JPS6238429B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention can be applied to, for example, carbon steel, cast iron, cast steel, stainless steel, nickel, chromium, aluminum,
This relates to a nitriding prevention method for protecting the parts that are not nitrided when nitriding the surfaces of materials such as alloy steel containing vanadium, molybdenum, etc., and machines and structural parts made from these materials in a salt bath. . Improving the fatigue strength, wear resistance, corrosion resistance, and heat resistance of machinery and structural parts made from carbon steel, cast iron, cast steel, stainless steel, and alloy steels containing nickel, chromium, etc. This is extremely important for improving performance and saving materials. Various surface hardening methods have been put into practical use for this purpose, and one of them is the salt bath nitriding method (tuftride method). This method was developed around 1954 and involves immersing the material to be nitrided in a salt bath containing KCN, KCNO, etc. as the main ingredients at 570°C. However, salt bath nitriding the entire part
On the contrary, it may impair other functions of the part or the convenience of post-processing necessary for manufacturing. The following are a few examples of cases where inconveniences may occur when the entire part is subjected to salt bath nitriding treatment. For example, salt bath nitriding treatment of a crankshaft for a small cooler can significantly improve the fatigue strength and wear resistance of the crankshaft, but it often breaks at the threaded part for attaching the pulley at the end of the shaft. be. This is because, although the nitriding depth is constant in all parts, it becomes relatively large in threaded parts with small diameters, making those parts brittle and weakened. A similar phenomenon occurs with other shafts and pins with large steps. As another example, when press stamped parts are subjected to salt bath nitriding treatment and then assembled by spot welding, weldability is often significantly impaired due to the nitrogen compound layer formed on the surface. By the way. As a method for preventing nitriding of such areas where salt bath nitriding is not desirable due to functional, processing, or structural reasons, conventional methods have been used to nickel or chrome plate those areas. however,
These plating methods involve complicated pre-treatment and post-treatment processes, require masking of parts to prevent salt bath nitriding, and also require secondary processing such as welding to be performed reliably without sacrificing quality. In order to perform this process, it is necessary to remove the plating after salt bath nitriding treatment, and furthermore, consideration must be given to pollution and the environment, such as treatment of plating waste liquid containing harmful cyanide, etc., and workability is reduced. , had more problems in terms of cost. In view of the current situation, the present inventors have investigated various methods for preventing nitridation in a salt bath that can be easily used, and have found that the method for preventing nitridation according to the present invention is effective in preventing nitridation to the extent that there is no problem in practical use. The present invention was completed based on the discovery that removal of the inhibitor after bath nitriding treatment is extremely easy. That is, the present invention applies an inorganic compound having a film-forming ability to a part of a workpiece to be subjected to salt bath nitriding treatment where the treatment is to be prevented, and then heats it to form a film to cover the part. , the object to be treated is subjected to salt bath nitriding treatment, and a nitride compound layer of a required thickness is formed on the parts to be subjected to the salt bath nitriding treatment, and the thickness is reduced on the parts covered with a film where the treatment is not desired. A nitride compound layer having a thickness of 3 μm or less is formed, and then the object to be treated is rapidly cooled at a temperature at least 400° C. lower than the temperature of the salt bath nitriding bath to destroy the film and separate it from the object to be treated. The present invention relates to a method for preventing local salt bath nitridation, which is characterized by removing the salt bath. The nitriding prevention method of the present invention will be explained in more detail below. Inorganic compounds that can be used in the present invention to form a thin layer of nitrided compounds of 3 μm or less by salt bath nitriding are combusted, carbonized, and It is necessary to have heat resistance without melting, and its constituent components include at least a binder that forms a film when heated, its solvent or dispersion medium, and inorganic aggregate. The binder must contain inorganic atoms in order to withstand the salt bath nitriding temperature of 570°C. Examples of such binders include water-soluble alkali metal silicates such as lithium silicate, sodium silicate, and potassium silicate; Water-dispersible silica sols such as colloidal silica, colloidal alumina, alumina, or water-soluble phosphates such as aluminum phosphate, magnesium phosphate, calcium phosphate, etc., or other materials that are not completely inorganic, such as ethyl silicate. These include alkyl silicates such as alkyl silicates and their partial hydrolysates, organic silicates such as ammonium silicates and guanidine silicates, and polyorganosiloxane compounds that form inorganic films as described above. Each of these binders has sufficient effects as a single compound, but a mixture of two or more compounds may also be used.Also, the activity of the binder can be appropriately adjusted to create a coating film with good adhesion and less cracks. Metal oxides such as zinc oxide, metal hydroxides such as aluminum hydroxide, fluorides such as calcium fluoride, sodium silicofluoride, borates,
Compounds such as phosphates and one of the aforementioned binder compounds
This range also includes so-called modified binders, which are prepolymerized by partially reacting one species or two or more species. These binders have such poor nitrogen permeability that when used in combination with the following solvents or dispersion media, the resulting film forms a thin nitride compound layer of 3 μm or less when the film is subjected to salt bath nitriding treatment. It needs to be something. The solvent or dispersion medium for dissolving or dispersing the binder is a necessary component for coating the inorganic compound according to the present invention, and the nitride compound layer after the salt bath nitriding treatment in the salt bath nitriding prevention section is 3 μm or less. It plays an important role in the formation of an anti-nitriding film with poor nitrogen permeability, such as a thin film. solvent or
Water is often used as the dispersion medium, but depending on the type of binder, alcohols such as ethanol, methanol, and isopropanol, ethers such as dioxane and tetrahydrofuran, and esters such as ethyl acetate may be used. Organic solvents such as hydrocarbon solvents and halogenated hydrocarbon solvents can also be used. Another important ingredient is inorganic aggregate. As an inorganic aggregate, it has a softening temperature of 600℃ or higher.
Refractory powder, metal powder, etc. are mainly used, and various shapes such as spherical, lumpy, acicular, fibrous, and scale-like are used. Further, it is desirable that the inorganic aggregate has no or low reactivity with the binder. When highly reactive aggregates are used, they react at the same time as they are mixed with the binder, causing difficulties in the application process, or causing problems such as hardening when stored as a single component as a nitriding prevention composition. Sometimes. However, a so-called one-liquid, one-powder mixture type in which a highly reactive aggregate is mixed with a binder immediately before being applied to the object to be treated can also be used in the present invention. The particle size and particle size distribution of the inorganic aggregate are not particularly limited, but can be used in the range of 0.001 μm to 1 mm, more preferably 0.1 μm.
Good results are obtained with aggregates having a weight average, average particle size in the range from m to 100 μm. (However, for fibrous or acicular aggregates, the length may exceed this range.) It is most preferable for the particle size distribution to follow the Andreasen's close-packed particle size curve, but if q in the Andreasen equation is Good results can be obtained in the range of 0.2 to 1. Specific compounds include oxides such as alumina, silica, zirconia, titania, and magnesia, composite oxides such as zircon and mullite, nitrides such as silicon nitride, boron nitride, and aluminum nitride, silicon carbide, and boron carbide. One or more of natural minerals such as carbide, silica sand, silica stone powder, kaolin, glass powder, metal powder, etc. are used. The blending ratio of binder and aggregate is 1 to 10,000 parts by weight, preferably 10 to 1,000 parts by weight, per 100 parts of aggregate. If the blending ratio of the binder is smaller than this range, the blend cannot be made into a slurry or paste form, which inhibits wetting of the object to be treated during application. On the other hand, if it is too large, the effectiveness of the aggregate cannot be expected, and foaming or
This may cause cracks, etc., and good results cannot be obtained. In addition to these, antisettling agents, surfactants, pigments,
A viscosity modifier, a curing agent, etc. are appropriately combined and blended as necessary. The inorganic compound that can be used in the present invention can be used in combination with the above-mentioned solvent, dispersion medium, aggregate, etc., and can be used in various forms, but it is usually in the form of a slurry, suspension, paste, etc. suitable for The viscosity varies depending on the application method, but the viscosity is 50~
Approximately 500,000 cps is good, but for spraying, a lower viscosity is better, and for brushing, dipping, etc., a higher viscosity is better for application workability. A readily available blend of inorganic compounds used to prevent salt bath nitridation of the present invention is the commercially available heat-resistant inorganic adhesive "Aron Ceramic" manufactured by Toagosei Chemical Industry Co., Ltd. The materials to be treated to which salt bath nitriding treatment can be applied are mainly general rolled or mechanical structural carbon steel,
In addition to cast iron, cast steel, carbon tool steel, etc., alloy steel (nitriding steel) containing components such as nickel, chromium, aluminum, vanadium, and molybdenum for efficient nitriding, and iron-based sintered metals. The coefficient of linear expansion is generally (9 to 18) x 10 ^-6 cm/cm
℃, the inorganic compound of the present invention also has an average linear expansion coefficient of (1 to 30) x 10 ^-6 cm/cm.
℃ range is preferable because good results can be obtained, and more preferably 25 to 600℃
Average coefficient of linear expansion of the film (4 to 23) x 10 ^-6 cm/cm℃
range of inorganic compounds, and they give better results. Particularly preferably, the difference in average linear expansion coefficient between the object to be treated and the film obtained from the inorganic compound at 25 to 570°C is (0.5 to 5) x 10 ^-6 cm/cm°C.
They are inorganic compounds that fall within the range of nitriding, and are particularly effective in preventing nitriding and removing unnecessary films after nitriding. If the difference between the coefficient of linear expansion and the coefficient of linear expansion is larger than this range, a film will be formed on the parts that should be subjected to salt bath nitriding treatment, and when immersed in a salt bath at 570 °C, the temperature will exceed 500 °C. Because of the heating caused by the temperature difference between 1 and 2, cracks, pinholes, foaming, peeling, and falling off occur in the film, making it useless to prevent nitriding. On the other hand, anti-nitriding agents whose coefficient of linear expansion is exactly the same as that of the base material at all temperatures from 25 to 570°C have excellent adhesion of the film to the object to be treated and good prevention effects, but after salt bath nitriding , it is difficult to peel off the film, which is not preferable. These inorganic compounds are applied to the object to be treated using a brush, spatula, spray, various coaters, etc. to the parts of the parts to be subjected to salt bath nitriding treatment where nitridation is to be prevented. Dust, rust,
If oil, rust preventive, etc. are attached, the adhesion of the inorganic compound to the object to be treated will be reduced, making it difficult to prevent uniform nitridation, so it is desirable to perform sanding and degreasing in advance. The thickness of the coating film is determined by the thickness of the nitride compound layer to be formed, but is usually controlled so that the film thickness is about 10 μm to 3 mm. Next, the applied inorganic compound is heated. This heating is necessary to evaporate the solvent or dispersion medium in the inorganic compound from the coating film, form a coating film, and form an anti-nitriding film.
Evaporation of the solvent or dispersion medium generates extremely small continuous pores, forming an anti-nitriding film with poor nitrogen permeability such that the nitride compound layer after salt bath nitriding of the preventive part becomes a thin layer of 3 μm or less. be done. The heating temperature for forming the film varies depending on the type of inorganic compound used, but usually requires heating at 50°C or higher, preferably 80-570°C, more preferably 100-300°C. If the heating temperature is low, the solvent or dispersion medium contained in the inorganic compound cannot be sufficiently removed by evaporation.
The film is destroyed. When the heating temperature is higher than the above range, it does not have a particularly bad effect, but it is not preferable from the point of view of energy saving. When heating, depending on the type of inorganic compound, rapid heating may cause foaming, inhibiting the nitridation prevention effect, so it is desirable to gradually raise the temperature to form a film. At first glance, this heating seems to require extra labor compared to plating, etc., but for precision parts etc., rapid immersion in a salt bath nitriding bath is 0.01~
In order to bring about a deformation of about 0.1 mm, preheating at 300°C for about 1 hour is usually required, so this step can also serve as heating of the inorganic compound. Next, the entire part is immersed in a bath heated to 570°C containing cyanide and cyanate as main components for the required time to perform salt bath nitriding treatment. After salt bath nitriding, removing the nitriding prevention film is particularly important for post-processing such as welding. Conventionally, the removal of the nitriding prevention film has been done by mechanical processing such as shot blasting and wire brushing, but this has poor workability and has worsened the working environment, but the nitriding prevention method according to the present invention One of its features is that it is extremely easy to remove the nitriding prevention film. In the present invention, the nitridation prevention film of the so-called prevention part, which is coated with an inorganic compound and heated, has such nitrogen permeability that the nitride compound layer becomes a thin layer of 3 μm or less under the film after salt bath nitriding treatment. Because of this, it can be easily and completely removed by applying rapid cooling with a temperature difference of at least 400° C. after the nitriding treatment. The hardness of the surface where a thin nitride compound layer of about 3 μm is formed is slightly higher than that of the raw material, but this does not pose a practical problem in post-processing such as weldability and machinability, but when it exceeds 3 μm, various problems arise. Therefore, inorganic compounds that form a film that provides such a nitride compound layer are not preferred. In the case of a film that does not have nitrogen permeability and does not form a compound layer at all, it cannot be completely peeled off by rapid cooling at 400°C, so inorganic compounds that form such a film are also not preferred. This can be understood from the fact that even when the nitriding prevention film according to the present invention was simply heated to 570° C. in an air atmosphere and then cooled with water, no abnormality was observed in the film. Therefore, it is presumed that the formation of an extremely thin nitride compound layer on the surface of the object to be treated facilitates the removal of the nitriding prevention film. In the present invention, a temperature difference of at least 400°C can be achieved by immediately placing the product in a cooled salt bath at 570°C or into oil or water at room temperature. It is important to cool the product in oil or water as soon as it is removed from the salt bath in order to avoid a decrease in temperature difference due to natural cooling between the time it is taken out of the salt bath and the time it is cooled in oil or water. This oil cooling or water cooling is a necessary step when performing the salt bath nitriding treatment because it traps nitrogen as a solid solution in the base material after the salt bath nitriding treatment, and according to the present invention, the nitriding process is performed in this step. It can also serve as the removal of the protective film. The nitridation prevention method of the present invention enables simple and inexpensive nitridation prevention. By preventing salt bath nitriding on parts that should not be subjected to salt bath nitriding due to the functionality of the parts, or on areas that are necessary for welding, machining, and other secondary processing (post-processing), we can make salt bath nitriding more effective. Applications will expand, and the performance and reliability of machines and structures will be greatly improved. Hereinafter, the present invention will be explained in more detail using examples of various materials. Example 1 Sample of carbon steel S45C for mechanical structure (vertical 150mm x horizontal
After degreasing and cleaning a 30 mm x 4 mm plate with trichlorethylene, we used alkali metal silicate as a binder, water as a solvent, and silica as the main inorganic bone to create a nitriding prevention film on half of its surface. Aron Ceramic HT (manufactured by Toagosei Kagaku Kogyo Co., Ltd., ingredients: 58% by weight of silica/3% by weight of alumina/sodium silicate)
14% by weight/25% by weight of water, viscosity 25℃cps) was applied with a brush at a rate of 450g/ ^m2 , and the temperature was increased from room temperature to 150℃.
It takes 60 minutes to gradually raise the temperature until it reaches that temperature.
A film was formed by heating and holding for 60 minutes. The film thickness after deposition is approx.
100μm, and the temperature of this nitriding prevention film is 25 to 600℃.
The average coefficient of linear expansion was 13×10 ^-6 cm/cm°C. Subsequently, nitriding treatment was carried out at 570°C for 90 minutes in a salt bath containing 35% NaCN, 25% Na ₂ CO ₃ and 40% NaOCN, and immediately the product was placed in water at 30°C to be rapidly cooled. The temperature difference during rapid cooling was approximately 520°C, and the nitriding prevention film was completely peeled off. In order to cut out a part of the sample and observe using a microscope the differences between S45C samples with and without nitriding treatment,
Figures 1 and 2 show photographs of the structure of a cross section of a sample that was tempered for 1 hour at °C. Figure 1 shows the part where the preventive film was not formed, and the surface layer has a thickness of 12.6 μm.
A nitride compound layer of about
Acicular nitride (Fe ₄ N) precipitates to a depth of about 0.3 mm, forming a diffusion layer. As shown in Figure 2, a slight nitride compound layer (2.0 μm) is observed in the area where the preventive film was formed, but almost no nitride precipitation was observed.
The tissue remains virtually unchanged from before treatment. This is clear from the cross-sectional hardness distribution in Figure 3, where the hardness of the salt bath nitrided area (solid line in the figure) and the preventive film formation area (broken line in the figure) are Hv662 and Hv278 at the surface, respectively. , the prevention effect is remarkable. As a result, necessary secondary processing such as machining on the nitriding prevention portion can be easily performed. Example 2 Comparative Example 1 The sample was a round bar of spheroidal graphite cast iron FCD50 (10 mmφ
After degreasing this with trichlorethylene,
Aronceramic C (manufactured by Toagosei Chemical Industry Co., Ltd., ingredients: 52% by weight of silica/5% by weight of alumina/14% by weight of sodium silicate/29% by weight of water, viscosity
70,000 cps) was applied to a part of the sample with a brush at a rate of 470 g/m ² . The sample of Example 2 was left at a room temperature of 20° C. for about 20 minutes, then heated at about 100° C. for 30 minutes, and then at 150° C. for 30 minutes to form a film. Then, it was directly immersed in a salt bath nitriding bath. The sample of Comparative Example-1 had a temperature of about 20℃ at room temperature of 20℃ after coating.
After being left for a minute, it was immediately immersed in a salt bath nitriding bath. The conditions of the salt bath nitriding bath were the same as in Example-1,
It was held at 570°C for 90 minutes. After completion of the treatment, the sample of Example-2 was immediately put into oil at 53°C and rapidly cooled. The anti-nitriding film was completely peeled off at the same time as oil cooling. When the sample of Comparative Example 1 was taken out of the salt bath nitriding bath, significant foaming was observed in the nitriding prevention film, and cracks were observed near the top of the foamed portion. During the observation, the temperature of the sample was allowed to cool to approximately 250℃, and it was placed in oil at 57℃, but the anti-nitridation film was strongly adhered to all areas other than the foamed areas and had to be removed with a wire brush. If so, it could not be removed. Observation was made by the same method as in Example 1, and a 7.4 μm nitride compound layer was formed on the surface layer where no nitriding prevention film was formed, and a Fe ₄ N nitride diffusion layer was further observed below. In the area where the protective film was formed, only a 1.6 μm thick nitride compound layer was observed. The hardness of the salt bath nitriding part and the nitriding prevention part is Hv454 and Hv242, respectively, and the prevention effect is obvious. The hardness of the foamed part of the sample of Comparative Example-1 was Hv390, which was virtually unchanged from the salt bath nitrided part, and almost no prevention effect was observed. Examples 3 to 4 Comparative Examples 2 to 3 In order to clarify the peeling effect of the nitriding prevention film on high carbon steel, carbon tool steel type 3 (SK3) with a carbon content of 1 to 1.1% and a coefficient of linear expansion was used. 9.8× ^10-6
This was carried out using a round bar sample (10 mmφ x 100 mm) with a temperature of cm/cm°C. After degreasing the sample and polishing it with sand vapor, alumina-based adhesive Aron Ceramic D (Toagosei Chemical Industry Co., Ltd.) was applied to prevent nitriding.
Composition: 60% alumina/5% silica/
12% by weight of sodium silicate/23% by weight of water) was applied at a rate of 220 to 300 g/m ² to give a dry film thickness of about 50 to 70 μm. Aronceramic D was mainly composed of alumina and had a viscosity of 50,000 cps and an average linear expansion coefficient of 8 x 10 ^-6 cm/cm°C from 25 to 600°C. After coating, it was left at room temperature for 4 hours, and the temperature was raised to 150°C at a rate of 150°C/hr, and salt bath nitriding treatment was performed under the same conditions as in Example 1. Immediately after the salt bath nitriding treatment, the specimens were placed in oil heated to various temperatures to examine the degree of peeling of the nitriding prevention film.

[Table] As shown above, it can be seen that by applying rapid cooling with a temperature difference of 400°C or more, the nitriding prevention film can be sufficiently removed. In addition, the hardness of the salt bath nitriding part is
Hv495, and the hardness of the protective film forming part is Hv307.
A clear difference is recognized, and the nitridation prevention method of the present invention can be fully used even in high carbon steel. Example 5 Round bar of chromium molybdenum steel SCM415 (10mmφ×
100 mm) sample was steam-cleaned with trichlorethylene, and then the silica-based heat-resistant inorganic adhesive "Aron Ceramic High Viscosity C" (manufactured by Toagosei Kagaku Kogyo Co., Ltd., ingredients: 56% by weight of silica/4% by weight of alumina/sodium silicate) 13% by weight/27% by weight of water, viscosity at 25°C
100000cps25~600℃ average linear expansion coefficient 13×10 ^-6 cm/
430 cm by dipping half of the sample into 430 cm
It was applied at a rate of g/m ² . After leaving at room temperature for about 10 minutes
A film was formed by heating at 180°C for 60 minutes, and then allowed to cool.
The thickness of the protective film when dry was approximately 100 μm. The sample was treated for 180 minutes in the same salt bath as in Example 1, and then immediately put into 30°C water for quenching. The temperature difference during rapid cooling was approximately 500°C, and the anti-nitriding film could be easily removed. The composition was observed under a microscope and the hardness was measured in the same manner as in Example-1. In the area where the preventive film was not formed, a 10.6 μm nitride compound layer was observed and its hardness was Hv672, whereas the thickness of the nitride compound layer in the preventive film formed area was approximately 1.4 μm, and the hardness was Hv672. Even if the bath nitriding treatment time became longer, the thickness of the nitride compound layer did not change much.

[Brief explanation of the drawing]

FIG. 1 is a microscopic photograph showing changes in the structure of a material subjected to salt bath nitriding treatment, and FIG. 2 is a microscopic photograph of a material subjected to nitriding prevention using the nitriding prevention method according to the present invention. FIG. 3 is a graph showing the depth from the surface and the micro-Vickers hardness (Hv) in the cross section of the material of the salt bath nitriding part and the preventing part. 1: Surface, 2: Compound layer, 3: Diffusion layer.

Claims (1)

[Claims] 1. In the object to be subjected to salt bath nitriding treatment,
An inorganic compound having film-forming ability is applied to the part where the treatment should be prevented, and heated to form a film to cover the part, and then the object to be treated is subjected to salt bath nitriding treatment. A nitride compound layer of the required thickness is formed on the area where the treatment is to be applied, and a 3μ thick layer is formed on the area covered with the film to prevent the treatment.
After that, the object to be treated is rapidly cooled at a temperature at least 400°C lower than the temperature of the salt bath nitriding bath to destroy the film and separate it from the object to be treated. A local salt bath nitridation prevention method characterized by removing.