JPS58144450A - Preparation of high strength and high toughness sintered material having cast iron structure - Google Patents

Abstract

PURPOSE:To enable the preparation of a high strength and high toughness sintered material having a cast iron structure , by a powder metallurgical method wherein a specific amount of an Fe-Si powder and graphite powder are uniformly mixed in an iron powder and a resulting mixture is molded into a pressed powder body to be sintered in a reductive atmosphere. CONSTITUTION:To an iron powder usually used as the stock material for powder metallurgy, an Fe-Si powder with Si content of 15-75% and an average particle size of 1-10mum is added in an amount of 0.8-2.5% on the basis of Si content and, at the same time, a graphite powder with an average particle size of 20mum or less is added in an amount of 1-2% to carry out sufficient mixing. The resulting mixed powder is pressurized and molded in a mold under pressure of 4-6ton/cm<2> and the molded body is heated and sintered at 1,050-1,180 deg.C in a reductive atmosphere comprising, for example, an NH3 decomposition gas. By this method, a high strength and high toughness sintered material having a structure consisting of free graphite, a ferrite phase and a perlite phase, excellent in anti-wear property, cutting property and vibration absorbability and having the same characteristics as usual cast iron is obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a high-strength, high-toughness sintered material having a cast iron structure by a powder metallurgy method.

Currently, ordinary cast iron has excellent properties such as wear resistance, machinability, and vibration absorption ability, and is also cheap and has a wide range of uses as machine parts. It is known that the excellent properties are mainly due to free graphite uniformly dispersed in the matrix of ferrite and pearlite phases. For example, in the case of sliding materials made of cast iron, free graphite acts as a solid lubricant on the sliding surface to reduce friction, and the remaining pores of free graphite serve as oil reservoirs for oil retention. Also, during cutting, the finely distributed free graphite acts as a chip breaker and plays the role of improving rigidity.

However, although cast iron has excellent properties as a material for machine parts, on the other hand, if the casting capacity is small, the cooling rate after casting is fast, so it becomes white.
There was a problem in that small parts could not have the characteristics inherent to cast iron.

Furthermore, since the production of cast iron parts had to rely on the casting method, it also had the essential problem of being inferior in mass productivity compared to the powder metallurgy method.

Up until now, various attempts have been made to research sintered materials that have the excellent properties of cast iron and the mass productivity of powder metallurgy, and their manufacturing methods.
It was not successful for the following reasons. That is, (a) when producing iron-based sintered materials, if a large amount of graphite powder is blended as a raw material powder, cementite will precipitate on the base material during sintering, harden it, and deteriorate mechanical properties. 9. On the other hand, if the sintering temperature is lowered, cementite precipitation can be prevented, but strength cannot be obtained.

(b) A method to prevent the precipitation of heptamentite by adding a graphitization-stable element such as Si is considered, but generally the temperature is about 1.200°C to diffuse and form a solid solution of Sl in Fe.
This heating temperature is significantly higher than the sintering temperature of ordinary iron-based sintered materials, and therefore, manufacturing costs will increase if sintering is performed at this heating temperature. The sintering atmosphere must be strictly controlled to prevent oxidation of Sl.

Therefore, the present inventors took into account the problems in producing cast iron as described above, and under the normal production conditions of iron-based sintered materials, free graphite was formed in a matrix consisting of a pearlite phase and a ferrite phase. As a result of conducting research to obtain a sintered material with a uniformly distributed cast iron structure, we found that (1) F as a raw material powder when manufacturing iron-based sintered materials;
e-Si alloy powder is used, and the Si source is Fe-
When the particle size of the Si alloy powder is adjusted to a specific range, the 81 component is evenly distributed on the surface of the iron powder that is also used as the raw material powder, and as a result, the 81 component is not oxidized during sintering (Fe In addition to stabilizing the alpha phase of Fe by forming a solid solution in it, it also accelerates the diffusion between Fe particles, promoting sintering and improving mechanical properties.

(2) If the C and S1 contents in the raw material powder are appropriately selected and the particle size of graphite powder is below a certain value, C becomes Fe.
The C around these particles becomes easily diffused and dissolved in the Fe particles, and in the cooling process, due to the graphitization promoting effect of 81, the carbon precipitates out of the vacancies and undissolved graphite as nuclei.
The final result is a so-called cast iron structure in which there is an 81-ferrite phase around the free graphite and a pearlite phase on the outside, and this cast iron structure has a relatively small amount of free graphite, so it has high strength and Must have high toughness.

The findings shown in (1) and (2) above were obtained.

This invention was made based on the above findings, and contains 81 of 15 to 75 inches and has an average particle size of 1 to 75.
1Oμm Fe-Si alloy powder: 08 to 2.5 in S1 amount
%, graphite powder with an average particle size of 20 μm or less: less than 1 to 2%, iron powder: remainder. After forming the green compact into a compact at
The present invention is characterized by a method for manufacturing a high-strength, high-toughness sintered material having a cast iron structure consisting of a ferrite phase and a pearlite phase.

Furthermore, in producing the sintered material of the present invention, the iron powder used as the raw material powder may be one that is normally used as a raw material for powder metallurgy, and the compacting pressure of the green compact is also within the usual range of 4 to 6.
It may be about ton/cn, and as the reducing atmosphere, for example, ammonia decomposition gas is suitable.

Next, the reason why the manufacturing conditions are limited as described above in the method for manufacturing a sintered material of the present invention will be explained.

(a) 81 content in Fe-Si alloy powder When the Si content is less than 15%, the Fe-Si alloy powder becomes too soft and difficult to grind, while when the S1 content exceeds 75%, Since the total amount of Fe-Si alloy powder blended is relatively too small and it becomes difficult to uniformly coat the surface of the iron powder, the Si content 6- was set at 15 to 75%.

(b) Blending amount of Fe-Si alloy powder Fe-Si alloy powder is blended in order to improve the strength and stiffness by incorporating 81 into the sintered material. If the Si content is less than 0.8%, the desired strength and stiffness cannot be achieved, while if the Si content exceeds 25%, the strength will decrease in the sintered material. The amount was determined to be 08 to 25 inches in terms of Sl amount.

(C) Amount of graphite powder blended If the blended amount is less than 1%, it will not be possible to obtain a cast iron structure in which a predetermined amount of free graphite exists in the sintered material matrix, and as a result, the stiffness will be reduced. On the other hand, if more than 2% is added, the strength and toughness tend to deteriorate, so the amount added is limited to less than 1 to 2.

(d) Average particle size of Fe-Si alloy powder If the average particle size is less than 1 μm, it will be easily oxidized and difficult to handle. On the other hand, if the average particle size exceeds 10 μm, diffusion into iron powder will be slow. As a result, the formation of the α phase is delayed and the mechanical properties are deteriorated, so the average particle size is set at 1 to 10 μm.

(e) Average particle size of graphite powder If the average particle size exceeds 20 μm, the specific surface area becomes small and diffusion into the iron powder becomes slow, so the average particle size was determined to be 20 μm or less. Note that the average particle size is preferably 15 μm or less.

(f) Sintering temperature If the sintering temperature is less than 1050°C, the resulting sintered material cannot be expected to have sufficient strength. On the other hand, if the sintering temperature exceeds 1180°C, a liquid phase will begin to appear and the sintered material will deteriorate. The temperature was set at 1050 to 1180°C since deformation occurs.

Next, the method of the present invention will be explained by comparing examples and comparative examples.

Example As raw material powder, average particle size: 2 μm.

Graphite powder having average particle diameters of 10 μm, 18 μm, and 30 μm, respectively, average particle diameters: 1.2 μm, 22 μm,
9.8 pm and 12.4 μm, and Si
Fe-Si alloy powder with a content of 16%, average particle size: 2 μm, Fe-Si alloy powder with Si content of 74%, average particle size: 2.2 μm, S1 content is 7
9% Fe-Si alloy powder, and particle size: -100m
esh reduced iron powder was prepared, and these raw material powders were blended into the composition shown in Table 1, mixed, and 4 ton/cr
After molding into a green compact at a pressure of I, the powder is sintered in an ammonia decomposition gas atmosphere at the temperatures shown in Table 1. Inventive sintered materials 1 to 7 and comparative sintered materials 1 to 8 with dimensions of lo Hz x 5 og were produced, respectively. Note that Comparative Sintered Materials 1 to 8 were manufactured under conditions in which any of the manufacturing conditions (those marked with * in Table 1) were outside the scope of the present invention.

Next, the resulting sintered materials 1 to 7 of the present invention and comparative sintered materials 1 to 8. Furthermore, for the purpose of comparison, conventional cast iron (Fe12 manufactured by the melting method) was prepared separately, and its tensile strength and impact value were measured, and a friction and wear test and a cutting test were conducted.

The friction and wear test was conducted using a bottle-on-disk type testing machine, using pins molded from the various sintered materials mentioned above, and a disk made from 545C' Cr-plated material.
The friction coefficient and specific wear amount were measured at a speed of 1 ml sec and a load of 4 kgf/d.

In addition, in the machining test, the above various sintered materials with a plate thickness of 5.5 mm were penetrated using a high-speed steel awl with a diameter of 3.2 gφ at a rotation speed of 400 rpm and a pressing force of 5 kgf. The penetration time was measured. These measurement results are also shown in Table 1.

From the results shown in Table 1, the sintered materials 1 to 7 of the present invention are
It is clear that all of these not only have excellent friction and wear characteristics and machinability equivalent to conventional cast iron, but also have much higher strength and toughness than conventional cast iron. In contrast, as seen in Comparative Sintered Materials 1-8, when the manufacturing conditions fall outside the scope of this invention, the strength, toughness, and friction and wear properties deteriorate.

It is clear that at least one of the characteristics of 11- and machinability becomes inferior.

A microscopic microscopic photograph (magnification: 500x) of the sintered material 2 of the present invention is shown in Fig. 1, which shows that the sintered material 2 has a cast iron structure in which free graphite is distributed in the matrix of the toshiferite phase and pearlite phase. That is clear.

As described above, according to the present invention, a high-strength, high-toughness sintered material having a cast iron structure can be manufactured at a low cost with relatively simple operations, and therefore mechanical parts that require the characteristics of cast iron can be manufactured. Of course, it brings about industrially useful effects such as making it possible to mass produce small mechanical parts with complex shapes.

[Brief explanation of the drawing]

FIG. 1 is a microscopic photograph of the structure of the sintered material of the present invention. Applicant Mitsubishi Metals Co., Ltd. Agent Tomi 1) Kazuo 12-

Claims (1)

[Claims] Contains 15 to 75% of 81 and has an average particle size of 1 to 10
μm Fe-8i alloy powder = Sl amount of 0.8 to 2.5 μm, graphite powder with an average particle size of 20 μm or less: 1 to less than 2%, iron powder: the remainder, and a blending composition consisting of the following (weight %) After uniformly mixing raw material powders having a
A method for producing a high-strength, high-toughness sintered material having a cast iron structure consisting of free graphite, a ferrite phase, and a pearlite phase, characterized by sintering at a predetermined temperature within the temperature range of °C.