JP2001152316A - Plasma carburizing method - Google Patents

Abstract

PROBLEM TO BE SOLVED: To obtain excellent wear resistance, low coefficient of friction, and improved corrosion resistance while maintaining the strength of a base material before carburizing. SOLUTION: In this plasma carburizing method, a titanium metal or a heat resistant steel such as austenitic heat resisting steel is heated to 350 deg.C to <700 deg.C in an atmosphere containing fluorine-type gas such as nitrogen fluoride gas to undergo surface fluoridizing treatment and then subjected to plasma carburizing treatment in a gaseous-hydrocarbon-containing atmosphere of 10-20,000 Pa and 350-<700 deg.C. Plasma carburizing can be applied to the surface of the activated (fluoridized) metal under the conditions of prescribed pressure and prescribed relatively low temperature, and a carburized layer consisting of carbonized metal layer can be formed in the surface layer part of the titanium metal or heat resisting steel while maintaining the strength of the base material.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a plasma carburizing method for improving corrosion resistance, friction coefficient and wear characteristics on the surface of heat-resistant steel or titanium metal.

[0002]

2. Description of the Related Art Generally, titanium metal has high heat resistance,
It is known that the strength is almost equal to that of carbon steel and that an oxide film is formed on the surface, so that it has excellent characteristics of corrosion resistance.

[0003] Pure titanium is used for all metals, especially copper,
It is possible to make alloys with tin, iron, aluminum, vanadium, chromium, cobalt, molybdenum, tungsten, etc., and to improve their workability and mechanical strength in various ways.

[0004] As applications of such titanium metal made of pure titanium or titanium alloy, corrosion-resistant containers, corrosion-resistant screws and bolts, eyeglass frames, and medical and dental materials are known.

[0005] Heat-resistant steel (JIS G 4311)
Are roughly classified into heat-resistant austenitic steel and heat-resistant ferritic steel. For example, SUH660 (A-286) is a precipitation hardening type austenitic Fe-Ni-Cr steel.
It is made of an alloy and is used as a material for screw parts for aircraft.

[0006] Such heat-resistant steel (SUH660) has a
It has excellent strength and corrosion resistance up to 04 ° C, but has a thermal conductivity of about 1/3 and a thermal expansion coefficient of about 1.5 compared to alloy steel.
When it is used as a screw part, it can cause "seizure" (the screw surface is thermally expanded and becomes non-rotatable) when tightening with a power tool or manual wrench. Therefore, seizure is prevented by electroplating such as cadmium or nickel, or by coating a resin containing a lubricant on the screw surface.

[0007] In addition, the inventors of the present application have disclosed in Japanese Patent Application Laid-Open No. 7-9 / 1990 that plasma carburizing treatment is particularly performed to reduce the coefficient of friction with titanium metal and improve wear resistance.
No. 0542.

According to the publication, as a pretreatment for removing the surface oxide film on the titanium metal surface during the carburizing treatment, a mixed gas of hydrogen gas and argon gas is turned into plasma to wash off the attached matter on the titanium metal surface. Cleaning process is performed.

[0009] Carburizing treatment and cleaning treatment for such titanium metal and the like are performed at a temperature lower than 700 ° C.
The process is performed at a high temperature of 700 ° C. or more because the surface is not activated and activated carbon cannot sufficiently enter.

[0010]

However, as described above, when performing the carburizing treatment and the cleaning treatment performed at a high temperature of 700 ° C. or more, there is a problem that the softening of the metal base material is inevitable.

In particular, when carburizing treatment is performed using titanium metal as a base material, the base material is usually subjected to precipitation hardening by aging at a temperature of about 500 to 700 ° C. When the heat treatment is performed, the effect of the precipitation hardening due to the aging treatment is lost, and the strength may be reduced.

Accordingly, an object of the present invention is to solve the above-mentioned problems and to provide excellent wear resistance, a low coefficient of friction, and improved corrosion resistance while maintaining the strength of the base material before carburizing. Is to get it.

[0013]

In order to solve the above-mentioned problems, according to the present invention, titanium metal or heat-resistant steel is heated to 350 ° C. or more and 700 ° C. in an atmosphere containing a fluorine-based gas.
After heating to less than the surface and fluorinating the surface, the titanium metal or heat-resistant steel is mixed with a hydrocarbon gas containing 10-200.
The plasma carburizing method of performing plasma carburizing in an atmosphere of 0 Pa, 350 ° C. or more and less than 700 ° C. was adopted.

As the fluorine-based gas, it is preferable to use a nitrogen fluoride gas. Further, as the heat-resistant steel,
Austenitic heat-resistant steel can be used.

In the plasma carburizing method of the present invention comprising the above-described steps, first, a fluorine-based gas is supplied to titanium metal or heat-resistant steel heated to 350 ° C. or more and less than 700 ° C. in a vacuum in a furnace used for plasma carburizing. Blow and hold.

At this time, the oxide formed on the surface of the titanium metal or the heat resistant steel is completely removed and replaced with a fluoride film. Since this fluoride film is in a very active state, the subsequent carburizing process is performed efficiently.

The surface of the metal that has undergone such an activation treatment (fluorination treatment) is subjected to plasma carburization under a predetermined pressure and a predetermined temperature condition. At this time, the activated carbon ions enter between the metal molecules, Alternatively, the metal atoms that fly out of the metal surface combine with the activated carbon ions and adhere to the metal surface and diffuse inside, or the carbon ions accelerated near the cathode are directly injected into the metal Rarely, a carburized layer composed of a metal carbide layer is formed on the surface of titanium metal or heat-resistant steel.

According to the plasma carburizing method according to the present invention, a carburized layer can be formed on a metal surface to a thickness of, for example, 20 μm or more, and a clear boundary is not formed with a non-carburized portion inside the metal. It is possible to form a durable surface treatment layer which is hard to peel off, reduces lubricity and friction coefficient by carbides, and improves wear resistance and metal corrosion resistance.

[0019]

DETAILED DESCRIPTION OF THE INVENTION The titanium metal used in the present invention is:
Either pure titanium or an alloy of titanium and another metal component may be used, and the composition of the alloy is not particularly limited. The purity of titanium, which is an industrial material titanium metal, is about 99.9 to 99.5%, but such pure titanium can also be used.

When a titanium alloy is used, other metal components include, for example, copper, tin, iron, aluminum, vanadium, chromium, cobalt, molybdenum, tungsten and the like.

The heat-resistant steel used in the present invention is heat-resistant steel (JI
SG 4311), such as SUH660 (A-286), ferritic heat-resistant steel such as low chromium heat-resistant steel, high chromium heat-resistant steel, and high aluminum heat-resistant steel, and austenitic heat-resistant steel including a heat-resistant alloy.

It is preferable that the surface of the titanium metal or heat-resistant steel to be treated is immersed in an organic solvent or cleaned by ultrasonic waves or the like before the plasma carburizing treatment.

The fluorine-based gas used in the present invention is fluorine
(F) or a fluorine compound gas containing fluorine, or
Nitrogen (N_Two) Containing fluorine mixed with inert gas
A mixed gas, the most suitable of which is nitrogen fluoride (N
F_Three). Also, as examples of other fluorine-based gases,
HF, BF_Three, CF_Four, SF₆, ClF₆, C _Two
F₆, WF₆, CHF_Three, SiF_FourAnd the like.

The hydrocarbon-based gas used for carburizing is charcoal.
A generic term for gases consisting only of hydrogen and hydrogen,
However, any compound of a cyclic hydrocarbon may be used. chain
As a typical example of the formula hydrocarbon, a general formula C_nH_{2n + 2}Indicated by
Methane hydrocarbons and ethylene hydrocarbons (general
Formula C_nH_2n), Acetylene-based hydrocarbons (general formula C_nH
_2n-2), Which may be linear or have a side chain.
No. In particular, methane, ethane, propane,
Tongues are preferred because they do not require vaporization equipment for use.
It can be said that it is a new thing. Also, as a cyclic hydrocarbon
Is an aromatic compound or an alicyclic compound,
A typical example of the aromatic compound is benzene (C
₆H₆).

Here, the pressure of the hydrocarbon gas under the conditions of the plasma carburizing treatment is 10 to 2000 Pa. The pressure of such a hydrocarbon gas is set so that a treatment layer mainly composed of TiC is formed on the surface layer of titanium metal, or the carbon-austenite solidification is formed on the surface layer of heat-resistant steel. At a low pressure of less than 10 Pa, the amount of carbon in the treated layer is small and the sliding characteristics are not sufficiently improved at a low pressure of less than 10 Pa. Also, 2000Pa
At a high pressure exceeding, the carbon content of the carburized layer becomes a saturated value, the carburizing effect is not further improved, and practicality is lost.

The plasma carburizing of the present invention can be performed by the following operation using a known carburizing apparatus (manufactured by JEOL Ltd.).

The operation of the plasma carburizing treatment will be described in more detail. First, a molded article made of titanium metal or heat-resistant steel is charged into a processing chamber, evacuated, and then heated by a heater.
Heat to 0 ° C. or more and less than 700 ° C., for example, introduce nitrogen gas containing nitrogen fluoride gas, and
Then, the oxide film formed on the surface of titanium metal or heat-resistant steel is replaced with an active fluoride film.

Thereafter, the nitrogen gas containing the nitrogen fluoride gas is detoxified and exhausted using a well-known exhaust gas treatment device.

Next, hydrocarbon gas and hydrogen gas are
Introduced into the furnace in the range of
The plasma carburizing process is performed by applying a DC high voltage of 0V.
In the plasma gas, ionized activated carbon C ⁺ is generated and adheres to the metal surface and diffuses further inside, or the carburizing reaction proceeds by the action of sputtering or implantation.

The ambient temperature of the plasma carburizing treatment in the present invention is 350 ° C. or more and less than 700 ° C., preferably 35 ° C.
0 to less than 600 ° C. At a low temperature lower than the above-mentioned predetermined range, the fluorination of the metal surface, that is, the substitution reaction from the metal oxide to the metal fluoride becomes difficult to occur, so that activated hydrogen cannot enter and diffuse into the metal. Further, at a high temperature exceeding the above-mentioned predetermined range, when the metal to be treated is titanium metal, the strength is reduced, and the heat-resistant steel is more likely to cause a change in the size of the compact due to thermal expansion. It is.

[0031]

[Example 1] Titanium alloy (Ti-6Al) which had been subjected to a solution treatment (water cooling treatment at 950 ° C for 1 hour) and an aging treatment (treatment of maintaining a heating state at 540 ° C for 8 hours) in advance.
-4V) was machined into a flat plate having a length of 25 mm, a width of 35 mm, and a thickness of 3 mm, and a JIS Z2201 metal tensile test specimen No. 4 (diameter: 14 mm, gauge length: 50 mm, parallel part length: about 60 mm, shoulder: 60 mm) (Radius of 16 mm), and after polishing (1000 emery), ultrasonically cleaned in acetone, and plasma carburized by the following apparatus and conditions.

That is, a processing chamber surrounded by a heat insulating material such as graphite fiber is provided in a heating furnace, and the processing chamber is heated by a heating element made of rod graphite, and a positive electrode of a DC glow discharge is provided above the processing chamber. connection,
In addition, a cathode was connected to the mounting table of the processed product, and a carburizing apparatus (manufactured by JEOL Ltd.) capable of introducing a process gas by appropriately switching and introducing a process gas at a key point in the processing chamber was used.

First, the processing chamber is evacuated, and 500
Heated to ° C. Then, a nitrogen gas containing a nitrogen fluoride (NF ₃ ) gas was introduced into the processing chamber, held for 30 minutes, and fluoridation treatment for replacing the oxide film on the surface with the fluoride film was performed.

Next, plasma carburizing conditions are as follows: gas composition is 10% propane gas, 90% hydrogen gas, pressure 100 Pa, processing time 3 hours, processing temperature 50
After the temperature was set to 0 ° C., nitrogen gas was injected into the processing chamber and cooled to room temperature.

With respect to the plasma carburized product of the embodiment subjected to the above treatment and the untreated product as a blank, a friction / wear test and a metal material tensile test (JIS Z2241) were performed under the following equipment and conditions. The results are shown in FIG. 1 and Table 1.

That is, by the counter weight,
A pin type sliding mating material (bearing steel: SUS) is provided on the lower surface of one end of the arm supported so that a load of 1.96 N is applied to one end.
J2) is fixed, and the sample is forcibly moved in a direction perpendicular to the longitudinal direction of the arm at a speed of 20 mm / sec.
Was reciprocated with a stroke of. At this time, the frictional force acting between the sliding partner and the sample piece was detected as strain generated in the arm by a strain gauge, and the amount of the strain was input to a computer via an A / D converter to calculate a friction coefficient. . In this experiment, the friction time was 2 hours, and the relative humidity was 50 to
This was performed at room temperature at 60%.

With respect to the wear characteristics, the wear weight or the wear volume was determined from the cross-sectional shape of the wear mark of the sample piece measured by the surface roughness meter or the weight change of the sample piece before and after the experiment.
The specific wear (mm ³ / Nm) after the friction test was determined, and the results are also shown in the figure.

[0038]

[Table 1]

Example 2 Instead of titanium used in Example 1, heat-treated steel (solution heat treatment (900 ° C. for 1 hour with water cooling) and aging treatment (720 ° C. for 1 hour)) J
Except for using SUH660 (A-286) of IS G 4311), a fluorination treatment and a plasma carburization treatment were performed under exactly the same conditions as in Example 1 to produce a test piece.

The friction and abrasion tests performed on the first embodiment were also performed on the second embodiment, and the results are shown in FIG.
And Table 1.

[Comparative Example 1] A test piece was placed in the processing chamber used in Example 1 and heated to 500 ° C in a vacuum. Then, fluoridation was not performed, and hydrogen gas 200 ml / min and argon gas 24 ml / The cleaning treatment was carried out at a current value of 0.3 A and a voltage of 430 V by adjusting the inside of the furnace to 65 Pa by introducing a mixed cleaning gas for one minute. Thereafter, plasma carburizing was performed in exactly the same manner as in Example 1.

The same experiment as in Example 1 was performed on the obtained test piece of Comparative Example 1, and the results are shown in FIG. 1 and Table 1.

Comparative Example 2 A test piece was produced in the same manner as in Comparative Example 1, except that the temperatures of the cleaning treatment and the plasma carburizing treatment were changed to 800 ° C., and the same experiment was performed. The results are shown in FIG. 1 and Table 1.

[Comparative Example 3] A test piece was placed in the processing chamber used in Example 2 and heated to 500 ° C in a vacuum. Thereafter, fluorination was not performed, and hydrogen gas 200 ml / min and argon gas 24 ml / Then, the inside of the furnace was adjusted to 65 Pa by introducing a mixed cleaning gas for 3 minutes, and a cleaning process was performed at a current value of 0.3 A and a voltage of 430 V. Thereafter, plasma carburizing treatment was performed in exactly the same manner as in Example 2.

The same experiment as that of Example 2 was performed on the obtained test piece of Comparative Example 3, and the results are shown in FIG. 2 and Table 1.

Comparative Example 4 A test piece was produced in the same manner as in Comparative Example 3, except that the temperatures of the cleaning treatment and the plasma carburizing treatment were changed to 800 ° C., and the same experiment was performed. The results are shown in FIG. 2 and Table 1.

As is clear from the results shown in FIGS. 1 and 2, the carburized products of the titanium alloys or heat-resistant steels of Examples 1 and 2 were compared with those of Comparative Examples 1, 2, 3, and 4. And the friction and wear properties were comparable or better.

As is clear from the results in Table 1,
The carburizing treatment of the titanium alloy or the heat-resistant steel of Example 1 and Example 2 was performed in Comparative Example 2, which was cleaned at a high temperature.
The result in the metal material tensile test was superior to the treated product of No. 4.

As described above, Examples 1 and 2 exhibited excellent properties in both friction and wear characteristics and mechanical strength of the base material.

[0050]

As described above, the present invention provides a method of subjecting a surface of titanium metal or heat-resistant steel heated to a predetermined temperature to a fluorination treatment in a fluorine-based gas atmosphere and then performing a plasma carburization treatment in a predetermined temperature range. In addition, there is an advantage that excellent wear resistance, a low coefficient of friction and corrosion resistance can be obtained without lowering the mechanical strength of the metal base material as compared with before the treatment.

The above advantages can be obtained more reliably when nitrogen fluoride gas is used as the fluorine-based gas used for the fluorination treatment.

Further, even when the material of the metal to be treated is heat-resistant steel and austenitic heat-resistant steel, the above-mentioned advantages can be obtained more reliably.

[Brief description of the drawings]

FIG. 1 is a table showing a coefficient of friction and a specific wear amount for bearing steels of an example and a comparative example.

FIG. 2 is a table showing a coefficient of friction and a specific wear amount with respect to bearing steel in Examples and Comparative Examples.

 ──────────────────────────────────────────────────続 き Continuing on the front page (72) Shinichi Tanaka, Inventor 5-5-24-404 Kitabebe, Higashisumiyoshi-ku, Osaka (72) Inventor Noriyoshi Tsuji 4-18-14-904F, Ikenohara, Osaka Sayama-shi Terms (reference) 4K028 BA03 BA12 BA14 BA22

Claims (3)

[Claims] 1. Titanium metal or heat-resistant steel is heated to 350 ° C. or more and less than 700 ° C. in an atmosphere containing a fluorine-based gas to fluorinate the surface, and then the titanium metal or heat-resistant steel contains hydrocarbon gas. 10 to 2000 Pa, 3
A plasma carburizing method comprising performing plasma carburizing in an atmosphere of 50 ° C. or more and less than 700 ° C. 2. The plasma carburizing method according to claim 1, wherein the fluorine-based gas is a nitrogen fluoride gas. 3. The plasma carburizing method according to claim 1, wherein the heat-resistant steel is an austenitic heat-resistant steel.