Iron powder and mixed powder for powder metallurgy as well as method of producing iron powder

Abstract

This invention is to control the diffusion of C (carbon) from added graphite to particles of iron powder in the sintering to thereby improve the accuracy of dimensional change in the sintered body by using iron powder for powder metallurgy and a mixed powder thereof as a starting material in the production of sintered mechanical parts by adding the iron powder with Cu powder and graphite powder and companying and sintering them, in which 0.008-0.5 wt % in total of at least one element having a value of standard free energy of formation of oxide at 1000.degree. C. of not more than -120 kcal/l mol of O.sub.2 is included in the iron powder and not less than 20% of the element is rendered into an oxide, or 0.01-0.20 wt % in total of an oxide powder of at least one element having a value of standard free energy of formation of oxide at 1000.degree. C. of not more than -120 kcal/l mol of O.sub.2 is added to the mixed powder.

Description

TECHNICAL FIELD

Iron powder used for powder metallurgy is roughly divided into two types pure iron powder and alloying steel powder.

This invention relates to iron powder and mixed powder for powder metallurgy belonging to the above former pure iron powder as well as a method of producing such iron powder.

BACKGROUND ART

Iron powder for powder metallurgy is used in the production of a sintered part having usually a density of 5.0-7.2 g/cm.sup.3. The part is made by adding and mixing iron powder with Cu powder, graphite powder and the like, shaping into a green compact in a mold, sintering and, if necessary, sizing a sintered body for dimensional correction.

However, the sintered body produced by adding Cu powder, graphite powder or the like to the iron powder is high in the strength, so that it has a drawback that the dimensional correction can not be conducted to a satisfactory extent due to spring-back of the sintered body even if the sizing for dimensional correction is conducted.

As a method of ensuring a desired dimensional accuracy without sizing, therefore, JP-B-56-12304 proposes a technique of enhancing the accuracy of dimensional change by improving particle size distribution of the starting powder, and JP-A-3-142342 proposes a technique of controlling a given size by predicting the dimensional change during sintering from the shape of powder.

However, the iron powder for powder metallurgy is added with Cu powder, graphite powder, lubricant and the like, or mixed for the uniformization of properties in the steps from powder formation to the shaping, or further transferred for replacement with a new vessel, so that the properties such as particle size distribution, shape and the like are apt to be changed at these steps Also, the position change of ingredients due to segregation of Cu powder or graphite powder added to the iron powder occurs and consequently the dimensional accuracy can not necessarily be obtained to a satisfactory extent.

DISCLOSURE OF INVENTION

The invention advantageously solves the above problems and to provides iron powder and mixed powder for powder metallurgy capable of providing a dense sintered body with a high accuracy by enhancing the accuracy of dimensional change in the sintering (concretely green density: about 6.90 g/cm.sup.3, scattering width of dimensional change: within 0.10%, preferably 0.06%) without impairing compressibility as well as a method of advantageously producing such iron powder.

The inventors have made various studies with respect to the composition of iron powder and the compounding ratio of additives in order to achieve the above object and found the following:

(1) The dimensional change in the sintered body is strongly correlated to the amount and particle size of graphite added to iron powder; (2) Even when the amount and particle size of graphite changes, if an oxide of a particular element is existent on the surface of the iron powder at a constant quantity or more, the scattering width of dimensional change or the fluctuating width of dimensional change reduces; and (3) As the scattering width of the oxide quantity becomes small, the fluctuating width of dimensional change is small.

The invention is based on the above knowledges.

That is, the essential points and construction of the invention are as follows. 1. Iron powder for powder metallurgy consisting of 0.008-0.5 wt % in total of at least one element having a value of standard free energy of formation of oxide at 1000.degree. C. of not more than -120 kcal/l mol of O.sub.2, not more than 0.30 wt % of oxygen and the remainder being Fe and inevitable impurities, in which not less than 20% of the above element forms an oxide.

2. Iron powder for powder metallurgy consisting of 0.008-0.5 wt % in total of at least one element having a value of standard free energy of formation of oxide at 1000.degree. C. of not more than -120 kcal/l mol of O.sub.2, not more than 0.30 wt % of oxygen and the reminder being Fe and inevitable impurities, in which not less than 20% of the above element forms an oxide and a scattering width of oxidation ratio is not more than 50%. 3. Iron powder for powder metallurgy according to paragraph 1 or 2, wherein the element having a value of standard free energy of formation of oxide at 1000.degree. C. of not more than -120 kcal/l mol of O.sub.2 is selected from the group consisting of Cr, Mn, V, Si, Ti and Al. 4. A mixed powder, characterized in that 0.01-0.20 wt % in total of oxide powder of at least one element having a value of standard free energy of formation of oxide at 1000.degree. C. of not more than -120 kcal/l mol of O.sub.2 is added to a mixed powder said mixed powder being formed from 0.5-0.8 wt % graphite powder or a mixture of 0.5-0.8 wt % graphite powder and 1.5-2.0 wt % Cu powder and the remainder being iron powder. 5. A mixed powder according to paragraph 4, wherein the oxide powder of at least one element having a value of standard free energy of formation of oxide at 1000.degree. C. of not more than -120 kcal/l mol of O.sub.2 is selected from the group consisting of Cr.sub.2 O.sub.3, MnO, SiO.sub.2, V.sub.2 O.sub.3, TiO.sub.2 and Al.sub.2 O.sub.3. 6. A method of producing iron powder for powder metallurgy, characterized in that iron powder having a composition consisting of 0.008-0.5 wt % in total of at least one element having a value of standard free energy of formation of oxide at 1000.degree. C. of not more than -120 kcal/l mol of O.sub.2, and the remainder being Fe and inevitable impurities is subjected to an oxidation treatment at a temperature of 100.degree.-200.degree. C. in a nitrogen atmosphere having an oxygen concentration of 2.5-15.0 vol % and then subjected to a selective reduction treatment for oxidized Fe in a reducing atmosphere at 800.degree.-1000.degree. C. 7. A method of producing iron powder for powder metallurgy according to paragraph 6, wherein the oxidation treatment of iron powder is conducted with stirring.

The invention will be described concretely based on experimental results originating in the invention.

The inventors have totally examined various experimental results and confirmed that the rate of dimensional change in the sintered body is strongly correlated to the amount and particle size of graphite added, and particularly, the scattering width of dimensional change (i.e. fluctuating width of dimensional change) tends to become large as the amount of graphite becomes large.

However, it is occasionally confirmed that the fluctuating width of dimensional change becomes small even though the amount of graphite added is large.

As a result of investigations on such a cause that the fluctuating width of dimensional change is small even if the amount of graphite added is large, it has been confirmed that this is due to the fact that a relatively large amount of oxide is existent on the surface of the iron powder.

However, when the oxide is existent on the surface of iron powder, the fluctuating width of dimensional change becomes not necessarily small.

Then, there has been considered a common point that each oxide could control the fluctuating width of dimensional change to a small extent. As a result, it has been elucidated that a good result is obtained when using all elements each having a value of standard free energy of formation of oxide at 1000.degree. C. of not more than -120 kcal/l mol of O.sub.2.

In Table 1 are shown a value of standard free energy of formation of oxide at 1000.degree. C. of each element, a composition of the resulting oxide, and a judgment on accuracy of dimensional change when each oxide is formed on the surface of the iron powder (oxide quantity: 0.1-0.2 wt %).

TABLE 1______________________________________ Standard free energy of formation of oxide atElement 1000.degree. C. (Kcal/l mol of O.sub.2) Oxide Judgment______________________________________Cu -37 Cu.sub.2 O XNi -57 NiO XCr -126 Cr.sub.2 O.sub.3 .largecircle.Mn -140 MnO .largecircle.V -148 V.sub.2 O.sub.3 .largecircle.Si -156 SiO.sub.2 .largecircle.Ti -165 TiO.sub.2 .largecircle.Al -203 Al.sub.2 O.sub.3 .largecircle.______________________________________ .largecircle. Flucutating width of dimensional change: slight X: Fluctuating width of dimensional change: large

As seen from Table 1, good accuracy of dimensional change is obtained when an oxide is made from an element having a value of standard free energy of formation of oxide at 1000.degree. C. of not more than -120 kcal/l mol of O.sub.2.

Although the reason why the accuracy of dimensional change is improved by existing the above oxide on the surface of iron powder is not yet clear, it is considered as follows.

When the aforementioned oxide exists on the surface of iron powder to a certain extent, the diffusion of C (carbon) from graphite added to particles of iron powder during the sintering is controlled and hence the amount of C invaded and diffused into the iron powder is held at an approximately constant value even if the amount and particle size of graphite added change, whereby a so-called Cu growth is stabilized to finally control the fluctuating width of dimensional change to a small range as compared with the fluctuating width of the amount of graphite added.

The above state is illustrated as shown in FIG. 1.

That is, when using the conventional iron powder has no oxide existing on its surface, as shown by a curved line 1 in the above figure, the quantity of dimensional change largely varies with the change of C amount, while when an adequate quantity of oxide exists on the surface of the iron powder, as shown by a curved line 2, the inclination of the curved line becomes small, so that even if the C amount changes, the quantity of dimensional change is not so varied.

Even when the amount of graphite added varies as mentioned above, in order to effectively reduce the rate of dimensional change, it is necessary that 0.008-0.5 wt % of an element having a value of standard free energy of formation of oxide at 1000.degree. C. of not more than -120 kcal/l mol of O.sub.2 (hereinafter referred to as adequate element simply) is included into the iron powder and not less than 20 wt % of the above element is rendered into an oxide.

When the amount of the adequate element is less than 0.008 wt %, the fluctuating width of dimensional change in the sintered body can not be reduced to the fluctuating width of graphite added, while when it exceeds 0.5 wt %, the compaction in the shaping rapidly lowers. Further, when the quantity of oxide is less than 20 wt %, as shown in FIG. 1, the inclination of a curve between amount of graphite and quantity of dimensional change is still large and hence the fluctuating width of dimensional change in the sintered body to the fluctuating width of graphite added can not be reduced.

As the adequate element, Cr, Mn, V, Si, Ti and Al are advantageously adaptable. Even in the case of adding these elements alone or in admixture, when the amount is within a range of 0.008-0.5 wt % in total, the same effect can be obtained. Moreover, a preferable range of each element added alone is as follows:

______________________________________ Cr: 0.05-0.5 wt %, V: 0.008-0.5 wt %, Ti: 0.008-0.5 wt %, Mn: 0.01-0.3 wt %, Si: 0.008-0.5 wt %, Al: 0.008-0.5 wt %______________________________________

Moreover, it is observed by EPMA that the oxide is dispersedly existent in the vicinity of the surface of the iron powder (about 10 .mu.m from the surface) and in particles thereof. In the invention, it has been confirmed that a desired effect is obtained when the oxide-forming ratio is not less than 20 wt %, and the effect becomes large when the position of existing the oxide is locally existent near the surface.

Furthermore, it is important to control the concentration of oxygen in iron powder to not more than 0.30 wt %. When oxygen is contained in an amount exceeding 0.30 wt %, the compressibility during the compact shaping lowers, which brings about degradation of strength in the product.

As mentioned above, when a given amount of the adequate element is included in the iron powder and not less than 20 wt % thereof is rendered into an oxide, the fluctuating width of dimensional change in the sintered body can largely be reduced as compared with the conventional case. As a result of the inventors' further studies, it is elucidated that it is effective to reduce the scattering width of oxidation ratio of the adequate element to not more than 50% (preferably not more than 30%) in order to further improve the accuracy of dimensional change in the sintered body.

That is, the quantity of dimensional change in the sintered body varies in accordance with the oxidation ratio of the adequate element as shown in FIG. 2. This tendency is conspicuous when the oxidation ratio is small. For example, in case of SiO.sub.2, when the oxidation ratio is not more than 20%, the fluctuating width of dimensional change becomes fairly large. Therefore, when the scattering width of the oxidation ratio is large (particularly the oxidation ratio is small), the scattering width of dimensional change becomes large. Inversely, when the scattering width of the oxidation ratio is small, the fluctuating width of dimensional change is effectively mitigated.

In Table 2 are shown results measured on fluctuating width of dimensional change and green density in the sintered body when Si as an adequate element is included into iron powder at various amounts and the scattering width of oxidation ratio of Si are variously varied.

TABLE 2______________________________________Sym- Scattering Scattering Fluctuatingbol range of width of width ofof oxidation oxidation dimensionaliron Si ratio in ratio in change in Greenpow- content Si content Si content sintered body densityder (wt %) (%) (%) (%) (g/cm.sup.3)______________________________________A 0.004 5.about.100 95 0.60 7.00B 0.007 5.about.95 90 0.56 6.99C 0.008 30.about.40 10 0.06 6.98D 0.016 35.about.45 10 0.06 6.98E 0.025 45.about.50 5 0.04 6.97F 0.027 55.about.65 10 0.06 6.92G 0.050 25.about.80 55 0.10 6.90H 0.20 30.about.50 20 0.05 6.89I 0.50 20.about.80 60 0.10 6.88J 0.60 60.about.80 20 0.06 6.77______________________________________

As seen from this table, when Si is included within a proper range and the oxidation ratio thereof is not less than 20 wt % and also the scattering width of the oxidation ratio is controlled to not more than 50%, there is obtained a very good accuracy of dimensional change that the fluctuating width of dimensional change in the sintered body is not more than 0.06%.

Moreover, all of the sintered bodies used in the above experiment are obtained by adding 2 wt % of Cu powder, 0.8 wt % of graphite powder and 1 wt % of zinc stearate as a lubricant to water-atomized iron powder reduced in a reducing atmosphere having a dew point of 10.degree.-60.degree. C., shaping into a green compact having a density of 6.9 g/cm.sup.3 and then sintering in RX gas having a CO.sub.2 content of 0.3% at 1130.degree. C. for 20 minutes. The scattering of dimensional change is evaluated by a fluctuating width of dimensional change in the sintering based on the green compact having a given outer diameter with respect to 100 ring-shaped specimens having an outer diameter of 60 mm, an inner diameter of 25 mm and a height of 10 mm. Furthermore, the green density is measured when the same iron powder as mentioned above is added and mixed with 1 wt % of zinc stearate and shaped under a shaping pressure of 5 t/cm.sup.2.

A preferable production method of the iron powder according to the invention will be described below.

At first, the production method of iron powder is not particularly restricted, so that the conventionally well-known methods such as water atomizing method, a reducing method and the like are adaptable. Among them, the water atomizing method is particularly advantageous in order to efficiently produce iron powder having a desired particle size, in which an average particle size of iron powder is preferably within a range of about 50-100 .mu.m.

Then, it is necessary that at least 20 wt % of adequate element included is rendered into oxide by subjecting the iron powder to an oxidation treatment in a proper oxidizing atmosphere. For this purpose, it is important that the oxidation treatment is carried out at a temperature of 100.degree.-200.degree. C. in a nitrogen atmosphere having an oxygen concentration of 2.5-15.0 vol %.

When the concentration of oxygen in the atmosphere is less than 2.5 vol %, it is difficult to ensure an oxide content of not less than 20%, while when it exceeds 15.0 vol %, the oxygen content in the iron powder can not be controlled to not more than 0.30 wt % even by a reduction treatment as mentioned later and the compressibility lowers. The reason why the essential ingredient of the atmosphere is oxygen is due to the fact that it is easy to control the oxygen concentration in the atmosphere and also there is no risk of explosion as in hydrogen or the like and the economical merit is large as compared with the case of using inert gas such as Ar or the like.

Moreover, in order to control the scattering width of the oxidation ratio in the formation of the oxide by the above oxidation treatment to not more than 50%, it is enough to conduct the oxidation treatment under stirring of powder. As the stirring apparatus, a rotary kiln and an agitating dryer are advantageously adaptable.

Now, not less than 20% of the adequate element is rendered into an oxide by the aforementioned oxidation treatment, during which iron itself is oxidized to form an iron oxide. Since such an iron oxide undesirably deteriorates the compressibility, it is necessary to reduce the iron oxide.

In the method according to the invention, therefore, only the oxidized Fe is selectively reduced by subjecting to a reduction treatment in a reducing atmosphere at 800.degree.-1000.degree. C. after the above oxidation treatment. In the selective reduction treatment of the oxidized Fe, the reason why the treating temperature is limited to the range of 800.degree.-1000.degree. C. is due to the fact that when the treating temperature is lower than 800.degree. C., it is difficult to reduce the oxygen content in the iron powder to not more than 0.30 wt %, while when it exceeds 1000.degree. C., the oxide of the adequate element is also oxidized and it is difficult to ensure the adequate quantity of not less than 20 wt %. Moreover, the treating time is sufficient to be about 20-60 minutes.

Although the above explains the technique of enhancing the accuracy of dimensional change in the sintered body by modifying the iron powder itself, even when ordinary iron powder is used, the accuracy of dimensional change in the resulting sintered body can be improved by the application of the above technique.

That is, the aforementioned technique lies in that a given adequate element is included in the iron powder and a part thereof is rendered into an oxide. On the other hand, even if a given quantity of oxide powder of the adequate element is mixed with the ordinary iron powder as a starting powder for the sintered body, there is substantially no difference in view of the effect.

As the oxide powder of the adequate element, Cr.sub.2 O.sub.3, MnO, SiO.sub.2, V.sub.2 O.sub.3, TiO.sub.2, Al.sub.2 O.sub.3 and the like are advantageously adaptable. The same effect as in case of modifying the iron powder itself can be obtained by adding at least one of these oxides at a quantity of 0.01-0.20 wt % in total.

The reason why the quantity of the oxide powder is limited to the range of 0.01-0.20 wt % is due to the fact that when the quantity is less than 0.01 wt %, the fluctuating width of dimensional change in the sintered body is still large, while when it exceeds 0.20 wt %, the green density and hence the strength of the sintered body rapidly lower.

In the case of such a mixed powder, there is caused a fear of deteriorating the accuracy due to segregation of the oxide powder based on nonuniform mixing. This is the same as in the scattering of oxidation ratio in the iron powder itself. Even if the segregation is somewhat caused, there is caused no segregation exceeding the upper limit of the oxidation ratio in the iron powder itself of 50%, so that there is substantially no problem.

On the contrary, the quantity of the oxide can strictly be controlled in the mixed powder, so that if uniform mixing is satisfied, the fluctuating width of dimensional change can be controlled with a higher accuracy and hence the quantity of dimensional change in the sintered body can freely be adjusted within a certain range.

In Table 3 are shown green density, dimensional change rate of the sintered body and transverse rupture strength of the sintered body when Al.sub.2 O.sub.3 powder is added in various quantities as an oxide powder.

Moreover, the dimensional change in the longitudinal direction of the sintered body is measured before and after the sintering on 100 sintered bodies, each of which bodies is produced by adding and mixing water-atomized iron powder with 1.5 wt % of Cu powder, 0.9 wt % of graphite powder, 1 wt % of a solid lubricant (zinc stearate) and 0.01-0.25 wt % of fine alumina powder, shaping into a green compact having a length of 35 mm, a width of 10 mm and a height of 5 mm at a green density of 7.0 g/cm.sup.3 and then sintering with a propane-modified gas at 1130.degree. C. for 20 minutes.

Furthermore, the green density is measured when the same iron powder as mentioned above is added and mixed with 1 wt % of zinc stearate and shaped under a shaping pressure of 5 t/cm.sup.2.

TABLE 3______________________________________ Quantity of Transverse dimensional Fluctuating rupture change in width of strength ofAddition Green sintered dimensional sinteredamount of density body change bodyAl.sub.2 O.sub.3 powder (g/cm.sup.3) (%) (%) (Kgf/mm.sup.2)______________________________________0 6.90 0.09 0.20 800.01 6.89 0.15 0.06 800.05 6.89 0.20 0.05 790.10 6.88 0.23 0.04 790.20 6.87 0.25 0.04 790.25 6.85 0.26 0.04 73______________________________________

The quantity of dimensional change in the sintered body is based on the dimension of the green compact.

As seen from this table, the dimensional change tends to expand with the increase in the quantity of fine Al.sub.2 O.sub.3 powder added. When the quantity is 0.1 wt %, the expansion of about 0.2% was caused as compared with the case of adding no fine powder, in which there is substantially no scattering of dimensional change.

Thus, when the quantity of Al.sub.2 O.sub.3 powder added is within a range of 0.01-0.20 wt %, the quantity of dimensional change in the sintered body can exactly be changed by a given value in accordance with the quantity of Al.sub.2 O.sub.3 powder added without decreasing the strength of the sintered body.

In such a mixed powder, therefore, when the quantity of the oxide powder added is properly adjusted, the dimension of the sintered body can optionally be adjusted. For instance, it is possible to produce plural kinds of the sintered bodies having different dimensions from a single shaping mold.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing a relation between amount of graphite added and quantity of dimensional change in sintered body; and

FIG. 2 is a graph showing a relation between oxidation ratio and quantity of dimensional change in sintered body.

BEST MODE FOR CARRYING OUT THE INVENTION

EXAMPLE 1

Various iron powders having a composition as shown in Tables 4-1 to 4-3 (average particle size: 50-100 .mu.m) were produced through a water atomization method and subjected to an oxidation treatment and further to a reduction treatment under conditions shown in Table 5.

The resulting iron powder is added and mixed with 2.0 wt % of Cu powder, 0.8 wt % of graphite powder and 1.0 wt % of zinc stearate as a lubricant, shaped into a green compact under a shaping pressure of 5.0 t/cm.sup.2 and then sintered in a propane-modified gas at 1130.degree. C. for 20 minutes.

The oxidation ratio of the added element after the reduction treatment, scattering width of oxidation ratio, green density and the fluctuating width of dimensional change and tensile strength of the resulting sintered body were measured to obtain results as shown in Tables 4-1 to 4-3.

Moreover, the fluctuating width of dimensional change was evaluated by a scattering width of dimensional change rate in the sintering on 100 ring-shaped specimens having an outer diameter of 60 mm, an inner diameter of 25 mm and a height of 10 mm based on the green compact having the same outer diameter. On the other hand, the green density was measured when the same iron powder as mentioned above was added and mixed with 1 wt % of zinc stearate and shaped under a shaping pressure of 5 t/cm.sup.2.

TABLE 4-1__________________________________________________________________________ ##STR1## ##STR2## ##STR3## ##STR4## ##STR5## ##STR6## ##STR7## ##STR8## ##STR9##__________________________________________________________________________1 0.08 0.10 68(40.about.95) 55 54 0.10 6.91 38 Acceptable Example 12 0.15 0.15 43(20.about.75) 55 54 0.08 6.93 40 Acceptable Example 23 0.35 0.15 50(20.about.80) 60 75 0.06 6.89 40 Acceptable Example 34 0.26 0.10 25(15.about.35) 20 45 0.04 6.91 39 Acceptable Example 45 0.20 0.10 50(38.about.62) 24 24 0.03 6.90 39 Acceptable Example 56 0.21 0.10 15(0.about.30) 30 55 0.19 6.91 42 Comparative Example 17 0.005 0.11 45(15.about.75) 60 55 0.21 6.93 36 Comparative Example 28 0.60 0.20 66(36.about.96) 60 44 0.11 6.75 35 Comparative Example__________________________________________________________________________ 3 ##STR10## ##STR11## ##STR12## ##STR13## ##STR14## ##STR15## ##STR16## ##STR17## ##STR18##__________________________________________________________________________9 0.008 0.10 44(16.about.72) 56 49 0.09 6.91 42 Acceptable Example 610 0.10 0.12 50(21.about.79) 58 77 0.06 6.92 40 Acceptable Example 711 0.40 0.25 62(30.about.94) 64 48 0.08 6.89 38 Acceptable Example 812 0.08 0.13 52(40.about.64) 24 55 0.03 6.89 40 Acceptable Example 913 0.08 0.13 24(20.about.28) 8 60 0.03 6.91 39 Acceptable Example 1014 0.11 0.14 15(0.about.30) 30 50 0.20 6.90 42 Comparative Example 415 0.005 0.10 40(13.about.67) 54 53 0.18 6.91 36 Comparative Example 516 0.60 0.40 62(36.about.88) 52 47 0.11 6.72 33 Comparative Example__________________________________________________________________________ 6 TABLE 4-2__________________________________________________________________________ ##STR19## ##STR20## ##STR21## ##STR22## ##STR23## ##STR24## ##STR25## ##STR26## ##STR27##__________________________________________________________________________17 0.03 0.10 45(18.about.72) 54 76 0.06 6.93 39 Acceptable Example 1118 0.15 0.15 47(20.about.74) 54 50 0.09 6.93 40 Acceptable Example 1219 0.25 0.15 57(28.about.86) 58 49 0.07 6.90 39 Acceptable Example 1320 0.18 0.14 50(41.about.59) 18 70 0.04 6.89 40 Acceptable Example 1421 0.20 0.13 35(18.about.52) 34 45 0.04 6.89 41 Acceptable Example 1522 0.21 0.13 13(0.about.26) 26 54 0.19 6.92 42 Comparative Example 723 0.005 0.11 47(17.about.77) 60 55 0.21 6.93 36 Comparative Example 824 0.60 0.17 60(30.about.90) 60 40 0.11 6.76 35 Comparative Example__________________________________________________________________________ 9 ##STR28## ##STR29## ##STR30## ##STR31## ##STR32## ##STR33## ##STR34## ##STR35## ##STR36##__________________________________________________________________________25 0.008 0.10 41(15.about.66) 51 50 0.09 6.92 39 Acceptable Example 1626 0.03 0.11 47(21.about.73) 52 68 0.08 6.90 39 Acceptable Example 1727 0.41 0.24 55(20.about.90) 70 42 0.06 6.90 40 Acceptable Example 1828 0.06 0.13 43(32.about.54) 22 60 0.03 6.90 39 Acceptable Example 1929 0.05 0.11 26(20.about.32) 12 58 0.03 6.90 39 Acceptable Example 2030 0.21 0.14 16(0.about.32) 32 51 0.20 6.92 42 Comparative Example 1031 0.003 0.10 32(5.about.59) 54 40 0.20 6.90 38 Comparative Example 1132 0.60 0.39 64(28.about.100) 72 49 0.11 6.70 31 Comparative Example__________________________________________________________________________ 12 TABLE 4-3__________________________________________________________________________ ##STR37## ##STR38## ##STR39## ##STR40## ##STR41## ##STR42## ##STR43## ##STR44## ##STR45##__________________________________________________________________________33 0.008 0.10 57(27.about.87) 60 49 0.09 6.90 40 Acceptable Example 2134 0.08 0.11 58(32.about.84) 52 70 0.06 6.91 40 Acceptable Example 2235 0.40 0.14 38(10.about.66) 56 45 0.08 6.91 41 Acceptable Example 2336 0.10 0.13 24(20.about.28) 8 55 0.04 6.91 40 Acceptable Example 2437 0.10 0.13 54(44.about.64) 20 69 0.03 6.91 39 Acceptable Example 2538 0.10 0.13 15(0.about.30) 30 49 0.19 6.92 43 Comparative Example 1339 0.003 0.10 35(5.about.65) 60 35 0.19 6.93 39 Comparative Example 1440 0.55 0.30 60(29.about.91) 62 35 0.10 6.72 32 Comparative Example__________________________________________________________________________ 15 ##STR46## ##STR47## ##STR48## ##STR49## ##STR50## ##STR51## ##STR52## ##STR53## ##STR54##__________________________________________________________________________41 0.008 0.10 37(10.about.64) 54 40 0.10 6.90 39 Acceptable Example 2642 0.07 0.11 60(30.about.90) 60 74 0.06 6.91 39 Acceptable Example 2743 0.39 0.15 57(31.about.82) 51 45 0.08 6.90 40 Acceptable Example 2844 0.11 0.13 75(54.about.96) 42 62 0.04 6.93 39 Acceptable Example 2945 0.08 0.12 27(20.about.34) 14 70 0.04 6.92 39 Acceptable Example 3046 0.009 0.12 10(0.about.20) 20 63 0.19 6.92 42 Comparative Example 1647 0.003 0.10 32(5.about.59) 54 45 0.21 6.91 38 Comparative Example 1748 0.55 0.20 71(43.about.99) 56 50 0.11 6.76 31 Comparative Example__________________________________________________________________________ 18 TABLE 5__________________________________________________________________________ Oxygen Oxidation ReductionTreating concentration temperature temperature Reducingconditions (vol %) (.degree.C.) (.degree.C.) atmosphere Stirring__________________________________________________________________________Acceptable 3 150 950 H.sub.2 (Dry) noneExample 1Acceptable 5 150 970 H.sub.2 (Dry) noneExample 2Acceptable 2.8 150 850 H.sub.2 (Dry) noneExample 3Acceptable 10 150 880 H.sub.2 (Dry) conductedExample 4Acceptable 7 150 1000 H.sub.2 (Dry) conductedExample 5Acceptable 12 150 950 H.sub.2 (due noneExample 6 point = 30.degree. C.)Acceptable 5 150 830 H.sub.2 (due noneExample 7 point = 30.degree. C.)Acceptable 5 130 920 H.sub.2 (Dry) noneExample 8Acceptable 3 170 950 H.sub.2 (due conductedExample 9 point = 30.degree. C.Acceptable 3 170 950 H.sub.2 (Dry) conductedExample 10Acceptable 3 150 950 H.sub.2 (Dry) noneExample 11.about.13Acceptable 3 150 950 H.sub.2 (Dry) conductedExample 14.about.15Acceptable 5 170 900 H.sub.2 (Dry) noneExample 16.about.18Acceptable 5 170 900 H.sub.2 (Dry) conductedExample 19.about.20Acceptable 3 170 970 H.sub.2 (Dry) noneExample 21.about.23Acceptable 3 170 970 H.sub.2 (Dry) conductedExample 24.about.25Acceptable 5 170 970 H.sub.2 (Dry) noneExample 26.about.28Acceptable 5 170 970 H.sub.2 (Dry) conductedExample 29.about.30Comparative 1 170 950 H.sub.2 (Dry) conductedExample 1, 4, 7Comparative 3 150 1050 H.sub.2 (Dry) conductedExample 10, 13, 16other compar- 3 150 950 H.sub.2 (Dry) noneative examples__________________________________________________________________________

As shown in Table 4, all of iron powders containing a given range of an adequate element and subjected to the oxidation treatment and the reduction treatment according to the invention contain not less than 20% of oxide of the added adequate element. When the sintered body is produced by using such an iron powder, the fluctuating width of dimensional change in the sintered body is not more than 0.1%, which is considerably excellent as compared with the conventional one. Furthermore, the green density and tensile strength are as high as about 6.9 kg/mm.sup.3 and about 40 kg/mm.sup.2, respectively. When the stirring is particularly conducted in the oxidation treatment (Acceptable Examples 4-5, 9-10, 14-15, 19-20, 24-25, 29-30), the scattering width of oxidation ratio of the added adequate element is suppressed to not more than 50% and hence the fluctuating width of dimensional change is not more than 0.05%, whereby a more excellent accuracy of dimensional change is obtained.

On the contrary, in Comparative Examples 1, 4 and 7, the oxygen concentration in the atmosphere for the oxidation treatment is 1%, so that the oxidation ratio of the added adequate element is less than 10%, while in Comparative Examples 10, 13 and 16, the temperature in the reduction treatment exceeds 1000.degree. C., so that the oxidation ratio of the added adequate element is less than 20%. In these Comparative Examples, a good accuracy of dimensional change is not obtained. In Comparative Examples 2, 5, 8, 11, 14 and 17 in which the amount of the adequate element added is less than the lower limit, even if the production conditions are adequate, the fluctuating width of dimensional change is as large as about 0.20%, while in Comparative Examples 3, 6, 9, 12, 15 and 18 in which the amount of the adequate element added is excessive, rapid decrease of compressibility and hence the decrease of strength in the sintered body was observed.

Moreover, when the oxygen concentration in the atmosphere for the oxidation treatment exceeded 15%, or when the temperature of the oxidation treatment exceeded 200.degree. C., the oxygen content after the treatment became too large and a long time was taken in the reduction treatment. Further, when the temperature in the reduction treatment was lower than 800.degree. C., a long reducing time was undesirably taken.

EXAMPLE 2

Iron powders having a composition as shown in Table 6 (average particle size: 50-100 .mu.m) was produced through water atomization method and then subjected to an oxidation treatment and reduction treatment under conditions shown in Table 7.

Then, green compacts and sintered bodies were produced in the same manner as in Example 1.

The oxidation ratio of the added adequate element after the reduction treatment, scattering width of oxidation ratio, green density and the fluctuating width of dimensional change and tensile strength of the resulting sintered body were measured to obtain results as shown in Table 6.

TABLE 6__________________________________________________________________________ Scattering Oxidation width of oxi- Fluctuating ratio of dation ratio width of Composition of iron powder elements of elements dimensional Green TensileRun (%) added added change density strengthNo. Cr Si Mn Al Ti V (%) (%) (%) (g/cm.sup.3) (kg/mm.sup.2) Remarks__________________________________________________________________________49 0.10 -- -- 0.02 -- -- 43(20.about.75) 55 0.10 6.92 40 Acceptable Example 3150 -- 0.07 -- -- -- 0.03 25(15.about.35) 20 0.03 6.93 40 Acceptable Example 3251 -- -- 0.20 -- 0.05 -- 50(38.about.62) 24 0.03 6.90 40 Acceptable Example 3352 0.08 0.08 0.20 -- -- -- 55(28.about.82) 54 0.09 6.91 39 Acceptable Example 3453 -- 0.08 0.15 0.03 -- -- 52(26.about.78) 52 0.06 6.91 39 acceptable Example 3554 -- -- 0.10 -- 0.10 0.06 70(40.about.100) 60 0.08 6.90 39 acceptable Example 3655 0.05 0.07 0.20 0.04 -- -- 28(38.about.62) 24 0.03 6.89 38 Acceptable Example 3756 0.06 0.10 0.15 -- 0.05 0.05 68(40.about.95) 55 0.08 6.89 39 Acceptable Example 3857 0.05 0.08 0.16 0.05 0.04 0.05 72(58.about.86) 28 0.03 6.89 38 Acceptable Example__________________________________________________________________________ 39 TABLE 7__________________________________________________________________________ Oxygen Oxidation ReductionTreating concentration temperature temperature Reducingconditions (vol %) (.degree.C.) (.degree.C.) atmosphere Stirring__________________________________________________________________________Acceptable 4 150 950 H.sub.2 (Dry) noneExample 31Acceptable 3 150 970 H.sub.2 (Dry) conductedExample 32Acceptable 3 150 850 H.sub.2 (Dry) conductedExample 33Acceptable 8 150 880 H.sub.2 (Dry) noneExample 34Acceptable 5 150 1000 H.sub.2 (Dry) noneExample 35Acceptable 5 150 950 H.sub.2 (due noneExample 36 point = 30.degree. C.)Acceptable 5 150 830 H.sub.2 (due conductedExample 37 point = 30.degree. C.)Acceptable 5 130 920 H.sub.2 (Dry) noneExample 38Acceptable 5 170 950 H.sub.2 (due conductedExample 39 point = 30.degree. C.__________________________________________________________________________

As shown in Table 6, even when a mixture of various adequate elements was added, if the amount of the mixture added was proper and the oxidation and reduction treatments were conducted according to the invention, not less than 20% of each added adequate element in the resulting iron powder was rendered into an oxide. When such iron powder was used to form a sintered body, the fluctuating width of dimensional change in the sintered body was as small as not more than 0.1%, and the green density and tensile strength were as high as about 6.9 kg/mm.sup.3 and about 40 kg/mm.sup.2, respectively.

Particularly, when stirring was conducted in the oxidation treatment (Acceptable Examples 32-33, 37, 39), the scattering width of oxidation ratio of the added adequate element was suppressed to not more than 50% and hence the fluctuating width of dimensional change was 0.03% and a very excellent accuracy of dimensional change was obtained.

EXAMPLE 3

Iron powder (purity: 99.9%, particle size: 80 .mu.m) was added with a given quantity of an oxide shown in Table 8 and added and mixed with 2.0 wt % of Cu powder, 0.8 wt % of graphite powder and 1.0 wt % of zinc stearate as a lubricant, shaped into a green compact under a shaping pressure of 5 t/cm.sup.2 and then sintered with a propane-modified gas at 1130.degree. C. for 20 minutes.

The fluctuating width of dimensional change and tensile strength of the resulting sintered body and the green density of the green compact were measured to obtain results as shown in Table 8.

Moreover, the fluctuating width of dimensional change was evaluated by a scattering width of dimensional change in the sintering on 100 ring-shaped specimens having an outer diameter of 60 mm, an inner diameter of 25 mm and a height of 10 mm based on the green compact having the same outer diameter. And also, the green density was measured when the same iron powder as mentioned above was added and mixed with 1 wt % of zinc stearate and shaped under a shaping pressure of 5 t/cm.sup.2.

TABLE 8__________________________________________________________________________ Addition Fluctuating width Green Tensile amount of of dimensional density strengthNo. oxide (%) change (%) (g/cm.sup.3) (kg/mm.sup.2) Remarks__________________________________________________________________________ 1 Cr.sub.2 O.sub.3 0.02 0.05 6.91 40 Acceptable Example 1 2 Cr.sub.2 O.sub.3 0.18 0.03 6.90 40 Acceptable Example 2 3 Cr.sub.2 O.sub.3 0.005 0.19 6.92 42 Comparative Example 1 4 Cr.sub.2 O.sub.3 0.30 0.11 6.77 34 Comparative Example 2 5 SiO.sub.2 0.02 0.04 6.90 40 Acceptable Example 3 6 SiO.sub.2 0.18 0.04 6.89 39 Acceptable Example 4 7 SiO.sub.2 0.005 0.19 6.90 42 Comparative Example 3 8 SiO.sub.2 0.30 0.11 6.75 29 Comparative Example 4 9 MnO 0.02 0.05 6.92 41 Acceptable Example 510 MnO 0.18 0.04 6.90 40 Acceptable Example 611 MnO 0.005 0.19 6.92 42 Comparative Example 512 MnO 0.30 0.10 6.77 34 Comparative Example 613 Al.sub.2 O.sub.3 0.02 0.04 6.91 40 Acceptable Example 714 Al.sub.2 O.sub.3 0.18 0.02 6.89 39 Acceptable Example 815 Al.sub.2 O.sub.3 0.005 0.18 6.91 41 Comparative Example 716 Al.sub.2 O.sub.3 0.30 0.10 6.78 30 Comparative Example 817 TiO.sub.2 0.02 0.05 6.91 41 Acceptable Example 918 TiO.sub.2 0.18 0.03 6.90 39 Acceptable Example 1019 TiO.sub.2 0.005 0.19 6.91 42 Comparative Example 920 TiO.sub.2 0.30 0.10 6.78 35 Comparative Example 1021 V.sub.2 O.sub.3 0.02 0.04 6.91 41 Acceptable Example 1122 V.sub.2 O.sub.3 0.18 0.03 6.90 40 Acceptable Example 1223 V.sub.2 O.sub.3 0.005 0.19 6.90 42 Comparative Example 1124 V.sub.2 O.sub.3 0.30 0.10 6.78 33 Comparative Example 1225 Cu.sub.2 O 0.1 0.18 6.90 42 Comparative Example 1326 NiO 0.1 0.20 6.91 41 Comparative Example 14__________________________________________________________________________

As shown in Table 8, when the sintered body was produced by using the mixed powder according to the invention in which the adequate elements were added at a given amount, the fluctuating width of dimensional change in the sintered body was not more than 0.05% and was considerably lower as compared with the conventional one, and also the green density and tensile strength were as high as about 6.9 kg/mm.sup.3 and about 40 kg/mm.sup.2, respectively.

On the contrary, when the quantity of the oxide powder added exceeded the range defined in the invention, rapid decrease of compressibility and hence decrease of strength in the sintered body were observed as in Comparative Examples 2, 4, 6, 8, 10 and 12. Further, when the quantity of the oxide powder added was less than the adequate quantity, the fluctuating width of dimensional change was as large as about 0.2% as in Comparative Examples 1, 3, 5, 7, 9 and 11.

In Comparative Examples 13 and 14 using Cu.sub.2 O or NiO powder having a value of standard free energy of formation of oxide at 1000.degree. C. of not less than -120 kcal/l mol of O.sub.2, the fluctuating width of dimensional change was not small.

EXAMPLE 4

Table 9 shows a chemical composition of iron powder used. The iron powder was obtained by water-atomizing molten steel to form a green powder, subjecting the green powder to an oxidation treatment in a nitrogen atmosphere containing 3 vol % of oxygen at 140.degree. C. for 60 minutes, reducing in a hydrogen containing atmosphere at 750.degree.-1050.degree. C. for 20 minutes and then pulverizing and sieving it.

In the analysis of Cr, Mn as an oxide, these elements were extracted as an inclusion through the alcoholic iodine method and calculated in the form of Cr.sub.2 O.sub.3 and MnO.

The fluctuating width of dimensional change and tensile strength when the sintered body was produced by using the above iron powder, the oxidation ratio of the added adequate element after the reduction treatment and the green density of the green compact were measured to obtain results as shown in Table 10.

As to the dimensional change of the sintered body, the influence of graphite quality was examined by a difference between Fe-2.0% Cu-0.8% graphite (hereinafter abbreviated as Gr) and Fe-2.0% Cu-1.0% Gr obtained by mixing graphite powder and copper powder with iron powder. The difference between both was measured with respect to 20 specimens. Each specimen had a ring shape having an outer diameter of 60 mm, an inner diameter of 25 mm and a height of 10 mm and was obtained by shaping into a green compact having a green density of 6.85 g/cm.sup.3 and then sintering in a nitrogen atmosphere at 1130.degree. C. for 20 minutes.

Furthermore, the compressibility was evaluated by a green density when the iron powder was added with 1 wt % of zinc stearate (Fe-1.0% ZnSt) and shaped into a tablet of 11 mm.phi..times.10 mm under a shaping pressure of 5 t/cm.sup.2.

Moreover, the strength was evaluated by a tensile strength when the iron powder was mixed with graphite powder and copper powder so as to have a composition of Fe-2.0% Cu-0.8% Gr, shaped into a JSPM standard tensile testing specimen (green density: 6.85 g/cm.sup.3) and sintered in a nitrogen atmosphere at 1130.degree. C. for 20 minutes.

TABLE 9__________________________________________________________________________Reduction Composition oftemperature Reducing iron powder (%)No. (.degree.C.) atmosphere Mn Cr O Remarks__________________________________________________________________________1 950 H.sub.2 (Dry) 0.15 0.10 0.22 Acceptable Example 12 970 H.sub.2 (Dry) 0.18 0.15 0.20 Acceptable Example 23 850 H.sub.2 (Dry) 0.20 0.26 0.19 Acceptable Example 34 880 H.sub.2 (Dry) 0.10 0.18 0.26 Acceptable Example 45 1000 H.sub.2 (Dry) 0.10 0.40 0.15 Acceptable Example 56 950 H.sub.2 (dew 0.14 0.35 0.21 Acceptable point = 30.degree. C.) Example 67 830 H.sub.2 (dew 0.14 0.20 0.20 Acceptable point = 30.degree. C.) Example 78 920 H.sub.2 (dew 0.13 0.21 0.28 Acceptable point = 45.degree. C.) Example 89 950 H.sub.2 (dew 0.10 0.15 0.18 Acceptable point = 45.degree. C.) Example 910 1050 H.sub.2 (Dry) 0.19 0.21 0.11 Comparative Example 111 1040 H.sub.2 (Dry) 0.16 0.11 0.10 Comparative Example 212 970 H.sub.2 (Dry) 0.003 0.003 0.12 Comparative Example 313 970 H.sub.2 (Dry) 0.17 0.60 0.24 Comparative Example 414 970 H.sub.2 (Dry) 0.40 0.20 0.19 Comparative Example 515 750 H.sub.2 (dew 0.16 0.15 0.40 Comparative point = 30.degree. C.) Example 6__________________________________________________________________________ TABLE 10__________________________________________________________________________ ##STR55## ##STR56## ##STR57## ##STR58## ##STR59## ##STR60## ##STR61##__________________________________________________________________________1 50 54 0.10 6.91 38 Acceptable Example 12 45 54 0.11 6.93 40 Acceptable Example 23 32 45 0.11 6.91 39 Acceptable Example 34 71 57 0.12 6.89 40 Acceptable Example 45 25 70 0.10 6.89 45 Acceptable Example 56 43 75 0.08 6.90 44 Acceptable Example 67 37 74 0.06 6.89 40 Acceptable Example 78 39 71 0.05 6.90 42 Acceptable Example 89 51 88 0.07 6.91 41 Acceptable Example 910 15 55 0.18 6.91 42 Comparative Example 111 18 54 0.19 6.91 40 Comparative Example 212 21 45 0.21 6.93 36 Comparative Example 313 76 44 0.11 6.75 37 Comparative Example 414 60 56 0.11 6.76 31 Comparative Example 515 79 57 0.10 6.72 34 Comparative Example 6__________________________________________________________________________

As seen from Table 10, all of iron powders satisfying the requirements according to the invention exhibited an accuracy of dimension change having a fluctuating width of not more than 0.12%. Furthermore, in the acceptable examples, there were shown good values on the compressibility (evaluated by green density under the shaping pressure of 5 t/cm.sup.2) and the strength (evaluated by tensile strength).

On the contrary, in Comparative Examples 1 and 2, the quantity of oxidized Cr among Cr content was not more than 20%, so that the fluctuating width exceeded 0.15% and the properties were deteriorated. In Comparative Example 3, the quantities of Cr and Mn are 0.006%, which were below the lower limit of the adequate range, so that the fluctuating width of dimensional change in the sintered body to the fluctuation of the amount of graphite added exceeded 0.15%. In Comparative Example 4, the quantity of Cr+Mn exceeds 0.5 wt %, so that the compressibility was poor and the strength was low. Similarly, since the quantity of Cr+Mn exceeds 0.5 wt % in Comparative Example 5 and the oxygen concentration exceeded 0.3 wt % in Comparative Example 6, the compressibility was lowered and the strength is was low.

EXAMPLE 5

Water-atomized green iron powder having a composition of 0.05-0.5 wt % of Cr, 0.01-0.3 wt % of Mn and the reminder being Fe and inevitable impurity was subjected to an oxidation treatment in a nitrogen atmosphere by varying an oxygen concentration and then reduced in a pure hydrogen atmosphere at 930.degree. C. for 20 minutes, and thereafter a relation between oxygen concentration in the atmosphere and ratio of oxidized Cr was measured to obtain results as shown in Table 11.

TABLE 11______________________________________Composi- Composition oftion of Oxygen finished irongreen concentra- powder (%)powder tion in ratio of(%) nitrogen oxidizedNo. Mn Cr (vol %) O Cr Remarks______________________________________16 0.22 0.20 5 0.21 54 Acceptable Example 1017 0.20 0.15 14 0.25 65 Acceptable Example 1118 0.19 0.20 1 0.17 12 Comparative Example 719 0.20 0.15 21 0.41 73 Comparative Example 8______________________________________

As seen from this table, in all acceptable examples in which the oxygen concentration in the nitrogen atmosphere satisfied the range defined in the invention, the oxygen content in the finished iron powder was not more than 0.3 wt % and the oxidation ratio of Cr per total Cr is not less than 20%. On the other hand, in Comparative Example 7 in which the oxygen concentration in the nitrogen atmosphere did not satisfy the lower limit according to the invention, the oxygen content in the finished iron powder was not more than 0.3 wt %, but the ratio of oxidized Cr was not more than 20%, while in Comparative Example 8 in which the oxygen concentration in the nitrogen atmosphere exceeded the upper limit according to the invention, the oxygen content in the finished iron powder exceeded 0.3 wt %.

EXAMPLE 6

Each of iron powders containing various contents of Si as shown in Table 12 was added and mixed with 1.5 wt % of Cu powder, 0.5 wt % of graphite powder and 1 wt % of zinc stearate as a lubricant, shaped into a ring-shaped green compact having an outer diameter of 60 mm, an inner diameter of 25 mm and a height of 10 mm and a green density of 6.9 g/cm.sup.3, and then sintered in an RX gas having a CO.sub.2 content of 0.3% at 1130.degree. C. for 20 minutes.

The fluctuating width of dimensional change in the resulting sintered body was measured to obtain results as shown in Table 12 together with results measured on the oxidation ratio of elementary Si in the iron powder and the scattering width of the oxidation ratio.

The fluctuating width of dimensional change was evaluated by a scattering width of dimensional change in the sintering on 100 specimens based on the green compact having the same outer diameter.

As seen from this table, in all acceptable examples according to the invention containing an adequate amount of Si, not less than 20% of which being rendered into an oxide, good accuracy of dimensional change was obtained, while in the comparative examples, the fluctuating width of dimensional change in the sintered body was still large.

TABLE 12______________________________________Sym- Scattering Fluctuatingbol Si width of width ofof con- Oxidation oxidation dimensionaliron tent ratio of ratio in change inpow- (wt Si Si content sintered bodyder %) (%) (%) (%) Remarks______________________________________A 0.004 15.about.85 70 0.56 Comparative Example 1B 0.007 17.about.80 63 0.52 Comparative Example 2C 0.008 25.about.40 15 0.04 Acceptable Example 1D 0.016 30.about.40 10 0.04 Acceptable Example 2E 0.025 35.about.45 10 0.02 Acceptable Example 3F 0.027 55.about.75 20 0.04 Acceptable Example 4______________________________________

EXAMPLE 7

According to the same manner as in Example 6, each of iron powders having various amounts of Si shown in Table 13 were added and mixed with 2.0 wt % of Cu powder, 0.8 wt % of graphite powder and 1 wt % of zinc stearate as a lubricant, shaped into a ring-shaped green compact having an outer diameter of 60 mm, an inner diameter of 25 mm and a height of 10 mm and a green density of 6.9 g/cm.sup.3, where-by 100 specimens were produced. Then, these specimens were sintered in an AX gas at 1130.degree. C. for 20 minutes, and the quantity of dimensional change in the sintering based on the green compact having the same outer diameter was measured to examine the fluctuating width thereof.

The results measured on the fluctuating width of dimensional change in the sintered body are also shown in Table 13 together with results measured on the oxidation ratio of elementary Si in the iron powder and the scattering width of the oxidation ratio.

As seen from this table, in all acceptable examples according to the invention containing an adequate amount of Si, not less than 20% of which being rendered into an oxide, good accuracy of dimensional change was obtained, while in the comparative examples, the fluctuating width of dimensional change in the sintered body was still large.

TABLE 13______________________________________Sym- Scattering Fluctuatingbol Si width of width ofof con- Oxidation oxidation dimensionaliron tent ratio of ratio in change inpow- (wt Si Si content sintered bodyder %) (%) (%) (%) Remarks______________________________________A 0.004 15.about.85 70 0.50 Comparative Example 3B 0.007 17.about.80 63 0.46 Comparative Example 4C 0.008 25.about.40 15 0.02 Acceptable Example 5D 0.016 30.about.40 10 0.02 Acceptable Example 6E 0.025 35.about.45 10 0.02 Acceptable Example 7F 0.027 55.about.75 20 0.04 Acceptable Example 8______________________________________

EXAMPLE 8

Each of green powders obtained by water atomizing molten steels having various amounts of Si and Mn was subjected to an oxidation treatment in a nitrogen atmosphere having different oxygen concentrations at 140.degree. C. for 60 minutes and then subjected to a reduction treatment in a pure hydrogen atmosphere at 930.degree. C. for 20 minutes to produce iron powders (average particle size: 80 .mu.m) having a chemical composition, quantity of oxide and scattering width of oxidation ratio shown in Table 14.

Then, the fluctuating width of dimensional change when the sintered body was produced by using these powders and the green density of the green compact were measured to obtain results as shown in Table 14.

The fluctuating width of dimensional change in the sintered body was evaluated as a scattering width determined from a quantity of dimensional change in the sintering based on the green compact having the same outer diameter with respect to 100 sintered specimens obtained by adding and mixing iron powder with 1.5 wt % of copper powder, 0.5 wt % of graphite powder and 1 wt % of zinc stearate as a lubricant, shaping into a ring-shaped green compact having a density of 6.9 g/cm.sup.3, an outer diameter of 60 mm, an inner diameter of 25 mm and a height of 10 mm and sintering in a propane-modified gas having a CO.sub.2 content of 0.3% at 1130.degree. C. for 20 minutes.

And also, the green density was measured when the same iron powder as mentioned above was added and mixed with 1 wt % of zinc stearate and shaped under a shaping pressure of 5 t/cm.sup.2.

Moreover, the scattering width of oxidized Si ratio in the Si content was determined from a scattering width obtained by dividing the iron powder into 10 parts and analyzing a ratio of SiO.sub.2 quantity to total Si amount per each part.

TABLE 14__________________________________________________________________________ Scattering Scattering Fluctuating range of width of width of oxidation oxidation Oxygen dimensionalComposition of ratio in ratio in concentration change in Greeniron powder (%) Si content Si content in atmosphere sintered body densityNo. Si Mn O (%) (%) (vol %) (%) (g/cm.sup.3) Remarks__________________________________________________________________________1 0.008 0.04 0.12 20.about.30 10 5 0.06 6.97 Acceptable Example 12 0.010 0.24 0.13 30.about.40 10 5 0.06 6.98 Acceptable Example 23 0.016 0.10 0.13 35.about.45 10 2.5 0.05 6.98 Acceptable Example 34 0.016 0.10 0.25 40.about.60 20 5 0.04 6.97 Acceptable Example 45 0.016 0.10 0.25 45.about.75 30 7.5 0.04 6.97 Acceptable Example 56 0.020 0.30 0.14 45.about.50 5 5 0.04 6.97 Acceptable Example 67 0.025 0.03 0.14 45.about.50 5 5 0.03 6.95 Acceptable Example 78 0.004 0.003 0.12 5.about.100 95 5 0.60 7.00 Comparative Example 19 0.30 0.35 0.30 45.about.55 10 5 0.11 6.77 Comparative Example 210 0.07 0.10 0.29 0.about.20 20 1 0.55 6.96 Comparative Example 311 0.016 0.10 0.12 1.about.54 54 1 0.70 6.98 Comparative Example 4__________________________________________________________________________

As seen from this table, all of Acceptable Examples 1-7 contained adequate amounts of Si and Mn, in which not less than 20% of Si and Mn amounts was rendered into an oxide and the scattering width thereof was not more than 50%, so that there was obtained an excellent accuracy of dimensional change of not more than 0.06%, which was lower than the typical lower limit of the dimensional accuracy after the correction of dimensional change through the conventional sizing. Further, the compressibility was very good.

On the contrary, all of the comparative examples are the case that the chemical composition, ratio of Si+Mn amount as an oxide and further oxygen concentration in the atmosphere do not satisfy the adequate ranges defined in the invention, so that the satisfactory results were not obtained in the accuracy of dimensional change in the sintered body and the compressibility.

EXAMPLE 9

Each of green powders obtained by water atomizing molten steels having various amounts of Si and Mn was subjected to an oxidation treatment in a nitrogen atmosphere having different oxygen concentrations at 140.degree. C. for 60 minutes and then subjected to a reduction treatment in a pure hydrogen atmosphere at 930.degree. C. for 20 minutes to produce iron powders (average particle size: 70 .mu.m) having a chemical composition, quantity of oxide and scattering width of oxidation ratio shown with Table 15.

Then, the fluctuating width of dimensional change when the sintered body was produced by using these powders and the radial crushing strength measured to obtain results as shown with Table 15.

The state of Si oxide on the particle surface of iron powder was observed by Auger analysis.

The fluctuating width of dimensional change in the sintered body was determined from a quantity of dimensional change before and after the sintering when pure iron powder was added and mixed with 0.8 wt % of two kinds of graphites having average particle sizes of 34 .mu.m and 6 .mu.m, shaped into a ring-shaped green compact of Fe-2% Cu-0.8% graphite having an outer diameter of 60 mm, an inner diameter of 25 mm, a height of 10 mm and a green density of 6.80 g/cm.sup.3 and sintered in a propane-modified gas having a CO.sub.2 content of 0.3% at 1130.degree. C. for 20 minutes.

Moreover, the radial crushing strength of the sintered body was measured with respect to a sintered body obtained by sintering a ring-shaped green compact having the same composition and green density as mentioned above and an outer diameter of 38 mm, an inner diameter of 25 mm and a height of 10 mm in a propane-modified gas having a CO.sub.2 content of 0.3% at 1130.degree. C. for 20 minutes.

TABLE 15__________________________________________________________________________ Scattering width of Presence of Difference ofOxygen Oxidation oxidation island-like dimensional Radialconcentration Composition of iron ratio of ratio in Si, Si oxide on change in crushingin atmosphere powder (%) Si and Mn Mn content surface of sintered body strengthNo. (vol %) Si Mn O (%) (%) iron powder (%) (N/mm.sup.2) Remarks__________________________________________________________________________1 5 0.026 0.05 0.12 30.about.60 30 presence 0.06 735 Acceptable Example 12 5 0.10 0.06 0.15 45.about.78 33 presence 0.06 730 Acceptable Example 23 5 0.21 0.26 0.17 35.about.60 25 presence 0.05 730 Acceptable Example 34 10 0.34 0.11 0.17 40.about.60 20 presence 0.03 720 Acceptable Example 45 5 0.024 0.008 0.14 30.about.90 60 none 0.10 740 acceptable Example 56 5 0.50 0.30 0.28 40.about.50 10 presence 0.03 600 Comparative Example 17 5 0.21 0.35 0.18 40.about.95 55 presence 0.11 660 Comparative Example 28 2.0 0.08 0.20 0.10 0.about.20 20 none 0.30 735 Comparative Example 39 18 0.21 0.26 0.34 50.about.100 50 presence 0.12 650 Comparative Example 410 5 0.62 0.08 0.40 50.about.70 20 presence 0.03 665 Comparative Example__________________________________________________________________________ 5

As seen from this table, when using the iron powder according to the invention (Acceptable Examples 1-5), the fluctuating width of dimensional change was not more than 0.1%. Particularly, when Si oxide was distributed on the particle surface of the iron powder in island form (Acceptable Examples 1-4), even if the average particle size of graphite powder added was largely different between 34 .mu.m and 6 .mu.m, the fluctuating width of dimensional change in the sintered body was as very low as not more than 0.06%, and also the radial crushing strength was as high as not less than 700 N/mm.sup.2.

On the other hand, all of the comparative examples are the case that the chemical composition and the ratio of Si quantity as an oxide did not satisfy the adequate ranges defined in the invention, so that a good accuracy of dimensional change in the sintered body was not obtained as mentioned below.

In Comparative Examples 1 and 2, the Si+Mn amount was not less than 0.50% exceeding the defined upper limit, so that the radial crushing strength was lower than 700 N/mm.sup.2.

In Comparative Example 3, the oxygen concentration in the atmosphere when water-atomized powder was dried was 2.0 vol % lower than the defined value, so that the fluctuation of dimensional change was large.

In Comparative Examples 4 and 5, the O content is 0.34 wt % and the Si content is 0.62 wt %, which exceeded the defined upper limits, respectively, so that only the radial crushing strength of lower than 700 N/mm.sup.2 was obtained.

EXAMPLE 10

Water-atomized iron powder (average particle size: 70 .mu.m) was added with not more than 0.3 wt % of various oxide powders shown in Table 16 (average particle size: 5 .mu.m) and added and mixed with 1.5 wt % of electrolytic copper powder (average particle size: not more than 44 .mu.m), 0.9 wt % of graphite powder (average particle size: not more than 10 .mu.m) and 1 wt % of a solid lubricant, shaped at a green density of 7.0 g/cm3 into a test specimen for transverse rupture strength having a length of 35 mm, a width of 10 mm and a height of 5 mm and then sintered in a propane-modified gas at 1130.degree. C. for 20 minutes.

The fluctuating width of dimensional change in the longitudinal direction of the sintered body before and after the sintering and the transverse rupture strength was measured to obtain results as shown in Table 16.

TABLE 16(a)__________________________________________________________________________ Dimensional change Fluctuating width Transverse Addition Green of sintered body of dimensional rupture Oxide amount density based on green change strengthNo. added (wt %) (g/cm.sup.3) compact (%) (kgf/mm.sup.2) Remarks__________________________________________________________________________ 1 -- 0 6.90 0.09 0.21 80 Comparative Example 1 2 Al.sub.2 O.sub.3 0.01 6.89 0.15 0.07 80 Example 1 3 Al.sub.2 O.sub.3 0.05 6.89 0.20 0.06 79 Example 2 4 Al.sub.2 O.sub.3 0.10 6.88 0.25 0.05 79 Example 3 5 Al.sub.2 O.sub.3 0.20 6.87 0.25 0.04 75 Example 4 6 Al.sub.2 O.sub.3 0.30 6.85 0.26 0.04 73 Comparative Example 2 7 TiO.sub.2 0.01 6.89 0.14 0.10 82 Example 5 8 TiO.sub.2 0.05 6.88 0.19 0.07 80 Example 6 9 TiO.sub.2 0.10 6.88 0.25 0.07 79 Example 710 TiO.sub.2 0.20 6.86 0.25 0.05 73 Example 811 TiO.sub.2 0.30 6.84 0.26 0.05 71 Comparative Example 312 SiO.sub.2 0.01 6.89 0.15 0.09 80 Example 913 SiO.sub.2 0.05 6.89 0.19 0.07 79 Example 1014 SiO.sub.2 0.10 6.88 0.25 0.06 78 Example 1115 SiO.sub.2 0.20 6.86 0.25 0.03 76 Example 1216 SiO.sub.2 0.30 6.84 0.25 0.03 72 Comparative Example 417 V.sub.2 O.sub.3 0.01 6.90 0.15 0.11 81 Example 1318 V.sub.2 O.sub.3 0.05 6.89 0.20 0.07 81 Example 1419 V.sub.2 O.sub.3 0.10 6.88 0.26 0.07 79 Example 1520 V.sub.2 O.sub.3 0.20 6.87 0.26 0.05 77 Example 1621 V.sub.2 O.sub.3 0.30 6.85 0.26 0.05 74 Comparative Example 5__________________________________________________________________________ TABLE 16(a)__________________________________________________________________________ Dimensional change Fluctuating width Transverse Addition Green of sintered body of dimensional rupture Oxide amount density based on green change strengthNo. added (wt %) (g/cm.sup.3) compact (%) (kgf/mm.sup.2) Remarks__________________________________________________________________________22 MnO 0.01 6.89 0.14 0.11 82 Example 1723 MnO 0.05 6.88 0.20 0.08 81 Example 1824 MnO 0.10 6.88 0.26 0.06 81 Example 1925 MnO 0.20 6.87 0.26 0.06 72 Example 2026 MnO 0.30 6.85 0.26 0.05 75 Comparative Example 627 Cr.sub.2 O.sub.3 0.01 6.89 0.14 0.09 82 Example 2128 Cr.sub.2 O.sub.3 0.05 6.89 0.21 0.07 82 Example 2229 Cr.sub.2 O.sub.3 0.10 6.89 0.25 0.06 80 Example 2330 Cr.sub.2 O.sub.3 0.20 6.87 0.25 0.06 78 Example 2431 Cr.sub.2 O.sub.3 0.30 6.85 0.25 0.04 74 Comparative Example 732 NiO 0.01 6.89 0.02 0.21 80 Comparative Example 833 NiO 0.05 6.89 0.00 0.20 84 Comparative Example 934 NiO 0.10 6.88 -0.03 0.20 80 Comparative Example 1035 NiO 0.20 6.88 -0.09 0.21 79 Comparative Example 1136 Cu.sub.2 O 0.01 6.89 0.12 0.20 73 Comparative Example 1237 Cu.sub.2 O 0.05 6.88 0.14 0.20 82 Comparative Example 1338 Cu.sub.2 O 0.10 6.88 0.18 0.21 83 Comparative Example 1439 Cu.sub.2 O 0.20 6.87 0.25 0.20 79 Comparative Example 15__________________________________________________________________________

As seen from this table, in all acceptable examples adding adequate amounts of oxides, the quantity of dimensional change in the sintered body was constant and the scattering thereof was very small. Further, the transverse rupture strength was substantially constant up to 0.1 wt %.

On the other hand, when using Cu.sub.2 O powder or NiO powder (average particle size: 5 .mu.m) in which a value of standard free energy of formation of oxide at 1000.degree. C. is smaller than -120 kcal/l mol of O.sub.2, the dimension tended to expand with the increase of the amount of Cu.sub.2 O added, or NiO tended to contract the dimension. In any case, the fluctuating width of dimensional change made little difference to the case of changing the dimension.

Furthermore, when the addition amount was less than 0.01 wt %, the quantity of adjusting dimensional change was small, while when it exceeded 0.20 wt %, the green density and the transverse rupture strength of the sintered body was rapidly lowered.

INDUSTRIAL APPLICABILITY

The iron powder for powder metallurgy and mixed powder thereof according to the invention considerably reduce the fluctuating width of dimensional change in the sintered body irrespectively of the amount of graphite added and particle size in the sintering after the addition of Cu and graphite as compared with the conventional iron powder for powder metallurgy, whereby there can be obtained the accuracy of dimensional change equal to or more than that after the conventional sizing step and also the radial crushing strength of the sintered body is stably obtained. Therefore, the design and production of sintered parts having a high strength can easily be attained without conducting the sizing.

Particularly, the oxidation ratio can strictly be controlled in the mixed powder, whereby the dimensional fluctuating width can be controlled with a higher accuracy. Moreover, the quantity of dimensional change of the sintered parts can freely be adjusted by adjusting the quantity of the oxide added.

Claims

1. A powder for powder metallurgy comprising 0.01-0.20 wt % in total of oxide powder of at least one element having a value of standard free energy of formation of oxide at 1000.degree. C. of not more than -120 kcal/l mol of O.sub.2 and a mixed powder, said mixed powder including 0.5-1.0 wt % graphite powder or a mixture of 0.5-1.0 wt % graphite powder and 1.5-2.0 wt % Cu powder, and the remainder being iron powder.

2. The powder according to claim 1, wherein the oxide powder of at least one element having a value of standard free energy of formation of oxide at 1000.degree. C. of not more than -120 kcal/l mol of O.sub.2 is selected from the group consisting of Cr.sub.2 O.sub.3, MnO, SiO.sub.2, V.sub.2 O.sub.3, TiO.sub.2 and Al.sub.2 O.sub.3.