Method of making metal piece having high density from metal powder

Abstract

A method of making a metal piece having a high density is disclosed in which metal powder is compacted into compacts and these compacts are stacked one upon the other in a metallic container having an upper open end and then the metallic container together with the compacts stacked therein are heated and hot forged in atmospheric air. A perforated carburizing protecting plate and solid reducing agent are disposed on the compacts in the container and the upper open end of the container is closed by a cover plate having a degassing gap.

Description

This invention relates to a method of making a metal piece having a high density from metal powder by heating and hot forging compacts obtained from the metal powder and stacked in a metallic container in atmospheric air without oxidizing the metal powder.

Heretofore, it has been proposed to make a metal piece having a high density from metal powder by hot forging the metal powder by the following three methods:

1. Method of sintering and forging metal powder.

2. Method of hot compressing metal powder by static pressure, and

3. Method of rolling metal powder.

The first method (1) of sintering and forging metal powder has rapidly been closed up as an interesting technique for recent powder metallurgy. This method, however, is limited to production of small machine parts which are required to be subjected to a number of cutting steps and of which weight is on the order of at most 4 to 5 Kg. In addition, this method is required to use a forging metallic die. This method is intended to be replaced for the conventional mechanical cutting method of making a metal piece for the purpose of providing a material increase in yield and saving materials and mechanical power, thereby reducing the cost of the metal piece. This method of sintering and forging metal piece, however, has the disadvantage that the forging must be effected in a limited space in the metallic die, that a compact must be heated in a reducing atmosphere, that a large compact could not be forged owing to the presence of the metallic die, and that use must be made of exclusively pressing machine or forging machine.

The second method (2) of hot compressing metal powder by static pressure also has the disadvantage that provision must be made of a special apparatus, that a special atmosphere or pressure medium must be used for compressing the metal powder and that it is difficult to forge a large compact. In addition, the apparatus used is difficult in handling. The method is only applicable to a special metal piece. Moreover, the metal piece becomes expensive and could not be made in a mass-production scale.

The third method (3) of rolling metal powder has the disadvantage that it is difficult to make a continuous compact by means of rolls and that a reducing atmosphere must also be used for heating the compact.

As stated hereinbefore, the conventional methods (1) to (3) could not satisfy those conditions which are required in the method of making a metal piece from metal powder, and could not satisfy such conditions that heating and hot forging of the compacts can be effected in atmospheric air, that all of the steps can be effected in a less expensive and easy manner, that various types of forging machines can easily be used, and that large machine parts can easily be manufactured in a mass-production scale.

An object of the invention, therefore, is to provide a method by which a large and macroscopically homogeneous metal piece can be obtained.

Another object of the invention is to provide a method which is capable of heating and hot forging compacts in atmospheric air in less expensive manner without oxidizing these compacts.

A further object of the invention is to provide a method by which large machine parts can easily be manufactured by using various types of forging machines.

A feature of the invention is the provision of a method of making a metal piece having a high density from metal powder, comprising the steps of compacting metal powder to obtain compacts, stacking said compacts one upon the other in a metallic container having an upper open end, disposing a perforated carburizing protecting plate and solid reducing agent in succession on said stacked compacts, closing said upper open end of said container by means of a cover plate made of metal and having a degassing gap, heating said stacked compacts together with said container at a temperature from 1,000.degree.C to 1,300.degree.C in atmospheric air, hot forging said stacked compacts together with said container in atmospheric air to deform them, and finally cutting off said deformed container to obtain a metal piece having a high density.

The invention makes it possible to achieve not only heating of the compact in atmospheric air but also hot forging in atmospheric air by using various types of forging machines while preventing the metal powder from being oxidized. As a result, the metal piece having a high density can be made in an extremely less expensive manner. Thus, the method according to the invention may be applied to various fields of powder metallurgy.

In the method according to the present invention, the compact is subjected to a pretreatment such that the compact can be hot forged in atmospheric air prior to its after treatment. The invention permits of easily obtaining a large macroscopically homogeneous metal piece. For example, the use of the same kind of metal powder ensures production of metal piece having no segregation. If at least two kinds of metal powders are used, these powders are fully mixed to each other prior to packing of the compact in a metallic container, and as a result, a homogeneous metal piece having no macroscopic segregation can be obtained, thereby improving the yield of the metal powder.

The invention will now be described in greater detail with reference to the accompanying drawings, wherein:

FIG. 1 is a flow sheet diagram showing successive steps of the method according to the invention;

FIG. 2 is a plan view of one embodiment of a metallic container used in the method according to the invention;

FIG. 3 is its front elevation partly in section;

FIG. 4 is a plan view of another embodiment of the metallic container; and

FIG. 5 is its front elevation partly in section.

Referring to FIG. 1, A designates a metal powder mixing step, B a compacting step, C a sintering step and D a step of stacking compacts in a metallic container. As shown in FIG. 1, the sintering step C may be omitted and carry out the successive steps A-B-D instead of A-B-C-D.

In the step A, metal powder is mixed with graphite powder, if necessary, and then compacted at the step B into compats which are then with or without sintering stacked in the metallic container one upon the other in the step D. The compact is required to have a density ratio of at least 70 percent of its theoretical density. A compact having a density ratio not higher than 70 percent becomes excessively contracted when it is hot forged at the step G, and as a result, it is impossible to sufficiently achieve the hot forging through the deformation of metallic container.

The compacts with or without subjected to the sintering step C may be stacked in the metallic container. But, it is preferable to stack the sintered compacts in the container for ease of handling. In case of compacting iron powder, carbon may be added to the iron powder for the purpose of adjusting the compositions of the metal piece. In this case, if carbon is alloyed beforehand with the iron, the compacting property of such iron-carbon alloy becomes deteriorated, so that it is preferable to mix the iron powder with graphite powder in the mixing step A and then to compact the mixed powder. As a result, it is preferable to make use of the sintering step C in order to fully diffuse the mixed components to each other. But, the presence or absence of the sintering step C is not the essential part of the present invention. In order to obtain the homogeneous metal piece, it is important to fully mix the metal powder prior to the compacting step B or to the stacking step D. It is also important how to compact the mixed powder. But, such treatments do not constitute the essential parts of the present invention.

One of the features of the invention consists in the packing step D in which the compacts obtained from the metal powder by the compacting step B are stacked in the metallic container. In this case, the metallic container may be made of any materials which do not hinder the hot forging subjected to the stacked compacts. For example, the metallic container may be made of mild steel in case of hot forging stainless steel powder. In case of selecting the materials by which the metallic container is made, materials are selected which are less expensive in materials per se and forging cost, which can easily be plastically deformed without being broken and which are easy in handling. But, in general, the metallic container may be made of mild steel. The metallic container may be manufactured in any suitable shape by means of customary methods such as pressing, extrusion, casting, welding, pressure bonding and the like by taking the shape of the metal piece and the compacts to be packed therein into consideration.

The metallic container constituting one of the essential parts of the invention will plays the following roles.

1. The metallic container prevents the graphite powder and the compacts from being fallen away therefrom. 2. During the heating step F or the hot forging step G, the outer wall surface of the metallic container becomes oxidized, thereby preventing the compacts from being oxidized. As a result, in the case of a metal powder having a highly oxidizing property, the hot forging step G can be achieved without oxidizing the metal powder with the aid of oxidation protecting agent also packed in the metallic container, as will hereinafter be described.

3. When the hot forging step G is subjected to the compacts together with the metallic container, the metallic container per se is deformed while confining the compacts therein, and as a result, the metallic container plays a role of a sort of a die, thereby bringing the compacts into their highly densed state. At the same time, several compacts are closely bonded to each other and made integral into one compact.

As stated hereinbefore, the metallic container plays its special role over all steps inclusive of the packing step D, heating step F and forging step G.

The above described first role of the metallic container, which is extremely important in the invention, will now be described in greater detail.

As described above, it is preferable to stack the compacts in the metallic container after its density has been made high. In this case, if use is made of large compacts each of which is large in cross section and long in length, it is impossible to make these large compacts integral into one compact by means of die and pressing machine. For example, pressure of about 4 tons/cm.sup.2 must be applied to iron powder having an apparent density on the order of 2.5 grams/cm.sup.3 for the purpose of making such density to a density on the order of 6.5 grams/cm.sup.3. The existing pressing machine for use in powder metallurgy, however, has its upper limit of 1,000 tons which is only capable of compacting iron powder into a rectangular compact having a cross section of 250 cm.sup.2 or a disc-like compact having a diameter of about 18 cm. On the other hand, it is difficult to manufacture a large die.

Under the above circumstances, if it is desired to make a large metal piece from metal powder, it is necessary to use several compacts each of which is made from the metal powder and stack these compacts one upon the other in the metallic container. In this case, in order to prevent the compacts from being fallen away from their superimposed condition and handle these compacts as one compact, it is quite important to pack these compacts in the metallic container.

As described above with reference to the role of the metallic container, when the hot forging step G is subjected to the compacts stacked in the metallic container, the metallic container serves to confine the compacts therein, and as a result, these compacts are closely bonded to each other and made integral into one compact.

Microscopic examinations have yielded the surprising result that no defect to the compact occurs, in other words, the several compacts are firmly and uniformly bonded to each other and made integral into one compact having a high mechanical strength. This fact will be understood if referred to practical examples to be described later.

The metal powder may be compacted into a compact having any desired shape. Even when the compacts having a gap therebetween are subjected to the hot forging step, the metal piece thus obtained has no defect at all, which proves that the method according to the invention is very useful for making the metal piece from the metal powder.

It is preferable to avoid the presence of a large gap between the container and the packed compacts and to make the inner surface of the container clean.

It is not necessary to define the wall thickness of the container to a given value as it is associated with the material constituting the container. In the case of hot forging a large compact at a high temperature, the metallic container may be made thick in thickness, while in the case of hot forging a small compact at a low temperature, the metallic container may be made thin in thickness.

If use is made of a container made of mild steel and packed with a compact composed of iron powder having a weight of 20 Kg and this container is subjected to the hot forging, it is sufficient to make the thickness of the wall of the container at least 5 mm. In this case, several compacts each having a density of 6.5 g/cm.sup.3 are packed in the container and subjected to the hot forging with a forging ratio of 4. The density ratio of the compact thus forged becomes 99.7 percent, the particulars of which will be described later with reference to the practical examples.

As will be understood from the following practical examples, the method according to the invention is capable of easily providing a metal piece having a high density. The density ratio of the compact to be forged is dependent upon the heating condition, forging condition, kind of metal powder and the like so that the wall thickness of the container must be adapted to the property of the metal piece by taking the after heat treatment step H into consideration. The condition of determining the wall thickness of the container, however, does not constitute the essential part of the invention.

The second essential part of the invention consists in a step E of packing an oxidation protecting agent in the container in order to prevent the packed compacts from being oxidized by atmospheric air.

The oxidation protecting agent packed in the container plays a role of interrupting the atmospheric air from the compacts or of converting oxygen in the atmospheric air into a non-oxidizinng atmosphere so as to prevent the oxygen from being made contact with the compact. As such oxidation protecting agent, use may be made of a suitable substance in dependence with the heating temperature and the kind of materials for the container. It is preferable to use a solid reducing agent such as graphite, carbon or high molecular hydrocarbon and the like. The amount and kind of the solid reducing agent must be selected such that when the solid reducing agent is heated the solid reducing agent continues to generate a reducing gas during the heating thereof. Powder-like or particle-like reducing agent may be used for ease of handling, but use may also be made of a plate-shaped or wire-shaped reducing agent.

It is most preferable to use as the oxidation protecting agent graphite powder owing to the following reasons (1) to (5).

1. The graphite powder is easily available in market and is simple in handling.

2. Iron is easily alloyed with carbon to form steel, and as a result, even when the iron is carburized no defect occurs to the iron.

3. When the graphite powder is heated at an elevating temperature, the graphite powder is burnt to form a reducing atmosphere, and as a result, not only the compact packed in the container is prevented from being oxidized, but also less oxygen content in the steel piece is obtainable.

4. In the case of packing the graphite powder into the container as will be described later, the combustion of the graphite powder is effected at a rate which is slower than the combustion of another oxidation protecting agents so that the graphite powder can not be exhausted even after a long heating at a high temperature, thereby fully preventing the compact in the container from being oxidized.

5. If the graphite powder is heated at a high temperature, the higher the heating temperature is the more intensely reducing atmosphere is obtained.

A method of packing the oxidation protecting agent into the metallic container will now be described.

Referring to FIGS. 2 and 3, reference numeral 1 designates a metallic container and 2 compacts each obtained by compacting metal powder and stacked one upon the other in the container 1. on the uppermost compact 2 is disposed a perforated carburizing protecting plate 3 whose surface is purified and made of a suitable metal. A proper amount of oxidation protecting agent 4 such as graphite powder and the like is packed in a space remained above the carburizing protecting plate 3. Reference numeral 5 designates a cover plate secured to the upper peripheral edge of the container 1 and covering the upper surface of the graphite powder 4.

In the embodiment shown in FIGS. 2 and 3, the cover plate 5 is provided at its center with a degassing hole 6. The cover plate 5 may be secured to the upper peripheral edge 7 of the container 1, for example, by welding.

The cover plate 5 shown in FIGS. 4 and 5 is of blind one whose diameter is slightly smaller than the inner diameter of the container 1 and which is dropped onto the oxidation protecting agent 4 so as to sandwich the oxidation protecting agent 4 between the blind cover plate 5 and the perforated carburizing protecting plate 3. The blind cover plate 5 may be secured at its two or three peripheral portions 8 to the upper inner wall of the container, for example, by spot welding.

The perforated carburizing protecting plate 3 serves to prevent the oxidation protecting agent 4 from being directly touched with the compacts 2.

If the oxidation protecting agent 4 such as graphite powder is heated under such condition that its overall surface is exposed to the atmospheric air, a sudden combustion of the oxidation protecting agent 4 occurs and the compacts 2 become oxidized. The cover plate 5 causes the oxidation protecting agent 4 to be slowly burnt on the one hand and substantially closes the container 1 so as to maintain the reducing atmosphere in the container 1 on the other hand.

If the container 1 is completely closed, the gas generated therein could not be escaped from the container 1. As a result, it is essential that the cover plate 5 is provided at its center with the degassing hole 6 or the cover plate 5 is freely dropped into the container 1 so as to cause the gas to be escaped through gaps formed between the periphery of the cover plate 5 and the inner wall surface of the container 1.

The kind and amount of the oxidation protecting agent 4 must be selected and the dimension of the degassing hole 6 and the gaps formed between the periphery of the cover plate 5 and the inner wall surface of the container 1 must be adjusted such that the oxidation protecting atmosphere is maintained in the container 1 during heating of the compacts 2 packed therein.

If the compacts 2 are stacked one upon the other in the container 1 as shown in FIGS. 3 and 5, the boundary portions between adjacent compacts 2 are closely bonded to each other so that subsequent rolling or forging elongation can be effected in the lengthwise direction of the compacts 2 to obtain a desired metal piece.

As explained hereinbefore, the use of the measures of protecting the compacts 2 against oxidation and of hot forging the compacts 2 together with the container 1 in the atmospheric air ensure deformation of the container 1 and make it integral with the compacts 2 without oxidizing them. The container 1 is made of material which is less expensive and easily forgeable and which is different in material from the compacts 2. As a result, it is the common practice to finally remove the container 1 from the compacts 2. Alternatively, the container 1 may be removed from the compacts 2 after the compacts 2 have been forged into a slab-like or billet-like piece whose inside density is high. That is, at the intermediate step, the container 1 may be removed from such piece. Eventually, the container 1 may be remained as it is. It is a matter of course that the portion of the container 1 in which the oxidation protecting agent is packed is cut off after the hot forging step G has been completed.

In the step F, it is preferable to heat the stacked compacts together with the metallic container at a temperature from 1,000.degree.C to 1,300.degree.C in atmospheric air. The experimental tests have shown that if the heating temperature is lower than 1,000.degree.C, it is difficult to deform the stacked compacts and the metallic container, and that if the heating temperature is higher than 1,300.degree.C, the solid reducing agent 4 becomes suddenly gassified and escaped out of the container so that its reducing effect could not effectively and economically be use.

This invention will now be described with reference to practical examples.

EXAMPLE 1

A. powder used:

a. Chemical composition

Table 1 ______________________________________ Chemical composition of atomized pure iron powder (%) C Si Mn P S O ______________________________________ 0.008 0.011 0.14 0.006 0.003 0.15 ______________________________________

b. Particle size distribution

Table 2 ______________________________________ Particle size distribution of atomized pure iron powder ______________________________________ Particle 100 150 200 250 size +100 to 150 to 200 to 250 to 325 -325 (Mesh) Weight (%) 1.4 18.3 35.6 14.9 17.2 22.6 ______________________________________

B. compacting and sintering conditions:

Table 3 ______________________________________ Compacting and sintering conditions of atomized pure iron powder (for use in rolling) Compacting Green Dimension Sintering pressure density of compact condition (t/cm.sup.2) (g/cm.sup.3) (mm) ______________________________________ 1,050.degree.C.times.1h 3.5 6.54 60.sup.W .times.200.sup.L .times.30.sup.T in H.sub.2 atmosphere ______________________________________ (W: Width, L: Length, T: Thickness, Hereinafter these abbreviations are also used).

C. shape of container and weight of sintered compact:

(a) Shape and dimension of container. Shape: Box type, Assembled by Welding, Thickness of Carburizing Protecting Plate 5 mm. Inner dimension: 120.times. 200.times.75 (mm) Degassing hole: 10 mm Dia. (b) Material and wall thickness of container. Material: Mild steel Wall thickness: 5 mm (c) Weight of sintered compact (inclusive of weight of container) 12.8 Kg (container is packed with four rectangular sintered compacts each having the dimension shown

D. amount of graphite powder and heating condition of compact and graphite powder:

(a) Amount of graphite powder 80 g (b) Heating condition of compact and graphite powder. Heating furnace: Heavy oil is used as fuel Atmosphere: Atmospheric air Heating temperature: About 1,200.degree.C

E. hot forging condition:

(a) Forging method: Rolling (b) Dimension after rolling: 200W.times.about 400L.times.20T (mm)

F. heat treatment:

Heat treatment is not subjected to the rolled piece.

G. mechanical properties, etc.:

(a) Density ratio of metal piece: 99.8% (b) Analytical value of oxygen: 0.11% (c) Mechanical property.

Table 4 __________________________________________________________________________ Mechanical properties of rolled metal piece composed of atomized pure iron powder Mechanical Tensile strength test Impact test property (JIS No. 4) (JIS No. 4) Yield Tensile Elonga- Reduction Impact value strength strength tion of area (Room Material (Kg/mm.sup.2) (Kg/mm.sup.2) (%) (%) temperature Kg.sup.. m/cm.sup.2 __________________________________________________________________________ Rolled metal piece made by the method 16.2 38.8 36.3 62.4 12.8 according to the invention *Sintered body ** ** *** (Density -- 14.6 8.1 -- 1.1 6.5 g/cm.sup.3) __________________________________________________________________________ *Sintering is effected at 1,050.degree.C for 1 hour in H.sub.2 atmosphere. **Standard test piece defined by JSPM (Japan Society of Powder Metallurgy). ***No notch test piece of 10 mm.sup.2 .times. 55 L(mm).

EXAMPLE 2

A. powder used:

Atomized pure iron powder which is the same as that used in the Example 1, but mixed with 0.44% by weight of graphite powder.

B. compacting and sintering conditions:

Table 5 ______________________________________ Compacting and sintering conditions of atomized pure iron powder (for use in forging) Compacting Green Dimension Sintering pressure density of compact condition (t/cm.sup.2) (g/cm.sup.3) (mm) ______________________________________ Compact is not 3.5 6.51 120Dia.times.30T subjected to sintering ______________________________________

C. shape of container and weight of non-sintered compact:

(a) Shape and dimension of container. Shape: Cylinder Type, Assembled by Welding, Thickness of Carburizing Protecting Plate 5 mm. Inner dimension: 120 Dia .times. 285L (mm) Degassing hole: 10 mm Dia (b) Material and wall thickness of container. Material: Mild steel Wall thickness: 5.5 mm (c) Weight of non-sintered compact. (inclusive of weight of container) 23.7 kg (container is packed with nine disc-like non-sintered compacts each having the dimension shown in Table 5).

D. amount of graphite powder and heating condition:

(a) Amount of graphite powder: 55 g (b) Heating condition of compact and graphite powder. Heating furnace: Heavy oil is used as fuel. Atmosphere: Atmospheric air Heating temperature: About 1,200.degree.C

E. hot forging condition:

(a) Forging method: Forging (b) Upsetting ratio: 1.2 (c) Forging ratio: 4 (d) Dimension after forging: 60 Dia .times. about 1,000L (mm)

F. heat treatment:

880.degree.C .times. 2 hours Air cooling (Normalizing treatment)

G. mechanical properties, etc.:

(a) Density ratio of metal piece: 99.7% (b) Analytical value of oxygen: 0.05% (c) Analytical value of carbon: 0.28% (d) Mechanical properties.

Table 6 __________________________________________________________________________ Mechanical properties of forged metal piece composed of Fe-C powder Mechanical Tensile strength test Impact test property (JIS No. 4) (JIS No. 4) Yield Tensile Elonga- Reduction Impact value strength strength tion of area (Room Material (Kg/mm.sup.2) (Kg/mm.sup.2) (%) (%) temperature Kg.sup.. m/cm.sup.2 __________________________________________________________________________ Forged metal piece made by the method 23.3 44.2 26.1 48.6 6.3 according to the invention *Sintered body ** ** *** (Density -- 21.5 8.3 -- 0.7 6.5 g/cm.sup.3) __________________________________________________________________________ *Sintering is effected at 1,050.degree.C for 1 hour in RX gas. Analytica value of C 0.26%. **Standard test piece defined by JSPM (Japan Society of Powder Metallurgy). ***No notch test piece of 10mm.sup.2 .times. 55L (mm).

EXAMPLE 3

A. powder used:

a. Chemical composition.

Table 7 ______________________________________ Chemical composition of atomized low alloy steel powder (%) C Si Mn P S Ni Cr Mo O ______________________________________ 0.08 0.036 1.47 0.008 0.005 0.51 0.54 0.53 0.14 ______________________________________

Use was made of a compact mainly consisting of the powder whose chemical composition shown in the above Table 7 and mixed with 0.35 percent by weight of graphite powder.

b. Particle size distribution

Table 8 ______________________________________ Particle size distribution of atomized low alloy steel powder ______________________________________ Particle 100 150 200 250 size +100 to 150 to 200 to 250 to 325 -325 (Mesh) Weight (%) 0.3 16.8 27.9 14.4 15.2 25.4 ______________________________________

B. compacting and sintering conditions:

Table 9 ______________________________________ Compacting and sintering conditions of atomized low alloy steel powder Compacting Green Dimension Sintering pressure density of compact condition (t/cm.sup.2) (g/cm.sup.3) (mm) ______________________________________ 1,050.degree.C.times.1h in 4 6.32 60 Dia .times. 30T RX gas atmosphere ______________________________________

C. shape of container and weight of sintered compact:

(a) Shape and dimension of container. Shape: Cylinder type, Assembled by Welding, Thickness of Carburizing Protecting Plate 5 mm. Inner dimension: 60 Dia .times. 135L (mm) Degassing hole: 5 mm Dia. (b) Material and wall thickness of container. Material: Mild steel Wall thickness: 5 mm. (c) Weight of sintered compact (inclusive of weight of container) 3.4 Kg (container is packed with four disc-like sintered compacts each having the dimension shown in Table 9).

D. amount of graphite powder and heating condition of compact and graphite powder:

(a) Amount of graphite powder: 20 g (b) Heating condition of compact and graphite powder. Heating furnace: Heavy oil is used as fuel. Atmosphere: Atmospheric air. Heating temperature: About 1,100.degree.C.

E. hot forging condition:

(a) Forging method: Forging. (b) Upsetting ratio: 1.2 (c) Forging ratio: 4 (d) Dimension after forging: 30 Dia .times. about 600L (mm)

F. heat treatment:

880.degree.C .times. 1h Air cooling (Normalizing treatment) 640.degree.C .times. 11/2h Water cooling tempering.

G. mechanical properties, etc.:

(a) Density ratio of metal piece: 99.8% (b) Analytical value of oxygen: 0.08% (c) Analytical value of carbon: 0.26% (d) Mechanical properties.

Table 10 __________________________________________________________________________ Mechanical properties of forged metal piece composed of atomized low alloy steel powder Mechanical Tensile strength test Impact test property (JIS No. 4) (JIS No. 4) 0.2% Tensile Elonga- Reduction Impact yield strength tion of area value Material strength (Kg/mm.sup.2) (%) (%) (Kg.sup.. m/cm.sup.2) (Kg/mm.sup.2) __________________________________________________________________________ Forged metal piece made by the method 63.3 88.7 19.8 46.3 7.8 according to the invention *Sintered body ** *** (Density -- 46.1 -- -- 0.3 6.28 g/cm.sup.3) __________________________________________________________________________ *Sintering is effected at 1,050.degree.C for 1 hour in RX gas. **Standard test piece defined by JSPM (Japan Society of Powder Metallurgy). ***No notch test piece of 10 mm.sup.2 .times. 55L (mm).

In the above Example 1, use was made of pure iron powder obtained by customary water atomizing method and the pure iron powder was compacted and sintered. The sintered compact was packed into a container made of mild steel into which was also packed graphite powder as the oxidation protecting agent. The container was then heated in atmospheric air and finally hot rolled in atmospheric air.

In the Example 2, use was made of the same iron powder as that used in the Example 1 and the iron powder was mixed with 0.44 percent by weight of graphite powder. This mixed iorn powder was compacted and directly packed into a container made iron mild steel. Thus, in the present Example 2 the compact was not sintered. Into the container was packed graphite powder and then heated in atmospheric air and finally hot forged into a round bar in atmospheric air and finally hot forged into a round bar in atmospheric air.

In the Example 3, use was made of low alloy steel powder obtained by customary water atomizing method and the low alloy steel powder was compacted and sintered. The sintered compact was packed into a container made of mild steel into which was packed graphite powder. The container was then heated in atmospheric air and finally hot forged into a round bar in atmospheric air. The inventors have found out that substantially the same results as those obtained by the low alloy steel by using high allow steel having the following chemical composition.

__________________________________________________________________________ C Si Mn P S Cr Mo W V O __________________________________________________________________________ 0.73 0.18 0.21 0.020 0.022 4.16 4.90 6.08 1.74 0.21 __________________________________________________________________________

As seen from the Tables 4, 6 and 10, the mechanical propoerties of the metal piece obtained by the method according to the inventions are compared with those of the sintered body without subjected to the packing and hot forging steps according to the invention. These Tables concretely show that the metal piece made by the method according to the invention is far superior in mechanical properties to the sintered body and that the compact in the container is fully prevented from being oxidized.

As stated hereinbefore, the method according to the invention is capable of heating and hot forging metal powder is atmospheric air without oxidizing it and without necessitating any specially complex device. Thus, the method according to the invention has the advantage that a metal piece having a high density can be obtained in an extremely less expensive and easy manner if compared with the conventional method, that a large machine part can be manufactured, and that new utility of powder metallurgy can be developed.

Claims

What is claimed is:

1. A method of making a macroscopically homogeneous metal piece having a high density from metal powder, comprising the steps of compacting metal powder to obtain compacts, stacking said compacts one upon the other in a metallic container having an upper open end, disposing a perforated carburizing protecting plate and a solid reducing agent in succession on said stacked compacts so that said solid reducing agent does not directly touch said metal compacts, said solid reducing agent upon heating generates and maintains an oxidation protecting atomosphere, closing said upper open end of said container by means of a cover plate made of metal and having a degassing gap so that the conainer is otherwise sealed, heating said stacked compacts together with said container at a temperature from 1,000.degree.C. to 1,300.degree.C in atmospheric air, hot forging said stacked compacts together with said container in atmospheric air to deform them, and finally cutting off said deformed container to obtain the metal piece.

2. A method of making a metal piece having a high density from metal powder as claimed in claim 1, wherein said solid reducing agent is graphite powder.

3. A method of making a metal piece having a high density from metal powder as claimed in claim 1, wherein said compact has a density ratio of at least 70 percent of the theoretical density thereof.

4. A method of making a metal piece having a high density from metal powder as claimed in claim 3, wherein said metal powder is selected from iron group powder consisting of pure iron powder and alloy steel powder.

5. A method of making a metal piece having a high density from metal powder as claimed in claim 1, wherein said metallic container is made of iron, preferably, mild steel.

6. A method of making a metal piece having a high density from metal powder as claimed in claim 1, wherein said step of maintaining oxidation protecting atmosphere in said metallic container comprises adjusting the amount and selecting the kind of said solid reducing agent.

7. A method of making a metal piece having a high density from metal powder as claimed in claim 1, wherein said step of maintaining oxidation protecting atmosphere in said metallic container comprises adjusting the degassing gap.