EP0334968B1 - Zusammengesetztes legierungsstahlpulver und gesinterter legierungsstahl - Google Patents

Zusammengesetztes legierungsstahlpulver und gesinterter legierungsstahl Download PDF

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Publication number
EP0334968B1
EP0334968B1 EP88908393A EP88908393A EP0334968B1 EP 0334968 B1 EP0334968 B1 EP 0334968B1 EP 88908393 A EP88908393 A EP 88908393A EP 88908393 A EP88908393 A EP 88908393A EP 0334968 B1 EP0334968 B1 EP 0334968B1
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Prior art keywords
powder
less
alloy steel
strength
sintered
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EP88908393A
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English (en)
French (fr)
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EP0334968A4 (de
EP0334968A1 (de
Inventor
Shigeaki C/O Kawasaki Steel Corp. Takajo
Osamu C/O Kawasaki Steel Corp. Furukimi
Kuniaki C/O Kawasaki Steel Corp. Ogura
Keiichi C/O Kawasaki Steel Corp. Maruta
Teruyoshi C/O Kawasaki Steel Corp. Abe
Ichio C/O Kawasaki Steel Corp. Sakurada
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JFE Steel Corp
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Kawasaki Steel Corp
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Priority claimed from PCT/JP1988/001007 external-priority patent/WO1989002802A1/ja
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    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C33/00—Making ferrous alloys
    • C22C33/02—Making ferrous alloys by powder metallurgy
    • C22C33/0257—Making ferrous alloys by powder metallurgy characterised by the range of the alloying elements
    • C22C33/0264—Making ferrous alloys by powder metallurgy characterised by the range of the alloying elements the maximum content of each alloying element not exceeding 5%
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C33/00—Making ferrous alloys
    • C22C33/02—Making ferrous alloys by powder metallurgy
    • C22C33/0207—Using a mixture of prealloyed powders or a master alloy
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C33/00—Making ferrous alloys
    • C22C33/02—Making ferrous alloys by powder metallurgy
    • C22C33/0257—Making ferrous alloys by powder metallurgy characterised by the range of the alloying elements

Definitions

  • the present invention relates to an alloy powder to be used for the production of sintered parts by powder metallurgy and also to a high-strength sintered alloy steel to be produced by heat treatment after sintering.
  • Iron-based sintering materials are in general use for automotive parts and others. Recently, these parts are required to be lighter and stronger than before.
  • a further attempt to increase the strength of sintering materials by increasing the amount of the alloying element has been made according to Japanese Patent Laid-open No. 231102/1986.
  • a disadvantage of such high alloy steel (containing more than 7% of Ni) is that it is high in production cost and it has such high hardness after sintering that it does not permit sizing and cutting.
  • An additional disadvantage is that it contains a large amount of residual austenite after sintering. This makes it necessary to perform special heat treatment such as sub-zero treatment if a high strength (in terms of tensile strength higher than 130 kgf/mm2) is to be obtained. This leads to a cost increase.
  • the residual austenite decomposes with the lapse of time, causing the deformation of parts.
  • Japanese Patent Publication No. 9649/1970 a low alloy steel powder which provides high-strength sintered bodies which undergo less dimensional changes during heat treatment performed after molding.
  • This low alloy steel powder is obtained by heating a mixture of iron powder and compounds of Ni, Mo, and Cu, thereby attaching by diffusion the alloy components, crushing agglomerated particles, and annealing thus obtained particles.
  • a disadvantage of this low alloy steel powder is that it contains 0.50-2.00 wt% of Cu, which segregates on the grain boundary, forming an ⁇ -Cu brittle layer, which aggravates the mechanical properties.
  • One of the present inventors proposed a composition for composite alloy steel powder which is suitable in the case where the sintered body is used as such without any post heat-treatment. (See Japanese Patent Laid-open No. 89601/1988.)
  • This alloy steel powder contains Ni and/or Cu and Mo, and the alloy composition imparts high hardness and dimensional stability to the sintered body. However, it does not affords any sintered body which has a tensile strength higher than 130 kgf/mm2.
  • This alloy steel powder does not provide any sintered body which has a tensile strength higher than 130 kgf/mm2. This is because the sintered body does not undergo strain-induced transformation from austenite to martensite when it is pulled, and it does not have a high density due to insufficient compressibility.
  • Sintered parts are given high internal toughness, high surface abrasion resistance, and high fatigue strength by carburizing quenching. Therefore, they exhibit their great practical value when used as high-strength parts such as gears. However, it is difficult to obtain high strength (in terms of tensile strength higher than 130 kgf/mm2) by the simple carburizing quenching of conventional sintered bodies.
  • the heat treatment after sintering is becoming important more and more.
  • the sintered body becomes so hard that it is difficult to correct its dimensions by machining or sizing.
  • machining and sizing should be performed prior to heat treatment.
  • the sintered body should have a low hardness (hence a low strength) before heat treatment and have a high hardness (hence a high strength) after heat treatment.
  • the present invention realizes mutually contradictory technologies machining or sizing the sintered body easily before heat treatment and imparting high strength and high toughness to the sintered body after heat treatment.
  • the present inventors carried out extensive studies on how to increase the strength and toughness of sintered bodies. As the result, it was found that the strength and toughness of sintered bodies are greatly affected by the composition of steel powder and the density of sintered bodies.
  • the present inventors found that Mo is less liable than Ni to make the sintered body hard in the absence of carbon. Therefore, it is possible to increase the amount of Mo more freely than the amount of Ni without adverse effect on the machining and sizing of the sintered body before heat treatment. On the other hand, Mo contributes more than Ni to the increase of strength after carburizing. With this in mind, the present inventors expected that very good results would be obtained if the composition of the conventional Ni-Mo series composite alloy steel powders is modified such that the amount of Mo is much greater than the amount of Ni. (The composition includes the one in which Mo is replaced by W.)
  • the present inventors carried out experiments as mentioned in the following to see how the strength and toughness of the sintered body are related with the composition and density of the sintered body.
  • Fig. 1 is a graph showing the relation between the density and the tensile strength of the heat-treated sintered bodies of different compositions.
  • Fig. 2 is a graph showing the relation between the density and the Charpy impact value of the same heat-treated sintered bodies as shown in Fig. 1.
  • Fig. 3 is a graph showing the relation between the C content and the tensile strength of the heat-treated sintered bodies containing Ni and Mo in amounts within the limit specified in the present invention.
  • Fig. 4 is a graph showing the relation between the C content and the Charpy impact value of the same heat-treated sintered bodies as shown in Fig. 3.
  • the green compacts underwent sintering (at 1250°C for 30 minutes in an ammonia decomposition gas), followed by oil quenching (after heating at 870°C for 60 minutes in an inert gas) and tempering at 180°C for 60 minutes.
  • the resulting sintered bodies have the tensile strength and Charpy impact value which vary according to the density as shown in Figs. 1 and 2. It is noted that the sintered bodies of compositions (X) and (Y) have a high tensile strength (higher than 130 kgf/mm2) and high toughness when the density is higher than 7.0 g/cm3. In addition, it is also noted that the tensile strength exceeds 150 kgf/mm2 when the density is higher than 7.3 g/cm3.
  • the present invention was completed on the basis of the above-mentioned findings.
  • the gist of the present invention resides in alloy steel powders for sintering as defined in claim 1 and in sintered alloyed steel products as claimed in present claims 2 and 3.
  • composite alloy steel powder as used in the present invention means steel powder composed of iron powder particles and alloy elements (e.g., Ni, Mo, and W) attached by diffusion to part of the surface of the iron powder particles.
  • alloy elements e.g., Ni, Mo, and W
  • Mo as the basic alloy component may be partly or entirely replaced by twice as much (by weight) W as Mo.
  • the reason why twice as much W as Mo is used is that W is one half as effective as Mo in changing the properties of the sintered steel.
  • the composite alloy steel powder containing Ni and Mo in the above-specified amounts was incorporated with a different amount of graphite so that the final product contains 0.1 to 1.0 wt% of C and also with 1 wt% of zinc stearate as a lubricant.
  • the resulting steel powder was molded and sintered, and the sintered body underwent oil quenching at 870°C for 30 minutes and tempering at 180°C for 60 minutes.
  • the thus obtained heat-treated sintered steel was tested for tensile strength and Charpy impact. The results are shown in Figs. 3 and 4. It is noted that the high strength and high toughness are obtained when the C content is in the range of 0.3 to 0.8 wt%.
  • C can be accomplished in different manners depending on the intended use of the finished parts.
  • graphite powder is added to the alloy steel powder at the time of sintering, and in another instance, carburizing quenching is performed after sintering.
  • the C content may vary along the cross-section of the part; but it is not always necessary that the C content should be in the above-mentioned range all over the cross-section. It is only necessary that the carburized part should contain as much C as specified.
  • the alloy steel powder as the raw material should have a high compressibility.
  • This requirement is met with the so-called composite alloy steel powder composed of iron powder particles and powders of Ni and Mo and/or W attached by diffusion to the surface of the iron powder particles.
  • completely uniform pre-alloy steel powder is not adequate to make high-density sintered products because usually it has a low compressibility.
  • the resulting sintered body does not have a sufficient strength because the alloy elements do not diffuse completely during sintering. Even in the case of composite alloy steel powder, the resulting sintered body lacks strength unless the diffusion alloying takes place completely.
  • the degree of diffusion alloying is indicated by "the degree of diffusion segregation" as an index. It is expressed by the ratio of the content of Ni or Mo + 1 ⁇ 2W in the alloy steel powder of particle size smaller than 45 »m to the average content of Ni or Mo + 1 ⁇ 2W in the entire steel powder, respectively.
  • the degree of diffusion segregation is higher than 4.2 for Ni or Mo + 1 ⁇ 2W, the resulting heat-treated sintered body has a low strength and compressibility.
  • the degree of diffusion segregation is smaller than 2.0, the resulting heat-treated sintered body lacks sufficient compressibility and tensile strength because austenite does not change into martensite through the strain-induced transformation. Therefore, the degree of diffusion segregation should be in the range of 2.0 to 4.2. This is achieved by controlling the particle size of the iron powder and alloy components and also by controlling the heating temperatures.
  • the composite alloy steel powder should be composed of 0.50-3.50 wt% of Ni, 0.65-3.50 wt% of Mo + 1 ⁇ 2W, and the remainder of Fe and inevitable impurities, according to the desired composition of the sintered body.
  • the composite alloy steel powder should have such a particle size that the fraction of the powder having a particle size larger than 180 »m accounts for less than 10 wt%.
  • the sintered body undergoes heat treatment so that the final product has a high strength.
  • the sintered body undergoes carburizing quenching and tempering.
  • the composite alloy steel powder is incorporated with graphite at the time of sintering and the sintered body undergoes ordinary quenching and tempering.
  • the heat treatment forms the texture of tempered martensite which is responsible for high strength and high toughness.
  • the quenching temperature should preferably be 800-930°C. With lower than 800°C, the quenching does not provide the uniform structure of austenite, and the final product lacks sufficient strength and toughness. With higher than 930°C, the quenching provides coarse austenite, and the final product lacks strength and toughness.
  • the tempering temperature should preferably be 100-250°C. Tempering at temperatures lower than 100°C will not provide sufficient toughness. Tempering at temperatures higher than 250°C will not provide sufficient strength.
  • the molding and sintering may be repeated more than once to increase the density. This may be accomplished by a recompression method which involves the steps of molding, sintering, and coining (sizing), or the steps of molding, preliminary sintering, coining (sizing), and final sintering.
  • the composite alloy steel powder as the starting material was produced in the following manner.
  • Atomized pure iron powder under 80 mesh
  • nickel oxide powder under 325 mesh
  • molybdenum trioxide powder under 325 mesh
  • the mixture was heated at 800°C for 120 minutes in hydrogen gas for the reduction of nickel oxide and molybdenum trioxide.
  • composite alloy steel powder composed of iron powder particles and Ni and Mo attached by diffusion to the surface of the iron powder particles.
  • the alloy steel powder was made into a sintered body having a density of 7.28-7.51 g/cm3 by the steps of molding under a pressure of 7 tons/cm2, preliminary sintering in hydrogen gas at 900°C for 30 minutes, coining under a pressure of 7 tons/cm2, and final sintering in hydrogen gas at 1250°C for 90 minutes.
  • the thus obtained sintered body was carburized with carbon potential of 0.8 wt% at 900°C for 6.5 hours, immediately followed by oil quenching, and tempered at 180°C for 120 minutes. Strength was measured using a tensile test piece having a parallel part 5 mm in diameter. The results are shown in Table 1. It is noted from Table 1 that the sintered bodies have a high strength so long as the degree of diffusion segregation is in the range of 2.0-4.2.
  • the sintered bodies have a tensile strength higher than 130 kgf/mm2 so long as the Ni content is 0.50-3.50 wt% and the Mo content is 0.65-3.50 wt% and the degree of diffusion segregation is adequate. Desirable results are obtained particularly in the case where the Mo content is higher than 0.85 wt%.
  • the compact underwent the primary sintering (preliminary sintering or ordinary sintering) in hydrogen gas at a prescribed temperature for 60 minutes.
  • the sintered body underwent the secondary forming (coining or sizing) under a prescribed pressure.
  • the sintered body underwent the secondary sintering (final sintering) in hydrogen gas at 1300°C for 60 minutes.
  • the sintered body underwent carburizing and quenching, and tempering under the same conditions as in Example 1.
  • the specimens were tested for tensile strength.
  • the results are shown in Table 3. It is noted from Table 3 that the sintered bodies have a tensile strength higher than 130 kgf/mm2 so long as their density is higher than 7.0 g/cm3, preferably 7.3 g/cm3.
  • Composite alloy steel powders having a particle size of 80 mesh or less were prepared in the following manner from water-atomized pure iron powder, carbonyl nickel powder, molybdenum trioxide (MoO3) powder, and tungsten trioxide (WO3) powder. All of the alloy component powders have a particle size of 325 mesh or under.
  • the iron powder has the following chemical composition.
  • the alloy steel powder was incorporated with 1 wt% of zinc stearate and then formed under a pressured of 6 tons/cm2.
  • the compact was sintered in an ammonia decomposition gas at 1250°C for 60 minutes. To examine the workability before heat treatment, the sintered body was tested for tensile strength.
  • the sintered body underwent heat treatment which includes carburizing with carbon potential of 0.85% at 880°C for 200 minutes and oil quenching, and tempering at 180°C for 60 minutes.
  • the heat-treated specimen was tested for tensile strength as an index of strength.
  • compositions of the composite alloy steel powders are shown in Table 4.
  • the composite alloy steel powders in Examples 25 to 31 and Comparative Examples 8 to 13 have the composition which is within or close to the range specified in the present invention.
  • the composite alloy steel powder in Comparative Example 14 has the conventional standard composition.
  • the composite alloy steel powders were evaluated. The results are shown in Table 5.
  • the composite alloy steel powder with a high Mo content and low Ni content according to the present invention provides sintered bodies having a tensile strength of 107-126 kgf/mm2 when formed under a pressure of 6 tons/cm2.
  • the sintered body before heat treatment can undergo machining and sizing without difficulties if it has a tensile strength lower than about 40 kgf/mm2.
  • Composite alloy steel powders were prepared from iron powder (80 mesh or less), nickel powder (325 mesh or less), and molybdenum oxide (MoO3) powder (325 mesh or less) in a prescribed ratio, by heating for reduction in a hydrogen gas atmosphere at 1000°C for 1 hour, annealing, and crushing.
  • the thus formed composite alloy steel powders have the chemical composition and the degree of diffusion segregation as shown in Table 6.
  • each of the composite alloy steel powders was formed under a pressure of 7 tons/cm2.
  • the compact was sintered in an ammonia decomposition gas at 850°C for 30 minutes.
  • the sintered body underwent compression forming again under a pressure of 7 tons/cm2.
  • the compact was sintered again in an ammonia decomposition gas at 1250°C for 30 minutes.
  • the thus obtained sintered body was heated in an inert gas at 870°C for 60 minutes, followed by oil quenching.
  • the sintered body further underwent quenching and tempering by heating in an oil bath at 180°C for 60 minutes and air cooling.
  • the sintered body was finally tested for tensile strength and Charpy impact strength.
  • the sintered body has the chemical composition, density, tensile strength, and impact value as shown in Table 7.
  • the sintered bodies have a tensile strength higher than 150 kgf/mm2 and a Charpy impact value higher than 4 kgf.m/cm2 if they have the chemical composition and density as specified in the present invention.
  • each of the composite alloy steel powders shown in Table 6 was formed under a pressure of 7 tons/cm2.
  • the compact was sintered in an ammonia decomposition gas at 1250°C for 30 minutes.
  • the sintered body was heated in an inert gas at 870°C for 60 minutes, followed by oil quenching.
  • the sintered body further underwent quenching and tempering by heating in an oil bath at 180°C for 60 minutes and air cooling.
  • the sintered body was finally tested for tensile strength and Charpy impact strength.
  • the results are shown in Table 8. It is noted from Table 8 that the sintered bodies have a tensile strength higher than 130 kgf/mm2 and a Charpy impact value higher than 3.5 kgf.m/cm2 if they have the chemical composition as specified in the present invention.
  • the heat-treated sintered steel of the present invention has both an extremely high strength and an extremely high toughness. Therefore, it is useful as sintered parts which need high strength and high toughness.
  • the alloy steel powder of the present invention meets the requirement for sintered parts having high strength. It exhibits outstanding properties in the case where both high density and workability are required. Therefore, it is expected to permit the easy production by powder metallurgy of mechanical parts of complex shape to be used under high loads.

Claims (3)

  1. Zusammengesetztes Legierungsstahlpulver für die Pulvermetallurgie, zusammengesetzt aus Eisenpulverteilchen und pulverförmigen Legierungskomponenten aus Ni und Mo und/oder W, welche durch Diffusion an einem Teil der Oberfläche der Eisenpulverteilchen angelagert sind, wobei das Legierungspulver zusammengesetzt ist aus 0,50 bis 3,50 Gewichts-% Ni, 0,65 bis 2,50 Gewichts-% Mo und/oder 1/2 W und der Rest aus Eisen und unvermeidlichen Verunreinigungen in den folgenden Gehaltsbereichen besteht:
    C:   weniger als 0,03 Gewichts-%, vorzugsweise weniger als 0,01 Gewichts-%;
    Si:   weniger als 0,1 Gewichts-%, vorzugsweise weniger als 0,05 Gewichts-%;
    Mn:   weniger als 0,4 Gewichts-%, vorzugsweise weniger als 0,15 Gewichts-%;
    Cr:   weniger als 0,3 Gewichts-%;
    Cu:   weniger als 0,3 Gewichts-%;
    Al:   weniger als 0,1 Gewichts-%;
    P :   weniger als 0,02 Gewichts-%;
    S :   weniger als 0,02 Gewichts-%;
    O :   weniger als 0,25 Gewichts-%, vorzugsweise weniger als 0,15 Gewichts-%;
    N :   weniger als 0,01 Gewichts-%, vorzugsweise weniger als 0,002Gewichts-%.
    wobei der Gehalt an Ni und der Gehalt Mo und/oder W in dem Stahlpulver mit einem speziellen Durchmesser von weniger als 45 »m im Bereich des 2,0- bis 4,2fachen des mittleren Gehaltes in dem gesamten Stahlpulver liegt, so daß eine hohe Zugfestigkeit und eine hohe Zähigkeit durch eine spannungsinduzierte Martensitumwandlung nach dem Carburieren, Abschrecken und Anlassen nach dem Sintern erreicht werden.
  2. Hochfester gesinterter legierter Stahl, hergestellt durch Carburieren, Abschrecken und Anlassen nach dem Ausbilden und Sintern des in Anspruch 1 definierten zusammengesetzten Legierungsstahlpulvers, wobei die Legierung zusammengesetzt ist aus 0,3 bis 0,8 Gewichts-% C, 0,50 bis 3,50 Gewichts-% Ni, 0,65 bis 3,50 Gewichts-% Mo und/oder 1/2 W, wobei der Rest aus Eisen und unvermeidbaren Verunreinigungen besteht, und mit einer Dichte von mehr als 7,0 g/cm³ und einer Zugfestigkeit von mehr als 1274,9 N/mm² (130 kgf/mm²).
  3. Hochfester hochzäher gesinterter legierter Stahl, hergestellt durch Abschrecken und Anlassen nach dem Ausbilden und Sintern des zusammengesetzten Legierungsstahlpulvers gemäß Anspruch 1, wobei die Legierung zusammengesetzt ist aus 0,30 bis 0,80 Gewichts-% C, 0,50 bis 3,50 Gewichts-% Ni, 0,65 bis 3,50 Gewichts-% Mo und/oder 1/2 W, Rest Eisen und unvermeidliche Verunreinigungen, mit einer Dichte von mehr als 7,0 g/cm³ und einer Zugfestigkeit von mehr als 1274,9 N/mm² (130 kgf/mm²).
EP88908393A 1987-09-30 1988-09-30 Zusammengesetztes legierungsstahlpulver und gesinterter legierungsstahl Expired - Lifetime EP0334968B1 (de)

Applications Claiming Priority (5)

Application Number Priority Date Filing Date Title
JP244074/87 1987-09-30
JP24407487 1987-09-30
JP13740088 1988-06-06
JP137400/88 1988-06-06
PCT/JP1988/001007 WO1989002802A1 (en) 1987-09-30 1988-09-30 Composite alloy steel powder and sintered alloy steel

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EP0334968A1 EP0334968A1 (de) 1989-10-04
EP0334968A4 EP0334968A4 (de) 1989-12-28
EP0334968B1 true EP0334968B1 (de) 1995-02-08

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Publication number Priority date Publication date Assignee Title
SE9101819D0 (sv) * 1991-06-12 1991-06-12 Hoeganaes Ab Jaernbaserad pulverkomposition som efter sintring uppvisar god formstabilitet
WO1994013418A1 (en) * 1992-12-11 1994-06-23 Höganäs Ab Iron-based powder composition
JP3651420B2 (ja) 2000-08-31 2005-05-25 Ｊｆｅスチール株式会社 粉末冶金用合金鋼粉

Citations (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS497294B1 (de) * 1970-03-14 1974-02-19

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Publication number Priority date Publication date Assignee Title
GB1225582A (de) * 1968-03-29 1971-03-17
GB1305608A (de) * 1970-03-18 1973-02-07
DE3227549A1 (de) * 1982-07-23 1984-01-26 Schunk & Ebe Gmbh, 6301 Heuchelheim Werkstoff zur pulvermetallurgischen herstellung von ventilsitzringen

Patent Citations (1)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
JPS497294B1 (de) * 1970-03-14 1974-02-19

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