EP3744441A1

EP3744441A1 - Mixed powder for powder metallurgy

Info

Publication number: EP3744441A1
Application number: EP18902349.2A
Authority: EP
Inventors: Masaki Yoshida; Yohei TAKAMATSU
Original assignee: Kobe Steel Ltd
Current assignee: Kobe Steel Ltd
Priority date: 2018-01-25
Filing date: 2018-12-17
Publication date: 2020-12-02
Also published as: TWI694157B; JP2019127654A; TW201936942A; BR112020014533B1; CA3089506A1; US20210060640A1; BR112020014533A2; CN111344090B; BR112020014533B8; CN111344090A; EP3744441A4; JP6929259B2

Abstract

A mixed powder for powder metallurgy according to an embodiment of the present invention contains an iron-based powder as a main component and further contains a powder of at least one sulfide selected from CaS, MnS, and MoS<sub>2</sub>; and a powder wherein a percentage content of magnesium oxide is greater than or equal to 0.005% by mass and less than or equal to 0.025% by mass, wherein the magnesium oxide has an average particle size D50 of greater than or equal to 0.5 µm and less than or equal to 5.0 µm.

Description

[TECHNICAL FIELD]

The present invention relates to a mixed powder for powder metallurgy.

[BACKGROUND ART]

In a powder metallurgy method, for example, a sintered body with a complex shape such as a net shape or the like can be formed by sintering an iron-based powder. Such a sintered body is used, for example, as a structural component such as an automobile component or the like. With an increasing demand for higher dimensional accuracy of such a component, the dimensional accuracy needs to be improved by further cutting the sintered body.
Furthermore, a reduction in production cost of the component is also strongly demanded; therefore, cost reduction in a cutting process is also deemed to be important. In the cutting process, cost can be lessened by extending the lifetime of a cutting tool; however, a sintered body like that described above tends to have low machinability and shorten the lifetime of the cutting tool.
Therefore, it has been common practice to use a mixed powder for powder metallurgy in which an additive that improves machinability and extends the lifetime of the cutting tool is mixed into an iron-based powder. Specifically, as the additive that improves machinability (a machinability improving material), for example, a powder of manganese sulfide (MnS), sulfur (S), or the like is used. Such a machinability improving material serves as a lubricant that reduces resistance in cutting or as a starting point for dividing chips, thereby extending the lifetime of the cutting tool.
In general, as a content of the machinability improving material in the mixed powder for powder metallurgy is increased, the machinability of the sintered body to be formed is improved, and thereby the lifetime of the cutting tool is extended. However, when the content of the machinability improving material is increased, a problem arises in which a mechanical property such as crushing strength or the like of a sintered material is degraded or a dimensional change rate is changed by sintering, requiring an additional die. Consequently, in general, the content of the machinability improving material in the mixed powder for powder metallurgy is approximately 0.3% by mass to 0.5% by mass.
Furthermore, demands for cost reduction in the cutting process and improvement in productivity increase needs for higher cutting speed; however, an effect of the above-described machinability improving material is relatively low in high-speed cutting. In a case in which 0.3% by mass or more of a sulfide is added as the machinability improving material, there arises a problem in which sulfur evaporates during sintering, thereby dirtying the sintered body in appearance and polluting an inside of a sintering furnace, which is likely to damage the sintering furnace.
For example, Japanese Unexamined Patent Application Publication No. 1997-279204 proposes an iron-based mixed powder for powder metallurgy containing a powder of a CaO-Al₂O₃-SiO₂-based composite oxide at 0.02% by weight to 0.3% by weight. According to the above-described patent document, use of a composite oxide containing Ca as a main component can reduce degradation of mechanical properties of a sintered body, prevent staining of the sintered body and damage on a sintering furnace, and reduce abrasion of a cutting tool in high-speed cutting.
However, further increasing demands for cost reduction and improvement in dimensional accuracy of a component require a mixed powder for powder metallurgy that further excels in machinability.

[PRIOR ART DOCUMENTS]

[PATENT DOCUMENTS]

Patent Document 1: Japanese Unexamined Patent Application Publication No. 1997-279204

[SUMMARY OF THE INVENTION]

[PROBLEMS TO BE SOLVED BY THE INVENTION]

In view of the foregoing circumstances, an object of the present invention is to provide a mixed powder for powder metallurgy which can form a sintered material having excellent machinability.

[MEANS FOR SOLVING THE PROBLEMS]

A mixed powder for powder metallurgy according to an embodiment of the present invention made for solving the above-described problems contains an iron-based powder as a main component and further contains a powder of at least one sulfide selected from CaS, MnS, and MoS₂; and a powder wherein a percentage content of magnesium oxide is greater than or equal to 0.005% by mass and less than or equal to 0.025% by mass, wherein the magnesium oxide has an average particle size D50 of greater than or equal to 0.5 µm and less than or equal to 5.0 µm.
It is conceivable that, in the mixed powder for powder metallurgy, the sulfide serves as a lubricant and generates an oxide which causes a magnesium oxide particle having a relatively small particle size to attach to a surface of a cutting tool, thereby reducing wear of the cutting tool by a hard oxide or the like in a sintered body. Accordingly, a sintered material formed by sintering the mixed powder for powder metallurgy has excellent machinability and enables the cutting tool to have a relatively long lifetime.
In the mixed powder for powder metallurgy, a total content of the sulfide is preferably greater than or equal to 0.04% by mass and less than or equal to 0.20% by mass. This configuration can reduce degradation of mechanical properties and the like of the sintered material formed by sintering the mixed powder for powder metallurgy.
"Iron-based powder" as referred to herein means a pure iron powder, an iron alloy powder, or a mixed powder thereof; "to contain as a main component" as referred to herein means that a content is greater than or equal to 90% by mass. "Average particle size D50" as referred to herein means a particle size at which an accumulated volume in a particle size distribution measured by a laser diffraction scattering method reaches 50%.

[EFFECTS OF THE INVENTION]

As described above, the mixed powder for powder metallurgy of the present invention can form a sintered material having excellent machinability.

[DESCRIPTION OF EMBODIMENTS]

Hereinafter, embodiments of the present invention will be described in detail with reference to the drawings as appropriate.

Mixed Powder for Powder Metallurgy

A mixed powder for powder metallurgy according to an embodiment of the present invention contains an iron-based powder as a main component and further contains a powder of a sulfide and a powder of magnesium oxide (MgO). Furthermore, the mixed powder for powder metallurgy may further contain, for example, a copper powder, a graphite powder, a powder lubricant, or the like.

Iron-Based Powder

The iron-based powder, which is the main component of the mixed powder for powder metallurgy, is not particularly limited; for example, a reduced iron-based powder, an atomized iron-based powder, an electrolytic iron-based powder, or the like can be used.
Furthermore, the iron-based powder is not limited to a pure iron powder; for example, a steel powder obtained by pre-alloying alloy elements (a pre-alloyed steel powder), a steel powder obtained by partially alloying alloy elements (a partially alloyed steel powder), or the like can be used, or a mixture of a plurality of kinds thereof may also be used. As the alloy elements, for example, known elements that improve properties of a sintered body, such as copper, nickel, chromium, molybdenum, sulfur, and the like, can be contained.
The iron-based powder only needs to be of such a size as to be used as a main raw material powder for powder metallurgy, and the average particle size D50 of the iron-based powder is not particularly limited; for example, the average particle size D50 can be greater than or equal to 40 µm and less than or equal to 120 µm.

Powder of Sulfide

In the sintered body obtained by sintering the mixed powder for powder metallurgy, the sulfide remains in a form of original particles. Since the sulfide is softer than an iron base, which is a main component of the sintered body, machinability of the sintered body is improved; in addition, the sulfide has lubricity and reduces abrasion in cutting, thereby extending the lifetime of a cutting tool.
Furthermore, in a case of a high cutting speed, the sulfide in the sintered body is desulfurized by heat generated in the cutting, generating an oxide. It is conceivable that the oxide attaches to a surface of the cutting tool and forms a film that protects the cutting tool, and in addition, the oxide serves as a binder that causes the magnesium oxide, which is very hard, to attach to the surface of the cutting tool.
As the above-described sulfide that can efficiently improve machinability and enables attachment of the magnesium oxide, at least one of CaS, MnS, and MoS₂ is used.
The lower limit of a total content of the sulfide is preferably 0.04% by mass, and more preferably 0.06% by mass. Meanwhile, the upper limit of the total content of the sulfide is preferably 0.20% by mass, and more preferably 0.18% by mass. In a case in which the total content of the sulfide is less than the lower limit, machinability may fail to be sufficiently improved. Conversely, in a case in which the total content of the sulfide is greater than the upper limit, mechanical properties of the sintered body obtained by sintering the mixed powder for powder metallurgy may be degraded.
The lower limit of an average particle size D50 of the sulfide such as CaS, MnS, or the like is preferably 1.0 µm, and more preferably 1.5 µm. Meanwhile, the upper limit of the average particle size D50 of the sulfide is preferably 10 µm, and more preferably 8 µm. In a case in which the average particle size D50 of the sulfide is less than the lower limit, it may be difficult to uniformly disperse the sulfide in the mixed powder for powder metallurgy, and/or the mixed powder for powder metallurgy may become unduly expensive. Conversely, in a case in which the average particle size D50 of the sulfide is greater than the upper limit, machinability of the sintered body obtained by sintering the mixed powder for powder metallurgy may fail to be sufficiently improved.

Powder of Magnesium Oxide

Magnesium oxide is a chemically stable, hard material. For this reason, a powder of the magnesium oxide exists as micro particles even in the sintered body obtained by sintering the mixed powder for powder metallurgy. The micro particles of the magnesium oxide are attached to the surface of the cutting tool by the oxide attributed to the sulfide, thereby protecting the cutting tool and improving machinability of the sintered body.
The lower limit of a content of the magnesium oxide is 0.005% by mass, and preferably 0.010% by mass. Meanwhile, the upper limit of the content of the magnesium oxide is 0.025% by mass, and preferably 0.020% by mass. In a case in which the content of the magnesium oxide is less than the lower limit, wear of the cutting tool may fail to be reduced. Conversely, in a case in which the content of the magnesium oxide is greater than the upper limit, the dimensional change rate in sintering may increase, or mechanical properties such as crushing strength and the like of the sintered body may be insufficient.
The lower limit of an average particle size D50 of the magnesium oxide is 0.5 µm, and preferably 0.7 µm. Meanwhile, the upper limit of the average particle size D50 of the magnesium oxide is 5.0 µm, and preferably 3.0 µm. In a case in which the average particle size D50 of the magnesium oxide is less than the lower limit, an aggregate of the MgO is formed, and it may become more difficult to uniformly disperse the magnesium oxide in the mixed powder for powder metallurgy. Moreover, in a case in which a weight ratio is constant, the number of MgO particles becomes large, and the MgO existing at a boundary between iron powder particles increases, thereby inhibiting sintering. As a result, the dimensional change rate may increase or mechanical properties such as crushing strength and the like may be insufficient. Meanwhile, in a case in which the average particle size D50 of the magnesium oxide is greater than the upper limit, sintering may be inhibited, causing a decrease in strength, the cutting tool may be chipped and wear thereof may be accelerated, the magnesium oxide particles may fail to be attached to the cutting tool, shortening the lifetime of the cutting tool, and/or processing accuracy may be decreased. In other words, the magnesium oxide having a sufficiently small particle size can attach to the surface of the cutting tool, extending the lifetime thereof, without causing damage that accelerates wear of the cutting tool.

Copper Powder

The copper powder serves as a binder that bonds particles of the iron-based powder to one another, thereby improving the strength of the sintered body obtained by sintering the mixed powder for powder metallurgy.
The copper powder can be selected from a wide range of copper powders that are used for powder metallurgy; for example, an electrolytic copper powder, an atomized copper powder, or the like can be used.
The copper powder may be simply mixed into the iron-based powder, may be attached to a surface of the iron-based powder by use of a binder, or may be mixed into the iron-based powder and subjected to heat treatment to be attached to the surface of the iron-based powder in a dispersed manner.
The lower limit of a content of the copper powder depends on strength and hardness required for the sintered body, and is preferably 0.8% by mass, and more preferably 1.0% by mass. Meanwhile, the upper limit of the content of the copper powder is preferably 5.0% by mass, more preferably 3.0% by mass, and particularly preferably 2.0% by mass. In a case in which the content of the copper powder is less than the lower limit, an effect of improving the strength of the sintered body may be insufficient. Conversely, in a case in which the content of the copper powder is greater than the upper limit, carbon diffusion may be inhibited and the strength of the sintered body may be insufficient.
The lower limit of an average particle size D50 of the copper powder is preferably 5 µm, and more preferably 10 µm. Meanwhile, the upper limit of the average particle size D50 of the copper powder is preferably 50 µm, and more preferably 40 µm. In a case in which the average particle size D50 of the copper powder is less than the lower limit, it may be difficult to uniformly disperse the copper powder in the mixed powder for powder metallurgy, and/or the mixed powder for powder metallurgy may become unduly expensive. Conversely, in a case in which the average particle size D50 of the copper powder is greater than the upper limit, the strength of the sintered body obtained by sintering the mixed powder for powder metallurgy may fail to be sufficiently improved.

Graphite Powder

The graphite powder forms a hard pearlite phase by reacting with iron in sintering of the mixed powder for powder metallurgy, thereby increasing the strength of the sintered body to be obtained.
As the graphite powder, for example, a natural graphite powder, an artificial graphite powder, or the like can be used.
The graphite powder may be simply mixed into the iron-based powder or may be attached to the surface of the iron-based powder by use of a binder.
The lower limit of a content of the graphite powder is preferably 0.2% by mass, and more preferably 0.5% by mass. Meanwhile, the upper limit of the content of the graphite powder is preferably 1.5% by mass, and more preferably 1.0% by mass. In a case in which the content of the graphite powder is less than the lower limit, the effect of improving the strength of the sintered body may be insufficient. Conversely, in a case in which the content of the graphite powder is greater than the upper limit, toughness of the sintered body may be insufficient.
The lower limit of an average particle size D50 of the graphite powder is preferably 1 µm, and more preferably 3 µm. Meanwhile, the upper limit of the average particle size D50 of the graphite powder is preferably 30 µm, and more preferably 20 µm. In a case in which the average particle size D50 of the graphite powder is less than the lower limit, it may be difficult to uniformly disperse the graphite powder in the mixed powder for powder metallurgy, or the mixed powder for powder metallurgy may become unduly expensive. Conversely, in a case in which the average particle size D50 of the graphite powder is greater than the upper limit, segregation may occur in the sintered body obtained by sintering the mixed powder for powder metallurgy, and the strength of the sintered body may fail to be sufficiently improved.

Powder Lubricant

The powder lubricant reduces friction between particles when the mixed powder for powder metallurgy is compacted, thereby improving formability thereof and extending die lifetime. In sintering, the powder lubricant is eliminated through evaporation or thermal decomposition.
As the powder lubricant, for example, a powder of a metal soap such as zinc stearate, of a non-metallic soap such as ethylene bis-amide, or of the like is used.
The lower limit of a content of the powder lubricant is preferably 0.2% by mass, and more preferably 0.5% by mass. Meanwhile, the upper limit of the content of the powder lubricant is preferably 1.5% by mass, and more preferably 1.0% by mass. In a case in which the content of the powder lubricant is less than the lower limit, formability of a compact of the mixed powder for powder metallurgy may be insufficient. Conversely, in a case in which the content of the powder lubricant is greater than the upper limit, density of the sintered body obtained by sintering the mixed powder for powder metallurgy, which has been compacted, may be decreased and the strength of the sintered body may be insufficient.
The lower limit of an average particle size D50 of the powder lubricant is preferably 3 µm, and more preferably 5 µm. Meanwhile, the upper limit of the average particle size D50 of the powder lubricant is preferably 50 µm, and more preferably 30 µm. In a case in which the average particle size D50 of the powder lubricant is less than the lower limit, it may be difficult to uniformly disperse the powder lubricant in the mixed powder for powder metallurgy, and/or the mixed powder for powder metallurgy may become unduly expensive. Conversely, in a case in which the average particle size D50 of the powder lubricant is greater than the upper limit, the strength of the sintered body obtained by sintering the mixed powder for powder metallurgy may fail to be sufficiently improved.

Advantages

It is conceivable that, in the mixed powder for powder metallurgy, the sulfide serves as a lubricant and generates an oxide which makes a magnesium oxide particle having a small particle size attach to the surface of the cutting tool, thereby reducing wear of the cutting tool by a hard oxide or the like in the sintered body. Consequently, the sintered material formed by sintering the mixed powder for powder metallurgy has excellent machinability and enables the cutting tool to have a relatively lifetime.

Other Embodiments

The above-described embodiment does not limit the configuration of the present invention. Therefore, in the above-described embodiment, the components of each part of the above-described embodiment can be omitted, replaced, or added based on the description in the present specification and general technical knowledge, and such omission, replacement, or addition should be construed as falling within the scope of the present invention.

[EXAMPLES]

Hereinafter, the present invention will be described in detail by way of Examples; the present invention should not be construed as being limited to description in the Examples.
Mixed powders for powder metallurgy Nos. 1 to 15 were experimentally produced by mixing a copper powder, a graphite powder, a machinability improving material, magnesium oxide, and a powder lubricant into an iron-based powder at their respective proportions shown in Table 1 below. It is to be noted that "-" in the table indicates that the material is not contained.

It is to be noted that "300M," an atomized pure iron powder (available from Kobe Steel, Ltd.) having an average particle size D50 of 70 µm, was used as the iron-based powder. As the copper powder, "CuAtW-250," a water-atomized copper powder (available from Fukuda Metal Foil & Powder Co., Ltd.) having a sieve opening of 250 µm, was used. As the graphite powder, "CPB" (available from Nippon Graphite Industries, Co.,Ltd.), having an average particle size D50 of approximately 23 µm, was used. As CaS or MnS, respectively, calcium sulfide ("CaS" in the table) having an average particle size D50 of 4.9 µm, which was obtained by reducing calcium sulfate (CaSO₄) having an average particle size D50 of 2.4 µm in an atmosphere of a reducing gas such as hydrogen or the like, or manganese sulfide ("MnS" in the table) having an average particle size D50 of 4.9 µm was used. As the magnesium oxide, magnesium oxide having an average particle size D50 of 0.7 µm, magnesium oxide having an average particle size D50 of 2.5 µm, or magnesium oxide having an average particle size D50 with 3.2 µm was used. As the powder lubricant, an ethylene bis-amide-based wax having an average particle size D50 of 27 µm was used.

Table 1

Sample No.	Copper powder	Graphite powder	Machinability improving material		Magnesium oxide		Lubricant
Sample No.	Content [% by mass]	Content [% by mass]	Kind	Content [% by mass]	Particle size D50 [µm]	Content [% by mass]	Content [% by mass]
1	1.5	0.9	CaS	0.04	0.7	0.010	0.8
2	1.5	0.9	CaS	0.04	2.5	0.020	0.8
3	1.5	0.9	CaS	0.04	3.2	0.020	0.8
4	1.5	0.9	CaS	0.04	6.2	0.020	0.8
5	1.5	0.9	CaS	0.04	0.7	0.020	0.8
6	1.5	0.9	CaS	0.08	-	-	0.8
7	1.5	0.9	CaS	0.08	0.7	0.010	0.8
8	1.5	0.9	CaS	0.08	0.7	0.020	0.8
9	1.5	0.9	CaS	0.08	0.7	0.030	0.8
10	1.5	0.9	CaS	0.08	0.7	0.040	0.8
11	1.5	0.9	CaS	0.12	0.7	0.020	0.8
12	1.5	0.9	-	-	0.7	0.040	0.8
13	1.5	0.9	MnS	0.08	0.7	0.020	0.8
14	1.5	0.9	-	-	-	-	0.8
15	1.5	0.9	MnS	0.50	-	-	0.8

The mixed powders for powder metallurgy Nos. 1 to 15 were each compacted in a die, whereby ring-shaped compacts each having an outer diameter of 64 mm, an inner diameter of 24 mm, and a height of 20 mm were formed. It is to be noted that conditions for compacting were set so that each compact had a density of 7.00 g/cm³. The compacts obtained were sintered in a nitrogen gas atmosphere containing a hydrogen gas at 10% by volume at a temperature of 1,120°C for 60 minutes, whereby sintered bodies were obtained.
Dimensional change rates (green base and die base) in sintering, a crushing strength, and a Rockwell hardness (B scale) of each sample were measured. Furthermore, a turning test was performed in which a side surface of a stack of ten sintered bodies of each sample was processed by turning. As a cutting tool, "SNMN120408," a chip using "NX2525," a cermet (both available from Mitsubishi Materials Corporation), was used. The side surface was cut by 5,287 m using dry cutting under cutting conditions in which a peripheral velocity was 200 m/min, a cutting depth was 0.15 mm/pass, and a feed amount was 0.08 mm/rev.
A parallel wear width (flank wear width Vb) of a flank face of the cutting tool after the turning test was measured.
A surface roughness Ra (arithmetic mean roughness) and a surface roughness Rz (maximum height) of a cutting surface of the sintered bodies after the turning test were measured. These surface roughness values were measured at three points respectively by "SJ-410," a surface roughness meter (available from Mitutoyo Corporation), wherein cutoff values were λc = 0.8 mm and λs = 2.5 µm and a measurement length was 5.0 mm, and an average value of measured values at the three points was calculated.

Measured values of each of the above-described parameters are summarized in Table 2 below.

Table 2

Sample No.	Green density [g/cm³]	Dimensional change		Sintered density [g/cm³]	rate Rockwell hardness	Crashing strength [MPa]	Flank wear width [µm]	Surface roughness Ra	Surface roughness Rz
Sample No.	Green density [g/cm³]	Green base [%]	Die base [%]	Sintered density [g/cm³]	rate Rockwell hardness	Crashing strength [MPa]	Flank wear width [µm]	Surface roughness Ra	Surface roughness Rz
1	7.00	7.00	0.35	6.96	77.1	870	62.2	0.90	4.56
2	7.00	7.00	0.36	6.94	77.6	870	65.9	1.09	5.19
3	7.00	7.00	0.36	6.93	77.8	868	82.2	1.02	4.88
4	7.00	7.00	0.34	6.95	78.3	875	173.2	1.62	8.16
5	6.99	6.99	0.36	6.94	77.2	856	57.3	1.32	6.12
6	7.00	7.00	0.35	6.94	78.0	857	150.3	1.29	5.86
7	7.00	7.00	0.36	6.93	77.1	851	118.3	0.86	4.37
8	7.00	7.00	0.37	6.93	77.2	850	105.6	1.17	5.43
9	6.99	6.99	0.38	6.93	77.4	834	94.6	0.99	4.38
10	7.01	7.01	0.41	6.93	76.7	822	89.5	0.70	4.25
11	6.99	6.99	0.38	6.93	77.2	839	104.8	1.00	4.09
12	7.00	7.00	0.37	6.94	77.4	857	299.1	2.23	10.37
13	7.01	7.01	0.34	6.96	79.1	867	68.6	0.33	1.97
14	7.00	7.00	0.33	6.97	78.9	879	250.6	1.35	6.79
15	6.99	6.99	0.37	6.95	78.0	840	144.6	0.82	3.76

The sintered bodies obtained by sintering the mixed powders for powder metallurgy Nos.1 to 3, 5, 7, 8, 11, and 13, each of which contained a sulfide and magnesium oxide and in which a percentage content of the magnesium oxide was greater than or equal to 0.010% by mass and less than or equal to 0.020% by mass and an average particle size D50 of the magnesium oxide was greater than or equal to 0.5 µm and less than or equal to 5.0 µm, showed sufficient formability and sufficient mechanical strength, and caused little wear of the cutting tool.
Moreover, a drilling test was performed on the sintered bodies formed from the mixed powders for powder metallurgy Nos. 1, 5, 8, 13, 14, and 15. As a drill, "AD-4D," a coated carbide drill (available from OSG Corporation) having a diameter of 3.8 mm, was used. Processing conditions were as follows: a peripheral velocity of the drill was 2 m/min (4,358 rpm), a feed rate was 450 mm/min (0.103 mm/rev), and "Yushiroken EC50," a water-soluble fluid (available from YUSHIRO CHEMICAL INDUSTRY CO.,LTD.), was poured as a cutting oil onto the sintered bodies during cutting. To ensure a cutting distance, 180 non-through holes were formed with depths of 10 mm.

In the drilling test, a flank wear width (Vb) of the drill was measured upon forming every thirtieth non-through hole. Table 3 below shows measurement results.

Table 3

Sample No.	Cutting distance [mm]
Sample No.	0	300	600	900	1200	1500	1800
1	0	43	60	66	71	73	77
5	0	39	50	54	57	60	65
8	0	37	61	64	70	79	84
13	0	48	60	67	70	74	79
14	0	44	68	72	77	82	92
15	0	49	63	72	78	82	88

Flank wear width [µm]

The sintered bodies obtained by sintering the mixed powders for powder metallurgy Nos. 1, 5, 8, and 13, each of which contained the sulfide and the magnesium oxide, caused less wear of the cutting tool than that of the sintered body obtained by sintering the mixed powder for powder metallurgy No.14, which contained neither the sulfide nor the magnesium oxide, and the sintered body obtained by sintering the mixed powder for powder metallurgy No.15, which contained the sulfide but did not contain the magnesium oxide.
Mixed powders for powder metallurgy Nos. 16 to 32 were experimentally produced by mixing a copper powder, a graphite powder, a machinability improving material, magnesium oxide, and a powder lubricant into an iron-based powder at their respective proportions shown in Table 4 below. It is to be noted that "-" in the table indicates that the material is not contained.

It is to be noted that the iron-based powder, the copper powder, the graphite powder, the magnesium oxide, and the powder lubricant were of the same types and proportions those used for the mixed powders for powder metallurgy Nos. 1 to 15. As the machinability improving material, manganese sulfide of the same type and proportion to that in the mixed powders for powder metallurgy Nos. 1 to 15, sulfur ("S" in the table) having an average particle size D50 of 46.1 µm, and iron sulfide ("FeS" in the table) having an average particle size D50 of 13.5 µm, the sulfur and the iron sulfide having passed a 100-mesh wire screen, were used.

Table 4

Sample No.	Copper powder	Graphite powder	Machinability improving material		Magnesium oxide		Lubricant
Sample No.	Content [% by mass]	Content [% by mass]	Kind	Content [% by mass]	Particle size D50 [µm]	Content [% by mass]	Content [% by mass]
16	2.0	0.8	MnS	0.04	0.7	0.010	0.8
17	2.0	0.8	MnS	0.06	0.7	0.010	0.8
18	2.0	0.8	MnS	0.08	0.7	0.010	0.8
19	2.0	0.8	MnS	0.08	0.7	0.020	0.8
20	2.0	0.8	MnS	0.08	0.7	0.030	0.8
21	2.0	0.8	MnS	0.08	0.7	0.040	0.8
22	2.0	0.8	MnS	0.10	0.7	0.010	0.8
23	2.0	0.8	MnS	0.12	0.7	0.010	0.8
24	2.0	0.8	MnS	0.08	2.5	0.020	0.8
25	2.0	0.8	MnS	0.08	3.2	0.020	0.8
26	2.0	0.8	MnS	0.08	6.2	0.020	0.8
27	2.0	0.8	MnS	0.50	-	-	0.8
28	2.0	0.8	S	0.08	46.1	0.010	0.8
29	2.0	0.8	S	0.50	-	-	0.8
30	2.0	0.8	FeS	0.08	13.5	0.010	0.8
31	2.0	0.8	FeS	0.50	-	-	0.8
32	2.0	0.8	-	-	-	-	0.8

The mixed powders for powder metallurgy Nos. 16 to 32 were each compacted in a die in a manner similar to that of the mixed powders for powder metallurgy Nos. 1 to 15, whereby ring-shaped compacts were formed; the compacts obtained were sintered in a nitrogen gas atmosphere containing a hydrogen gas at 10% by volume at a temperature of 1,130°C for 60 minutes, whereby sintered bodies were obtained.
Dimensional change rates (green base and die base) in sintering, a crushing strength, and a Rockwell hardness (B scale) of each sample were measured. Furthermore, a turning test was performed in which a side surface of a stack of ten sintered bodies of each sample was processed by turning. As a cutting tool, "2NU-CNGA120408LF," a chip using "BN7500," cubic boron nitride, (both available from Sumitomo Electric Industries, Ltd.), was used. The side surface was cut by 2,735 m by dry cutting under cutting conditions in which a peripheral velocity was 200 m/min, a cutting depth was 0.1 mm/pass, and a feed amount was 0.1 mm/rev.
A parallel wear width (flank wear width Vb) of a flank face of the cutting tool after the turning test was measured.

Measured values of each of the above-described parameters are summarized in Table 5 below.

Table 5

Sample No.	Green density [g/cm³]	Dimensional change rate		Sintered density [g/cm³]	Rockwell hardness	Crushing strength [MPa]	Flank wear width [µm]	Surface roughness Ra [µm]	Surface roughness Rz [µm]
Sample No.	Green density [g/cm³]	Green base [%]	Die base [%]	Sintered density [g/cm³]	Rockwell hardness	Crushing strength [MPa]	Flank wear width [µm]	Surface roughness Ra [µm]	Surface roughness Rz [µm]
16	7.01	0.13	0.45	6.92	73.7	834	35.6	0.80	4.65
17	7.01	0.14	0.45	6.92	73.2	829	25.4	0.64	3.85
18	7.01	0.14	0.46	6.92	73.4	827	19.3	0.70	5.00
19	7.00	0.16	0.48	6.90	73.3	820	19.1	0.62	6.05
20	7.00	0.19	0.50	6.90	73.6	794	20.7	0.68	6.53
21	7.00	0.22	0.53	6.89	74.0	789	29.1	0.75	7.71
22	7.01	0.16	0.47	6.92	74.2	827	23.1	0.75	5.10
23	7.01	0.16	0.47	6.91	73.8	824	21.6	0.69	6.07
24	7.00	0.16	0.48	6.90	73.6	824	24.7	0.71	6.17
25	7.00	0.17	0.49	6.89	73.3	822	36.2	0.73	6.25
26	7.00	0.15	0.47	6.91	74.6	844	121.7	0.73	6.25
27	7.00	0.19	0.50	6.91	73.9	799	138.7	0.45	2.38
28	7.01	0.44	0.75	6.85	71.4	708	42.1	0.68	6.28
29	6.99	0.68	1.02	6.75	68.8	715	88.0	0.32	3.42
30	7.00	0.22	0.53	6.90	72.6	787	38.3	0.77	4.79
31	7.01	0.52	0.85	6.93	68.2	753	182.7	0.73	3.30
32	7.00	0.12	0.43	6.93	74.4	853	352.9	0.39	2.04

The sintered bodies obtained by sintering the mixed powders for powder metallurgy Nos. 16 to 19 and 22 to 25, each of which contained sulfides and magnesium oxide and in which a percentage content of the magnesium oxide was greater than or equal to 0.010% by mass and less than or equal to 0.020% by mass and an average particle size D50 of the magnesium oxide was greater than or equal to 0.5 µm and less than or equal to 5.0 µm, showed sufficient formability and sufficient mechanical strength, and caused little wear of the cutting tool.
Mixed powders for powder metallurgy Nos. 33 to 39 were experimentally produced by mixing a copper powder, a graphite powder, a machinability improving material, magnesium oxide, and a powder lubricant into an iron-based powder at their respective proportions shown in Table 6 below. It is to be noted that "-" in the table indicates that the material is not contained.

It is to be noted that the iron-based powder, the copper powder, the graphite powder, the magnesium oxide, and the powder lubricant were of the same types and proportions to those used for the mixed powders for powder metallurgy Nos. 1 to 15. As the machinability improving material, manganese sulfide of the same type and proportion to that in the mixed powders for powder metallurgy Nos. 1 to 15 and molybdenum disulfide ("MoS₂" in the table) having an average particle size D50 of 5.0 µm were used.

Table 6

Sample No.	Copper powder	Graphite powder	Machinability improving material 1		Machinability improving material 2		Magnesium oxide		Lubricant
Sample No.	Content [% by mass]	Content [% by mass]	Kind	Content [% by mass]	Kind	Content [% by mass]	Particle size D50 [µm]	Content [% by mass]	Content [% by mass]
33	2.0	0.8	MnS	0.08	-	-	0.7	0.010	0.8
34	2.0	0.8	-	-	MoS₂	0.08	0.7	0.010	0.8
35	2.0	0.8	MnS	0.04	MoS₂	0.02	0.7	0.010	0.8
36	2.0	0.8	MnS	0.04	MoS₂	0.04	0.7	0.010	0.8
37	2.0	0.8	MnS	0.06	MoS₂	0.04	0.7	0.010	0.8
38	2.0	0.8	-	-	MoS₂	0.50	-	-	0.8
39	2.0	0.8	-	-	-	-	-	-	0.8

The mixed powders for powder metallurgy Nos. 33 to 39 were each compacted in a die in a manner similar to that of the mixed powders for powder metallurgy Nos. 16 to 32, whereby ring-shaped compacts were formed; the compacts obtained were sintered under conditions similar to those of Nos. 16 to 32, whereby sintered bodies were obtained.
Dimensional change rates (green base and die base) in sintering, a crushing strength, and a Rockwell hardness (B scale) of each sample were measured. Furthermore, a turning test was performed in which a side surface of a stack of ten sintered bodies of each sample was processed by turning. As a cutting tool, "2NU-CNGA120408LF," a chip using "BN7500," cubic boron nitride, (both available from Sumitomo Electric Industries, Ltd.), was used. The side surface was cut by 2,735 m by dry cutting under cutting conditions in which a peripheral velocity was 200 m/min, a cutting depth was 0.1 mm/pass, and a feed amount was 0.1 mm/rev.
Parallel wear width (flank wear width Vb) of a flank face of the cutting tool after the turning test was measured.

Measured values of each of the above-described parameters are summarized in Table 7 below.

Table 7

Sample No.	Green density [g/cm³]	Dimensional change rate		Sintered density [g/cm³]	Rockwell hardness	Crushing strength [MPa]	Flank wear width [µm]	Surface roughness Ra [µm]	Surface roughness Rz [µm]
Sample No.	Green density [g/cm³]	Green base [%]	Die base [%]	Sintered density [g/cm³]	Rockwell hardness	Crushing strength [MPa]	Flank wear width [µm]	Surface roughness Ra [µm]	Surface roughness Rz [µm]
33	7.01	0.20	0.51	6.92	74.7	814	30.5	0.77	5.79
34	7.00	0.25	0.56	6.90	75.6	795	28.1	0.73	5.50
35	7.00	0.20	0.51	6.91	75.8	810	26.3	0.74	5.87
36	7.00	0.22	0.53	6.91	76.3	790	24.0	0.78	6.28
37	7.00	0.23	0.54	6.90	76.0	801	25.4	0.80	6.33
38	7.00	0.36	0.69	6.86	79.7	908	78.1	1.12	5.07
39	7.01	0.14	0.46	6.93	77.4	853	184.3	0.57	3.65

The sintered bodies obtained by sintering the mixed powders for powder metallurgy Nos. 33 to 37, each of which contained a sulfide and magnesium oxide and in which a percentage content of the magnesium oxide was greater than or equal to 0.010% by mass and less than or equal to 0.020% by mass, showed sufficient formability and sufficient mechanical strength, and caused little wear of the cutting tool. Furthermore, in comparison between No. 33 and No. 34, No. 34, in which the molybdenum disulfide was used as the sulfide, caused a smaller parallel wear width of the flank face than that of No. 33, in which the manganese sulfide was used as the sulfide. Moreover, Nos. 35 to 37, in each of which two kinds of sulfides (the manganese sulfide and the molybdenum disulfide) were used, caused smaller parallel wear widths of the flank faces than those of No. 33 and No. 34, in each of which only one of the manganese sulfide and the molybdenum disulfide was used as the sulfide. It is to be noted that, although No. 33 is identical to No. 18 in proportions of the components and No. 39 is identical to No. 32 in proportions of the components, it is conceivable that differences in the measured values were caused by a difference in batch of each material.

[INDUSTRIAL APPLICABILITY]

The mixed powder for powder metallurgy according to the present invention is suitably used for producing a high-precision component which requires cutting after sintering.

Claims

A mixed powder for powder metallurgy, comprising:
an iron-based powder as a main component;

a powder of at least one sulfide selected from CaS, MnS, and MoS₂; and

a powder wherein a percentage content of magnesium oxide is greater than or equal to 0.005% by mass and less than or equal to 0.025% by mass,

wherein the magnesium oxide has an average particle size D50 of greater than or equal to 0.5 µm and less than or equal to 5.0 µm.
The mixed powder for powder metallurgy according to claim 1, wherein a total content of the sulfide is greater than or equal to 0.04% by mass and less than or equal to 0.20% by mass.