JPS6111309B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a sliding member made of an iron-based sintered alloy that has excellent wear resistance. For example, materials for sliding parts used under harsh operating conditions, such as internal combustion engine cams, tabs, and oil pump housings, are required to have excellent wear resistance. The sliding surface layer is hardened by applying the following steps. However, general cast products do not have good dimensional accuracy and must be machined, but the above-mentioned cast iron does not have good machinability and has the problem of increasing processing costs. Sintered bodies manufactured by powder metallurgy have good dimensional accuracy, high yields, and have good oil retention due to their inherent pores, so sintered alloys are widely used as materials for machine parts. Iron-based sintered alloys with various alloying elements added to improve their wear resistance are being used, but their wear resistance deteriorates under harsh usage conditions. It is not as good as that of cast iron or wear-resistant alloy cast iron. In addition, nitriding is an effective means of improving the wear resistance of steel materials, but nitriding progresses quickly in sintered alloys due to the inherent pores. If chromium is contained in an iron-based sintered alloy, it will expand during the nitriding treatment, resulting in large dimensional changes, so these alloying elements cannot be contained. Therefore, a nitrided layer having sufficient hardness cannot be obtained, and no great effect can be expected in improving the wear resistance of iron-based sintered alloys by nitriding treatment. The present invention aims to solve the above-mentioned problems and provide a sliding member made of a sintered alloy with good dimensional accuracy and improved wear resistance. , copper 3-10%, chromium 0.5-5
%, the balance is essentially iron, and the pores are sealed by triiron tetroxide (Fe ₃ O ₄ ) formed on the inner walls of the pores. The present invention relates to a sliding member characterized in that a nitrided layer mainly composed of a γ' (Fe ₄ N) phase and an ε (Fe _2-3 N) phase is formed on at least the sliding surface. As a result of research, the inventor found that by nitriding a copper-containing iron-based sintered alloy, a large amount of γ' (Fe ₄ N) phase is generated, and the outermost layer contains ε along with the γ' phase.
A nitrided layer containing a large amount of (Fe _2-3 N) phase is formed, which significantly improves wear resistance, and the γ′ phase generation effect of copper is not impaired by the inclusion of chromium. The severe expansion caused by the above-mentioned nitriding treatment of the iron-based sintered alloy containing
This can be prevented by sealing the pores by applying steam treatment to form triiron tetroxide (Fe ₃ O ₄ ) on the inner walls of the pores, and reducing the thickness of the nitrided layer to about 0.1 to 0.5 mm. We have found that the nitrided layer obtained by steam-treating an iron-based sintered alloy containing copper and chromium and then nitriding it has extremely high hardness and even better wear resistance. The reason why the thickness of the nitrided layer is controlled by the steam treatment and significant expansion of the sintered body is prevented is considered to be due to the following reason. In other words, the pores of the sintered alloy are three-dimensionally interconnected, and during the nitriding process, the nitriding gas enters the pores within the sintered body, causing nitriding not only from the surface of the sintered body but also from the internal pores. progresses. Since chromium is an element that has a strong affinity with nitrogen, when an iron-based sintered body containing chromium is subjected to nitriding treatment, the nitridation occurs deep within a short period of time, causing severe expansion of the sintered body. Prior to nitriding, the sintered body is steam-treated to form a film of triiron tetroxide on the inner walls of the pores to seal the pores. The nitrided layer may become unnecessarily thick as the nitriding layer may be reduced, and then enter the pores and nitridation may proceed from the pores, or the reduction of triiron tetroxide and nitridation from the pores may proceed simultaneously. It is considered that this prevents the sintered body from expanding significantly. Gas nitriding or gas soft nitriding is suitable for the nitriding treatment in the present invention. According to gas nitriding or gas nitrocarburizing, atmospheric gas penetrates into the pores existing in the sintered alloy to form a sufficiently deep nitrided layer.
In the ion nitriding method, it is not possible to nitride from the inner walls of the pores below the surface, but only from the surface, so only an extremely shallow nitrided layer is formed, similar to nitriding in melted materials. In addition, in the salt bath nitriding method, it is difficult to remove the salt bath that has entered the pores after the nitriding process.
Afterwards, it rusted severely, making it unsuitable for use as a mechanical part. Regarding the chemical composition of the base material of the sliding member of the present invention, if the carbon content is less than 0.3%, the structure of the sintered alloy becomes mostly soft ferrite, resulting in insufficient mechanical strength, and if it exceeds 20%, free cementite forms. As the amount increases, it becomes brittle, so the content should be reduced from 0.3 to
A range of 2.0% is preferable. Copper is an essential element for forming the nitride layer mainly consisting of the γ' phase as described above, and it is
If it is less than 10%, the γ' phase will be little or not formed, and the effect of improving wear resistance will be insufficient. If it exceeds 10%, the Cu phase will increase in the structure of the sintered alloy, causing mechanical problems. The content is preferably in the range of 3 to 10% since the strength is significantly reduced. A particularly preferred range is 5 to 8%. Chromium, together with copper, significantly increases the hardness of the nitride layer,
This element is effective in further improving wear resistance; if it is less than 0.5%, the hardness of the nitrided layer will not increase sufficiently, and if it exceeds 5%, chromium-enriched areas will occur in the sintered alloy. This prevents the formation of a uniform nitrided layer with an appropriate thickness during the nitriding process.
Its content is preferably in the range of 0.5 to 5%. Alloying elements other than carbon, copper and chromium include:
Nickel, molybdenum, vanadium, titanium, niobium, and tungsten are effective in improving the mechanical strength of the sintered alloy base material and increasing the hardness of the nitrided layer to further improve wear resistance. Next, an experimental example will be explained. -100 mesh atomized iron powder, -325 mesh natural graphite powder, -325 mesh electrolytic copper powder and JIS G
Ferrochrome powder, which is made by crushing low carbon Ferrochrome No. 2 (FCrL2) specified in 2303 "Ferochrome" to -200 mesh, is blended as shown in Table 1, and zinc stearate 1 is added as a lubricant.
% and mixed and compression molded in a mold at a compression pressure of 5t/em ² to form a plate-shaped green compact with a thickness of 10mm and an outer diameter of 60mm.
A cylindrical green compact with an inner diameter of 40 mm and a height of 20 mm was obtained.
These green compacts were then heated at 1150°C in an RX gas atmosphere.
A sintered body was obtained by heating and sintering for 90 minutes. 1st
In the table, sample B is a sintered material for the sliding member of the present invention, and the other samples are comparative sintered materials. The table also includes analytical values for the ingredients. 〓〓〓〓〓

[Table] These sintered bodies were heated to 520℃ in a steam atmosphere for 120 minutes.
After steaming for 1 minute to generate triiron tetroxide (Fe ₃ O ₄ ) on the inner walls of the pores, equal amounts of ammonia gas and RX gas containing 0.6% carbon dioxide by volume were added. In a mixed gas atmosphere created by mixing
Gas nitrocarburizing treatment was performed by heating at 570°C for 120 minutes, and the following tests were conducted. Structure Observation The results of microscopic structure observation of the plate-shaped sample B of the present invention material from the surface toward the inside are shown in FIGS. 1 to 3. Figure 1 is a micrograph at 100x magnification showing the structure in a cross section perpendicular to the surface of the surface layer, and Figure 2 is a micrograph at 400x magnification showing the structure at a depth of 0.17 to 0.36 mm from the surface. FIG. 3 is a micrograph at a magnification of 400 times showing the structure in a depth range of 0.5 to 0.75 mm from the surface. The structure at a depth of 5 mm from the surface, ie, the center, was substantially the same as the structure shown in FIG. From FIG. 1, it can be seen that the material of the present invention has a nitrided layer 2 formed from the surface 1 to a depth of approximately 0.3 mm. The nitrided layer 2 was determined to have γ′ by the X-ray diffraction test described below.
It has been confirmed that this is a nitrided layer consisting mainly of (Fe ₄ N) phase and ε (Fe _2-3 N) phase. It can be seen from FIG. 2 that a gray-looking triiron tetroxide (Fe ₃ O ₄ ) film 6 is formed on the inner wall of the pores 5 of the base sintered alloy, and the pores 5 are sealed.
No triiron tetroxide was observed on the inner walls of the pores 3 in the nitrided layer 2, and it is thought that the triiron tetroxide was reduced and disappeared during the nitriding process. From FIG. 3, a film 6 of triiron tetroxide is observed on the inner wall of the pores 5 of the base sintered alloy, as in FIG. 2. 4 is the base of the base material sintered alloy, which exhibits a fine pearlite structure. X-ray diffraction test The component phases of the plate-shaped sample were examined by X-ray diffraction from the surface toward the inside, and the results are shown in Table 2. In the table, ε is Fe _2-3 N, γ' is Fe ₄ N, and α is ferrite. The mark ◎ indicates that a large amount was detected, the mark ○ indicates that a small amount was detected, and the mark x indicates that it was not detected.

[Table] The table shows that in all samples, triiron tetroxide is reduced to a depth of 0.1 mm from the surface and is no longer recognized during nitriding. Except for sample D, the ε phase is produced in large quantities up to a depth of 0.2 mm from the surface. In Sample B (material of the present invention) and Sample C, the γ' phase exists in a small amount on the surface, in a large amount in the range of 0.1 to 0.2 mm from the surface, and is almost entirely composed of the base material at a depth of 0.5 mm from the surface. Therefore, only small amounts of both the ε and γ' phases are detected. Sample A and sample〓〓〓〓〓
In D, no γ' phase was detected. α is the detected ferrite in the pearlite base of the base material, but except for sample D, the value is 0.1 from the surface.
It is not observed up to a depth of 0.2 mm, a little remains at a depth of 0.2 mm from the surface, and a large amount remains from a depth of 0.5 mm from the surface to the center because it is not nitrided. The above test results for Sample B (material of the present invention) are consistent with the results of the above-mentioned structure observation. Hardness test 200g from the surface to the inside of a plate-shaped sample
The micro-Vickers hardness was measured under a load of 100 mL, and the changes in hardness were determined. The results are shown in Figure 4. From the same figure, sample B (material of the present invention) has a high hardness of the nitrided layer, and the decrease in hardness is slight from the surface to a depth of 0.3 mm. In other samples, the hardness of the nitrided layer is not high (Sample A and Sample C), or the hardness is 0.2
It can be seen that the hardness suddenly decreases at a depth of mm (sample A and sample D), and that the material of the present invention has excellent wear resistance. Wear test: The above plate sample and cast iron as a comparison material other than the above.
A 3 mm thick test piece was taken from a conventional sliding member manufactured by forcibly chilling FC30 using a cold metal, and the surface layer was ground by 0.1 mm to reduce the surface roughness to a center line average roughness of 0.10. The surface layer of these specimens was subjected to a wear test using an Okoshi type rapid abrasion tester. The test conditions are as follows. Final load: 6.3Kg, friction speed: 1.14m/sec,
Friction distance: 600m, lubricating oil: 1 drop/sec of motor oil #30 at 60°C, mating motor material: H _R
Chrome molybdenum steel with hardness of C30
SCM435. The test results are shown in FIG. From the same figure, it can be seen that sample B (inventive material) has significantly less friction than conventional chilled cast iron sliding members, as well as other comparison materials made of nitrided sintered alloys, and has excellent wear resistance. It can be seen that the performance has been significantly improved. Dimensional change measurement For a cylindrical sample with an outer diameter of 60 mm, an inner diameter of 40 mm, and a height of 20 mm, the change in the outer diameter of the sample after nitriding treatment with respect to the inner diameter of the mold (die) used for compacting was measured. Average value for each 50 samples
() and range (R) were determined. The results are shown in Table 3. For reference, the same table also includes the results of similar measurements when the sintered material was subjected to nitriding treatment without being subjected to steam treatment. The average value is the average value of sample outer diameter - mold inner diameter / mold inner diameter x 100%, and the range R is expressed as the range of (sample outer diameter - mold inner diameter) mm.

[Table] As shown in the table, if nitriding treatment is performed without steam treatment, not only will a large dimensional change occur regardless of the composition of the sintered body material, but the dimensional variation will also be very large. When the body material is steam-treated to seal the pores with triiron tetroxide, and then nitrided, the dimensional changes and dimensional variations are small.
In particular, it can be seen that the material of the present invention (sample B) has excellent dimensional accuracy. The dimensional change of the above-mentioned material of the present invention is smaller than the dimensional change due to the usual heat treatment (quenching, tempering, etc.) of molten steel materials, indicating that the material of the present invention has good dimensional accuracy. As explained above, it contains copper and chromium,
The sliding member of the present invention, which is made by nitriding an iron-based sintered alloy whose pores are sealed with triiron tetroxide, has good dimensional accuracy despite containing chromium, and the nitrided layer has a γ′ It is mainly composed of phase and ε phase,
It has high hardness because it contains copper and chromium.
In addition, it has sufficient thickness and has good oil retention properties due to the presence of pores, so it exhibits excellent abrasion resistance even under severe usage conditions, and has great industrial utility value.

[Brief explanation of the drawing]

Fig. 1 is a micrograph at 100x magnification showing a typical structure of the surface layer of the sliding member of the present invention, Fig. 2 is a micrograph at 400x magnification, and Fig. 3 is the base material of the sliding member of the present invention. Magnification showing typical structure of sintered alloy〓〓〓〓〓
This is a 400x photomicrograph. In the figure, 1 is the surface, 2 is the nitrided layer, 3 is the pores in the nitrided layer, 4 is the base of the base material sintered alloy, 5 is the pores in the base material, and 6 is triiron tetroxide. FIG. 4 is a graph showing the change in hardness from the surface toward the inside, and FIG. 5 is a graph showing the results of an abrasion test using an Okoshi rapid abrasion tester. 〓〓〓〓〓

Claims (1)

[Claims] 1 Carbon 0.3-2.0%, Copper 3-10%, Chromium 0.5-5
%, the balance is essentially iron, and the pores are sealed by triiron tetroxide (Fe ₃ O ₄ ) formed on the inner walls of the pores. 1. A sliding member, characterized in that a nitrided layer mainly consisting of a γ' (Fe ₄ N) phase and an ε (Fe _2-3 N) phase is formed on at least the sliding surface.