KR100217261B1 - The spring with excellent nitrization property and the method thereof - Google Patents

Abstract

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a spring that requires excellent fatigue characteristics, such as an engine valve spring, and a method of manufacturing the same, wherein an unevenness in surface hardness and depth of hardening does not occur when nitriding the spring. would, in prior nitriding in that configuration, the electrolytic polishing, the thickness of the oxide film surface of the spring or the like below the 1.5㎛, haedunda and the residual stress of the spring below the surface -5kgf / mm ² or more 5kgf / mm ^2, This makes it possible to obtain a spring having a high surface hardness after the nitriding treatment and a deep nitriding side depth.

Description

Spring excellent in nitriding properties and its manufacturing method

The present invention relates to a spring and a method for producing the same, which require excellent fatigue characteristics such as an engine valve spring.

The spring steel wire which has been quenched and tempered has an oxide film having a thickness of 2 to 5 µm in order to improve lubrication between the spring steel wire and the coiling tool during spring forming.

In the case of performing nitriding treatment on a spring formed of such spring steel wire, spring forming, low temperature annealing, descaling, and nitriding treatment are performed sequentially.

Here, the low temperature annealing is to remove the residual stress of the spring generated by the spring forming. In addition, descale is a process for effectively removing the oxide film and later performing nitriding treatment, and usually shot blasting is performed.

However, the spring which performed the shot blast had a problem that there exist nonuniformity in the hardness and hardened layer depth after nitriding treatment.

MEANS TO SOLVE THE PROBLEM As a result of performing various examination in order to solve said subject, the present inventors obtained the following knowledge.

① When nitriding, hardening is hindered if residual stress exists on the spring surface.

(2) Residual stress occurs on the surface of the spring due to the short blast for descaling, and the nonuniformity causes the nonuniformity of hardness after hardening and depth of hardened layer.

For this reason, in order to carry out an efficient nitriding treatment with less unevenness, it is considered important to remove the oxide film and to reduce the residual stress as much as possible, and thus, to achieve the present invention.

That is, the gist of the present invention, in prior to nitriding, the thickness of the oxide film surface of the spring below the 1.5㎛, and may I haedu the residual stress of the spring below the surface -5kgf / mm ² or more 5kgf / mm ^2.

The following method is mentioned as a means of obtaining such a characteristic.

① Spring-formed steel wire is spring-formed, held at 400-450 ° C for 10-40 minutes to perform low temperature annealing, and then the oxide film is removed to below 1.5㎛ by chemical and / or electrical means.

(2) After forming the spring steel wire for 10-40 minutes at 400-450 ° C for low temperature annealing, remove the oxide film to a thickness of 1.5 µm or less by mechanical means, and then remove it in an inert gas atmosphere or under vacuum. Low temperature annealing is performed by holding at -450 degreeC for 10 to 40 minutes.

(3) After removing the oxide film of the spring steel wire to a thickness of 1.5 µm or less, performing spring molding, and performing low temperature annealing at 400 to 450 ° C for 10 to 40 minutes in an inert gas atmosphere or vacuum.

After these steps, the nitriding treatment may be performed.

The reason for limiting the above conditions is described below.

(Oxide film: 1.5㎛ or less)

This is because if the thickness exceeds 1.5 µm, the diffusion of nitrogen in the nitriding treatment is prevented by the oxide film. It is preferable that there is no oxide film.

(Residual stress: -5kgf / mm ² or more, 5kgf / mm ² or less)

This is because out of this range results in slow diffusion of nitrogen, which makes it impossible to perform an efficient nitriding treatment.

[Means for removing oxide film]

The oxide film must be removed because it interferes with the nitriding treatment. However, if the oxide film is removed by a conventional shot blast, residual stresses are generated, which lowers the nitriding treatment efficiency. Therefore, it is necessary to remove the oxide film by a method in which no residual stress is issued or to remove the oxide film by removing the oxide film by a method in which residual stress is generated. As a method in which residual stress does not occur, there are chemical means such as pickling and electrical means such as electropolishing, and these may be performed alone or in combination.

When the oxide film is removed by a method of generating residual stress, for example, a shot blast, it is necessary to perform low temperature annealing to remove the generated residual stress. At this time, annealing needs to be performed in an inert gas atmosphere such as vacuum or Ar so as not to generate an oxide film again.

All of these means are methods for removing the oxide film after performing spring molding, but the oxide film may be removed before spring molding. In this case, the means for removing the oxide film is not particularly limited, and residual stress may be generated. After the spring molding, the residual stress is removed by low temperature annealing in an inert gas atmosphere or vacuum to prevent the oxide film from forming again.

Hereinafter, embodiments of the present invention will be described.

Example 1

Oil-tempered wire with a diameter of 4 mm is spring-formed, and springs with different thicknesses and residual stresses are produced by removing the oxide film on the surface and by low temperature annealing. Nitriding treatment was performed for 4 hours.

And for each spring, the hardness of the depth of 20 micrometers was measured from the spring surface, and this was made into surface hardness. In addition, the nitriding treatment efficiency was evaluated by making the depth where the hardness becomes the same as the hardness of the line center portion as the nitride layer depth. The results are shown in Table 1. The higher the surface hardness and the deeper the nitride layer, the better the nitriding treatment efficiency. In addition, all the hardness of the spring line center part after nitriding was Hv = about 470.

As shown in Table 1, it was confirmed that the thinner the oxide film and the smaller the residual stress, the higher the surface hardness, the deeper the nitride layer depth, and the Examples were excellent in the nitriding treatment efficiency.

Example 2

Next, three types of steel wires having different thicknesses of the oxide film as shown below are spring-formed, and each treatment shown in Table 2 is carried out, and the thickness of the oxide film and the residual stress are measured before nitriding. After the treatment, the surface hardness and the depth of the nitride layer were measured.

These results are shown in Table 2 and Table 3. In addition, low temperature annealing conditions are 20 minutes at 450 degreeC.

Steel wire for spring I: Anodized thickness = 0㎛

Steel wire II for spring: Oxide thickness = 1.1 µm

Steel wire for spring III: Anodized film thickness = 4.2m

As shown in Table 2 and Table 3, it can be seen that in each of the Examples, both the surface hardness and the depth of the nitride layer are performing better and more efficient nitriding treatment than the comparative example.

As described above, according to the method of the present invention, the nitriding treatment can be efficiently carried out by preventing residual stress from remaining in the spring before nitriding treatment as much as possible. Moreover, the surface hardness of a spring and the nonuniformity of the nitride layer depth can be reduced.

Claims (4)

In prior to the nitriding treatment, the thickness of the oxide film surface of the spring is not more than 1.5㎛, the residual stress of the spring surface -5kgf / mm ² or more excellent spring characteristics, characterized in that the nitriding 5kgf / mm ² or less. After forming the spring steel wire, holding it at 400 to 450 ° C for 10 to 40 minutes for low temperature annealing, removing the oxide film to a thickness of 1.5 µm or less by chemical and / or electrical means, and performing nitriding treatment. A method for producing a spring having excellent nitriding characteristics. After forming the spring steel wire and holding it for 10 to 40 minutes at 400 to 450 ° C for low temperature annealing, the oxide film was removed to a thickness of 1.5 µm or less by mechanical means, and then 400 to 450 in an inert gas atmosphere or vacuum. A method for producing a spring excellent in nitriding characteristics, characterized in that it is subjected to low temperature annealing at 10 캜 for 40 minutes, followed by nitriding. After removing the oxide film of the spring steel wire to a thickness of 1.5 µm or less, performing spring molding, and performing a low temperature annealing at 400 to 450 ° C. for 10 to 40 minutes in an inert gas atmosphere or vacuum to carry out nitriding treatment. A method for producing a spring having excellent nitriding characteristics.