TECHNICAL FIELD

This disclosure relates to a mixed powder for powder metallurgy, and particularly to a mixed powder for powder metallurgy that does not need to use a stain-causing metal soap and has excellent ejection properties and compressibility. In addition, this disclosure relates to a mixed powder for powder metallurgy that can achieve excellent fluidity, ejection properties and compressibility in the case of further adding carbon black. Further, this disclosure relates to a lubricant for powder metallurgy.

BACKGROUND

Powder metallurgy technology is a method with which parts having complex shapes can be compacted in a shape that is extremely close to the shape of the product and can be produced with high dimensional accuracy. Further, the powder metallurgy technique can significantly reduce cutting costs. Therefore, powder metallurgical products are widely used as all kinds of machines and parts.

In powder metallurgy, a mixed powder (hereinafter referred to as “mixed powder for powder metallurgy” or simply “mixed powder”) is used, where the mixed powder is obtained by mixing an iron-based powder, which is a main raw material, with, if necessary, an alloying powder such as copper powder, graphite powder and iron phosphide powder, a powder for improving machinability such as MnS, and a lubricant.

The lubricant contained in the mixed powder for powder metallurgy plays an extremely important role when such a mixed powder for powder metallurgy is subjected to compaction to produce a product. The effects of the lubricant will be described below.

First, the lubricant has a lubrication effect when the mixed powder is subjected to compaction in a die. The lubrication effect is further roughly divided into the following two. One is the effect of reducing the friction between particles contained in the mixed powder. During the compaction, the lubricant enters between the particles to reduce the friction, thereby promoting the rearrangement of the particles. The other is the effect of reducing the friction between the die used for compaction and the particles. The lubricant on the surface of the die enters between the die and the particles, thereby reducing the friction between the die and the particles. With these two effects, the mixed powder can be compressed to a high density during the compaction.

In addition, the lubricant also has a lubrication effect when a green compact, which is formed by subjecting the mixed powder to compaction in the die, is taken (ejected) out of the die. Generally, a green compact is ejected out of a die by pushing it out with a punch, where large frictional resistance is generated due to the friction between the green compact and the surface of the die. In this case, some of the lubricant contained in the mixed powder on the surface of the die reduces the frictional force.

As described above, the lubricant contained in the mixed powder for powder metallurgy plays a very important role during the compaction. However, the lubricant is only required during the compaction and the ejection out of the die and is unnecessary after the ejection. Further, it is desirable that the lubricant disappears during the sintering of the green compact, so that no lubricant remains in a final sintered body.

In addition, since the lubricant generally has a stronger adhesive power than the iron-based powder, it deteriorates the fluidity of the mixed powder. Further, since the lubricant has a smaller specific gravity than the iron-based powder, the density of the green compact is lowered when a large amount of lubricant is added.

Furthermore, the lubricant used in the mixed powder for powder metallurgy is required to function as a binder in some cases. The binder here refers to a component that allows an alloying powder and other additive components to adhere to the surface of the iron-based powder which is a main component. A common mixed powder for powder metallurgy is obtained by simply mixing an iron-based powder with additive components such as an alloying powder, a powder for improving machinability, and a lubricant. However, each component may segregate inside the mixed powder in this state. Particularly for graphite powder, which is generally used as an alloying powder, it tends to segregate when the mixed powder is flowed or vibrated because it has a smaller specific gravity than other components. In order to prevent such segregation, it has been proposed that the additive components be adhered to the surface of the iron-based powder via a binder. Such a mixed powder for powder metallurgy in which additive components are adhered to the surface of the iron-based powder via a binder is also referred to as a segregation prevention treatment powder. The segregation prevention treatment powder has the additive components adhered to the iron-based powder, thereby preventing the above-described segregation of additive components.

The binder used in such a segregation prevention treatment powder usually is a compound that also functions as a lubricant. This is because, by using a binder also having lubricating ability, the total amount of the binder and the lubricant added to the mixed powder can be reduced.

Generally, such a mixed powder for powder metallurgy is subjected to press forming at a pressure of 300 MPa to 1000 MPa to obtain a green compact having a predetermined part shape, and then the green compact is sintered at a high temperature of 1000° C. or higher to obtain a final product (such as a machine part). In this case, the total amount of the lubricant and the binder contained in the mixed powder is generally about 0.1 parts by mass to 2 parts by mass with respect to 100 parts by mass of the iron-based powder. In order to increase the green density, i.e., the density of the green compact, the amount of the lubricant and the binder added is preferably small. Therefore, the lubricant is required to exhibit excellent lubricating ability while being added at a small amount.

Conventionally, metal soaps such as zinc stearate are widely used as the lubricant. However, metal soaps cause stains on surfaces of furnaces, workpieces, sintered bodies, etc. during the process of sintering a green compact. For this reason, various lubricants have been proposed to replace the metal soap.

For example, JP H06-506726 A (PTL 1) proposes using diamide wax as a lubricant. In the technique proposed in PTL 1, the diamide wax also serves as a binder. In addition, WO 2005/068588 A1 (PTL 2) proposes using polyhydroxycarboxylic acid amide as a lubricant.

Further, in order to improve the fluidity of the mixed powder for powder metallurgy containing a lubricant, it is proposed further adding a powder for improving fluidity to the mixed powder for powder metallurgy.

For example, JP 2003-508635 A (PTL 3) proposes adding a fluidity improver such as silica to a mixed powder containing a lubricant such as diamide wax to improve the fluidity. In addition, JP 2010-280990 A (PTL 4) proposes adding carbon black to a mixed powder containing a lubricant such as diamide wax to improve the fluidity and the apparent density.

CITATION LIST

Patent Literature

PTL 1: JP H06-506726 A

PTL 2: WO 2005/068588 A1

PTL 3: JP 2003-508635 A

PTL 4: JP 2010-280990 A

SUMMARY

Technical Problem

However, the polyhydroxycarboxylic acid amide proposed in PTL 2 must be synthesized by an amidation reaction using polyhydroxycarboxylic acid or its equivalent and an aliphatic amine as raw materials, which is not readily available.

In addition, the diamide wax used as a lubricant in PTL 1 and other documents does not have sufficient ejection properties.

Further, when particles such as silica or carbon black are added to a conventional lubricant to improve the fluidity, as proposed in PTL 3 and PTL 4, the compressibility of the mixed powder is deteriorated. When the compressibility deteriorates, spring back increases during the compaction. As a result, the ejection properties deteriorate.

It could thus be helpful to provide a mixed powder for powder metallurgy that contains a readily available compound as a lubricant, does not need to contain a stain-causing metal soap, has excellent ejection properties and compressibility, and can exhibit excellent fluidity without deteriorating the ejection properties or the compressibility even in the case of further containing carbon black.

Solution to Problem

As a result of intensive study, we found that the problem can be solved when an ester of disaccharide and fatty acid, which is readily available as a commercial product, is used as a lubricant. The present disclosure is based on the findings, and the primary features thereof are as follows.

1. A mixed powder for powder metallurgy comprising an (a) iron-based powder and

a (b) lubricant, wherein

the (b) lubricant is an ester of disaccharide and fatty acid represented by R—COOH, and the R is an alkyl group having 11 or more carbon atoms or an alkenyl group having 11 or more carbon atoms.

2. The mixed powder for powder metallurgy according to 1., wherein the R is an alkyl group having 11 to 21 carbon atoms or an alkenyl group having 11 to 21 carbon atoms.

3. The mixed powder for powder metallurgy according to 1. or 2., wherein the lubricant has a melting point of 40° C. or higher.

4. The mixed powder for powder metallurgy according to any one of 1. to 3., further comprising one or both of an (c) alloying powder and a (d) machinability improver.

5. The mixed powder for powder metallurgy according to 4., wherein one or both of the (c) alloying powder and the (d) machinability improver are adhered to a surface of the (a) iron-based powder via a (e) binder.

6. The mixed powder for powder metallurgy according to 5., wherein the binder (e) is the ester of disaccharide and fatty acid.

7. The mixed powder for powder metallurgy according to 5. or 6., further comprising (f) carbon black in an amount of 0.01 parts by mass to 0.3 parts by mass with respect to 100 parts by mass of the (a) iron-based powder.

8. A lubricant for powder metallurgy, which is an ester of disaccharide and fatty acid represented by R—COOH, wherein the R is an alkyl group having 11 or more carbon atoms or an alkenyl group having 11 or more carbon atoms.

Advantageous Effect

The mixed powder for powder metallurgy of the present disclosure can exhibit extremely excellent ejection properties and compressibility without containing any stain-causing metal soap. In addition, the mixed powder for powder metallurgy can exhibit excellent fluidity without deteriorating the ejection properties or the compressibility even in the case where hard fine particles such as carbon black are added to improve the fluidity. Further, the ester of disaccharide and fatty acid used as a lubricant in the present disclosure is readily available as a commercial product, which is also advantageous in terms of production and costs.

