JPH02149652A - Method for surface finishing of titanium difficult to flow and having prolonged service life of gloss - Google Patents

Abstract

PURPOSE:To permit the surface finishing of titanium having excellent surface uniformity and corrosion resistance by coating titanium with nickel having specific thickness, holding it by heating at a specific temp. for specific time, thereafter cooling it at a specific speed, removing the surface part and regulating the layer of a beta-Ti phase having specific thickness to the utmost surface. CONSTITUTION:The surface of titanium is applied with nickel coating having 3 to 30mm thickness. The titanium is held by heating at 850 to 950 deg.C for 0.5 to 30min and is thereafter cooled to <=200 deg.C at >=40 deg.C/sec cooling speed. Then, residual Ni on the surface layer and an intermetallic compound layer are removed to regulate a layer constituted of a beta-Ti layer having 20 to 150mum thickness to the utmost surface. In this way, surface finishing of titanium having excellent surface uniformity and corrosion resistance can be executed.

Description

DETAILED DESCRIPTION OF THE INVENTION (Industrial Application Field) The present invention relates to a method for hardening the surface of titanium, and particularly to a method for finishing the surface of titanium, which is hard to scratch, has excellent corrosion resistance, and has a long gloss life.

(Prior art) Titanium, especially pure titanium, has various advantages and characteristics such as being lighter and having better corrosion resistance than stainless steel sheets, and has a lower coefficient of thermal expansion. However, on the other hand, it is relatively soft and is easily scratched. However, it also has disadvantages such as inferior abrasion resistance and surface gloss compared to other metal materials. Therefore, although it has potential for a variety of applications such as daily necessities, electrical appliances, accessories, and various mirror-finished members 2 such as curved mirrors, it has not been widely used.

As a way to improve this problem, various methods have been investigated to make the surface of titanium harder.

It has been proposed. There are three known methods for hardening the surface of titanium, including the nitriding method, the boriding method, and the coating metal diffusion method, but the former two require long-term heat treatment at high temperatures to form a hardened layer. However, the cured layer is also thin, on the order of several μm, and has various drawbacks, such as being difficult to form uniformly. The latter coating metal diffusion method involves coating titanium with metals such as nickel, cobalt, tin, or chromium in advance by plating or other methods, and then heat-treating the titanium to diffuse these coated metals into the titanium. This type of surface hardening method is, for example, disclosed in Japanese Patent Application Laid-Open No. 56-81665.
There are proposals in Japanese Patent Laid-Open No. 58-91165, etc. These conventional proposals for diffusion methods have in common that the main factor for surface hardening is intermetallic compounds.

These intermetallic compounds, when the coating metal is nickel,
Usually, TiN1x, T1Ni, and TiJi are formed in this order from the outermost surface. These intermetallic compounds are formed in layers, that is, have interfaces, are brittle, and have different properties such as corrosion resistance. The growth rate is also slow, for example, 1 at 900°C.
During diffusion annealing for a certain period of time, these compounds grow only a few μm at most, and heating at a high temperature for a long time is required to obtain the required thickness.

Therefore, peeling at the interface of intermetallic compounds, deterioration of corrosion resistance, disturbance of surface roughness and color tone due to partial appearance of different structures on the surface, uneven gloss, and reflectance when used with mirror finishing. The conventional technology includes various problems in manufacturing and practical aspects, such as a decrease in

(Problems to be Solved by the Invention) The present invention aims to solve the various problems mentioned above in the conventional titanium surface hardening method. The object of the present invention is to establish a new surface finishing method that enables versatile use of titanium, which has characteristics such as excellent quality, long gloss life, high uniformity of quality, and ease of mass production.

(Means and effects for solving the problems) In order to solve these problems, the present inventors investigated in detail the relationship among heat treatment conditions, metal structure, corrosion resistance, etc., and found that intermetallic compounds such as NiTi play a role in surface hardening. From the standpoint of expecting this, we can hope for a fundamental solution to this problem.In fact, we have learned that it is effective to suppress the growth of intermetallic compounds and to harden the structure of the β-Ti phase formed below. By obtaining this, the present invention was constructed.

That is, the gist of the present invention is to provide a nickel coating with a thickness of 3 to 30 μm on the surface of titanium, and to coat the surface of titanium with a nickel coating of 3 to 30 μm in thickness at a temperature of 850 to 950°C.
．． After heating and holding for 5 to 30 minutes, it was cooled to a temperature of 200°C or less at a cooling rate of 40°C/sec or more, and then the residual Ni on the surface layer and the intermetallic compound layer were removed, and the thickness was reduced to 20°C.
A method for finishing the surface of titanium, which is resistant to scratches and has a long luster life, is characterized by forming a layer consisting of a βTi phase with a thickness of ~150 μm on the outermost surface.

The present invention will be explained in detail below.

The type of titanium used in the present invention and its metal structure are pure titanium or alloyed titanium, in other words, α phase or β-phase.
It may be titanium having either a phase or a mixed structure of these. In addition, as for the shape and the history of heat treatment, etc., it is possible to provide a plate shape, a bar shape, a block shape, etc., a cold rolled material, a warm/hot processed material, a cast material, a forged material, etc.

Nickel coating on these materials is carried out by electroplating or electroless plating after careful degreasing using an organic solvent or the like and, if necessary, immersion in a nitric-fluoric acid mixed aqueous solution or a hydrofluoric acid-hydrogen peroxide solution. .

The coating may be applied by a thermal spraying method, a pressure bonding method, or the like, but an electroplating method is preferable because it provides good coating uniformity, makes it easy to homogenize the cured layer, and allows heat treatment in the atmosphere. In this case, it is generally considered that plating titanium is extremely difficult, but according to the results of various studies conducted by the present inventors regarding plating pretreatment methods, it was found that N a OH and H20
Heat immersion in the mixed aqueous solution of □ for around 10 minutes (60 to 70℃)
) treatment is effective in improving plating adhesion and can be recommended as one measure.

The coating thickness of nickel varies depending on the desired thickness of the hardened layer, but in the case of the present invention, unlike conventional methods, hardening by an intermetallic compound layer is not intended, so a large amount of Ni is not required. If the coating thickness is too thick, a large amount of Ni will remain in the outermost layer after heat treatment, requiring extra effort to remove it, while if the coating thickness is too thin, it will be difficult to form a hardened layer. From this point of view, in the present invention, the thickness of the nickel coating layer is set to 3 to 30 mm.

In the heat treatment after nickel coating, the treatment conditions are selected from the viewpoint of suppressing the formation of intermetallic compounds as much as possible and promoting the development and hardening of the β-Ti phase directly under the intermetallic compound layer.

If the heating holding temperature is less than 850°C, it takes time for Ni to diffuse into the titanium, and β-transformation of the Ni diffusion layer may also be difficult to occur, resulting in an insufficient thickness of the β-Ti phase that forms the hardened layer. On the other hand, if the heating temperature exceeds 950"C, Ni will diffuse and permeate into the titanium too much, forming a pro-eutectoid α phase along the grain boundaries in the central layer, resulting in a decrease in ductility. The concentration gradient of Ni toward the inside becomes gentle, making it difficult to form a hard and high-quality hardened layer on the surface.The heating retention time is longer than that of the conventional method because the present invention does not use intermetallic compounds as a curing factor. However, if the heating time is less than 0.5 minutes, the diffusion of Ni will be insufficient and hardening will not be achieved, and if it is held for more than 30 minutes, Ni will diffuse and penetrate into the titanium. If it is too much, the pro-eutectoid α phase is likely to be formed, or the internal crystal grains become coarse, resulting in a decrease in ductility.

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

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

The reason for this is mainly due to the acicular α formed directly under the β-Ti phase.
The purpose is to promote the formation of a homogeneous β-Ti phase by suppressing phase development. If the cooling rate is less than 40"C/sec or the cooling end temperature is higher than 200°C, the acicular α phase will erode the β-Ti phase and grow.
The i-phase becomes inhomogeneous, and the thickness of the β-Ti phase layer decreases, resulting in problems such as disturbance of surface color tone or the occurrence of defects due to insufficient curing when used as a final product. Here, in the present invention, the thickness of the β-Ti phase layer is 20 to 150 μm.
However, if the thickness is less than 20 μm, it is difficult to remove the upper layer of intermetallic compounds etc. so that only the hardened layer is on the surface, and it is easily scratched by sand and dust.
Furthermore, wear resistance is not sufficient, and if the thickness exceeds 150 nm, Ni will diffuse into the titanium too much, and the crystal grains inside will become coarse and brittle. Here, the thickness of the β-Ti phase layer of 20 to 150 μm can be easily achieved by the heat treatment conditions described above, and the hardness of the β-Ti phase formed in this way is 20 to 150 μm.
As a result of synergistic effects such as precipitation dispersion strengthening by ultrafine precipitates such as izNi, it has a Pinkers hardness of 450 to 600. In addition, if it is necessary to harden the − layer, β
- 300-6, which is well known as a means of curing the Ti phase.
It is also possible to perform aging treatment at a temperature of 0.00"C.Also, it is desirable to use a non-oxidizing gas such as helium gas for the atmosphere and cooling during heat treatment, but 1. Conditions for the nickel plating layer to remain after heat treatment. By selecting , it is possible to perform treatments such as atmospheric conditions and water cooling.

After heat treatment, the remaining Ni layer and intermetallic compound layer are removed by chemical polishing, electric polishing, mechanical polishing such as puffing, etc. to expose the β-Ti phase on the surface, and the desired surface roughness is achieved depending on the application. Adjust the surface condition and use it.

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

[Example] After degreasing a pure titanium (JISI type) cold-rolled plate with a thickness of 0.8 mm with an organic solvent, NaOH was added at 20 g/l and Hz (h
Pre-plating treatment was carried out by immersing the sample in a mixed aqueous solution containing 7 g/42% at 65°C for 10 minutes, followed by nickel plating treatment using a known Watt bath at a current density of 2A/dm'', followed by heating in an argon atmosphere. Treatment, cooling treatment, and mechanical polishing to give the finished surface a specific texture are performed. Table 1 shows the processing conditions for the various samples thus prepared.

In order to investigate the characteristics such as the difficulty of surface scratches and gloss deterioration of such samples, the surface micro-Vickers hardness (load of 20 g) was measured.
In addition, the as-puffed sample and the sample with 5 mm Erichsen overhang were immersed in 5% saline solution at 50°C for 10 days, and the surface condition such as gloss was evaluated on a 5-grade scale (rating (The more the better) 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 heat treatment, it is possible to create a titanium surface finish that is resistant to scratches, has excellent corrosion resistance and surface gloss, and is resistant to gloss deterioration even in a corrosive environment. It is clear that it can be done.

〔Effect of the invention〕

According to the method of the present invention, it is possible to easily apply a titanium surface finish that does not easily cause peeling of the hardened layer, has excellent surface homogeneity, wear resistance, and corrosion resistance, and does not easily cause gloss deterioration, and is useful in related industrial fields. The benefits are extremely large.

Claims (1)

[Claims] A nickel coating with a thickness of 3 to 30 μm is applied to the surface of titanium, heated and held at a temperature of 850 to 950°C for 0.5 to 30 minutes, and then cooled to a temperature of 200°C or less at a cooling rate of 40°C/sec or more. Then, the residual Ni on the surface layer and the intermetallic compound layer were removed, and the thickness was reduced to 20 to 150 μm.
- A method for finishing the surface of titanium, which is resistant to scratches and has a long luster life, characterized by using a layer consisting of a Ti phase as the outermost surface.