JPS649364B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a method for manufacturing a dense sintered iron member. Conventionally, the manufacturing method for densified sintered iron parts is as follows:
A recompression method, ie, a twice-molding and twice-sintering method, is commonly used. In this method, after one-time molding and sintering, lubricating oil, molybdenum disulfide, graphite,
After applying a lubricant such as soap, the compact is put into a closed mold, compression molded again, and fired to make the sintered iron member denser. However, when using the above-mentioned lubricants,
During compression molding, the lubricant coating is likely to break, so the surface of the molded product often comes into direct contact with the mold surface. This tendency becomes particularly noticeable when the compression molding pressure exceeds 8 ton/cm ² , causing damage to the surface of the molded product and the mold, or due to an increase in pressure loss due to increased frictional force between the molded product and the mold. This causes problems such as insufficient increase in the density of the molded body and non-uniform density. As a result of various studies to solve the above-mentioned problems, the inventors of the present invention succeeded in treating an annealed preform with subsalt phosphate and sodium stearate to produce zinc phosphate and zinc stearate. It has been shown that the purpose can be fully achieved if an extremely thin multi-layer coating is formed, and the preform with this multi-layer coating is compression-molded in a closed mold and then sintered. I found it. The present invention was completed based on such new knowledge. Hereinafter, the method of the present invention will be explained in detail in the order of steps. (i) First, a mixed powder mainly composed of iron powder or iron or iron alloy powder is preformed under a pressure of about 4 to 8 tons/cm ² . When using a mixed powder, the iron content is preferably 70% by weight or more in order to form a zinc phosphate film well. As the alloy component and/or mixed component, manganese, chromium, molybdenum, graphite, nickel, copper, etc. are appropriately selected and used depending on the use and purpose of the product. The particle size of the powder may be similar to that of the powder normally used as a raw material for sintered alloys, and a mold lubricant such as zinc stearate powder may be added in a conventional manner. If the pressure during preforming is less than 4ton/ ^cm2 , the strength of the molded product will be insufficient and it will easily break during transportation after molding.
If it exceeds 8 ton/cm ² , the frictional force between the molded body and the mold surface increases, making it easy to damage the mold surface. (ii) Next, the obtained preform is heated at 500 to 1000° C. in a non-oxidizing atmosphere and then cooled in a furnace to remove distortion and soften the mold.
The atmosphere during this annealing process may be either neutral or reducing, and the atmospheric gas is a reducing gas such as hydrogen gas, ammonia gas, or hydrocarbon modified gas, and an inert gas such as nitrogen or argon. used as. When the heating temperature is less than 500℃,
Since the preform is not sufficiently smoothed and softened, sufficient densification cannot be achieved during subsequent compression molding. On the other hand, when the heating temperature exceeds 1000°C, carbon components contained in the preform or in the atmospheric gas tend to increase the hardness of the preform as it forms an alloy with iron. This makes it difficult to achieve sufficient densification during subsequent compression molding. After heating, the preform is preferably cooled in a furnace at a rate of 3000° C./hr or less, more preferably about 300 to 3000° C./hr. Cooling rate is 300℃/hr
If it is less than 1, it is preferable because the annealing effect is large in terms of physical properties, but it is economically disadvantageous because it takes a long time to cool down. On the other hand, the cooling rate is 3000
When the temperature exceeds °C/hr, the iron base tends to harden due to the cooling effect, making it difficult to achieve sufficient densification during compression molding as described below. (iii) The annealed preform is then treated with zinc phosphate to form a zinc phosphate film on its surface. This zinc phosphate film and the zinc stearate film formed in the subsequent process significantly reduce the friction between the molded body and the mold during compression molding, increasing the density of the molded body and improving the surface of the molded body. This prevents scratches from occurring due to friction on the mold surface. The thickness of the zinc phosphate film should be within a range that allows the zinc stearate film to adhere uniformly to the entire surface of the molded product in the next step and that the zinc phosphate film does not adversely affect the properties of the iron member after sintering, that is, 0.5~ It is preferable to set the thickness to about 2 μm. Incidentally, this thickness is only about 1/5 to 1/2 of the thickness of the zinc phosphate film of the cold forging material. The zinc phosphate treatment is carried out in substantially the same manner as in the case of cold forging materials, both in terms of treatment liquid composition and treatment conditions, and an example thereof is as follows. For example, zinc phosphate [Zn(PO ₄ ) ₂ 4H ₂ O] 80g,
Add 10 g of zinc nitrate [Zn(NO ₃ ) ₂ 6H ₂ O] and 80 c.c. of phosphoric acid (H ₃ PO ₄ ) to water and add the preform to a solution of 1000 c.c. Soak at about ℃. After annealing, the surface of the preform is usually free of fat, rust, etc., so degreasing and
There is no need to perform pickling or the like. However, in order to prevent the zinc phosphate treatment solution from entering the pores of the preform, a sealing process such as shot blasting should be performed, or the preform should be immersed in water to fill the pores with water. Further improved material properties can be obtained by taking measures such as keeping the material in place. After the zinc phosphate treatment, wash with water to remove the treatment solution. (iv) The preform on which the zinc phosphate film is formed is
It is then subjected to treatment with sodium stearate. Treatment with sodium stearate is 70-90
This is carried out by immersing the preform on which the zinc phosphate film has been formed into a sodium stearate aqueous solution having a concentration of about 4 to 8% heated to about .degree. Thus, the zinc phosphate and sodium stearate on the preform react to form a further zinc stearate film on the zinc phosphate film. The zinc stearate film may be formed evenly over the entire surface, and as a result, the zinc phosphate film and the zinc stearate film are sequentially formed on the surface of the preform so that the total thickness is about 0.5 to 2 μm. becomes. After the sodium stearate treatment is completed, the molded body is washed with hot water to remove the treatment agent from the pores and surface of the molded body, and then dried. If the treatment agent remains, it may inhibit the increase in the density of the compact during compression molding or may deteriorate the properties of the sintered iron member. (v) Next, the preform on which the zinc phosphate film and the zinc stearate film have been formed is compression molded in a closed mold at a pressure of 8 to 18 tons/cm ² .
When charging the preform into a closed mold prior to compression molding, care must be taken to ensure that the preform does not come into contact with the mold and become distorted or work hardened. A preform that has undergone work hardening has poor compressibility and an insufficient increase in density.
However, if the dimensions of the die are made significantly larger than the dimensions of the compact in order to completely prevent contact between the preform and the die, the processing rate during compression molding will increase and the zinc phosphate There is a noticeable tendency for the film to be destroyed and the surfaces of the molded body and mold to be damaged, or for processing strain to increase and to promote dimensional variation during sintering. Therefore, the mold dimensions should be slightly larger than the preform to the extent that the mold does not cause distortion when the preform is inserted into the mold, and such that the processing rate of the mold is as small as possible. It is better to set it as a degree. Specifically, the clearance between the mold wall and the preform is 0.005 to 0.10.
It is preferable to make it about mm. If the compression molding pressure is less than 8 ton/cm ² , the compact will not be sufficiently densified, while if it exceeds 18 ton/cm ² , there is a risk that the compression molding mold will be destroyed. (vi) The compact formed by compression molding above is sintered in a non-oxidizing atmosphere according to a conventional method. As the atmospheric gas, the same gas as used in the annealing process described above is used. The sintering temperature varies depending on the type of raw material, the dimensions of the compact, the performance required for the product, etc., but is usually within a range of about 1000 to 1300°C. According to the method of the present invention, the major advantage is that damage to the surface of the compact and the mold during compression molding can be prevented, and a sintered iron member with high density and excellent dimensional accuracy can be obtained. Examples will be shown below to further clarify the features of the present invention. Example 1 Commercially available water spray iron powder (particle size...100 mesh passed)
A mixed powder containing 1% stearic acid powder as a mold lubricant was applied at a pressure of 5 tons/cm ² to a mold with an outer diameter of 16 mm and an inner diameter.
It was molded into a cylindrical shape of 10 mm and 10 mm in height, and after heating it at 900℃ for 45 minutes in a hydrogen gas atmosphere, it was heated to about 1000℃.
Cool slowly at a rate of °C/hr. After soaking the obtained preform in water to allow water to penetrate into the internal pores of the preform, a zinc phosphate treatment solution (zinc phosphate) at a temperature of about 80°C was added.
80g/and phosphoric acid 80c.c./and zinc nitrate 10g/
(containing aqueous solution) to form a 1 μm thick zinc phosphate film. After removing the treatment liquid by washing with water, the preform is immersed in a saturated sodium stearate aqueous solution at a temperature of about 90°C for 30 minutes to form a uniform zinc stearate film on the entire surface, and then thoroughly washed with hot water, dry. The preformed body on which the two-layer film is formed is slightly larger (clearance) than the preformed body.
0.01~0.05mm) and compression molded with a pressure of 8ton/ ^cm2 , then in a decomposed ammonia gas atmosphere.
Sinter at 1150°C for 45 minutes. Regarding the obtained cylindrical sintered member, the density of the preform, the extraction force when taking out the compact after compression molding from the mold, the presence or absence of scratches on the surface of the compression molded product, and the density and hardness of the sintered member. When the radial crushing strength and the radial crushing strength were determined, the results shown in Table 1 were obtained. Comparative Examples 1 to 3 The results for sintered members obtained in the same manner as in Example 1, except that the following lubrication treatment was performed instead of the lubrication treatment of the molded body with zinc phosphate and zinc stearate, were as follows. As shown in the table.

【table】

[Table] Katsuta.
From the results shown in Table 1, it is clear that the coating according to the present invention has excellent lubricity, does not damage the compact, and increases the hardness of the sintered member. Example 2 Commercially available AISI standard 4100 series alloy powder (manganese 0.6%,
Chromium 1.1%, molybdenum 0.25%, residual iron; particle size...passed 100 meshes) and graphite powder (particle size 44 μm or less)
A mixed powder containing 0.5% zinc stearate powder and 1% zinc stearate powder was heated to a pressure of 6 tons/cm ² with an outer diameter of 16 mm and an inner diameter of 10 mm.
mm, height 10 mm, and heated at 850°C for 45 minutes in a hydrogen gas atmosphere, then heated at approximately 1000°C/hr.
Cool slowly at a cooling rate of . The obtained preform was then treated in the same manner as in Example 1 to form a film of zinc phosphate and zinc stearate, placed in a mold slightly larger (clearance 0.01 to 0.05 mm) than the preform, and heated to 14 tons. After compression molding with a pressure of /cm ² , sintering was performed at 1150°C for 45 minutes in a decomposed ammonia gas atmosphere. The obtained sintered body was subjected to the same measurements as in Example 1, and the results shown in Table 2 were obtained. Example 3 Commercially available mill scale reduced iron powder (particle size...passed 100 meshes) was mixed with 4% nickel powder (particle size 10 μm or less).
Molybdenum powder (particle size 10μm or less) 2%, copper powder (particle size 44μm or less) 3%, graphite powder (particle size 44μm or less)
A powder containing 0.2% zinc stearate powder and 1% zinc stearate powder was preformed in the same manner as in Example 2, annealed, a film was formed, and compression molded. Sinter for minutes. The obtained sintered body was subjected to the same measurements as in Example 1, and the results shown in Table 2 were obtained. Comparative Example 4 After obtaining a preform in the same manner as in Example 2, it is annealed and sintered in the same manner as in Example 2 without performing zinc phosphate and sodium stearate treatment and compression molding. . The obtained sintered body was subjected to the same measurements as in Example 1, and the results shown in Table 2 were obtained. Comparative Example 5 After obtaining a preform in the same manner as in Example 3, it is annealed and sintered in the same manner as in Example 3 without performing zinc phosphate and sodium stearate treatment and compression molding. . The obtained sintered body was subjected to the same measurements as in Example 1, and the results shown in Table 2 were obtained.

【table】

[Table] From the results shown in Table 2, according to the method of the present invention,
It is clear that a sintered iron member with high hardness and high strength can be obtained without damaging the surface of the compact.

Claims (1)

[Claims] 1 (i) A step of preforming a mixed powder mainly composed of iron powder or iron or iron alloy powder at a pressure of 4 to 8 tons/cm ² , (ii) Non-oxidizing the preform 500-1000 in sexual atmosphere
(iii) treating the preform with zinc phosphate; (iv) treating the preform with sodium stearate; (v) placing the preform in a closed mold. 8~18ton/
compression molding at a pressure of cm ² ; (vi) A step of sintering the obtained compact in a non-oxidizing atmosphere. A method for manufacturing a densified sintered iron member, comprising: