JPH0312140B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to a method for modifying a metal surface by gas complex permeation.

[Conventional technology] Methods for modifying the surface of metal such as machine parts, tools, jigs, metals, etc. can be roughly divided into carburizing, nitriding, sulfurizing, and boronizing, etc. Penetration and diffusion of modifying medium elements into the metal surface layer. There are two methods: methods in which the surface is modified through surface hardening, plating, chemical conversion treatment, overlay, etc., and methods in which the surface layer is physically and chemically reacted and modified through surface hardening, plating, chemical conversion treatment, overlay, etc. The latter requires complex and advanced technology, is expensive, has limited base materials, and has many difficulties in work and quality control. The former osmotic modification method is divided into two types depending on the modification medium as follows.

That is, there are two methods: one uses liquid/solid as the reforming medium, and the other uses gas.

Conventional methods that use liquids, solids, and gases as reforming media have the following problems.

(1) For example, the most commonly used salt bath reforming method is to use a mixed salt of NaCN and NaCNO as a charging salt, melt it in an external heating furnace, and reform it at a predetermined temperature for a predetermined period of time. Since salts are extremely poisonous, there is a pollution problem, and the social responsibility is called for, which requires a large amount of money for pollution prevention equipment, and at the current state of technology, a perfect prevention method has not yet been established.

(2) Modification methods such as carburization have problems in that the modification temperature is high, damaging the base material and wasting heating energy. This method usually requires holding the product at a high temperature of 900°C or higher for several hours, which damages the base material and requires post-modification heat treatment, which causes deformation and dimensional changes in the product and requires a large number of man-hours. On the other hand, the surface hardness is also low.

(3) For example, the reforming method by nitrogen gas infiltration is
There is a problem that the reforming time is long. After the initial air replacement in the furnace, the cycle of heating, holding, heating, long-term holding, and cooling in the furnace takes more than 100 hours, and even short ones require more than 30 hours. As a result, heat energy is large, a large amount of gas is consumed, large operating costs are incurred, and delivery times are long. This is because the nature of the gas requires sufficient time to penetrate into the metal.

(4) For example, in the above-mentioned salt bath modification method, the surface modification layer is limited to a compound layer, so the thickness of the surface modification layer is extremely thin, on the order of 1/100 mm, and the layer thickness cannot be adjusted and controlled. The problem is that it cannot withstand high pressure loads.

(5) There is a problem that some shapes of the base material whose surface is to be modified cannot be modified. For example, in a reforming method using ion nitriding, the treated product is heated by the ion attack. Since ions travel in a straight line, there are areas where surface modification cannot be performed in the shadow areas where ions cannot travel. That is, it is extremely difficult to form a uniform layer on narrow grooves, the inner surfaces of micropores, the bottom surfaces, the inner surfaces of long objects, and the like.
Furthermore, it is not possible to form a uniform layer on objects with complex shapes, or to perform simultaneous processing on objects with large differences in surface area or mass. Furthermore, rapid cooling using water or oil cooling is not possible.

(6) There is a problem that the base material is limited. Among the base materials, there are many materials such as metals such as Al and Al alloys in which the desired modified layer cannot be obtained. A material containing compound (nitrogen) forming elements such as Cr and Mo is essential.

(7) There is a problem of deterioration of surface roughness. In particular, the salt bath modification method causes surface deterioration and reduces surface roughness.

(8) There is a problem in that the base material expands and contracts due to the thermal effects of processing, causing dimensional deviations. In addition, some of them are defective due to an increase in size due to the thickness of the layer deposited on the surface layer.

(9) In the nitriding method, as in the specification of Patent No. 106622, in which a metal material is heated in a high-temperature vacuum and then nitrogen is applied, heating must be performed at a high temperature of 900°C, which is costly and time-consuming. Not only does this increase costs, but the metal material becomes annealed and its hardness and mechanical properties deteriorate.
If quenching is performed again, the material will deteriorate further, and gas will be included in the material again, returning it to its initial state.

In addition, since vacuum heating relies only on radiant heat, its heating rate is less than half that of convection heating, resulting in extremely poor thermal efficiency and inefficient nitriding, which involves flowing nitrogen gas in a vacuum. Therefore, a large amount of nitrogen gas is required, resulting in insufficient nitriding.

Furthermore, the total processing time was as long as 24 hours.

(10) With the method of reforming by permeating a simple gas, it is difficult to adjust the surface properties of metal materials depending on the application.

[Problems to be Solved by the Invention] The purpose of the present invention is to solve the problems of the surface modification method as described above, and to provide a method that has good thermal efficiency, requires less expensive gas, is inexpensive, and can be done in a short time. The object of the present invention is to provide a gas composite permeation reforming method that provides stable wear-resistant, heat-resistant, and corrosion-resistant surface properties and allows the surface properties to be freely adjusted.

[Means for Solving the Problems] In order to solve the above problems, the gas composite permeation reforming method of the present invention involves inserting a metal base material into a permeation reforming furnace, degassing it at room temperature, and then applying N ₂ is injected into the furnace, heated up from room temperature, then heated to 400-580℃ for 10-90 minutes in a mixed gas of _N2 and ammonia, and then heated in a mixed gas of ammonia, _N2 and _CO2. The mixture was maintained at 400 to 580 °C for 10 to 240 minutes, then held for 10 to 120 minutes in a mixture of increasing CO ₂ concentration and decreasing N ₂ concentration, and further while circulating N ₂ . It is characterized by cooling.

Note that the mixed gas further contains H ₂ , Ar,
It is also possible to add one or more of ENDO, EXO, etc., and it is also possible to achieve better results by performing the continuous heating, holding in the mixed gas, and further heating and holding in multiple stages.

[Effects of the Invention] According to the gas composite permeation reforming method of the present invention, the furnace is evacuated at room temperature to create a vacuum to the extent that the gas contained in the metal base material is not released, and then gas is injected and heated. , removes oxygen and exhaust gases that impair the decomposition of ammonia gas, etc., creating an optimal nitriding gas atmosphere in the furnace, reducing the consumption of _N2 gas, and cleaning the surface of metal materials by vaporization to prevent nitriding. It is activated to be suitable for metal materials, can also remove furnace wall stains, does not require a high degree of vacuum compared to the method of removing gases contained in metal materials by vacuum heating, can be heated at low temperatures, and does not have any negative effects on the materials. There is no need to worry about giving.

Furthermore, since heating is performed using convection in a gas atmosphere, thermal efficiency is high and heating time is short, and by adjusting the type and ratio of the mixed gas, the thickness and properties of the surface compound can be freely adjusted and blended.

Furthermore, according to the present invention, the processing time is 12 hours or less, and the hardness of the surface layer of the base material can be increased to 900 or higher at a temperature of 600°C or less, and the surface layer is modified to improve hot strength and wear resistance. It has excellent effects such as improved properties, reduced wear resistance, improved fatigue strength, creep strength, and corrosion resistance, and improved surface roughness, and is of great industrial value.

[Example] Hereinafter, an example of the present invention will be described based on the drawings.

FIG. 1 is a block diagram of an apparatus used in the gas composite permeation reforming method of the present invention.

In FIG. 1, a osmotic reforming furnace 1 uses a vacuum tank and is connected to a vacuum pump 2 via pipes and valves (not shown). The interior of the permeation reforming furnace 1 is equipped with an external heating retort and the like to purify the atmosphere, prevent oxidation, and keep moisture and oxygen concentrations low at all times. Cylinders A, B, C, D, and E contain ammonia, N ₂ , CO ₂ , H ₂ , and protective gas, respectively.
Ar, ENDO, EXO, etc. are sealed, and after venting the inside of the permeation reforming furnace 1, N ₂ is added manually or automatically via pipes and valves (not shown) depending on the reaction stage.
The gas alone or a mixture of other gases such as _N2 gas is injected into the furnace. This gas is selected depending on the material, size, and shape of the metal base material, and the properties of the modified layer.

Further, the furnace 1 is equipped with a blower to circulate the atmosphere uniformly with high efficiency. For example, N ₂ can be injected from a cylinder B and circulated to cool the furnace and improve refining and efficiency.

In addition, the relationship between temperature and time is 400 for 0.5 to 10 hours.
The temperature is raised linearly from ℃ to 580℃, held for several hours, and then rapidly or gradually cooled.

The procedure of the invention is shown below with respect to a typical example.

(1) Insert the metal base material into the furnace and vent the air. The degree of vacuum in the furnace is 800 to 10 ^-2 mbar.

(2) Inject 2-12Nm ³ /hr (total amount 2-22Nm ³ ) of N ₂ and then heat from room temperature to 300-450℃ for 20-230℃.
Let the temperature rise in minutes.

(3) 400-580℃ for 10-90 minutes in a mixed gas of 1-8Nm ³ /hr nitrogen and 1-8Nm ³ /hr ammonia.
Continuous heating is applied. (Total amount: N ₂ 1-6Nm ³ , ammonia 1-6Nm ³ ) (4) Ammonia 1-8Nm ³ /hr, N ₂ 1-8Nm ³ /
hr, and held at 400-580°C for 10-240 minutes in a mixture of _CO2 ^0.1-2Nm3 /hr. (Total amount: ammonia 2 to 16Nm ³ , N _{2 2} to 16Nm ³ , CO ₂ 0.1Nm ³ to 3N
m ³ ) (5) Hold for an additional 10-120 minutes to increase the CO ₂ concentration and decrease the N ₂ concentration. In that case, the gas supply amount is as follows.

N ₂ 1-6Nm ³ /hr, ammonia 1-4Nm ³ /
hr, CO ₂ 0.1-2Nm ³ /hr (6) 2-12Nm ³ /hr of N ₂ is circulated for displacement and cooling tempering.

Although the above is a case of one-stage processing, multi-stage processing such as two-stage or three-stage processing can also be performed. This makes it easy to mix individual gases and select temperature and time, and performs effective surface modification through optimal and efficient combinations.
A single phase or a mixed phase (eg γ'-F _e4 N, ε-F _e4~3 N) can be obtained. For example, in a two-stage process, the temperature is linearly raised to 500°C over 0.5 to 10 hours, held for a predetermined time, then raised to 550°C, held for a predetermined time, and then rapidly cooled and slowly cooled.

Utilizing a vacuum system has several advantages. First is the improvement of atmosphere purity. In the conventional method, initial air evacuation is insufficient, so atmosphere purity cannot be obtained and the desired modified layer cannot be obtained in many cases, but this can be solved by adjusting the degree of vacuum.

Next, the conventional method of replacing gas to atmosphere may take several hours depending on the properties of the gas flow, but the present invention has made it possible to significantly shorten the operating time. Additionally, gas consumption was reduced.

Furthermore, compared to other heating furnaces, this osmotic reforming furnace has a smaller amount of heat storage and radiated heat in the furnace body, so the rate of increase in furnace temperature is faster.

In addition, in the conventional method, after treatment, the base material must be moved to separate equipment and heated and cooled, but in the method of the present invention, cooling and other tempering can be performed as is in the latter half of the treatment, resulting in improved quality. In addition, the processing time can be shortened. In particular, the influence of post-processing cooling conditions on the fatigue strength of materials is significant; in low-alloy steels, gradual cooling produces precipitation of γ′ (F _e4 N) phase, granular α″ (F _e16 N ₂ ), etc. Decrease fatigue strength.

Furthermore, according to the present invention, cooling characteristics can be improved. Cooling with gas depends on the pressure and flow rate of the cooling gas. Therefore, from atmospheric pressure
Utilizes the jet flow in the burl area to improve cooling, reduce time, and improve the quality of the base material and surface layer. Therefore, solid solution formation of the element diffusion layer is promoted and completed.

Next, there is the excellent brightness. Ti in the base material,
Materials containing active metals such as W, Mo, and V undergo embrittlement due to gas absorption and oxidation due to affinity with O ₂ , so this method is effective in preventing these problems.

Note that according to the present invention, surface roughness can be improved. The main purpose of this type of surface modification is closely related to surface roughness. The degree of this is an important factor in the evaluation.

This effect is noticeable in die steel and high-speed steel, which are the mainstream in this process.
According to experimental values, the roughness (R _nax ) was improved by a factor of 2 to 4.

The above-mentioned osmotic reforming furnace is equipped with an automation system that uses various sensors and microprocessors to ensure even more thorough quality stabilization.

The metal base material is placed in a basket, and the basket is inserted into the permeation reforming furnace.The dimensions of the basket are, for example, length x length x width 1100mm x 650mm x 650mm.
mm.

According to the gas composite permeation reforming method of the present invention, the elements cause a chemical reaction on the surface of the metal, and the generated elements diffuse into the metal to form a hard and tough surface layer.

A tough compound is formed in the surface layer, and fine granular and acicular precipitates are formed in the diffusion layer, showing effective properties.

The properties obtained by this modification method are as follows.

(1) Improvement in hot strength The surface obtained by this modification method shows high hot strength retention. Figure 2 shows an example, where the horizontal axis shows temperature and the vertical axis shows hardness (Bitzker's).
There is a certain amount. In the figure, the solid line and the two-dotted chain line indicate the one-stage treatment and the multi-stage treatment of the present invention, respectively, and the dotted line indicates the one subjected to other osmotic modification methods. Hardening is obtained.

(2) Improved wear resistance Figure 3 shows an example of a comparison of SKD61 with other osmotic modification methods, where the horizontal axis shows the friction time and the vertical axis shows the amount of wear. In the figure, the solid line and the two-dot chain line indicate the one-stage treatment and the two-stage treatment of the present invention, respectively, and the dotted line indicates the one subjected to another osmotic modification method. According to the present invention, the wear resistance is greatly improved as shown in the figure. Furthermore, since the penetration depth is large, the total wear amount is even larger.

(3) Reduction of wear resistance According to the present invention, powder wear resistance performance is significantly improved. This means that the wear coefficient is 0.3 to 0.3 for non-penetration.
This can also be explained by the fact that the present invention improves the value from 0.4 to 0.1 to 0.2. This prevents temperature rises during use and significantly improves product accuracy and lifespan. Figure 4 shows an example of SKD61, with the horizontal axis representing the test load and the vertical axis representing the temperature rise. In the figure, the solid line and the double-dashed line indicate the one-stage treatment and the two-stage treatment of the present invention, respectively, and the dotted line indicates the other osmotic modification method.

(4) Improving fatigue strength The present invention can be applied uniformly to products with complex shapes such as grooves. Elements dissolved in the matrix improve fatigue strength under repeated stress, bending stress, and rotational bending stress. FIG. 5 shows an example of SKD61, in which the horizontal axis shows processing time and the vertical axis shows fatigue strength. In the figure, the solid line and the two-dot chain line indicate the one-stage processing and the two-stage processing of the present invention, respectively. The present invention exhibits a significant improvement in fatigue strength.

(5) Improvement in creep strength Figure 6 shows the creep properties of S15C, SNCM3, and SKD61 with non-permeable products (indicated by dotted lines), one-stage treatment (indicated by solid lines), and two-stage treatment (indicated by two-dot chain lines) of the present invention. This shows a comparison of strength.
According to the present invention, a remarkable improvement in creep strength can be seen in any steel type.

(6) Improved corrosion resistance Significant improvement compared to other treated products. The improvement was proven in various corrosion tests such as salt water test, acid test, and ammonia test.

(7) Improvement of surface roughness As mentioned above, the surface roughness (R _nax ) was improved.

[Brief explanation of the drawing]

Fig. 1 is a block diagram of the equipment used in the gas composite osmotic reforming method of the present invention, Fig. 2 is a diagram showing changes in hardness with respect to temperature for the present invention and the conventional method, and Fig. 3
Figure 4 shows the change in wear amount with respect to wear time for the present invention and the conventional method. Figure 4 shows the change in temperature rise with respect to the test load for the present invention and the conventional method. Figure 5 shows the change in the temperature rise with respect to the test load for the present invention and the conventional method. FIG. 6 is a diagram showing changes in fatigue strength with respect to processing time, and is a diagram showing a comparison of creep strength between the present invention and the conventional method. 1... Permeation reforming furnace, 2... Vacuum pump.

Claims (1)

[Claims] 1. A metal base material is inserted into a permeation reforming furnace, and after venting at room temperature, N ₂ gas is injected into the furnace, and the metal base material is
Heating to ~450°C in 20-230 minutes, then heating to 400-580°C in 10-90 minutes in a mixture of _N2 and ammonia, then in a mixture of ammonia, _N2 and _CO2. The mixture was held at 400 to 580°C for 10 to 240 minutes, then held at 400 to 580 °C for 10 to 120 minutes with increasing CO ₂ concentration and decreasing N ₂ concentration, and then
A gas mixture permeation reforming method characterized by cooling while circulating _N2 . 2. The gas composite permeation reforming method according to claim 1, wherein the heating and holding in the mixed gas are performed in multiple stages.