JPS643934B2 - - Google Patents

Description

[Detailed description of the invention]

The present invention relates to a sintered alloy with excellent heat resistance and wear resistance, and more specifically, the present invention relates to a sintered alloy that is suitable as a sealing material for use in turbocharger exhaust manifold side seal rings, internal combustion engine piston rings, etc. The present invention relates to a method for producing a sintered alloy. Generally, FC and
FCD-based cast iron or resin are often used, but even if they have a fair degree of wear resistance, they may lack heat resistance. On the other hand, sintered alloys tend to be used for piston rings, etc., but this is because sintered alloys have 10 to 20% of pores, which act as oil pockets to retain lubricating oil and improve wear resistance. It is intended to take advantage of the property of improving seizing resistance. However, the pores inherent in the sintered alloy reduce the effective cross-sectional area of the sintered sealing material, resulting in an increase in the actual stress of the sealing material and a deterioration in heat resistance. Reducing the pore volume ratio is effective in compensating for this drawback and improving the heat resistance of sintered alloys, but this requires the use of special techniques such as sinter forging and hot pressing, which increases the cost of sintered products. It is economically disadvantageous. Other techniques for improving the heat resistance of sintered sealants include Cr, Ni, and other common elements that improve heat resistance.
There is a method of pre-mixing powders such as Co, Mo, and W into iron powder, but since sintering generally uses a solid phase diffusion reaction, Ni and Co are excluded.
It is extremely difficult to uniformly diffuse and dissolve Cr, Mo, W, etc. into the Fe matrix of a sintered alloy. Therefore, the sintered alloy obtained by mixing the heat resistance improving element powder and iron powder as described above has Cr, Mo, and W.
These elements exist in a particle shape similar to that of a single element, and alloying with these elements has not reached a level where it can sufficiently contribute to improving heat resistance and wear resistance. The present invention provides a sintered alloy that can solve the above problems. Hereinafter, the required performance of the sealing material and the problems of the prior art will be specifically explained using a turbocharger exhaust manifold side seal ring as an example. In recent years, an increasing number of internal combustion engines are equipped with turbochargers as a means of improving fuel efficiency and increasing output of automobiles. Seal ring for turbocharger exhaust manifold side (hereinafter referred to as seal ring)
are exposed to high temperatures due to the influence of high-temperature exhaust gas, and must be sealed with lubricating oil at high temperatures. Therefore, maintaining tension is one of the important characteristics of a seal ring, and therefore, high heat resistance is required as a performance of the sealing material. Furthermore, since the rotational speed of the turbocharger's turbine is at a high speed of up to several hundred thousand rpm, it also has high performance as a sealing material in terms of wear resistance (including the property of not wearing out the mating material) and seizure resistance. required. FC and
FCD cast iron and resin clearly lack heat resistance for seal rings, so seal rings are currently made of high-speed steel, austenitic cast steel, high Cr cast steel, etc.
Molten materials such as stainless steel are commonly used. Although these melt-produced materials have excellent heat resistance, the seal ring has a small diameter, requires a large number of processing steps, and has the disadvantage that the material yield is extremely low. Furthermore, these melt-produced materials have problems in seizure resistance and wear resistance. On the other hand, since sintered alloys have a high degree of freedom in material composition and contain pores, improvements in heat resistance, wear resistance, etc. can be easily implemented. Moreover, since sintered alloys can be manufactured with extremely high dimensional accuracy, it is possible to greatly reduce the number of processing steps, and the material yield is also extremely good. However, when heat resistance is imparted to sintered alloys by adjusting the material composition as described above, the technique of simply mixing powder of heat-resistant elements with iron powder and then sintering the material results in a remarkable improvement in heat resistance. I can't expect that. In order to solve the problems of the prior art as described above and dramatically improve the heat resistance and wear resistance of a sintered alloy sealing material, the present inventors have determined that a structure that satisfies the following conditions can be obtained. We learned that this is important. Pre-alloyed Co-Mo-Cr-Si hard particles are dispersed in the sintered alloy matrix, and austenitic stainless steel powder is used as the main raw material to improve the heat resistance of the sintered alloy matrix. Furthermore, cobalt is dissolved in the matrix during sintering. Thus, the heat resistance is further improved than when near-elementary particles are dispersed in the sintered alloy matrix. The wear resistance of the sintered alloy is generally good due to the oil retaining effect of the internal pores, but it is further improved by the addition of the hard particles. That is, the hard particles, which are relatively harder than the sintered alloy matrix, form the primary sliding surface, while the relatively soft matrix forms a lubricating oil pool due to initial wear, similar to the internal pores. Therefore, not only the wear resistance but also the seizure resistance is further improved than if only due to the oil retaining effect of the pores. The hard particles mentioned above are
Co--Mo--Cr--Si particles whose composition is appropriately adjusted as described below are used. Fine chromium carbide precipitates at the grain boundaries of a sintered alloy matrix made of austenitic stainless steel (hereinafter abbreviated as austenite matrix), further improving wear resistance and heat resistance. The present invention, which satisfies the above conditions and,
Special alloy powder particles are dispersed in a matrix made of austenitic stainless steel containing Co as a solid solution, and have a structure in which fine chromium carbide is precipitated at the grain boundaries of the matrix, and the special alloy powder particles are In terms of weight ratio, it essentially consists of Mo: 25-40% Cr: 5-20% Si: 1-20% Co: 50-70%, and is contained in an amount of 1-20% by weight, and the austenitic stainless steel The overall composition of the matrix and the special alloy powder particles is as follows, in terms of weight ratio: C: 0.5-1.5% Cr: 15-25% Ni: 6-20% Mo: 0.3-10% Co: 2.5-25% A predetermined amount of austenitic stainless steel powder, cobalt powder, graphite powder, and the above-mentioned special alloy powder are mixed so that Si: 0.1 to 4% and the remainder substantially consists of Fe and unavoidable impurities. Provided is a method for producing a heat-resistant and wear-resistant sintered alloy, which comprises compacting, forming, and sintering in a reducing atmosphere. In the present invention, percentages are weight percentages unless otherwise specified. Hereinafter, the reasons for the limitations of the present invention will be described and further explained. In special alloy powder particles, Co increases its heat resistance and contributes to improved wear resistance at high temperatures. Mo, Si, and Cr combine with each other to form an intermetallic compound and are dispersed in a matrix of powder particles mainly composed of Co, which increases the hardness of the special alloy powder particles and contributes to improved wear resistance. In addition, Co, which is a special alloy powder particle, diffuses into the austenite matrix and improves its heat resistance. In order to ensure sufficient diffusion and solid solution of Co under normal sintering conditions, special alloy powder must be used before sintering.
It is necessary that the Co content is 50% or more.
The proportion of Co diffused into the austenite matrix as a solid solution and the proportion of elements (mainly iron) that diffuse in the opposite direction to this diffusion (diffusion from the austenite matrix to the hard alloy powder) are thought to be small.
If the Co content exceeds 70%, the hardness of the special alloy powder particles will decrease, so the Co content must be 70% or less. The composition of the special alloy powder particles in the sintered alloy of the present invention is, when the contents of Co, Mo, Cr, and Si are expressed in weight%, Mo = 25 to 40%, Cr = 5 to 20%.
%, Si=1 to 20%. When the composition is within this range, a Mo-Si-Cr intermetallic compound with a crystal structure called a Laves phase is generated, and wear resistance and heat resistance are improved. The special alloy powder particles within the above composition range have a hardness of Hv500 to 1500, which improves wear resistance and seizure resistance, and does not increase the wear of the mating material. Next, if the amount of special alloy powder particles is less than 1%, the wear resistance and heat resistance of the sintered alloy will be insufficient, and if it exceeds 20%, the compactability during production will be reduced. Therefore, the amount of special alloy powder particles is 1 to 20%. The reasons for limiting the overall composition of the sintered alloy are as follows. When C is less than 0.5%, the amount of Cr carbide is insufficient, and when it exceeds 1.5%, excessive Cr carbide precipitates at the grain boundaries of the austenite matrix, resulting in a tendency for the material to become brittle. If Cr is less than 15%, the Cr content of the austenite matrix will be less than 11% when the amount of special alloy powder particles and the Cr content are maximum within the claimed range, and the heat resistance of the austenite matrix will decrease. And also Cr
The amount of carbide is insufficient. On the other hand, when Cr exceeds 25%, when the amount of special alloy powder particles and Cr content are minimum within the claimed range, the Cr content of the austenite matrix exceeds about 25%, and the effect of Cr is saturated. Since this is disadvantageous, the upper limit was set at 25%.
If the Ni content is less than 6%, the austenite structure of the matrix will become unstable, resulting in insufficient heat resistance, while if the Ni content exceeds 20%, it is unfavorable from a pharmaceutical standpoint.
If Mo is less than 0.3%, the special alloy powder particles will not perform the intended function, while if it exceeds 10%, when the amount of special alloy powder and the amount of Mo are the minimum within the claimed range, Mo in austenite matrix
When the Mo content is about 10% or more, which is also the maximum, the Mo content in the austenite matrix is about 2% or more, which is economically unfavorable because the effect of Mo on improving the heat resistance of the austenite matrix is saturated. If Co is less than 2.5%, the amount of solid solution in the austenite matrix is insufficient, and if it exceeds 25%, the hardness of the sintered alloy tends to decrease.
If the Si content is less than 0.1%, the special alloy powder particles will not perform their intended function, and if it exceeds 4%, the sintered alloy will locally soften due to its melting point lowering effect, resulting in a decrease in strength. The solid solution of Co into the austenite matrix, which is one of the features of the present invention, is achieved by adding Co to the raw material of the sintered alloy in advance.
Co can be dissolved as a solid solution, but since many commercially available austenitic stainless steels do not contain Co, it is necessary to dissolve some of the Co in the Co powder or special alloy powder into the austenite matrix during sintering. is preferred. As mentioned above, the special alloy powder (particles) is a Co-Mo-Cr-Si alloy, and among the alloying elements, Co is the most easily diffused, and it diffuses into the austenite matrix and improves its heat resistance. At the same time, it also serves to increase the bonding strength between the special alloy powder particles and the austenite matrix during diffusion. One of the structural features of the present invention is that fine chromium carbides are precipitated at the grain boundaries of the austenitic matrix. must be reacted with during sintering. The composition of the austenitic stainless steel powder itself does not need to be particularly limited, and it may be any powder that does not cause problems in compressibility, fluidity, etc. in normal powder compaction. The sintering conditions in the present invention are that after compacting the mixed powder at 5 to 10 tons/ ^cm2 ,
It is desirable to adopt a condition of heating to 0.degree. C. in a reducing atmosphere (hydrogen gas or decomposed ammonia gas). Furthermore, if the proportion of pores in the sintered alloy exceeds 20%, the effective cross-sectional area of the sintered material will decrease, the actual stress will increase, and the heat resistance will decrease, so the relative density of the sintered material will be high. That's more preferable. However, it is generally difficult to reduce the porosity to 5% or less by cold forming and sintering, which are commonly used in the production of sintered alloys. The preferred composition of the sintered alloy of the present invention is C0.8-1.2
%, Cr16~18%, Ni8~12%, Mo2~5%, Co6
~10%, and Si0.1~1.0%. In addition, the particle size of special alloy powder (particles) is 150 microns or less, and the particle size of austenitic stainless steel powder (particles) is
Preferably 150 microns or less. Examples will be described below and a more detailed explanation will be added. Example 1 A predetermined amount of the various powders shown in Table 1 was weighed, mixed for 30 minutes with a V-type mixer, and then molded under a pressure of 7 tons/
^cm2 , and finally sintered at 1200°C for 1 hour in a decomposed ammonia gas atmosphere. However, the special alloy powder and austenitic stainless steel powder were set to -100 mesh (149 μm). In addition, graphite powder and cobalt powder are -325 mesh (44 μm)
And so. After sintering, the nominal diameter is 20mm, width is 1.6mm,
A seal ring with a thickness of 1.1 mm was made and a tension reduction test was conducted. Tension reduction tests were carried out by loading the seal ring into a cast iron cylinder with the same dimensions as the seal ring's nominal diameter, and testing it at 350℃, 400℃, and 450℃.
This was carried out by heating in Ar gas for 10 hours. The amount of change in the free gap between before and after the test was determined and taken as the tension reduction rate. Table 1 also shows each characteristic value and tension reduction rate after sintering.

【table】

[Table] From the results in Table 1, it is clear that the material of the present invention has excellent heat resistance. FIGS. 1 and 2 show the metal structure of the material A of the present invention shown in Table 1 (magnifications are 100 times and 500 times, respectively). In FIG. 2, a shows special alloy powder particles, b shows fine carbides precipitated at grain boundaries, c shows an austenitic stainless steel matrix, and d shows pores. Example 2 A predetermined amount of the various powders shown in Table 2 was weighed and mixed in a V-type mixer for 30 minutes, and a pin (wear test piece) was produced under the same molding and sintering conditions as in Example 1. The wear test was conducted using a rotor pin type wear tester shown in FIG. The material of rotor B as a mating material is JIS SUM43 by quenching and tempering.
It was set as HRC35. Rotor B of φ40mm and φ10
mm, length 15mm pin A is polished to approximately 1.
It had a finish roughness of ~2 μm RZ. We performed a wear test by applying a load in the direction of the arrow while lubricating it by dropping SAE #30 engine oil.
The wear amount of the pin is measured by the long axis of the wear scar, and the wear amount of the rotor as a mating material is measured by measuring the amount of dent with a roughness meter at a load of 2 kg, a friction speed of 150 m/min, and a friction distance of 5000.
It was measured under the conditions of m. Further, the friction speed was set to 200 m/min, the load was gradually increased, and the load at which seizure occurred was determined and the seizure limit load was determined. The results are also shown in Table 2. It is clear that, compared to the comparative example, the material of the present invention not only has less wear resistance of itself but also has less wear of the mating material, and has higher seizure resistance.

【table】

【table】 [Brief explanation of the drawing]

1 and 2 are metallurgical micrographs of the material A of the present invention shown in Table 1 of Example 1, and FIG. 3 is a diagram schematically showing a rotor pin type wear tester. A... Pin, B... Rotor, a... Special alloy powder particles, b... Fine chromium carbide, c... Austenite matrix, d... Holes.

Claims (1)

[Scope of Claims] 1. Special alloy powder particles are dispersed in a matrix made of austenitic stainless steel containing cobalt as a solid solution, and the matrix has a structure in which fine chromium carbide is precipitated at the grain boundaries of the matrix. The special alloy powder particles are substantially composed of Mo: 25-40%, Cr: 5-20%, Si: 1-20%, Co: 50-70%, and contain 1-20% by weight. Further, the overall composition of the matrix made of austenitic stainless steel and the special alloy powder particles is as follows in weight ratio: C: 0.5 to 1.5% Cr: 15 to 25% Ni: 6 to 20% Mo: 0.3 to 10% Co: 2.5 to 25% Si: 0.1 to 4%, and the austenitic stainless steel powder, cobalt powder, graphite powder, and the special alloy powder are mixed so that the balance substantially consists of Fe and unavoidable impurities. A method for producing a heat-resistant and wear-resistant sintered alloy, which comprises mixing a predetermined amount, forming a powder, and sintering in a reducing atmosphere.