JPS589139B2 - Materials made by powder hot forging - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a high-strength powder hot forged material with excellent abrasion resistance.

Powder hot forged parts have the excellent economical properties of powder metallurgy, i.e.
Due to the extremely good material yield, the ability to largely omit machining processes such as cutting, and the excellent material uniformity, they are starting to replace cast parts, ordinary forged parts, or machined parts.

However, powder hot forged parts do not have a balance between product cost (i.e. raw material cost and processing cost) and performance at the stage of industrialization, and in the end, they are not necessarily superior to molten materials in terms of price competitiveness in the market. This was a problem.

The main field in which hot powder forged products are being used is mechanical parts such as copper gears and cams.

The material properties required for such parts are, of course, first and foremost high tensile, compressive strength and toughness.

More important are the hardness and appropriate sliding properties necessary for wear resistance.

In particular, the latter property is not simply good if it is hard or strong, but can only be achieved by appropriate conditions of the fine metal structure.

Moreover, all of these material properties must be satisfied at the same time, which is extremely difficult to achieve using conventional materials, or is economically unsatisfactory.

For example, although cast iron materials have excellent abrasion resistance due to the action of graphite particles (flaky or spherical) contained in them, they are not strong due to the brittleness of the graphite particles contained and voids at the particle interface. , the toughness was inferior.

Furthermore, cast iron cannot avoid the non-uniformity and segregation of the casting structure, so it is not a suitable material.

In the case of carbon steel and low-grade, low-alloy steel, although strength and toughness can be satisfied through appropriate heat treatment, wear resistance and sliding resistance are inferior, and special surface treatment such as nitriding treatment is required.

In addition, the presence of non-metallic inclusions that remain in molten steel cannot be ignored during the steelmaking process, and the orientation of the material caused by forging and rolling processes can impair the isotropy of the strength of the part. There was a problem.

Similar problems exist in conventional powder hot forged parts, and materials with high strength and excellent abrasion resistance have not been obtained.

The present invention combines the characteristics of powder metallurgy, which is a technology to create a composite structure alloy that is difficult or impossible to manufacture with melting methods, and the characteristics of powder hot forging technology, which is high density and free of defects such as pores. By combining materials manufacturing techniques, we succeeded in creating a new, unprecedented material for high-performance mechanical parts.

The most important component of the material of the present invention is fine graphite particles uniformly dispersed and precipitated in the base phase.

In the past, the material in which graphite was dispersed and precipitated was of course known as cast iron, and spheroidal graphite cast iron was an excellent material in which spherical graphite particles were dispersed. However, the size of the graphite particles was 10 μm. Relatively large at ~100μm,
For this reason, the strength and toughness of the material were inferior.

It is difficult to finely control graphite particles using the casting method, and even changing the carbon content over a large range leads to the precipitation of a hard cementite phase, making it difficult.

However, in the powder metallurgy method, it has been found that fine graphite particles can be uniformly dispersed and precipitated very easily by selecting an appropriate composition of the raw material powder.

In other words, in the powder state, carbon that exists in various forms such as free graphite, carbide, and solid solution carbon diffuses at high temperatures without melting as an alloy, and by sintering, the austenite phase becomes a solid solution at high temperatures. Through powder hot forging, which consists of at least two steps: a first step to crush residual pores and adjust the shape by forging, fine graphite particles with a particle size of 0.5 to 20 μm are uniformly dispersed. It can be precipitated.

The powder hot forging process is an essential step, firstly, to increase matrix strength, and secondly, to crush pores that become the core of giant graphite precipitation.

The chemical composition to be selected for this purpose is most important for the presence of Si, which is a graphitizing element for carbon and a reinforcing element for iron alloys, and must be selected in a range of 2 to 3%.

This range was chosen because below the lower limit the above-mentioned effect is not effective, and above the upper limit the hardening progresses too much and becomes brittle.

Other essential elements include Mn, which is most effective in improving hardenability and is also a strengthening element for iron alloys.
．． The content is 9%.

Mn is also an effective element for stably existing graphite.

The upper and lower limits were similarly set based on the effective range and harmful range.

Needless to say, C is an element that becomes graphite, but at the same time it is an essential element for forming steel.

As mentioned above, if there are many large graphite particles, it will have an adverse effect on strength and toughness, and if there are too few, the effect of abrasion resistance will be reduced.

Further, since carbon in solid solution is essential for strengthening the base phase as eutectoid steel, about 0.8% is necessary as a solid solution content.

It has been found that the optimum total carbon content is 1 to 2% when added to the amount of carbon necessary for graphite particles.

More preferably, it is 1.4 to 1.8%.

Other elements such as P, S, and O are normally present as elements that cannot be avoided, but if their amount is less than 0.3%, no positive effect can be observed, so they are usually treated as impurities. There is no particular stipulation regarding the extent of contamination.

Furthermore, trace amounts of Mg, Al, Sn,
Transition elements such as Mo, Cr, and Cu are not particularly stipulated as they do not have any particularly positive effects as long as they exist as impurities.

By further heat-treating the material of the present invention, it is possible to harden the base phase, increase strength, and improve wear resistance without impairing sliding resistance.

In this case, the hardness of the base phase is most preferably adjusted to 400 to 600 mHv, and upper and lower limits are set to avoid embrittlement due to excessive hardening and to enhance the effect.

Example A powder having a composition of Fe-2.6Si-0.8Mn-1.7C was obtained by crushing, classifying, and decarburizing cast iron chips.

This powder mainly contained 1.7% of C as coarse graphite or flake graphite in the Fe-2.6Si-0.8Mn matrix.

The powder was formed in a mold, heated and sintered in a reducing gas, and immediately hot-forged in the mold.

By this process, the molded body has a density of 7.6g, which is almost the theoretical density.
/c. It became c.

Furthermore, this forged body was quenched at 900°C and tempered.

Photographs of the cross-sectional structure of each material are shown in Figures 2 and 3.

It is clearly observed that fine particles of graphite are uniformly dispersed and precipitated.

Table 1 shows comparative examples of the mechanical properties of this material.

The results of the wear test are shown in Figure 1.

Table 2 shows three examples and three comparative examples. From these results, it is clear that the material of the present invention has superior properties suitable for mechanical structural parts compared to conventional materials, and is useful for being widely used as a powder hot forging material. It is a good material.

[BRIEF DESCRIPTION OF THE DRAWINGS] FIG. 1 is a chart showing the performance of an embodiment of the present invention, which is the result of an Okoshi type wear test. The mating material is S-55C (HV-270), the pressure is ~10 kg/cm2, the distance is ~500 m, and the lubrication is ~none.The solid line shows S55C, the dotted line shows FC-25, and the chain line shows the product of the present invention. FIG. 2 shows the micrographs of the structure taken at 100 times magnification, and FIG. 3 shows the structure taken at 400 times magnification.

Claims (1)

[Claims] 1 Weight percent: Si2-3%, Mn0.2-0
．． 9%, C1.0-2.0%, the remainder is substantially iron,
A hot powder powder characterized in that 0.5 to 1.2% of the carbon content is uniformly dispersed and precipitated as fine graphite particles with a particle size of 0.5 to 2.0 μm by hot powder forging. Material by forging.