JP6720501B2 - Surface treatment method for steel parts - Google Patents

Description

The present invention relates to a surface treatment method for steel parts.

Patent Document 1 discloses a steel fixed pulley for a belt type continuously variable transmission. The fixed pulley includes a columnar shaft portion and a conical fixed sheave portion formed integrally with the shaft portion. The fixed pulley has a constant diameter from the tip of the shaft portion to the shaft root, and the diameter gradually increases from the shaft root along the inclined fixed sheave portion. Therefore, around the shaft root, a corner portion having a substantially L-shaped cross section is formed over the entire circumference.

A conical movable pulley is fitted into the shaft portion of the fixed pulley so that the fixed pulley can be slidably moved on the shaft portion to come into contact with and separate from the shaft portion. In the assembled state, a peripheral groove having a substantially V-shaped cross section surrounded by the fixed pulley and the movable pulley is formed, and a metal belt is wound around the peripheral groove. When the metal belt slides, bending is repeatedly applied to the corners, resulting in stress concentration.

Since the inclined contact portion of the fixed sheave portion is easily worn due to contact with the metal belt, in Patent Document 1, the entire surface of the fixed pulley is carburized and tempered to increase the carbon concentration of the entire surface. , The hardness of the entire surface is high.

Further, Patent Document 2 discloses a spur gear as a sintered cold forged component. This spur gear has residual pores acting as cutouts on the surface of the spur gear, and these residual pores cause stress concentration. In Patent Document 2, in order to reduce the effect of stress concentration due to residual pores, a cold forged sintered spur gear is carburized in a carburizing gas atmosphere having a predetermined carbon concentration, and then the predetermined carbon concentration is set. Decarburization is performed in a gas atmosphere having a lower carbon concentration. This increases the toughness of the entire surface of the spur gear.

JP 2000-130527 A JP, 2013-204080, A

In the fixed pulley of Patent Document 1, a portion where relatively high wear resistance is required, such as an inclined contact portion of a fixed sheave portion that comes into contact with a metal belt, and a corner where stress concentration due to repeated bending occurs Such as a part, where relatively high toughness is required.

As in Patent Document 1, in order to make the wear resistance of the contact portion of the fixed sheave portion relatively high, if the entire surface of the fixed pulley is carburized and tempered to increase the hardness of the entire surface, the corner portion is increased. The toughness of the surface of the is relatively low, and cracks may occur due to stress concentration.

On the other hand, when the decarburizing treatment of Patent Document 2 is applied to the fixed pulley of Patent Document 1 and the entire surface of the fixed pulley after the carburizing and tempering treatment is decarburized in a gas atmosphere, stress concentration occurs in the corner portion. Although the carbon concentration on the surface of the can be relatively low, the carbon concentration on the surface of the contact part of the fixed sheave part is also relatively low, and the wear resistance of the surface of the contact part increased by the above carburizing and tempering treatment is reduced. Will end up.

The present invention has a shaft portion and a large diameter portion that projects radially from the outer periphery of the shaft portion and a force acts by contact with a member, and a corner portion between the shaft portion and the large diameter portion. A method of surface treatment of a steel part provided with, a step of carburizing the entire surface of a steel part having a corner in a furnace, immersing the steel part in a liquid, the corner part in the liquid in comprising the step of decarburization by heating pressurization, and a surface treatment method of steel parts.

Therefore, only the corners on the entire surface of the steel part are locally decarburized in the liquid.

According to the present invention, the carbon concentration in the corner portion is lower than that in other portions, and the toughness of the corner portion is partially increased, so that the occurrence of cracks due to stress concentration is reduced.

It is sectional drawing of the fixed pulley to which this invention is applied. It is explanatory drawing of the carburizing process of the whole surface of a fixed pulley. It is explanatory drawing of the local decarburization process of the surface of a fixed pulley. It is a graph which shows the result of the bending fatigue strength of the conventional surface treatment method and the surface treatment method of one Example.

An embodiment of the present invention will be described below with reference to the drawings.

FIG. 1 shows a fixed pulley 1 of a belt type continuously variable transmission used in a vehicle, for example, as a steel part to which the present invention is applied. The fixed pulley 1 is made of case-hardening steel, for example, SCr420H, and includes a cylindrical shaft portion 2 and a conical fixed sheave portion 3 formed integrally with the shaft portion 2. As shown in the figure, the fixed pulley 1 has a constant diameter from the tip 4 of the shaft 2 to the shaft root 5, and the fixed sheave from the shaft root 5 to the outer periphery 6 of the fixed sheave part 3. The diameter gradually increases along the conical surface of the portion 3. Therefore, around the shaft root 5, a corner portion 7 having a substantially L-shaped cross section is formed over the entire circumference. Further, the fixed pulley 1 has the maximum diameter at the outer circumference 6 of the fixed sheave portion 3.

A conical movable pulley (not shown) is fitted into the shaft portion 2 of the fixed pulley 1 so as to be slidably moved on the shaft portion 2 with respect to the fixed pulley 1 so as to be detachable from and contactable with the fixed pulley 1. When the movable pulley is fitted in the shaft portion 2, a circumferential groove (not shown) having a substantially V-shaped cross section is formed between the fixed sheave portion 3 and the movable pulley. The width of the circumferential groove can be changed by sliding the movable sheave portion along the shaft portion 2 by hydraulic pressure or the like. In the assembled state, two pulley pairs consisting of the fixed pulley 1 and the movable pulley are arranged so that the two shaft portions 2 and 2 are parallel to each other, and a metal belt (not shown) is provided for each pulley pair. It is wound in each circumferential groove so as to come into contact with both the fixed sheave portion 3 and the movable pulley. Then, the width dimension of the circumferential groove continuously changes in response to an input from an engine (not shown), so that the running diameter of the metal belt changes.

When the metal belt slides, the surface 8a of the slanted contact portion 8 of the fixed sheave portion 3 is easily worn by contact with the metal belt. Therefore, by making the hardness of the surface 8a relatively high, the resistance of the surface 8a can be improved. High wear resistance is required. Therefore, in this embodiment, the wear resistance of the surface 8a is increased by increasing the carbon concentration of the surface 8a of the contact portion 8.

On the other hand, in the corner portion 7 around the shaft root 5 of the shaft portion 2, stress is concentrated due to repeated bending due to sliding of the metal belt, and a crack 9 as shown by a dashed arrow in FIG. Is likely to occur. Therefore, regarding the corner portion 7, it is required to reduce the occurrence of cracks 9 due to stress concentration by making the toughness of the surface 7a of the corner portion 7 relatively high. Therefore, in this embodiment, the toughness of the surface 7a is increased by making the carbon concentration of the surface 7a of the corner portion 7 relatively low.

The surface treatment method of the fixed pulley 1 in this embodiment will be described below with reference to FIGS. 2 and 3.

First, case hardening steel is forged or cast and then machined to form the fixed pulley 1 having the shaft portion 2 and the fixed sheave portion 3.

Next, as shown in FIG. 2, a plurality of fixed pulleys 1 are arranged in a carburizing treatment furnace 10 in a collective packing manner with the fixed sheave portion 3 being on the lower side, and a predetermined carburizing temperature, for example, 800 to The plurality of fixed pulleys 1 are carburized at 1000°C. This carburization is performed in a gas carburizing atmosphere 11 using a hydrocarbon such as acetylene. When carburizing, for example, the carburizing depth (ECD) from the surface 1a of the fixed pulley 1 is set to 0.9 mm, and the carbon concentration of the surface 1a of the fixed pulley 1 is set to 0.85% by weight. As a result, carbon permeates the surface 1a and the carbon concentration on the surface 1a becomes relatively high. Therefore, the hardness of the surface 1a is relatively high, and the wear resistance of the surface 1a is relatively high over the entire fixed pulley. On the other hand, carbon hardly penetrates into the inside 1b of the fixed pulley 1, and the hardness of the inside 1b is lower than the hardness of the entire surface 1a.

The metal structure of the fixed pulley 1 after carburizing is austenite.

Next, with reference to FIG. 3, a local decarburizing process of the fixed pulley 1 carburized in the carburizing process of FIG. 2 will be described.

First, as shown in FIG. 3, the liquid tank 13 is filled with the liquid 12 having a relatively low carbon potential. The liquid 12 is, for example, a mixed liquid of water and alcohol. For example, the carbon potential of water is 0% and the carbon potential of alcohol is 0.9%. In this example, the ratio of water to alcohol is adjusted so that the carbon potential is, for example, 0.2 to 0.3%. The carbon potential of the liquid 12 needs to be lower than the carbon concentration (0.85% by weight) of the surface 1a after the carburizing treatment. The carbon concentration of the surface 1a after the equilibrium is set to a target carbon concentration.

Next, the fixed pulleys 1 carburized in the carburizing step of FIG. 2 are immersed in the liquid tank 13 one by one, with the fixed sheave portion 3 facing downward as shown in FIG. The fixed pulley 1 is rapidly cooled by the liquid 12 in the liquid tank 13. As a result, the same effect as that obtained by performing the quenching process on the fixed pulley 1 can be obtained. During immersion, the metal structure of the fixed pulley 1 changes from austenite to martensite. Therefore, the hardness of the entire surface 1a of the fixed pulley 1 is relatively high, and the wear resistance of the entire surface 1a is relatively high.

The fixed pulley 1 may be quenched after the fixed pulley 1 is dipped.

Then, using the plus electrode 14 and the minus electrode 15 provided near the corner 7 as shown in FIG. 3, the surface 7a of the corner 7 is locally energized in the liquid 12 to a temperature at which decarburization is possible. To heat. Here, the temperature at which decarburization is possible is a temperature at which the temperature of the surface 7a of the corner portion 7 falls within a range of 930 to 1050°C, for example. The surface 7a of the corner 7 may be locally heated in the liquid 12 by a heater or high frequency heating. Further, this heating is performed under the condition of the heat transfer coefficient such that the heat transfer coefficient per second between the liquid 12 and the fixed pulley 1 is 2 kW/m ² K or more. The surface 7a of the corner portion 7 is locally heated so that the temperature of the surface 7a is in the range of 930 to 1050° C., but the surface 1a of the fixed pulley 1 other than the surface 7a of the corner portion 7 and the inside 1b. Is cooled by heat transfer with the liquid 12, and decarburization does not proceed. By the above local heating, the surface 7a of the corner portion 7 is partially decarburized, and the carbon concentration of the surface 7a becomes lower than the carbon concentration of the surface 1a of the fixed pulley 1 other than the corner portion 7. Desirably, the carburized depth (ECD) of the surface 7a of the corner portion 7 after decarburization is 0 mm, and the carbon concentration of the surface 7a is 0.2% by weight according to the carbon potential of the liquid 12. Therefore, the toughness of the surface 7 a of the corner portion 7 becomes higher than the toughness of the surface 1 a of the fixed pulley 1 other than the corner portion 7.

The metal structure of the surface 7a after decarburization becomes ferrite. The metallic structure of the surface 7a after decarburization may be martensite and bainite.

Then, the surface 7a of the corner portion 7 locally heated as described above is cooled at a cooling rate of about 10° C./second. Here, the metal structure of the surface 7a of the corner portion 7 after cooling becomes ferrite and pearlite.

FIG. 4 shows the test results of bending fatigue strength when the fixed pulley 1 was surface-treated by the conventional surface treatment method and the surface treatment method of this example. As shown in the figure, the bending fatigue strength of the corner portion 7 of the fixed pulley 1 subjected to the surface treatment by the conventional surface treatment method without decarburization is about 0.7 GPa, whereas the surface treated by the surface treatment method of the present embodiment. The bending fatigue strength of the processed corner portion 7 exceeds 1.2 GPa. Therefore, the bending fatigue strength of the corner portion 7 of the fixed pulley 1 can be increased by the surface treatment method of this embodiment.

As described above, in this embodiment, after the surface 1a of the fixed pulley 1 is carburized, the surface 7a of the corner portion 7 where stress concentration occurs is locally decarburized in the liquid 12, so that the surface 7a The carbon concentration becomes relatively low, and the toughness of the surface 7a becomes high. This reduces the occurrence of cracks on the surface 7a due to stress concentration.

Further, in this embodiment, since the surface 7a of the corner portion 7 is cooled at a cooling rate of about 10° C./second after heating, the metal structure becomes ferrite and pearlite, and the Young's modulus of the surface 7a decreases. .. Accordingly, even when the same bending amount acts on the corner portion 7, the stress generated in the corner portion 7 can be reduced.

Furthermore, in the present embodiment, the local heating of the surface 7a of the corner portion 7 is performed by the heat transfer such that the heat transfer coefficient per second between the liquid 12 and the fixed pulley 1 becomes 2 kW/m ² K or more. Since the heat treatment is performed under the condition of the rate, only the surface 7a of the corner portion 7 is heated while the portion other than the surface 7a of the corner portion 7 is cooled. As a result, the fixed pulley 1 is required to have partially different characteristics while maintaining the portion of the fixed pulley 1 (for example, the surface of the fixed sheave portion 3) where appropriate hardness, toughness and Young's modulus are obtained in the carburizing process. It is possible to change the hardness, toughness and Young's modulus only at the portion (corner portion 7) to be formed.

Further, in the above-mentioned embodiment, the carburizing treatment having a relatively long treatment time is collectively performed so that the plurality of fixed pulleys 1 have a collective packing form, and the local decarburizing treatment having a relatively short treatment time is performed individually. The fixed pulley 1 of 1. is individually performed. Therefore, the lead time per fixed pulley 1 in the carburizing process and the lead time of one fixed pulley 1 in the decarburizing process should be close to each other to improve the productivity of the plurality of fixed pulleys 1 as a whole. You can

In addition, although the fixed pulley 1 of the belt type continuously variable transmission is disclosed as an example of the steel parts in the above-described embodiment, the present invention can be applied to various other parts as the steel parts.

1...Fixed pulley 1a...Surface 2...Shaft 3...Fixed sheave 7...Corner 7a...Surface 8...Contact portion 8a...Surface 9... Crack 10... Carburizing furnace 12... Liquid 13... Liquid tank 14... Positive electrode 15... Negative electrode

Claims (6)

It has a shaft portion and a large diameter portion that projects radially from the outer periphery of the shaft portion and a force acts by contact with a member, and a corner portion is provided between the shaft portion and the large diameter portion. A surface treatment method for steel parts, comprising:
A step of carburizing the entire surface of the steel part having the corner portion in the furnace,
The steel component was dipped into a liquid, comprising the steps of decarburizing the corner portion by heating pressurized within the liquid,
A method for surface treatment of steel parts, comprising: The method for surface treatment of a steel component according to claim 1, further comprising a step of cooling the corner portion at a rate of 10°C/sec after the decarburization step. The steel part according to claim 1, wherein the decarburizing step is performed under a condition that a heat transfer coefficient per second between the liquid and the steel part is 2 kW/m ² K or more. Surface treatment method. The surface treatment method for steel parts according to claim 1, wherein a plurality of steel parts are carburized in the carburizing step, and individual steel parts are decarburized in the decarburizing step. The surface treatment method for a steel component according to claim 1, wherein the liquid has a carbon potential of 0.2 to 0.3%. The plus electrode and minus electrode directed at the corner portion was immersed in the liquid, according to claim 1 in which the above positive electrode and the negative electrode, the corner portion, characterized in that decarburization by heating pressurized in the liquid The method for surface treatment of steel parts according to.

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