 ℃, and then the heat treatment effect of the tubular shaft is a salt bath treatment step (S400) of cooling and maintaining constant temperature maintenance at 230 to 260° C. for 0.5 to 3 hours to improve; After the salt bath treatment step, the cleaning completion step (S500) of washing and removing foreign substances on the surface; characterized in that it comprises a.

Here, the average grain size of the hardened surface layer of the tubular shaft obtained through the heat treatment method for improving the torsional strength of the tubular shaft of the automobile constant velocity joint is 13 ~ 17㎛, AGS No 5 ~ 9 (Austenite Grain Size; austenite grain size) , 0.2mm from the surface layer is formed of bainite and martensite, the retained austenite is 15%, the surface hardness is Hv650 ~ 740, characterized in that, the tubular shaft torsional strength improvement heat treatment method of the automobile constant velocity joint The average grain size of the base material layer of the tubular shaft obtained through It is characterized in that the residual ferrite is 5%, and the core hardness is Hrc 37 to 45.

On the other hand, the breaking load of the tubular shaft that has gone through the washing completion step is 6300t N.m or more, and the yield strength is 4400t N.m or more, and the torsional strength is improved.

The present invention improves the torsional strength of the tubular shaft by improving the part for the conventional continuous heat treatment method for imparting torsional strength of the tubular shaft of the constant velocity joint of an automobile to the discontinuous heat treatment method through slow cooling preparation, thereby improving the torsional strength of the tubular shaft It has the advantage of increasing the service life of the tubular shaft and preventing safety accidents of automobiles, and the torsional strength required for the tubular shaft through the improved heat treatment method by reducing the defect rate of the tubular shaft due to heat treatment below the torsional strength required by the conventional heat treatment method By improving the above, there is an effect of minimizing the defect rate of the product, shortening the production time, and improving the productivity.

1 is a perspective view of a tubular shaft of a conventional plunging type automobile constant velocity joint.
2 is a cross-sectional view taken along line AA of FIG. 1 of a tubular shaft of a conventional plunging type automobile constant velocity joint.
3 is a graph of a treatment temperature of a heat treatment method of a conventional plunging type automobile constant velocity joint.
4 is a graph of the treatment temperature of the heat treatment method for improving the torsional strength of the tubular shaft of the automobile constant velocity joint according to the present invention.
5 is a block diagram of a heat treatment method for improving torsional strength of a tubular shaft of an automobile constant velocity joint according to the present invention.

Advantages and features of the present invention, and methods for achieving them, will become apparent with reference to the embodiments described below in detail in conjunction with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, but will be implemented in a number of different forms.

In the present specification, the present embodiment is provided to complete the disclosure of the present invention, and to completely inform those of ordinary skill in the art to which the present invention pertains to the scope of the present invention. And the invention is only defined by the scope of the claims. Accordingly, in some embodiments, well-known components, well-known operations, and well-known techniques have not been specifically described in order to avoid obscuring the present invention.

Like reference numerals refer to like elements throughout. In addition, the terms used (mentioned) herein are for the purpose of describing the embodiments and are not intended to limit the present invention. As used herein, the singular also includes the plural unless specifically stated otherwise in the phrase. In addition, elements and operations referred to as 'include (or include)' do not exclude the presence or addition of one or more other elements and operations.

Unless otherwise defined, all terms (including technical and scientific terms) used herein may be used with the meaning commonly understood by those of ordinary skill in the art to which the present invention belongs. Also, terms defined in commonly used dictionary are not to be interpreted ideally or excessively unless defined.

Hereinafter, technical features according to an embodiment of the heat treatment method for improving the torsional strength of the tubular shaft of a constant velocity joint for automobiles according to the present invention will be described in detail with reference to the accompanying drawings.

4 is a treatment temperature graph of the tubular shaft torsional strength improvement heat treatment method of a constant velocity joint of an automobile according to the present invention, and FIG. 5 is a block diagram of the tubular shaft torsional strength improvement heat treatment method of a constant velocity joint of an automobile according to the present invention.

The torsional strength improvement heat treatment method of the tubular shaft of an automobile constant velocity joint according to the present invention improves the conventional heat treatment method described above to always obtain the torsional strength of the tubular shaft above the required strength, thereby minimizing the defect rate of the tubular shaft and , through this, productivity can be improved and production efficiency can be increased. To this end, the torsional strength improvement heat treatment method of the tubular shaft of a constant velocity joint of an automobile according to the present invention is a heating step (S100), a carburizing heat treatment step (S200), a slow cooling preparation step (S300), as shown in FIGS. , through the salt bath treatment step (S400) and the washing completion step (S500), the torsional strength of the tubular shaft can be significantly improved.

The temperature raising step (S100) is

After the tubular shaft is charged into the heat treatment furnace to which the reducing gas is supplied, the temperature of the furnace is allowed to reach 900 ~ 950 °C. That is, in the temperature raising step (S100), as described above, after the tubular shaft is charged into the heat treatment furnace to which the reducing gas is supplied in the section for increasing the initial temperature, the furnace chamber internal temperature is 900 to 950 ° C. This is to adjust the product temperature so that carbon diffuses and penetrates the product surface in the carburizing process.

The carburizing heat treatment step (S200) is

After the temperature raising step, while supplying the carburizing gases such as propane, LNG, and benzene, the internal temperature is maintained for 4 to 20 hours in a temperature range of 900 to 950 ° C. handle Here, through the carburizing heat treatment step, the deformation due to the heat treatment of the inner periphery of the tubular shaft and the inner groove is minimized to show the strain within the required range of error from the center line diagram, and along with this, the inner shaft is It is positionally coupled between the inner and outer circumferential grooves by the traction ball in the blur shaft to obtain the required hardness and hardness that can have mechanical strength against fatigue from impacts caused by rotation and expansion/contraction coupling.

The slow cooling preparation step (S300) is

After the carburizing heat treatment step, it is slowly cooled to 500° C. or less within 30 minutes to 10 hours in an anti-oxidation protective gas atmosphere, and after finishing, it is prepared for preheating and temperature rise again. Here, in the slow cooling preparation step, it is important not to go through a continuous heat treatment step because the carburizing heat treatment step is previously performed, and then the slow cooling is performed to perform the primary finishing. That is, in the prior art, after the carburizing heat treatment step, the heat treatment is continuously performed in the step of immediately carrying out salt bath cooling. The required torsional strength can be obtained.

Therefore, if the salt bath cooling is performed continuously through the carburizing heat treatment step to the next step, the required torsional strength of the tubular shaft cannot be obtained, and the structure is not uniform.

The salt bath treatment step (S400) is

After the slow cooling preparation step, the temperature of the heat treatment furnace is again preheated and heated to a temperature of 850 ~ 920 ℃, and the temperature is maintained by adjusting the heating to be maintained at the elevated temperature of 850 ~ 920 ℃, and then the heat treatment effect of the tubular shaft is To improve, maintain constant temperature at 230~260℃ while cooling for 0.5~3 hours.

The washing completion step (S500) is

After the salt bath treatment step, foreign substances on the surface are washed and removed so as not to affect the quality of the product, thereby providing a quality tubular shaft with improved torsional strength.

The above and the heat treatment method according to the present invention finish the heat treatment while performing slow cooling in an anti-oxidation protective gas atmosphere, and the surface and deep tissue as shown in FIG. 2 of the tubular shaft 100 obtained through the following steps not continuously In this bainite structure state, both the inner peripheral groove and the deep part of the tubular shaft show the characteristics of the bainite structure well, as shown in Table 1 below.

Tubular shaft inner circumferential groove surface and deep tissue

In addition, the average grain size of the hardened surface layer of the tubular shaft 100 is 13 to 17 μm, AGS No 6 to 10 (Austenite Grain Size; austenite grain size), and the texture state of 0.2 mm from the surface layer is bainite and It is formed of martensite, retained austenite is 15%, is heat treated to a surface hardness of Hv650~740, and the average grain size of the base material layer of the tubular shaft is 8~10㎛, AGS No 5~11 (Austenite Grain Size; austenite) Knight grain size), and the structure state of 1/2T of the base material structure state is formed of bainite, the residual ferrite is 5%, and it is heat-treated with a core hardness of Hrc37~45. It is heat treated to suit the mechanical strength.

Furthermore, the breaking load required for the existing tubular shaft is 5,550-5780t Nm, and the yield strength is 3,700-4,080t Nm. The breaking load of the tubular shaft that has gone through the above cleaning step is 6300-7000t Nm, and the yield strength is 4400-5000t. With Nm, the torsional strength is improved over the required strength, and the heat treatment is improved.

As described above, the heat treatment method for improving the torsional strength of the tubular shaft of a constant velocity joint of an automobile according to the present invention does not proceed with continuous heat treatment until salt bath cooling as in the conventional carburizing heat treatment method, but after the carburizing heat treatment step, it is cooled slowly in an anti-oxidation protective gas atmosphere. After the primary finishing, by preheating and heating for salt bath cooling again, satisfactory results can be obtained that exceed the requirements for mechanical strength and torsional strength of the tubular shaft as described above.

In contrast, a tubular shaft was prepared as a sample, and the results of the superficial layer and deep tissue as shown in FIG. 2 of the tubular shaft 100 through the heat treatment method of the present invention could be confirmed through the table below.

Tubular shaft (hardened surface layer) drawing request Crystal grain size: A.G.S No. 5 or more Surface texture (surface layer 0.2mm)
:Bainite + Martensite organization photo
(X500)

result Average grain size 13~17㎛
AGS No. 9 level Martensite + Residual Austenite 15%
Surface hardness:
0.17mm-Hv 671 /1.05mm-Hv 595
*Hv 595=Hrc 55

As shown in [Table 2], the mechanical strength, durability, and abrasion resistance could be improved by obtaining the structure and hardness that raise the standard of the hardened surface layer required for the tubular shaft.

Tubular shaft (base material layer) drawing request Crystal grain size: A.G.S No. 5 or more Base material structure (1/2T)
:Bainite + Martensite organization photo
(X500)

result Average grain size 8-10㎛
AGS No. 10.5 level Bainite + Residual Ferrite (small amount)

As shown in [Table 3], it was possible to obtain the structure and hardness oriented toward the standard of the required base material layer of the tubular shaft, and to ensure the uniformity of the microstructure.

Through the results shown in Tables 2 and 3 as described above, the torsional strength (breaking load, yield strength) through the conventional carburizing heat treatment method and the heat treatment method of the present invention as Comparative Examples and Examples through the conventional carburizing heat treatment method through the tube shaft sample More improved torsional strength was obtained.

In common, the standard required value of torsional strength is 5,550 t N.m or more for breaking load and 4,080 t N.m or more for yield strength.

<Comparative example>

This is a torsional strength test result obtained through continuous heat treatment up to a salt bath without undergoing an annealing preparation step as in the heat treatment method of the present invention as a conventional carburizing heat treatment method. First, in the heat treatment, preheating and temperature rise in the heat treatment furnace for about 2 hours, and when the temperature is raised to 940°C, carburizing heat treatment is performed for about 14 hours through carburization at 940°C for 8 hours and diffusion at 940°C for 6 hours. Thereafter, the temperature was maintained at 850° C. while gradually decreasing the temperature for 90 minutes, and then cooled to 240° C., followed by salt bath cooling while maintaining the temperature for 150 minutes. Here, the heat treatment proceeded continuously from preheating to salt bath cooling, and then the washing process was performed separately to obtain the tubular shaft sample of the comparative example, and the strength test was performed to obtain the results shown in [Table 4].

Strength test result of comparative example tubular shaft
Comparative sample 1 Comparative sample 2 Comparative sample 3 Comparative sample 4 Comparative sample 5 breaking load
5779t N.m 5988t N.m 6032t N.m 5903t N.m 5996t N.m. yield strength
4589t N.m 4690t N.m. 4601t N.m 4544t N.m. 4470t N.m

<Example>

These are the torsional strength test results obtained through the discontinuous heat treatment entering the salt bath cooling step after the primary finishing of the carburizing heat treatment step through the slow cooling preparation step according to the present invention. First, in the heat treatment, preheating and raising the temperature in the heat treatment furnace for about 2 hours, and when the temperature is raised to 940°C, carburizing at 940°C for 8 hours and diffusion for 6 hours at 940°C for about 14 hours. proceed to After that, through the slow cooling preparation step, it is cooled to 500° C. or less by slow cooling in an anti-oxidation protective gas atmosphere for 5 minutes to 10 hours, and the first finishing is performed. Thereafter, the temperature was maintained by preheating and raising the temperature to 850 ~ 920 ℃ to maintain the heating, and then cooled to 240 ℃ to improve the heat treatment effect of the tubular shaft, and the constant temperature was maintained while cooling the temperature for 150 minutes. Here, after the heat treatment is finished from preheating to carburizing heat treatment and slow cooling preparation, salt bath cooling is discontinuous, and then the washing process is performed separately to obtain the tubular shaft sample of the embodiment according to the present invention, and the strength test [ Table 5] and the same results were obtained.

Example strength test result tubular shaft
Example Sample 1 Example Sample 2 Example Sample 3 Example Sample 4 Example Sample 5 breaking load
6301t N.m 6652t N.m 6511t N.m 6489t N.m. 6628t N.m. yield strength
4775t N.m 4763t N.m 4593t N.m 4765t N.m 4751t N.m

Comparing the result values in [Table 4] and [Table 5] according to the torsional strength obtained through the comparative example and the example, both of them can obtain the strength that can satisfy the standard requirement first, but in the case of the comparative example, it is close to the standard requirement Therefore, a problem that does not satisfy the standard requirement value may occur frequently, which may cause a safety accident of the vehicle.

However, the torsional strength obtained by the present invention shows a performance improvement of 20-25% compared to the standard required value, and there is no fear of being produced below the standard required value, so product defects due to torsional strength are minimized. The reliability of consumers and producers can be secured by providing a tubular shaft that cannot be the cause of safety accidents, and can be safer, durable, and have an increased service life. In addition, it is possible to produce close to zero product defect rate, thereby increasing productivity and production efficiency.

In the above, the present invention has been described in detail as one embodiment, but the scope of the present invention is not limited thereto, and it is clear that a number of variations and modifications are possible by those skilled in the art within the scope of the technical idea, Examples and practical equivalents will be included.

100: tubular shaft
S100: heating step S200: carburizing heat treatment step
S300: slow cooling preparation step S400: salt bath treatment step
S500: washing step

Claims (4)

In a heat treatment method for improving torsional strength of a tubular shaft of a constant velocity joint of an automobile,
a temperature raising step (S100) of charging the tubular shaft to a heat treatment furnace to which a reducing gas is supplied and then allowing the temperature of the furnace to reach 900 to 950°C;
After the temperature raising step, while supplying the carburizing gases such as propane, LNG, and benzene, the internal temperature is maintained for about 4 to 20 hours depending on the carburized hardened layer in the temperature range of 900 to 950 ° C. a carburizing heat treatment step (S200) of infiltrating and processing the gas;
After the carburizing heat treatment step, the slow cooling preparation step (S300) of preparing for preheating and temperature rise again after the annealing in the atmosphere of an anti-oxidation protective gas to 500° C. or less within 30 minutes to 10 hours in the air, and after finishing;
After going through the slow cooling preparation step, the temperature of the heat treatment furnace is again preheated and heated to a temperature of 850 to 920 ° C. a salt bath treatment step (S400) of maintaining constant temperature at 230 to 260° C. for 0.5 to 3 hours to improve;
After the salt bath treatment step, a cleaning completion step (S500) of washing and removing foreign substances on the surface; a heat treatment method for improving torsional strength of a tubular shaft of an automobile constant velocity joint, characterized in that it comprises a.
The method of claim 1,
The average grain size of the hardened surface layer of the tubular shaft obtained through the heat treatment method for improving the torsional strength of the tubular shaft of the automobile constant velocity joint is 13 to 17 μm, AGS No 6 to 10 (Austenite Grain Size; austenite grain size), and the surface layer The structure of 0.2mm from bainite and martensite is formed, the retained austenite is 15%, and the surface hardness is Hv650~740.
The method of claim 1,
The average grain size of the base material layer of the tubular shaft obtained through the heat treatment method for improving the torsional strength of the tubular shaft of the automobile constant velocity joint is 8 to 10 μm, AGS No 7 to 11 (Austenite Grain Size; austenite grain size), and the base material structure The structural state of 1/2T of the state is formed of bainite, the residual ferrite is 5%, and the core hardness is Hrc37-45.
The method of claim 1,
The breaking load of the tubular shaft that has undergone the washing completion step is 6300 t Nm or more, and the yield strength is 4400 t Nm or more.

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