JPS5839222B2 - Manufacturing method of wear-resistant sintered alloy - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method for producing a high carbon, high Cr sintered alloy having excellent wear resistance.

Many materials have been developed as wear-resistant sintered alloys,
Some of them are Varibsheet IJ for automobile engines.
It has been put to practical use in applications such as

Recently, high-speed steel is sometimes used especially in parts that require a high degree of wear resistance.

However, since high-speed steel contains large amounts of expensive elements such as W, Co, and Mo, it inevitably poses a major economic problem.

In order to generally improve wear resistance, macroscopic hardness is increased, and conventional methods for this purpose include utilizing the martensitic transformation of steel, and coating the surface with a hard phase such as nitride. Alternatively, a method of dispersing carbide or other hard particles in a matrix has been adopted.

The present invention adopts a method in which Cr-based carbide, which is relatively advantageous in terms of economy, is dispersed in the matrix as hard particles without using expensive alloying elements such as those used in high-speed steel, thereby creating a sintered alloy with excellent wear resistance. The purpose of this invention is to provide a method for manufacturing the product at low cost.

One of the inventors of the present invention has already proposed a method of using Fe-Cr-based sigma powder (Japanese Patent Publication No. 54-32732, etc.) as a method for producing Fe-Cr-C-based sintered alloys. This method yields a sintered alloy with higher density and better wear resistance than that produced by this method.

Next, the content of the present invention will be explained in detail.

Cr-based carbides include M3C type, M7C3 type, M23C
There are three types of 6 types, but M7C has the highest hardness.
Type 3>M23C6>Ma C type.

Therefore, in order to improve wear resistance, it is desirable to contain a large amount of M7C3 type carbide.

When the Cr content is less than 12 wt% VC, the amount of carbide is insufficient, resulting in a decrease in wear resistance.

Moreover, when Cr exceeds 22 wt%, the sigma (δ
) phase is generated and becomes brittle, so the maximum amount of Cr added is 22wt.
Must be %.

On the other hand, if the carbon content is less than 2 wt%, M23C- is generated, which reduces the M7C3 phase that contributes to wear resistance for the above-mentioned reason, which is not preferable.

Conversely, when the carbon content exceeds 3.5 wt%, the M3C phase increases, and for the same reason, the wear resistance decreases significantly.

The first feature of the present invention is that a large amount of highly hard M7C3 type carbide is dispersed in the matrix.

Figure 1 is a Fe-Cr-C equilibrium diagram of 17 wt% Cr, where 1 is the liquid phase (L), 2 is the L+γ phase, and 3 is the γ Wagyu M7
C3 phase and 4 are γ phase-liquidus line.

In the case of 17% Cr, the shaded area 5 in Fig. 1 is γ+M7C3.
This is the +L3 phase coexistence region, and by sintering in this temperature region, a large amount of M7C3 type carbide can be contained.

FIG. 2 is a diagram showing changes in sintering temperature and specific gravity of the sintered body in the case of 17% Cr, and each temperature range is as follows.

6: γ+M7C32 phase coexistence region 7: γ+M7C3+L 3 phase coexistence region 8: γ+L 2-phase coexistence region As can be seen from the diagram, sintering progresses rapidly in the 3-phase coexistence region of γ+M7C3+L.

Next, the features of the present invention regarding the sintering atmosphere will be described.

Conventionally, sintering of Cr-containing steel has had to be carried out using ammonia decomposition gas or hydrogen gas having a low dew point, since Cr is an easily oxidizable element.

However, even if a strong reducing gas with a low dew point is used, there are problems such as decarburization occurring during sintering and insufficient reduction of powder oxides.

For this reason, attempts have been made to sinter powders such as stainless steel in a vacuum, but when the powder contains a large amount of carbon as in the present invention, the oxygen (oxide) in the powder reacts with the added carbon, resulting in MO+C. -) M+CO Decarburization occurs according to the reaction formula, which again causes variation in carbon content.

In the present invention, we have found that a reduced-pressure CO gas atmosphere is effective as a method for achieving sufficient reduction through this reaction and for preventing decarburization by supplying active carbon atoms from the atmosphere.

Furthermore, as described above, it has been found that good sintering can be performed only in the three-phase coexistence region of γ (gamma phase), M7C3 carbide phase, and liquid phase (L) as the sintering temperature range.

As shown in FIG. 2, densification is slow and it is difficult to obtain high density only by solid-phase sintering of the gamma-phase M7C3 phase.

Furthermore, in the region of gamma phase + liquid phase, not only the carbides become coarse, but also the carbides grow in a so-called network shape at the grain boundaries, resulting in a decrease in the strength of the sintered body and an increase in sintering strain.

Figure 3 is a 400 times enlarged microscopic photograph of the sintered body sintered in this gamma phase and liquid phase coexistence region (high temperature).

Sintering in the three-phase coexistence region promotes densification due to the presence of a small amount of liquid phase, but prevents carbide growth and network formation, resulting in a good sintered body.

FIG. 3a is a photograph of the structure at the same magnification as b of a sintered body according to an example of the present invention, which will be described later.

The above three-phase coexistence temperature range differs depending on the content of Cr, and is 11,800
The temperature range is ~1250°C.

Next, the atmosphere during the cooling process, which is another feature of the present invention, will be described.

That is, the method of the present invention uses a reduced pressure N2 atmosphere for rapid cooling because it is cheaper than CO gas, but in particular,
This is to improve wear resistance by diffusing nitrogen atoms into the sintered body and to increase the cooling rate compared to the case of vacuum.

The specific gravity of the sintered body sintered by the manufacturing method according to the present invention is 7.5. ! It will be more than il/CC, but it will not necessarily be 7.5 El/CC depending on the powder state and embossing conditions.
In some cases, densification may not progress until

However, in order to produce a sintered alloy with excellent wear resistance, which is the object of the present invention, the powder, stamping conditions, and sintering temperature are adjusted to 7.5. ! ? It is necessary to select so that the specific gravity icf,L is equal to or higher than /Ce.

The reason why Cr gold-containing alloy powder is used as the raw material powder is as follows.

That is, if Cr is added alone, it has disadvantages such as a delay in homogenization, resulting in an inhomogeneous structure, and the generation of pores in places where carbides are formed. It is not preferable to add M23C6 type carbides during the decomposition process of the sigma phase or pores due to heterogeneous diffusion.

In the case of the present invention, these conventional drawbacks can be overcome by using Cr alloy powder as a raw material and sintering it in the above-mentioned temperature range. This results in a sintered alloy with excellent properties.

Next, an example will be explained.

Example Fe-16, 3'wt% carbon powder was added to and mixed with 5wt% Cr alloy powder, and then molded at a pressure of 6t/i.

After dewaxing this molded body, CO gas 100
It was sintered at 1230°C for 30 minutes in a reduced CO atmosphere at 500 Torr and quenched in a N2 atmosphere at 500 Torr.

The specific gravity of the obtained sintered body was 7.55 El/QC, and the hardness was HRc53.

Further, as shown in Table 1, the nitrogen content in the sintered body was more than three times as enriched as that of the same molded body sintered at the same temperature in vacuum.

The structure of the sintered body after surface polishing was a uniform grain size structure in which fine carbides were dispersed, as shown in FIG. 3a.

This sintered body is produced as it is without any special heat treatment.
It showed excellent wear resistance when used as wear-resistant parts.

[Brief explanation of the drawing]

FIG. 1 is a diagram for explaining the present invention in detail.
%, and Figure 2 shows the change in sintering specific gravity and the coexistence state of liquid phase and solid phase depending on the sintering temperature of Fe-Cr-C based sintered body. This is an example of the figure,
FIG.
This is a 0x magnification microscopic structure photograph, and Fig. 3 is a 400x magnified microscopic structure photograph of a sintered body obtained by sintering a molded body having the same composition in a two-phase coexistence region of gamma phase and liquid phase. 1... Liquid phase region, 2... γ+L2 phase region, 3... γ phase + M7C3 phase region, 4...
...γ-liquidus line, 5...γ+M7C3+L3 phase region, 6...γ+M7C3 coexistence region, 7...
...γ+M7C3+L coexistence region, 8...γ+
L coexistence region, 9... A line showing changes in sintering specific gravity depending on sintering temperature.

Claims (1)

[Claims] I A powder obtained by adding and mixing 2.0 to 3.5 weight % of carbon powder to an alloy powder containing 12 to 22 weight % of Cr, the remainder of which is essentially iron, is pressed into a predetermined shape. After that, sintering is performed in a reduced pressure CO gas atmosphere in a temperature range where three phases coexist in the Fe-Cr-C system equilibrium phase diagram: gamma phase, M7C3 carbide phase, and liquid phase, and the cooling process is quenched in a sterile EN2 gas atmosphere. The specific gravity after sintering is 7.51/C.
111 or more. 2. In claim 1, the sintering temperature is 118
A method for producing a wear-resistant sintered alloy, characterized in that the temperature range is from 00C to 1250C.