JP2929680B2 - Coil spring and manufacturing method thereof - Google Patents

Description

Description: BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a carbide precipitation hardening type Co alloy coil spring having excellent heat resistance, corrosion resistance, and wear resistance, and a method for producing the same.

[Conventional technology]

Conventionally, Fe-based, Ni-based, and Co-based superalloys are known as alloys having excellent heat resistance and corrosion resistance regardless of the use. Of these, Fe-based and Ni-based alloys are being developed as Fe-Cr-Ni alloys and Ni-Cr alloys with additional elements, etc. Has also been obtained.

On the other hand, Co alloys, which are said to be the most excellent in heat resistance and corrosion resistance among the above three systems, have been developed mainly by adding Cr, W and C.

[Problems to be solved by the invention]

However, although the Co alloy has the above-mentioned excellent properties,
Since it is a hard material, it is difficult to obtain a wire by plastic working such as drawing, and it is also difficult to obtain a coil spring. Further, even when a wire is obtained, precipitation of carbide, which is a key to improving the heat resistance of the Co alloy, causes a decrease in fatigue characteristics (there is a possibility that the carbide may be a starting point and break). Furthermore, since the material strength itself is low (Vickers hardness (Hv) is generally 400 to 500), there is a problem that the fatigue strength is inferior.

[Means for solving the problem]

Considering that the above-mentioned Co alloy is a hard material, it is difficult to draw, and in ordinary molten cast materials, carbide precipitates along the dendrite, and because it is very large, , Cracks during processing along the matrix and the precipitate. To improve this, carbide is reduced to 7 μm or less by a method such as solidifying the ingot at a cooling rate of 5 × 10 ° C./sec or more during casting or solidifying powder solidified at a cooling rate of 5 × 10 ° C./sec or more. It has been found that wire processing becomes possible if it is set to.

Considering the improvement of the fatigue strength of the Co alloy wire, the material before coiling is subjected to a processing of 10 to 20% at a reduction in area after annealing (1150 ° C.). When this material is coiled into the desired shape and then subjected to aging at 550 to 650 ° C x (30 to 120 minutes), the coil has a wire surface hardness of 550 or more and a surface hardness of Hv = 50 or more higher than the center. It turns out that a spring is obtained.

However, in the above-described coil spring, the starting point of fatigue may be the boundary between the carbide and the matrix, and it cannot be said that the coil spring is always excellent in characteristics.

This is because the material obtained by the conventional casting method has a large carbide exceeding 5 μm, for example, about 7 μm.
When the size is not more than m, the fracture starts from the inner surface of the coil spring and the fatigue characteristics are improved. In order to suppress the size of the carbide to 5 μm or less, an ingot with a cooling rate of 1 × 10 ² ° C./sec or more or a solidified powder may be used.

In this way, in order to process Co-based alloys into those that require excellent fatigue properties such as coil springs, in addition to the original characteristics of heat resistance, corrosion resistance, wear resistance, etc. It is important to suppress the growth, at the stage of casting the Co-based alloy or cooling the Co alloy powder, the cooling rate is set to 1 × 10 ² ° C / sec or more, and the carbide is suppressed to a size of 5 μm or less, This realizes wire drawing and coiling of the Co-based alloy.

Thus, the present invention has been made to solve the above problems, and provides a coil spring of a carbide precipitation hardening type Co alloy excellent in heat resistance, corrosion resistance, wear resistance and fatigue characteristics, and a method for manufacturing the same. is there.

That is, in the coil spring of the present invention, the component is 26.0 ≦ Cr in weight%.
≦ 33.0, 3.0 ≦ W ≦ 13.0, 0.2 ≦ C ≦ 0.6, Fe + Ni ≦ 6.0, the balance consisting essentially of Co, with carbide size of 5 μm
The hardness of the wire surface is less than or equal to 550 in Vickers hardness, and the hardness of the wire surface is 50 or more in the hardness than the center of the wire.
It is characterized by being higher.

Further, the method of manufacturing the coil spring is characterized in that the alloy lump having the above-mentioned composition is solidified at a cooling rate of 1 × 10 ² ° C./sec or more at the time of casting, or the powder having the same composition is made 1 × 10 ² ° C./sec.
The solidified material at a cooling speed of at least sec is processed by hot working or cold working while performing annealing to a desired wire diameter as necessary, and in the final step, a cold reduction of 10 to 20% is achieved. After performing the drawing process and coiling the drawn material, 55
Aging at 0 to 650 ° C for 30 to 120 minutes.

The Co alloy used in the coil spring of the present invention has the above-described composition. First, if Cr is less than 26.0%, the corrosion resistance and heat resistance are inferior, and if it exceeds 33.0%, it becomes too hard to work. Next, if W is less than 30%, the heat resistance is inferior. If C is less than 0.2%, the strength is reduced, and if it exceeds 0.6%, coiling becomes difficult. The content of Fe + Ni as an impurity is preferably 6% or less from the viewpoint of corrosion resistance. However, trying to reduce it will lead to higher costs.

(Test example)

By the way, an alloy ingot or a powder composed of such a composition is processed into a wire rod.Here, as a test example, an ingot is made by melting casting or powder extrusion method while changing a cooling rate, and each is drawn to 2 mmφ. (Wire material processing test). Table 1 shows the results.

From the above, carbide was set to 7 μm
It was confirmed that the size was as follows, and that a cooling rate of at least 5 × 10 ° C./sec was required.

Next, the surface reduction rate of 5,1
A wire drawing of 0, 15, 20, and 25% was performed, and coiling was performed to check the state of breakage (coiling test). The coil spring obtained by the above coiling has a wire diameter of 2 mm, a coil center diameter of 20 mm, a free length of 50 mm, and an effective number of turns of 3.5. Table 2 shows the test results.

From the above results, it was found that wire drawing with a reduction in area from 5% to 20% was suitable.

Next, with respect to the coil spring of the powder extruded 7 which had been subjected to the wire drawing with different area reduction rates, the cross-sectional hardness distribution was measured (hardness-wire drawing test). The aging temperature in this case is 600
° C and the aging time was 1 hour. The results are shown in the graph of FIG. As can be seen from the graph, the hardness decreases from the surface toward the center, and the hardness decreases as the coil spring has a lower area reduction rate.

Next, the relationship between the cross-sectional hardness distribution of the coil spring and the aging temperature was examined (hardness-aging temperature test). The coil spring used in this test was obtained by coiling a wire rod that had been subjected to wire drawing with a surface reduction rate of 10% on the powder extrusion 7, and had an aging time of 1 hour. The results are shown in the graph of FIG. As shown in the graph, it was confirmed that the hardness decreased from the surface toward the center, and that the hardness was particularly low at 500 ° C. and 700 ° C.

The relationship between the cross-sectional hardness distribution of the coil spring and the aging time was also examined (hardness-aging time test). The coil spring used here is also the same as the hardness-aging temperature test,
The aging temperature was 600 degrees. The results are shown in the graph of FIG. As shown in the graph, the hardness decreases from the surface toward the center as in FIG.
At 180 minutes, the surface hardness was found to be lower than Hv = 550.

In addition, tests were also conducted on fatigue characteristics, which are important characteristics of coil springs. The coil spring used had a powder extrusion of 7, a wire diameter of 2.0 mm, a coil center diameter of 20 mm, a free length of 40 mm, and an effective number of turns of 3.5. First, coil springs having the same hardness value were classified according to the size of carbide, and a test was performed with an average tightening force of 40 kg / mm ² (fatigue test 1). The results are shown in the graph of FIG. As is clear from this graph, it was found that the coil spring containing carbide having a maximum size of 7 μm had poor fatigue characteristics.

Therefore, a test was performed on the set amount of the coil spring containing carbide having a maximum size of 5 μm (fatigue test 2).
The test method is to measure the amount of set after repeated tightening with a tightening force of 40 ± 25 kg / mm ² and a tightening frequency of 1 × 10 ₇ times. The coil spring used is powder extruded 7, 600 ° C. × 1 hr aging material It is. The results are shown in the graph of FIG. As can be seen from this graph, it was confirmed that the surface hardness was at least Hv = 550 and the sag was small.

Finally, a test for corrosion resistance was performed. This is 1%
A polarization test was performed in an H ₂ SO ₄ solution to determine voltage-current characteristics. The composition and results of the coil spring used in the test are shown in Table 3, and a graph of the voltage-current characteristics is shown in FIG.

Fe + Ni is 5% as shown in Table 3 and the graph of FIG.
It was found that the corrosion resistance was inferior when the super content was less than 26.0%. In addition, I examined the amount of W and C by changing the amount.
There was no significant change in corrosion resistance in the range of 3.0% and C in the range of 0.2 to 0.6%. X shown in the graph of FIG. 6 is Fe-Cr-Ni
Alloy (SUS304), Y is Ni-Cr-Fe alloy (Inconel 71
8), which indicates that the developed material of the present invention has excellent corrosion resistance.

As a result of the various tests and examinations described above, the size of the carbide in the wire must be 5 μm or less from the fatigue test 1, and from this and the results of the wire processing test, it is possible to use any of the melting casting and powder extrusion methods. Cooling rate 1 × 10 ² ℃ / sec
And must be. Moreover, the surface hardness is higher than the fatigue test 2
It is necessary that Hv = 600 or more. From this result and the result of the hardness-aging temperature test, it was found that the aging temperature was optimally 550 ° C. In addition, comparison with the results of the hardness-aging time test confirmed that the optimum aging time was 30 to 180 minutes. Furthermore, in the wire drawing performed in the final step, good results have been obtained even in the case of a surface reduction rate of 5% in the coiling test. On the other hand, in the hardness-drawing test, the surface hardness of the wire having the area reduction rate of 5% is significantly lower than Hv = 550. From the above, it can be said that it is optimal to perform the wire drawing with the area reduction rate of 10 to 20%.

〔The invention's effect〕

As described above, the coil spring of the present invention is more excellent in corrosion resistance than Fe-based and Ni-based alloys, which are conventionally considered to be excellent in corrosion resistance, and can be effectively used in a corrosive environment such as seawater and acid. Can be planned.

In addition, since it also has heat resistance and abrasion resistance as other excellent characteristics, it can be used in an environment where these plural characteristics are required.

[Brief description of the drawings]

FIG. 1 is a graph showing the surface hardness of wires processed with different surface reduction rates, FIG. 2 is a graph showing the relationship between aging temperature and sectional hardness, and FIG. 3 is aging time and sectional hardness. FIGS. 4 and 5 are graphs showing the results of a fatigue test on coils having different surface hardnesses, and FIGS.
The figure is a graph showing voltage-current characteristics by a polarization test.

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-64-31594 (JP, A) JP-A-63-84791 (JP, A) JP-B-61-46529 (JP, B2) (58) Field (Int.Cl. ⁶ , DB name) C22C 19/07 B B21F 35/00 A C22F 1/10 J

Claims (2)

(57) [Claims] (1) The composition contains 26.0 ≦ Cr ≦ 33.0, 3.0 ≦ W ≦
13.0, 0.2 ≦ C ≦ 0.6, Fe + Ni ≦ 6.0, with the balance being essentially Co, the size of carbide is 5 μm or less, the hardness of the wire surface is 550 or more in Vickers hardness, and it is higher than the center of the wire. A coil spring, wherein the hardness of the wire surface is higher than the hardness by 50 or more. 2. The composition in which the components are in weight% 26.0 ≦ Cr ≦ 33.0,3.0 ≦ W ≦
13.0, 0.2 ≦ C ≦ 0.6, Fe + Ni ≦ 6.0, alloy solid consisting of the balance substantially Co was solidified at the cooling rate of 1 × 10 ² ° C./sec or more during casting, or powder composed of the above components was 1 × 10 ² The material solidified at a cooling rate of at least ℃ / sec is processed by performing hot working or cold working while performing annealing to a desired wire diameter as required, and in the final step, a surface reduction rate of 10 to 20%. After performing cold drawing and coiling the drawn material, 550-650 ° C
Aging for 30 to 120 minutes.