JPH0114310B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a gas nitrocarburizing treatment method for gear shift forks. A gear shift fork is a part of an automobile's power transmission system that transmits the movement of the shift lever to each gear of the transmission.As shown in Fig. 1, the gear shift fork 1 has fork shaft holes 4 and 5 in the center. It consists of arm parts 6, 7 provided on the left and right in a direction perpendicular to the shaft hole axis, and claw parts 2, 3 protruding from the tips of the arm parts, respectively. The inner circumferential surface of the gear shift fork 1 is approximately semicircular so as to slidably fit into a gear coupling sleeve (not shown). Ribs are provided as shown in Figure 1b. This gear shift fork 1 has conventionally been manufactured using spheroidal graphite cast iron, malleable cast iron, or machine structural steel such as S45C or S55C, and after producing a rough shape by casting, hot forging, etc., it is then machined into the shape shown in Fig. 1. Make it into the shape shown, and add this shift fork 1
The sliding surface of the gear coupling sleeve was subjected to surface treatments such as induction hardening, plating, and thermal spraying to impart wear resistance. However, the gear shift fork manufactured in this manner has the following drawbacks. for example,
In induction hardening, the martensite formed by hardening comes into microscopic metal contact with the sliding partner material, which increases the temperature of the contact surface and causes decomposition of the hardened martensite, resulting in a decrease in hardness and the possibility of sliding with the gear coupling sleeve. There are problems such as progressive wear on the moving surfaces. Also, when surface-treating the contact surface with abrasion-resistant plating such as chrome, the hardness of the base material is Hv250.
In the case of plating on something of the order of ~290, if the contact pressure on the sliding surface exceeds about 50Kg/ ^cm2 , peeling will occur at the interface between the substrate and the plating, and as a result, repeated contact loads will cause cracks in the plating layer. Occur. As this condition progresses, the plating layer peels off from the base material, resulting in a significant decrease in wear resistance. In addition, Fe-Mo alloy, Fe
-The method of coating with Cr alloy etc. is undesirable as it increases the cost.On the other hand, the method of coating with resin can be used if a material with good compatibility with oil is selected as the coating material. Sleeve fitted thereto (not shown)
However, when coated with resin, the perpendicularity of the sliding surfaces of the shift fork pawls 2 and 3 to the fork shaft holes 4 and 5 is sufficiently good. If this is not ensured, the coated resin will easily melt and be damaged due to contact with the pole piece, causing a problem. The present invention is intended to eliminate the drawbacks of the conventional gear shift fork surface treatment methods as described above, and is to impart wear resistance to the sliding surface of the gear shift fork and efficiently release residual stress in the shift fork. The present invention also provides a method for surface treatment of a gear shift fork, which can reduce heat treatment distortion. That is, in the gas nitrocarburizing treatment method for a gear shift fork of the present invention, the shape of the shift fork is made as symmetrical as possible in the front and back with respect to the fork shaft hole provided approximately in the center of the main body, and the claws provided at the tips of the two arm portions are By heating the shift fork held in a position facing the direction of gravity in a mixed gas atmosphere consisting of ammonia gas and nitrogen gas at a temperature range of 550°C to 650°C for a predetermined period of time, and then slowly cooling it. The present invention is characterized in that an iron-nitrogen compound layer with a thickness of 8 microns or more is formed on the sliding surface of the gear shift fork. Next, an embodiment of the method of the present invention will be described with reference to the drawings. The gear shift fork material in the present invention is spheroidal graphite cast iron (FCD), and its components include 3.2 to 4.0% C by weight and Mn.
0.2-0.7%, P 0.01-0.04%, S 0.01-0.02
%, Si is 2.8% or less, and the balance is Fe. Among these, the contents of C, Mn, P, and S can be appropriately selected within the above range, but as shown in Figure 2, Si may be added to the surface during gas nitrocarburizing depending on the component content in the material. Since the thickness (μ) of the resulting iron-nitrogen compound layer changes, the thickness (μ) of the iron-nitrogen compound layer is at least 8μ or more on the sliding surface of the gear shift fork.
To form a nitrogen compound layer, increase the Si content to 2.8%.
It is necessary to ensure the following: FIG. 2 is a graph showing the influence of Si on the thickness (μ) of the nitride layer when cast iron is subjected to gas nitrocarburizing treatment at 570° C. for 210 minutes. After processing the rough shaped material made of spheroidal graphite cast iron having the above configuration into a predetermined shape, it is subjected to gas nitrocarburizing treatment.
Gas nitrocarburizing treatment is performed by heating the rough material to 550 to 650°C in a mixed gas atmosphere containing ammonia gas and RX gas at a ratio of 1:1, holding it for 2 to 6 hours, and then slowly cooling it. . The mixing ratio of ammonia gas and RX gas is not limited to the above, and is, for example, between 2:3 and 3:2. In the gas nitrocarburizing treatment method of the present invention, if the heating temperature is 550°C or lower, it will take more than ten hours to form the iron-nitrogen compound layer on the surface of the rough shaped material to a thickness of several microns. On the other hand, if the temperature is higher than 650℃, the diffusion of nitrogen becomes faster, so an iron-nitrogen compound layer of more than ten micrometers can be formed on the cast iron surface in a short time, but the outermost surface of the compound layer becomes porous. Therefore, if used for a long period of time, the iron-nitrogen compound layer may peel off during sliding between the sleeve and fork, resulting in a decrease in wear resistance, which is undesirable. Also, rough shapes
When held at temperatures above 650°C for several hours, pearlite in the cast iron base structure decomposes, resulting in a decrease in strength. At the same time, carbon in the decomposed pearlite precipitates into graphite, causing expansion of the cast iron base and causing dimensional changes, resulting in product damage. This causes problems with poor accuracy. Therefore, the heating and holding temperature of the gear shift fork rough shape material in the method of the present invention is 550 to 650°C, particularly 570±10°C.
°C is optimal. Figure 3 shows the distribution of the Vickers hardness Hv (100 g) near the surface of the gear shift fork after heat treatment at 570°C and 650°C for 3 hours and slow cooling. The decrease in hardness is more significant when the heating temperature is 650°C. Next, products that have been heat-treated in a mixed gas are usually oil-quenched or slowly cooled, but products with complex shapes such as gear shift forks are oil-quenched as shown in Figure 4. Since dimensional changes are likely to occur in each part, gradual cooling is necessary to ensure dimensional accuracy. Figure 4 is a graph showing the influence of slow cooling and rapid cooling (oil quenching) on the perpendicularity of the shift fork pawl after gas nitrocarburizing the gear shift fork. The results obtained by holding a fork at 570℃ for 210 minutes in a mixed gas atmosphere of ammonia gas and RX gas at a ratio of 1:1, performing rapid cooling and slow cooling, and investigating the squareness before and after treatment. be. In addition, in the method of the present invention, it is necessary to consider the attitude of the shift fork during gas nitrocarburizing treatment as a countermeasure against distortion during heat treatment. During gas nitrocarburizing, the shift fork is held at a high temperature of around 600°C for a long time, so it is possible that the fork claw may bend due to its own weight. Therefore, in order to prevent this deflection, for example, a shift fork (roughly shaped material) 1
Place the support rods 8, 9, 10 in the direction perpendicular to the arms 6, 7 at the positions shown in Fig. 5 a, b on the sliding surfaces of the arm parts 6, 7 with the coupling sleeves. It is held suspended in the air with the tips of the claws 2 and 3 facing in the direction of gravity (downward in the figure), and gas nitrocarburizing treatment is performed in this state. The shape of the shift fork itself has a large effect on the distortion of the shift fork pawl. In the case of a shift fork having the shape shown in FIG. 5, that is, the shape of the ribs 11a and 11b being symmetrical with respect to the axis 12 of the arm part, the distortion due to gas soft nitriding treatment is small; As shown in Table 1, if the heights of the front, rear, left, and right ribs (portions 11c, 11d, and 11e in the figure) are asymmetrical with respect to the axis 12 of the arm, The amount of change due to soft nitriding treatment becomes considerably large. The values shown in Table 1 are obtained by measuring the heights y 1 and y ₂ from the center of the slotted pin hole 13 of the gear shift fork to the claw at the tip of each arm before and after gas nitrocarburizing treatment, as shown in Figures ₁ and 6. The amount of dimensional change is indicated by + and - signs depending on the direction of movement.

[Table] Therefore, as for the shift fork shape, the height of the left and right ribs should be symmetrical, and at the same time, the axis 12 of the arm part should be symmetrical.
However, it is necessary to make the height of the front and rear ribs the same. Next, the method of the present invention will be explained using examples. Example 1 Gear shift forks having the shape shown in FIG. 1 were manufactured using the materials shown in Table 2 below, and each was subjected to the surface treatment method shown in the table to obtain gear shift fork samples of the present invention and comparison. obtain.

[Table] Each shift fork sample was assembled into a transmission and a wear test was conducted. The test conditions are as follows. Sleeve rotation speed 800r.pm Synchro load 6-7Kg (on lever) Synchro durability 1st←→2nd Load cycle 0.5-0.6 seconds: 250000 times Oil temperature 55-60℃ (SAE #90 gear oil) Gear shift fork conducted under the above conditions Figure 7 shows the results of measuring the amount of wear on the sliding surface of the shift fork obtained in the wear test. Example 2 Using the gear shift fork samples manufactured in Example 1 above, they were installed in the same manner as in the operation of an actual transmission, and the sleeve was rotated to prevent wear by adding the same amount of oil as in the actual transmission. I conducted a test. The test conditions are as follows. Sleeve rotation speed 1600r.pm Synchro load 8.5℃ or 9.1Kg (on lever) Load cycle 0.9~1.2 seconds; 100,000 times Oil temperature 65~70℃ (SAE #90 gear oil) Test results for each sample obtained under the above conditions , the eighth
As shown in the figure. In the figure, the solid line represents the relationship between the shift rotation speed and the amount of wear for each shift fork when the synchro load is 9.1Kg, and the broken line represents the relationship between the shift rotation speed and the amount of wear for each shift fork when the synchro load is 8.5Kg. . Example 3 Using the sample, it was attached to an actual transmission in the same manner as in Example 2, and a wear test was conducted under the test conditions shown below. Sleeve rotation speed 4000r.pm Synchro load 5.0Kg (on lever) Load cycle 5.6~6.0 seconds; 10000 times Oil temperature 90~95℃ (SAE#90 gear oil) The results obtained from the above test are shown in Figure 9. . As is clear from Examples 1, 2, and 3 above, the shift fork surface-treated by the method of the present invention has significantly better wear resistance and durability than conventional shift forks. I understand. Therefore, according to the method of the present invention, a good shift fork with significantly improved initial conformability and wear resistance can be obtained by simple heat treatment, and dimensional accuracy can be improved by keeping the distortion generated during heat treatment as low as possible. You can get a high shift fork.

[Brief explanation of drawings]

Figure 1a shows a top view of the gear shift fork, and Figure 1b shows a side view. Figure 2 is a graph showing the relationship between nitride layer thickness and Si content during gas nitrocarburizing of cast iron. , a graph showing the relationship between temperature and internal hardness in cast iron gas nitrocarburizing treatment, FIG.
Graphs showing the effects of rapid cooling and slow cooling on the perpendicularity of the shift fork pawl after gas nitrocarburizing treatment of cast iron, Figures 5a and b show the posture of the shift fork when performing the gas nitrocarburizing treatment of the present invention. Top view and side view, Figure 6a is a top view of a shift fork with asymmetric ribs, Figure 6b is a side view, Figure 7 is a graph showing the results of the synchro durability test in Example 1, Figure 8 9 represents a graph showing the results of the synchro durability test in Example 2, and FIG. 9 represents a graph showing the results of the synchro durability test in Example 3. In the figure, 1... shift fork, 2, 3... claw part, 4, 5... shaft hole, 6, 7... arm part, 8, 9, 10... support rod, 11... rib part.

Claims (1)

[Claims] 1 Carbon (C) 3.2-4.0%, manganese (Mn) by weight
Made of spheroidal graphite cast iron consisting of 0.2 to 0.7%, phosphorus (P) 0.01 to 0.04%, sulfur (S) 0.01 to 0.02%, silicon (Si) 2.8% or less, and the balance iron (Fe),
Holding a gear shift fork whose shape is as symmetrical as possible in the longitudinal and horizontal directions with respect to the fork shaft hole provided in the approximate center of the main body, the claws provided at the ends of the two arm parts of the gear shift fork are held in a position where each of them faces the direction of gravity, The thickness of the gear shift fork sliding surface is increased by heat treatment at a temperature range of 500°C to 650°C for a predetermined period of time in a mixed gas atmosphere consisting of ammonia gas and nitrogen gas, followed by slow cooling.
A method for gas nitrocarburizing treatment of a gear shift fork, characterized by forming an iron-nitrogen compound layer of 8μ or more.