JPH0692631B2 - Titanium surface finishing method that does not easily scratch and has a long gloss life - Google Patents

Description

TECHNICAL FIELD The present invention relates to a surface hardening method for titanium, and more particularly to a surface finishing method for titanium, which is hard to be scratched, has excellent corrosion resistance, and has a long gloss life.

(Prior Art) Titanium, in particular pure titanium, has various advantages and characteristics such as lighter weight, superior corrosion resistance, and smaller coefficient of thermal expansion than stainless steel sheets, but it is relatively soft, but easily scratched. In addition, it also has drawbacks such as abrasion resistance and surface gloss which are inferior to other metal materials. Therefore, it has not been widely used, although it has various potential applications such as daily sundries, electrical products, accessories, and various mirror-finished members such as curved mirrors.

As one measure to improve this, various methods for hardening the surface of titanium have been studied and proposed. Nitrogen method, boride method, or diffusion method of coating metal is known as the surface hardening method of such titanium, but the former two require long-term heat treatment at high temperature to form a hardened layer. In addition, the hardened layer is as thin as several μm, and it has various drawbacks such that it is difficult to form it uniformly. The latter method of diffusing the coated metal is performed by coating titanium with a metal such as nickel, cobalt, tin, or chromium in advance by a method such as plating, and then heat-treating the coated metal to diffuse it into titanium. The surface is hardened, and this kind of surface hardening method is disclosed, for example, in JP-A-56-81665 or JP-A-56-81665.
There is a proposal based on the 58-91165 bulletin, etc. The conventional proposals for these diffusion methods are common in that the main factor of surface hardening is the intermetallic compound.

When the coating metal is nickel, such an intermetallic compound is
Usually, TiNi ₃ , TiNi, and Ti ₂ Ni are formed in this order from the outermost surface. These intermetallic compounds are formed in layers, that is, they have an interface, are brittle, and have different properties such as corrosion resistance. Also, the growth rate is slow, and for example, these compounds grow only a few μm at the maximum in diffusion annealing at 900 ° C. for about 1 hour,
To achieve the required thickness, heating at high temperature for a long time is required. Therefore, surface roughness and color tone disorder due to peeling at the interface of intermetallic compounds, deterioration of corrosion resistance, or partial appearance of different textures, uneven glossiness, and reflectance when used after mirror finishing are used. However, the conventional techniques include various problems in manufacturing and practical use.

(Problems to be Solved by the Invention) The present invention is intended to solve the various problems as described above in the conventional surface hardening method for titanium, is not easily scratched, does not peel off the hardened layer, and has a gloss. It is to establish a new surface finishing method that enables versatility of titanium, having features such as excellent, long gloss life, high quality homogeneity, and easy mass production.

(Means and Actions for Solving the Problems) The present inventors have conducted heat treatment conditions to solve these problems,
Examine the relationship between metal structure and corrosion resistance in detail,
From the standpoint of expecting the role of surface hardening in intermetallic compounds such as NiTi, a drastic solution to this problem cannot be expected, but rather the growth of intermetallic compounds is suppressed and the structure hardening of the β-Ti phase formed under this is hardened. The present invention has been achieved by finding that it is effective to achieve the above.

That is, the gist of the present invention is to coat the surface of titanium with nickel having a thickness of 3 to 30 μm, heat and hold at a temperature of 850 to 950 ° C. for 0.5 to 30 minutes, and then cool to 200 ° C. at a cooling rate of 40 ° C./sec or more. After cooling to the following temperature, the residual Ni on the surface layer and the intermetallic compound layer were removed, and a layer consisting of a β-Ti phase with a thickness of 20 to 150 μm was used as the outermost surface. It is a difficult and long-lasting titanium surface finishing method.

Hereinafter, the present invention will be described in detail.

The type of titanium used in the present invention and the metal structure thereof are pure titanium or titanium alloy, that is, α-phase or β.
It may be either titanium having a -phase or a mixed structure thereof. Further, as the shape and the previous history of heat treatment and the like, any of plate-like, rod-like or lump-like, cold-rolled material, hot / hot worked material, cast material, forged material and the like can be provided.

The nickel coating on these materials is carefully degreased with an organic solvent and, if necessary, immersed in a mixed aqueous solution of hydrofluoric acid or a hydrofluoric acid / hydrogen peroxide solution, and then electroplated or electroless plated. Although it may be applied by a thermal spraying method, a pressure bonding method, etc., the electroplating method is preferable from the viewpoints that the coating has good uniformity, the cured layer can be easily homogenized, and the heat treatment can be performed in the atmosphere. In this case, it is generally considered that plating on titanium is extremely difficult, but according to the results of various studies by the present inventors on the pretreatment method for plating, according to the results of about 10 minutes in a mixed aqueous solution of NaOH and H ₂ O _2. Heat-pickled (60
(~ 70 ℃) treatment is effective for improving plating adhesion and can be recommended as one measure.

The coating thickness of nickel differs depending on the desired thickness of the hardened layer, but in the case of the present invention, unlike the conventional method, it is not intended to be hardened by the intermetallic compound layer, so that it does not require a large amount of Ni. However, if the coating thickness is too thick, a large amount of Ni will remain in the outermost layer after heat treatment, and extra effort will be required to remove this, while if the coating thickness is too thin, it will be difficult to form a hardened layer. From such a viewpoint, in the present invention, the thickness of the nickel coating layer is 3 to 30 μm.

The heat treatment after nickel coating suppresses the formation of intermetallic compounds as much as possible, and the treatment conditions are selected from the viewpoint of developing and hardening the β-Ti phase immediately below the intermetallic compound layer. If the heating and holding temperature is less than 850 ° C, it takes time for Ni to diffuse into titanium, and the β transformation of the Ni diffusion layer may be difficult to occur, resulting in an insufficient thickness of β-Ti phase as a hardened layer. on the other hand,
On the contrary, when the heating temperature exceeds 950 ° C, Ni diffuses and penetrates too much into titanium and forms pro-eutectoid α phase along the grain boundary of the central layer, which deteriorates ductility.
The Ni concentration gradient also becomes gentle, making it difficult to form a high-quality hardened layer with high hardness on the surface layer. In the present invention, since the heating and holding time does not cause the intermetallic compound to be a factor for curing, it does not require a long heating time of several tens of hours as in the conventional method, but if the heating and holding time is less than 0.5 minutes, diffusion of Ni is unsatisfactory. If it is not sufficiently hardened, and if it exceeds 30 minutes, Ni diffuses and penetrates into titanium too much, which makes it easy to generate pro-eutectoid α phase, or the internal crystal grains become coarse and the ductility decreases. To do.

For this reason, in the present invention, the heating and holding temperature is 850 to 950 ° C. and the heating and holding time is 0.5 to 30 minutes.

Here, the present invention is at a cooling rate of 40 ° C / sec or more after heating and holding.
One of the features is that it is cooled to a temperature of 200 ° C or less.

The reason for this is mainly to promote the formation of a homogeneous β-Ti phase by suppressing the development of the acicular α phase formed immediately below the β-Ti phase. Cooling rate is less than 40 ℃ / sec, or
If the cooling end temperature is higher than 200 ° C, the needle-like α phase becomes β
-Si phase erodes and grows, the β-Ti phase becomes inhomogeneous, and the thickness of the β-Ti phase layer also decreases. It causes problems such as occurrence. Here, in the present invention, β-Ti
The thickness of the phase layer is 20 to 150 μm, but if the thickness is less than 20 μm, it is difficult to remove the intermetallic compound in the upper layer and leave only the hardened layer as the surface, and dust It is easy to get scratches due to, etc., and the wear resistance is not sufficient. On the other hand, if the thickness exceeds 150 μm, Ni diffuses too much into titanium and the crystal grains inside become coarse and tend to become brittle. . Here, the thickness of the β-Ti phase layer is 20 ~
Setting the thickness to 150 μm can be easily achieved by the above heat treatment conditions, and β formed in this way
-The hardness of the Ti phase has a Vickers hardness of 450 to 600 as a result of synergistic effects such as precipitation dispersion strengthening by ultrafine precipitates such as Ti ₂ Ni. When further hardening is required, it is well known as a means for hardening the β-Ti phase of 300 to 6
It is also possible to perform an aging treatment at a temperature of 00 ° C. Further, it is desirable to use a non-oxidizing gas such as helium gas for the atmosphere and the cooling during the heat treatment, but if the conditions under which the nickel plating layer remains after the heat treatment are selected, the atmosphere atmosphere and the water cooling can be performed.

After the heat treatment, the residual Ni layer and the intermetallic compound layer are removed by a method such as chemical polishing, electric polishing, or mechanical polishing such as buffing to expose the β-Ti phase on the surface. Adjust to the surface condition of before use.

Hereinafter, the effects of the present invention will be described in detail with reference to Examples.

〔Example〕

Pure titanium (JIS Class 1) cold-rolled sheet with a thickness of 0.8 mm is degreased with an organic solvent, and then plated by immersing it in a mixed aqueous solution containing 20 g / l of NaOH and 7 g / l of H ₂ O ₂ at 65 ° C for 10 minutes. Pretreatment is performed, then nickel plating treatment with a current density of ² A / dm ² by a known watt bath, heat treatment in an argon atmosphere,
It was cooled and mechanically polished so that the mirror-finished buffed surface had a specific structure. Table 1 shows the processing conditions for the various samples thus prepared.

The surface micro-Vickers hardness (load 20gr) is used to investigate the characteristics such as the surface scratch resistance and gloss deterioration of the sample.
The buffed as-polished sample and the 5 mm Erichsen bulge-treated sample were immersed in 5% saline solution at 50 ° C for 10 days to evaluate the surface condition such as gloss on a five-point scale. Good) Table 2 shows these results, including the overall score.

As a result, according to the method of the present invention, with a small amount of nickel plating and a short time heat treatment, it is possible to obtain a surface finish of titanium which is hard to be scratched, has excellent corrosion resistance and surface gloss, and is unlikely to cause gloss deterioration even in a corrosive environment. It is clear that it can be applied.

[Effects of the Invention] According to the method of the present invention, peeling of the hardened layer is unlikely to occur, surface homogeneity, wear resistance and corrosion resistance are excellent, and it is possible to easily perform a surface finish of titanium that is unlikely to cause gloss deterioration. , The advantages to the related industrial fields are extremely large.

Claims (1)

[Claims] 1. A titanium coating having a thickness of 3 to 30 μm is applied to the surface of titanium, which is heated and held at a temperature of 850 to 950 ° C. for 0.5 to 30 minutes and then cooled to a temperature of 40 ° C./sec or more and 200 ° C. or less. After cooling to a temperature, the residual Ni on the surface layer and the intermetallic compound layer are removed, and a layer of a β-Ti phase having a thickness of 20 to 150 μm is used as the outermost surface to prevent defects. Surface finishing method for titanium with long gloss life.