MANUFACTURING METHOD OF TRIVALENT CHROMIUM PLATED PART

Abstract

A manufacturing method of a trivalent chromium plated part in which a trivalent chromium plating film is formed on a sliding part of a device that seals fluid, the method includes: a step of forming the trivalent chromium plating film on a base material or on a base plating firm formed on a base material surface; a step of applying an external force to the trivalent chromium plating firm; and a step of applying a baking treatment to the trivalent chromium plating firm after the external force is applied.

Description

TECHNICAL FIELD

The present invention relates to a manufacturing method of a trivalent chromium plated part.

BACKGROUND ART

In recent years, the use of a trivalent chromium plating bath containing trivalent chromium, which has a lower environmental impact compared with hexavalent chromium, has been considered for chromium plating having high hardness and excellent wear resistance (for example, see JP 2022-3171 A).

SUMMARY OF INVENTION

At the time of film formation, trivalent chromium plating predominantly has an amorphous structure, and it tends to be brittle, and thus, cracks with wide widths are likely to be formed. In addition, in general, after a trivalent chromium plating film is formed on a base material, a baking treatment is performed to prevent occurrence of hydrogen embrittlement. However, the baking treatment can increase groove width of the cracks.

In a case in which the trivalent chromium plating film is formed on a sliding part of a device that seals fluid, if the groove width of the cracks in the trivalent chromium plating film is large, the fluid can leak outside through the cracks.

An object of the present invention is to suppress an increase in groove width of cracks in a trivalent chromium plating film.

According to one aspect of the present invention, a manufacturing method of a trivalent chromium plated part in which a trivalent chromium plating film is formed on a sliding part of a device that seals fluid, the method includes: a step of forming the trivalent chromium plating film on a base material or on a base plating film formed on a base material surface; a step of applying an external force to the trivalent chromium plating film; and a step of applying a baking treatment to the trivalent chromium plating film after the external force is applied.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a laser microscope photograph of a cross-section of a trivalent chromium plated part according to an embodiment of the present invention.

FIG. 2 shows laser microscope photographs of a surface of the trivalent chromium plating film according to a comparative example in which an external force has not been applied, and (a) shows the surface before a baking treatment, and (b) shows the surface after the baking treatment.

FIG. 3 shows laser microscope photographs of the surface of the trivalent chromium plating film to which the external force has been applied, and (a) shows the surface before the baking treatment, and (b) shows the surface after the baking treatment.

DESCRIPTION OF EMBODIMENTS

In the following, a manufacturing method of a trivalent chromium plated part according to an embodiment of the present invention will be described with reference to the drawings.

The trivalent chromium plated part is formed by forming a trivalent chromium plating film on a sliding part, which is a part of a device that seals fluid. The device that seals the fluid includes, for example, a fluid pressure cylinder that is used as an actuator, a shock absorber that is installed in a vehicle, and a front fork for motorcycles. The trivalent chromium plated part includes, for example, a cylinder tube or a piston rod that slides against the cylinder tube of the fluid pressure cylinder, a cylinder tube or a piston rod that slides against the cylinder tube of the shock absorber, and an outer tube or an inner tube that slides against the outer tube of the front fork.

In this embodiment, a case in which a hard trivalent chromium plating film having the plating thickness of, for example, about 10 to 20 μm is formed on a steel base material that is a base material of the piston rod and the inner tube will be described.

FIG. 1 is a laser microscope photograph of a cross-section of a trivalent chromium plated part 100. As shown in FIG. 1, a nickel plating film 2 is formed as a base plating film on a surface of a steel base material 1, and a trivalent chromium plating film 3 is formed on the surface of the nickel plating film 2.

The steel base material 1 is, for example, carbon steel, such as S15C, S20C, S25C, S30C, S35C, S40C, S45C, or the like, or chromium molybdenum steel, such as SCM430, SCM435, SCM440, or the like.

The nickel plating film 2 is formed for enhancing adhesion of the trivalent chromium plating film 3 to the steel base material 1 and for supplementing the corrosion resistance of the trivalent chromium plating film 3. The nickel plating film 2 is formed by a known method. Instead of the nickel plating film 2, other metal plating film such as copper plating film, etc. may also be employed as the base plating film. The base plating film is not an essential configuration, and the trivalent chromium plating film 3 may be formed on the surface of the steel base material 1 directly.

The trivalent chromium plating film 3 has an excellent sliding property and corrosion resistance, and by forming the trivalent chromium plating film 3 on the surface of the sliding part, the quality of the sliding part is improved. At the time of the film formation, the trivalent chromium plating predominantly has an amorphous structure and tends to be brittle, and so, cracks 4 having wide widths are likely to be formed in the trivalent chromium plating film 3. After the trivalent chromium plating film 3 is formed, although a baking treatment is applied in order to prevent occurrence of hydrogen embrittlement by removing hydrogen absorbed in the trivalent chromium plating film 3, as will be described later in detail, the groove width of the crack 4 is increased by the baking treatment. Because the surface of the trivalent chromium plating film 3 functions as a sliding surface, if the groove width D of the crack 4 is large, the fluid leaks out through the crack 4. Specifically, when the trivalent chromium plated part 100 is the piston rod of the fluid pressure cylinder, oil (the fluid) sealed in the cylinder tube leaks to the outside of the cylinder tube through the crack 4 in the surface of the piston rod. Especially, when the pressure of the fluid (working oil or working gas) inside the device is high as in the case of the fluid pressure cylinder, there is a high risk that the fluid leaks to the outside through the crack 4. The present invention has been made in order to prevent the fluid sealed in the device from leaking out through the crack 4 in the surface of the trivalent chromium plating film 3.

In the following, a manufacturing method of the trivalent chromium plated part 100 according to this embodiment, specifically, a method of forming the trivalent chromium plating film 3 will be described in detail.

First, before forming the trivalent chromium plating film 3 on the nickel plating film 2 formed on the surface of the steel base material 1, a pretreatment for cleaning the surface of the nickel plating film 2 is performed. The pretreatment includes, for example, a degreasing treatment for removing organic substances from the surface of the nickel plating film 2, and an acid cleaning treatment for removing oxides from the surface of the nickel plating film 2. When the nickel plating film 2 is not formed, the pretreatment for cleaning the surface of the steel base material 1 is performed.

Next, the trivalent chromium plating film 3 is formed on the surface of the nickel plating film 2. As the method of forming the trivalent chromium plating film 3, for example, a plating treatment is performed to a plating thickness of about 10 to 20 μm under plating conditions of a current density of 30 to 90 A/dm² in a trivalent chromium plating bath containing chromium chloride or chromium sulfate as a chromium source, formate or malate as a complexing agent, boric acid as a pH buffering agent, ammonium chloride, potassium nitrate, or sodium sulfate as a conductive agent, a trace amount of surfactant, and so forth.

Next, an external force is applied to the trivalent chromium plating film 3. The external force is applied by cold working. In this embodiment, by pressing an outer circumferential surface of a rotating roller against the surface of the trivalent chromium plating film 3 at room temperature, the external force is applied to the surface of the trivalent chromium plating film 3 to cause minute plastic deformation. As a result, the number of cracks 4 in the surface of the trivalent chromium plating film 3 is increased.

As a method of the cold working to apply the external force to the trivalent chromium plating film 3, in addition to the method using the roller, it may be possible to employ a shot peening in which a plurality of hard balls are caused to collide with the surface of the trivalent chromium plating film 3 at high speed to apply minute plastic deformation by applying the external force to the surface of the trivalent chromium plating film 3.

When the external force is to be applied to the trivalent chromium plating film 3, if heat is applied to the trivalent chromium plating film 3, the groove width of the crack 4 is increased. Therefore, the method of applying the external force by the cold working is preferable because the number of cracks 4 can be increased without increasing the groove width of the crack 4.

After the external force is applied to the trivalent chromium plating film 3, the baking treatment is performed on the trivalent chromium plating film 3. The baking treatment is performed for preventing occurrence of hydrogen embrittlement by removing hydrogen absorbed in the trivalent chromium plating film 3, and the heating is performed under the condition of about 200° C. for about 2 hours.

As described above, although the groove width of the crack 4 is increased by the baking treatment, by applying the external force to the trivalent chromium plating film 3 before performing the baking treatment, it is possible to suppress the increase in the groove width of the crack 4. This point will be described in detail with reference to FIGS. 2 and 3.

FIG. 2 shows laser microscope photographs of the surface of the e trivalent chromium plating film 3 according to a comparative example in which the external force has not been applied, and (a) shows the surface before the baking treatment and (b) shows the surface after the baking treatment. FIG. 3 shows laser microscope photographs of the surface of the trivalent chromium plating film 3 to which the external force has been applied, and (a) shows the surface before the baking treatment and (b) shows the surface after the baking treatment. In FIGS. 2 and 3, black lines represent the cracks 4.

It is known that the trivalent chromium plating film 3 before the baking treatment has an amorphous structure, and by applying the baking treatment, the amorphous structure is crystallized. During the crystallization, the trivalent chromium plating film 3 undergoes contraction, and so, the groove width of the crack 4 is increased. As can be seen from FIG. 2(a), when the external force is not applied, each region surrounded by the cracks 4 is large, and so, during crystallization caused by the baking treatment, an amount of contraction of each region is large, and the groove width of the crack 4 tends to be increased. As can be seen from the comparison between FIG. 2(a) and FIG. 2(b), the black lines expressing the cracks 4 are thicker in FIG. 2(b). On the other hand, as can be seen from FIG. 3(a), by applying the external force, the number of cracks 4 in the surface of the trivalent chromium plating film 3 is increased, and each region surrounded by the cracks 4 is smaller, and so, during the crystallization caused by the baking treatment, the amount of contraction of each region is smaller, and the increase in the groove width of the crack 4 is suppressed.

Thus, when the groove width of the crack 4 after the baking treatment is compared between the case in which the external force has not been applied (see FIG. 2(b)) and the case in which the external force has been applied (see FIG. 3(b)), the groove width of the crack 4 is smaller for the case in which the external force has been applied (see FIG. 3(b)) (the black lines expressing the cracks 4 are thinner). When the dimension of the groove width of the crack 4 was actually measured, the groove width of the crack 4 for the case in which the external force was applied (see FIG. 3(b)) was about two-thirds of the groove width of the crack 4 for the case in which the external force was not applied (see FIG. 2(b)).

After the baking treatment, surface finishing of the trivalent chromium plating film 3 is performed by a mechanical treatment. The mechanical treatment includes, for example, grinding processing and polishing processing. By applying the mechanical treatment on the surface of the trivalent chromium plating film 3, the surface of the trivalent chromium plating film 3 undergoes plastic flow, thereby closing the cracks 4. Because the increase in the groove width of the crack 4 after the baking treatment is suppressed by the application of the external force, the cracks 4 are easily closed by the surface finishing. As a result, it is possible to prevent the fluid inside the device from leaking outside through the cracks 4, and the corrosion resistance of the trivalent chromium plating film 3 is improved. In contrast, when the external force is not applied, the increase in the groove width of the crack 4 after the baking treatment cannot be suppressed, and so, it is not possible to completely close the cracks 4 by the surface finishing. As a result, there is a risk in that the fluid leaks out through the crack 4.

As described above, the trivalent chromium plating film 3 is formed, and the trivalent chromium plated part 100 is manufactured.

According to the embodiment mentioned above, the advantages described below are afforded.

By applying the baking treatment after the application of the external force to the trivalent chromium plating film 3, it is possible to suppress the increase in the groove width of the crack 4 due to the baking treatment. Thus, it is possible to prevent the leakage of the fluid to the outside through the crack 4, and the quality of the trivalent chromium plated part 100 is improved.

In the following, modifications of this embodiment will be described.

Instead of the method described in the above-mentioned embodiment, following three method may also be employed as the method for applying the external force to the trivalent chromium plating film 3.

(1) The surface of the trivalent chromium plating film 3 may be irradiated with laser light, which is light energy, to apply the external force to the surface of the trivalent chromium plating film 3 by an impact pressure of plasma generated on the surface.

(2) The external force may be applied to the surface of the trivalent chromium plating film 3 by immersing the trivalent chromium plated part 100 in a liquid and generating ultrasonic waves in the liquid.

(3) The external force may be applied to the trivalent chromium plated part 100 as positive and negative thermal energy by heating and rapidly cooling the trivalent chromium plated part 100. This causes stress inside the trivalent chromium plating film 3, and the number of cracks 4 is increased. In this method, the difference in thermal expansion coefficient between the steel base material 1 and the trivalent chromium plating film 3 also leads to an increase in the number of cracks 4.

In the following, the configurations, operations, and effects of the respective embodiments of the present invention will be collectively described.

The manufacturing method of the trivalent chromium plated part 100 in which the trivalent chromium plating film 3 is formed on the sliding part of the device that seals the fluid includes: a step of forming the trivalent chromium plating film 3 on the steel base material 1 (the base material) or on the nickel plating film 2 (the base plating film) formed on the surface of the steel base material 1 (the base material); a step of applying the external force to the trivalent chromium plating film 3; and a step of applying the baking treatment to the trivalent chromium plating film 3 after the external force is applied.

With this configuration, by applying the baking treatment after the external force is applied to the trivalent chromium plating film 3, it is possible to suppress the increase in the groove width of the crack 4 by the baking treatment.

In addition, the manufacturing method of the trivalent chromium plated part 100 further includes a step of performing the surface finishing on the trivalent chromium plating film 3 by the mechanical treatment after the baking treatment.

With this configuration, because the increase in the groove width of the crack 4 after the baking treatment is suppress by applying the external force to the trivalent chromium plating film 3, it is possible to easily close the cracks 4 by the surface finishing.

In addition, the external force is applied by the cold working.

With this configuration, by applying the external force by the cold working, it is possible to increase the number of cracks 4 without increasing the groove width of the crack 4.

In addition, the trivalent chromium plated part 100 is the cylinder tube or the piston rod of the fluid pressure cylinder, the cylinder tube or the piston rod of the shock absorber, or the outer tube or the inner tube of the front fork.

With this configuration, although the pressure of the fluid inside the fluid pressure cylinder, the shock absorber, and the front fork is high, it is possible to prevent leakage of the fluid to the outside through the cracks 4 in the surface of the trivalent chromium plating film 3 that is the sliding surface.

Embodiments of this invention were described above, but the above embodiments are merely examples of applications of this invention, and the technical scope of this invention is not limited to the specific constitutions of the above embodiments.

This application claims priority based on Japanese Patent Application No. 2022-52092 filed with the Japan Patent Office on Mar. 28, 2022, the entire contents of which are incorporated into this specification.

Claims

1. A manufacturing method of a trivalent chromium plated part in which a trivalent chromium plating film is formed on a sliding part of a device that seals fluid, the method comprising:

a step of forming the trivalent chromium plating film on a base material or on a base plating film formed on a base material surface;

a step of applying an external force to the trivalent chromium plating film; and

a step of applying a baking treatment to the trivalent chromium plating film after the external force is applied.

2. The manufacturing method of the trivalent chromium plated part according to claim 1, further comprising

a step of performing a surface finishing on the trivalent chromium plating film by a mechanical treatment after the baking treatment.

3. The manufacturing method of the trivalent chromium plated part according to claim 1, wherein

the external force is applied by cold working.

4. The manufacturing method of the trivalent chromium plated part according to claim 1, wherein

the trivalent chromium plated part is a cylinder tube or a piston rod of a fluid pressure cylinder, a cylinder tube or a piston rod of a shock absorber, or an outer tube or an inner tube of a front fork.

5. The manufacturing method of the trivalent chromium plated part according to claim 1, wherein

number of cracks in a surface of the trivalent chromium plating film is increased by the step of applying the external force to the trivalent chromium plating film.