Patent Application
20080202651
The technology herein relates to methods for manufacturing high-density iron-based compacts (compacted bodies) and high-density iron-based sintered bodies. More specifically, it relates to methods for increasing strength and density of iron-based sintered bodies by improving compactibility of sintered iron-based powder preforms.
Powder metallurgy technologies allow complex parts to be manufactured with near-net shapes and with high dimensional accuracy and can significantly decrease costs of cutting. Therefore, powder metallurgy products are widely employed in automobile parts.
Recently, highly strengthened powder metallurgy products have been desired for miniaturizing and reducing the weight of parts. In particular, iron-based powder products (iron-based sintered bodies) have been strongly desired to be highly strengthened.
A basic process for manufacturing an iron-based sintered component (sometimes referred to as an iron-based sintered body or, simply, a sintered body) is as follows:
A material prior to sintering is called a compact. In the above-mentioned basic process, the green compact serves as a compact.
Furthermore, a segregation-preventing treatment may be employed in a process for obtaining an iron-based mixed powder as disclosed in Japanese Unexamined Patent Application Publication No. 1-165701 or Japanese Unexamined Patent Application Publication No. 5-148505.
The resulting sintered body is subjected to sizing or cutting according to need for processing into a product. In addition, when the sintered body is required to have a high strength, a carburizing heat treatment or a bright heat treatment may be performed.
In the above-mentioned methods, the densities of the resulting compacts are about 6.6 to 7.1 Mg/m3 (Mg means megagrams) at the highest. Therefore, densities of sintered bodies prepared from these compacts are of similar levels.
In order to increase the strength of an iron-based powder product (or an iron-based sintered component), it is effective to highly increase the density of the sintered component (sintered body). The number of pores in a sintered component is decreased with an increase in the density, and thereby mechanical properties such as tensile strength, impact value, and fatigue strength are improved. In general, a high-density sintered body can be effectively obtained by increasing the density of a compact.
The following technologies are known methods for increasing the density of a compact:
A warm compaction technology, in which a metal powder is compacted while being heated, is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 2-156002, Japanese Examined Patent Publication No. 7-103404, U.S. Pat. No. 5,256,185, and U.S. Pat. No. 5,368,630.
For example, in a technology disclosed in U.S. Pat. No. 5,368,630, an iron-based mixed powder is prepared by mixing 0.6 mass % of a graphite powder and 0.6 mass % of a lubricant with a partially alloyed iron-based steel powder in which 4% Ni, 0.55% Mo and 1.6% Cu are contained. The iron-based mixed powder is compacted at a temperature of 150° C. or more at a pressure of 689 MPa to thereby yield a compact having a density of about 7.3 Mg/m3.
However, application of the warm compaction technique requires facilities for compacting powders by heating the powders to a predetermined temperature and strictly controlling the temperature. This has disadvantages in that manufacturing costs are increased and dimensional accuracy of parts is decreased due to warm compaction.
A high-velocity compaction technology, in which a compact with a high density is obtained by striking an upper punch (or an impact ram for striking an upper punch) of a compacting die at a high velocity when a powder is compacted, is disclosed in, for example, US Patent Publication No. 2002/0106298, International Patent Publication No. WO 99/36214, and “High Velocity Compaction of Metal Powders, a Study on Density and Properties,” by A. Skagerstrand, Hydropulsor A B, Sweden (hereinafter referred to as Skagerstrand's document).
For example, in a technology disclosed in US Patent Publication No. 2002/0106298, a high-density compact having a density of 7.7 Mg/m3 or more is obtained by applying an impact load generated by making an impact ram strike an iron-based mixed powder at a velocity of 2 m/s or more at least once.
However, in this method, only products with simple shapes which can be produced by a uniaxial press (single level compaction) are produced.
Furthermore, cracks readily occur in a high-density compact obtained by applying this method. Therefore, it is difficult to process the compact into parts with complicated shapes.
In addition, in International Patent Publication No. WO 99/36214, it is shown that a favorable result can be obtained by using a gas-atomized powder composed of almost spherical particles. However, in US Patent Publication No. 2002/0106298, it is shown that a high density can be achieved by using a water-atomized powder of irregularly shaped particles but the density of a compact prepared by using spherical particles is insufficient. Thus, effects of high-velocity compaction of powders are unclear.
A cold forging of sintered body (sinter cold forging technology) is a combination of powder metallurgy and cold forging and is a compacting and processing technology for obtaining a high-density end product as described below. A sintered preform (sometimes referred to as a preliminary sintered product or a preliminary sintered body or, simply, a preform) is prepared by pre-compacting and pre-sintering (preliminarily sintering) a metal powder. The sintered preform is subjected to cold reprocessing (forging or recompressing) and then to re-sintering to obtain a high-density end product. In this technology, the component after the preliminary compaction (a compacted preform) is a green compact, and the component prepared by reprocessing the preform is a compact.
In this method, the reprocessing of preforms is performed by cold forging accompanied with a high degree of deformation. Therefore, high densification can be advantageously achieved compared with simple recompression and objects having more complex shapes can be obtained.
As an example of a method employing recompression, Japanese Unexamined Patent Application Publication No. 1-123005 discloses a sinter cold forging method as follows. A liquid lubricant is applied to a surface of a sintered preform to be cold forged. The sintered preform is preliminarily compacted in a die. Then, a negative pressure is applied to the preform for absorbing and removing the liquid lubricant. The preform is finally compacted in the die and then is re-sintered. In this method, the liquid lubricant coated before the preliminary compaction and impregnated into the sintered preform is removed before the final compaction. Consequently, fine pores inside the sintered preform are crushed and disappear during the final compacting and thereby an end product with a high density can be obtained according to the document.
However, the density of the end product obtained by this method is about 7.5 Mg/m3 at the highest. Thus, there is a limitation to the strength that can be achieved.
As an example employing cold forging, for example, U.S. Pat. No. 4,393,563 discloses a technology for obtaining an end product (sintered component) by mixing an iron powder, a ferro-alloy powder, a graphite powder, and a lubricant; pre-compacting the resulting mixed powder to produce a green compact; pre-sintering (preliminarily sintering) the green compact; cold forging the pre-sintered compact to achieve a plastic working for at least 50 mass %; and then performing re-sintering, annealing, and roll forming. In this method, the preliminary sintering is carried out under conditions suppressing diffusion of the graphite. Consequently, a high deformation property is achieved in the subsequent cold forging process and thereby the compact load can be decreased.
Japanese Unexamined Patent Application Publication No. 2002-294388 discloses a technology for obtaining a high-density sintered body by improving the deformation property of a preliminary sintered body by decreasing the nitrogen content in a pre-sintering atmosphere or performing annealing after the preliminary sintering, and then conducting reprocessing and re-sintering. Specifically, a partially alloyed steel powder to which a pure iron powder and about 0.6 mass % of Mo are diffusively adhered is mixed with graphite (about 0.2 to 0.6 mass %) and a lubricant to provide of a compact with a density of about 7.4 Mg/m3 having a tablet shape. Then, the compact is pre-sintered under such conditions that the amount of free graphite (that does not diffuse into the base and remains in a graphite form) becomes 0.02 mass % or less. By backward-extrusion forging at a cross-section-decreasing ratio of 60% to 80% and final sintering, a final sintered body having a density of 7.8 Mg/m3 and a cup shape is obtained.
However, the cold forging process has a disadvantage in that the productivity is significantly low compared with that of a compression process.
In addition, according to our experiments, in the pre-sintering conditions (at 1100° C. for 15 to 20 minutes) disclosed in U.S. Pat. No. 4,393,563, the diffusion of graphite cannot be sufficiently prevented and thereby the deformation property of the pre-sintered body is not sufficiently improved. Further, the remaining free graphite is made to disappear by the final sintering and pores may be formed even if the diffusion of graphite is sufficiently suppressed.
Furthermore, Japanese Unexamined Patent Application Publication No. 11-117002 discloses a metal powder material for compaction (preform) having a microstructure in which graphite remains along grain boundaries of the metal powder. The preform is pre-pared by mixing graphite in an amount of not less than 0.3 mass % with an iron-based metal powder to prepare a metal mixed powder, compacting the metal mixed powder into a preliminary compact (compacted preform) having a density of 7.3 g/cm3 or more, and pre-sintering the preliminary compact at a temperature of, preferably, 700° C. to 1000° C. According to this technology, excessive hardening of the iron powder is prevented by solid-solving only carbon in a concentration necessary for increasing strength and allowing free graphite to remain. Thus, a material for compaction (preform) having a low compact load and a high deformation property can be obtained. Specifically, a sintered body having a density of 7.87 Mg/cm3 is exemplarily shown. However, according to the understanding of the present inventors, the technology includes a backward-extrusion process conducted after the preliminary sintering and therefore belongs to the category of cold forging technology.
In addition, the metal powder material for compaction formed by this method has a disadvantage in that the remaining free graphite disappears during the final sintering and elongated pores may be formed.
It could therefore be advantageous to provide a method for stably manufacturing, with a high productivity, an iron-based compact having a complex shape and a high density and further an iron-based sintered body having a high strength and a high density.
We found the fact that a sintered iron-based powder preform of which N and O contents are decreased to be as low as possible and which have a low hardness and a high plastic deformation property can be obtained by preliminary sintering a preliminary compact within an appropriate temperature range, preferably, in an atmosphere in which oxidation and nitridation are suppressed, and the fact that a compact having a high density of 7.65 Mg/m3 or more, 7.70 Mg/m3 or more in preferable condition, and not having elongated pores can be readily obtained without occurrence of cracking by subjecting the sintered iron-based powder preform to high-velocity compaction. The high-velocity compaction is preferably performed at a compaction energy density of 1.8 MJ/m2 or more, more preferably 2.2 MJ/m2 or more, and a ram velocity of 2 m/s or more.
In the high-velocity compaction, a ram may strike an upper punch twice or more. However, this method has an advantage in that the above-mentioned high density can be achieved by striking the upper punch once.
Thus, we provide:
The methods and sintered bodies will now be specifically described.
First, the reasons why the compositions of the sintered iron-based powder preform are limited to the above-mentioned ranges will be described.
The C content is adjusted within the range of about 0.10 to about 0.50 mass % depending on strength necessary for a sintered component in consideration of hardenability for carburizing quenching or bright quenching. When the C content is less than about 0.10 mass %, desired hardenability cannot be secured. On the other hand, when the C content is more than about 0.50 mass %, the hardness of a sintered preform is too much increased and the density after high-velocity compaction is decreased.
O is an element that is unavoidably contained in an iron-based metal powder. The hardness of a sintered preform increases with an increase in the O content, and the density after high-velocity compaction decreases with an increase in the O content. Therefore, a lower O content is preferable. When the O content is more than about 0.3 mass %, the density after high-velocity compaction is decreased. Therefore, the upper limit of the O content is determined to be about 0.3 mass %. In addition, since the lower limit of the O content for industrially stably manufacturing an iron-based metal powder is about 0.02 mass %, the lower limit of the O content in a sintered iron-based powder preform is preferably about 0.02 mass %.
N is an element that increases the hardness of a sintered preform, like C. The free graphite content is decreased to substantially zero by dissolving graphite in an iron-based metal powder. Therefore, to increase the density after high-velocity composition while maintaining the hardness of a sintered preform as low as possible without relying on a decrease in the C content, the N content is preferably decreased as low as possible. When the N content is more than about 0.010 mass %, the density after high-velocity composition is decreased. Therefore, in the present invention, the N content is limited to about 0.010 mass % or less, preferably about 0.0050 mass % or less.
The base component and other components of which contents should be controlled are described in above. When a composition is composed of the above-described components and the balance of Fe and unavoidable impurities, the composition is called a pure-iron based composition.
The sintered preform may optionally contain an element described below as an alloy component. A composition containing such an optional element is called an alloy-based composition.
One or more than one elements selected from Mn: about 1.2 mass % or less, Mo: about 2.3 mass % or less, Cr: about 3.0 mass % or less, Ni: about 5.0 mass % or less, Cu: about 2.0 mass % or less, and V: about 1.4 mass % or less
All of Mn, Mo, Cr, Ni, Cu, and V are elements that improve hardenability. The sintered preform may contain one or more than one of these elements according to need for securing the hardness of a sintered body. However, when the content of each element is more than the above-mentioned upper limit, disadvantageously, the hardness of a sintered preform is increased and the density after high-velocity compaction is decreased. In addition, Cr is likely to cause breakage in a sintered preform. Therefore, in order to stably perform high-velocity compaction, the Cr content is preferably about 1.0 mass % or less. Further, a sintered preform that substantially does not contain Cr is more preferable.
More preferably, the contents of Mn, Mo, Ni, and V are Mn: about 1.0 mass % or less, Mo: about 2.0 mass % or less, Ni: about 2.0 mass % or less, and V: about 1.0 mass % or less.
When any of these alloy elements is added, the content of each element is preferably about 0.1% or more.
In particular, Mn, Mo, and Ni are preferable elements and the balance is preferably Fe and unavoidable impurities.
The content conformation of these alloy elements does not have any limitation. That is to say, each element may be pre-alloyed to an iron-based metal powder, may be partially alloyed to an iron-based metal powder by partial diffusion and adhesion of the element, or may be mixed as a metal powder (alloy powder) with an iron-based metal powder. In addition, the alloying may be performed by a combination of these methods. For example, a part of an alloy component is pre-alloyed and then other part of the alloy component may be partially alloyed (hybrid alloying). However, in any case, if the content of each alloying component exceeds the predetermined level, namely, when the content is more than Mn: 1.2 mass %, Mo: 2.3 mass %, Cr: 3.0 mass % (preferably 1.0 mass % or less), Ni: 5.0 mass %, Cu: 2.0 mass %, and V: 1.4 mass %, the hardness of a sintered preform is increased and the density after high-velocity compaction is decreased.
The balance other than the above-mentioned components is preferably Fe and unavoidable impurities. As the unavoidable impurities, P: about 0.1 mass % or less, S: about 0.1 mass % or less, and Si: about 0.2 mass % or less are acceptable. In addition, the free graphite content is preferably controlled to about 0.02 mass % or less as described below.
The sintered iron-based powder preform is prepared by pre-compacting and pre-sintering an iron-based metal mixed powder prepared by mixing an iron-based metal powder with a graphite powder. It is preferable that the graphite powder diffuse to the matrix of the iron-based metal powder and that free graphite substantially do not exist.
The free graphite content of the sintered iron-based powder preform is controlled to about 0.02 mass % or less, namely, substantially zero by adjusting conditions for the preliminary sintering. Most of graphite powder is diffused into an iron-based metal powder by the preliminary compaction and preliminary sintering and is dissolved or precipitated as a carbide in the matrix. Thus, the graphite powder negligibly remains as free graphite. Here, when the free graphite content is more than about 0.02 mass %, a graphite elongated layer is prominently formed along a flow of the sintered preform during high-velocity compaction and the graphite diffuses and disappears in the iron-based metal-base matrix during re-sintering to generate elongated pores. Such elongated pores may affect a sintered body as a defect to decrease the strength. Therefore, the free graphite content is preferably limited to about 0.02 mass % or less.
The microstructure of the sintered iron-based powder preform preferably has a structure in which a ferrite (F) phase mainly exists and a pearlite (P) phase is mixed in regions where graphite has diffused. The hardness of the sintered preform can be adjusted to a level suitable for high-velocity compaction by controlling the conditions for the preliminary sintering within the after-mentioned range.
Further, it is important that the sintered iron-based powder preform has a density of about 7.2 Mg/m3 or more, preferably about 7.3 Mg/m3 or more. By adjusting the density to about 7.2 Mg/m3 or more and further preferably about 7.3 Mg/m3 or more, the contact area between iron-based metal particles is increased. Consequently, material diffusion via the contact area is extensively caused by the preliminary sintering. Therefore, a preform with an excellent elongation property and a high deformability can be obtained and thereby a compact with a high density can be obtained. More preferably, the density is about 7.35 Mg/m3 or more. It is preferable that a sintered preform has a higher density, but the upper limit is about 7.8 Mg/m3 from the restriction of costs such as life duration of a die. In addition, a practical density range is about 7.30 to about 7.55 Mg/m3.
Since the above-mentioned sintered preform is prepared by performing preliminary sintering and further C is dissolved in Fe, the hardness of the sintered preform is higher than that of a compact (green compact) prepared by compressing a powder. According to conventional knowledge, it is expected that a sintered preform with a high hardness has low processability and thereby a high density cannot be readily realized by subjecting the sintered preform to high-velocity compaction compared with a case that a powder is directly subjected to high-velocity compaction. However, contrary to the expectation, a sintered preform pre-pared under appropriate conditions and having a hardness higher than that of a powder compact can obtain a high density at a lower energy density. In addition, defects such as cracking do not occur.
Next, a method for manufacturing a sintered iron-based powder preform will be described.
As a raw material powder, an iron-based metal powder, a graphite powder, and optionally a lubricant are used.
A preferable iron-based metal powder has a composition containing C: 0.05 mass % or less, O: 0.3 mass % or less, and N: 0.010 mass % or less and the balance of Fe and unavoidable impurities. In addition, according to need, a steel powder prepared by pre-alloying, partially alloying, or hybrid alloying at least one of Mn: 1.2 mass % or less, Mo: 2.3 mass % or less, Cr: 3.0 mass % or less (preferably 1.0 mass % or less), Ni: 5.0 mass % or less, Cu: 2.0 mass % or less, and V: 1.4 mass % or less can be advantageously used. In addition, a mixed metal powder prepared by mixing an alloy powder composed of at least any one of the above-mentioned alloy elements with an iron powder or a steel powder may be used as the iron-based metal powder.
In every iron-based metal powder, when the C content is more than 0.05 mass %, when the O content is more than 0.3 mass %, or when the N content is more than 0.010 mass %, the compressibility of the powder is decreased. Therefore, it is difficult to increase the density of the sintered preform to 7.2 Mg/m3 or more. Furthermore, more preferably, the iron-based metal powder contains C: 0.05 mass % or less, O: 0.3 mass % or less, and N: 0.0050 mass % or less.
In addition, it is preferable that the O content be as low as possible from the view-point of compactibility. However, since O is an unavoidable element, the lower limit of the O content is desirably determined to 0.02 mass % that is an industrially practicable level but not being economically expensive. The O content is preferably 0.03 to 0.2 mass % from the viewpoint of industrial economic efficiency.
The particle diameter of the iron-based metal powder is not specifically limited, but an average particle diameter is preferably about 30 to about 120 μm, which can be industrially manufactured at a low cost.
Particle sizes are measured by a sieve method (using a sieve defined in JISZ8801-1) and the average particle diameter is determined as a median (d50) value of the weight integrated particle size distribution.
The graphite powder used as the raw material powder is preferably contained in an iron-based mixed powder within the range of about 0.03 to about 0.5 mass % with respect to the total amount of the iron-based metal powder and the graphite powder in order to secure a predetermined strength of a sintered body or to increase hardenability in heat treatment. When the graphite powder content is less than about 0.03 mass %, the effect on improvement in strength of a sintered body is insufficient. On the other hand, when the graphite powder content is more than about 0.5 mass %, the hardness of a sintered preform is increased and the density after high-velocity compaction is decreased. Therefore, the graphite powder content in the iron-based mixed powder is preferably 0.03 to 0.5 mass % with respect to the total amount of the iron-based metal powder and the graphite powder.
In addition, wax, spindle oil or the like may be added to the iron-based mixed powder in order to improve the adhesion of the graphite powder to the particle surfaces of the iron-based metal powder.
Further, a known segregation-preventing treatment (disclosed in, for example, Japanese Unexamined Patent Application Publication No. 1-165701 and Japanese Unexamined Patent Application Publication No. 5-148505) may be employed for improving the adhesion of the graphite powder to the particle surfaces of the iron-based metal powder.
The iron-based mixed powder may further contain, in addition to the above-mentioned raw material powder, a known lubricant in order to improve compaction density for compacting and to decrease ejection force from a die. Examples of the known lubricant include zinc stearate, lithium stearate, and ethylene bis(stearo)amide. The lubricant content is preferably about 0.1 to about 0.6 parts by mass with respect to 100 parts by mass of the total amount of the iron-based metal powder and the graphite powder.
In addition, the mixing of the iron-based mixed powder may be performed by a known method, for example, a mixing method using a Henschel mixer, a cone mixer or the like.
Then, the iron-based mixed powder having the above-described mixture ratio is subjected to preliminary compaction to produce a preliminary compact having a density of about 7.2 Mg/m3 or more. When the density of the preliminary compact is about 7.2 Mg/m3 or more, preferably about 7.3 Mg/m3 or more, the contact area between iron-based metal particles is increased. Consequently, in the subsequent process, i.e., in preliminary sintering, volume diffusion, surface diffusion, and melting extensively occur via the contact area. As a result, an excellent elongation property is obtained during high-velocity compaction and a high deformability can be realized.
The preliminary compaction may be performed by any known compaction method. For example, either a die-wall lubrication method or a warm compaction method is applicable. A compaction method disclosed in Japanese Unexamined Patent Application Publication No. 11-117002 or Japanese Unexamined Patent Application Publication No. 2002-294388 may be used. In addition, any combination of the above-mentioned methods is applicable.
Further, the compaction method disclosed in Japanese Unexamined Patent Application Publication No. 11-117002 uses a device including a compacting die having a compaction space, an upper punch, and a lower punch. The upper and lower punches are inserted into the compacting die for pressing a mixed powder. The compaction space includes a larger diameter portion in which the upper punch is inserted, a smaller diameter portion in which the lower punch is inserted, and a tapered portion connecting them. Either one or both of the upper punch and the lower punch are provided with a notch for increasing the volume of the compaction space at the outer circumferential edge of an end face facing the compaction space of the compacting die. By the use of this device, spring back after compaction or ejection force of a compact are suppressed, and therefore a compact with a high density can be readily manufactured.
In addition, a high-velocity compaction method disclosed in US Patent Publication No. 2002/0106298 may be used. However, this method should be applied to a compact with a simple shape for avoiding occurrence of cracking.
Then, the preliminary compact is subjected to preliminary sintering to be formed into a sintered preform.
The preliminary sintering is performed at a temperature of higher than about 1000° C. but not higher than about 1300° C. When the temperature for preliminary sintering is about 1000° C. or less, the residual amount of free graphite exceeds 0.02 mass % and therefore elongated pores are generated during the subsequent process, i.e., re-sintering. This acts as a defect in a member that is used under high stress and likely causes a decrease in strength. On the other hand, when the temperature for preliminary sintering is higher than 1300° C., the effect on improvement in compactibility is saturated. Therefore, the higher temperature rather causes a large increase in the manufacturing costs and thus is economically disadvantageous. Consequently, the temperature for preliminary sintering is limited to the range of higher than about 1000° C. but not higher than about 1300° C.
Preferably, the preliminary sintering may be conducted in a non-oxidative atmosphere of vacuum, Ar gas, or hydrogen gas at a nitrogen partial pressure of about 100 kPa or less, preferably about 30 kPa or less. A lower nitrogen partial pressure is advantageous for decreasing the N content in a sintered preform. Regarding this point, when the nitrogen partial pressure is higher than about 100 kPa, it is difficult to control the N content in a sintered preform to 0.010 mass % or less. For example, a hydrogen-nitrogen gas mixture having a hydrogen concentration of about 70 volmass % is a preferable atmosphere.
In addition, an annealing at a temperature lower than that of the preliminary sintering may be performed after the preliminary sintering of a preliminary compact, if necessary.
By the annealing after the preliminary sintering, the N content of the sintered preform is significantly decreased. Therefore, even if the nitrogen partial pressure of the atmosphere for the preliminary sintering is as high as 95 kPa, the N content in a sintered preform can be readily decreased to 0.010 mass % or less by conducting the annealing after the preliminary sintering. Thus, the gas costs can be advantageously reduced.
The annealing is preferably conducted within the temperature range of about 400° C. to about 800° C. When the annealing temperature is lower than about 400° C. or higher than about 800° C., the effect on reducing the N content is small. Further, a non-oxidative atmosphere is preferable for the annealing as in the preliminary sintering. By the annealing in a non-oxidative atmosphere, the N content in a sintered preform is further effectively reduced. It is not necessary that the nitrogen partial pressure level of the atmosphere for annealing is the same as that of the atmosphere for preliminary sintering.
The annealing time is preferably about 600 to about 3600 seconds. When the annealing time is shorter than about 600 seconds, the effect on reducing the N content is insufficient. On the other hand, when the annealing time is longer than about 3600 seconds, the effect is saturated and the productivity is decreased.
Further, without problems, the preliminary sintering and the subsequent annealing can be conducted sequentially without taking out the preform from a sintering furnace where the preliminary sintering has been performed. That is to say, a preform after the preliminary sintering may be cooled to about 400° C. to about 800° C. and then be annealed directly, or a preform after the preliminary sintering may be cooled to lower than about 400° C. and then be annealed by heating to about 400° C. to about 800° C. again.
Then, the sintered preform after preliminary sintering or preliminary sintering and annealing is subjected to high-velocity compaction to produce a compact.
The high-velocity compaction may be conducted using a compacting machine for high-velocity compaction, for example, manufactured by Hydropalsor A B in Sweden. In a high-velocity compaction machine, a striking stress is generally applied to an upper punch with a ram, but the high-velocity compaction means is not limited to this.
In
In this compaction method, the compaction energy density for applying a striking stress to the upper punch is preferably about 1.8 MJ/m2 or more, more preferably about 2.2 MJ/m2 or more.
However, when the sintered preform has a pure-iron based composition, which is relatively soft, the compaction energy density is preferably controlled to about 1.4 MJ/m2 or more, more preferably about 1.8 MJ/m2 or more. In addition, when the sintered preform has an alloy-based composition, the compaction energy density is preferably controlled to about 1.8 MJ/m2 or more, more preferably about 2.2 MJ/m2 or more.
When the compaction energy density is lower than about 1.8 MJ/m2 or, in a pure-iron based composition, lower than about 1.4 MJ/m2, it is difficult that a compact after the high-velocity compaction has a sufficiently high density.
In addition, the compaction energy density is preferably not higher than about 2.6 MJ/m2. According to the composition category, when the composition is a pure-iron based, the compaction energy density is preferably about 2.2 MJ/m2 or less. When the composition is an alloy-based, the compaction energy density is preferably about 3 MJ/m2 or less. This is because the effect on improving the density of a compact is small even if an energy density level is higher than these values and also die tool life duration is significantly decreased. Here, the compaction energy density is calculated by the following equality (1):
Compaction energy density=0.5 mv2/S (1)
where m represents mass of an impact ram, and v represents a ram velocity. Therefore, 0.5 mv2 represents compaction energy. S represents a cross-sectional area to be processed. As the cross-sectional area, any one of the upper punch cross-sectional area, the sintered pre-form cross-sectional area, and the compact cross-sectional area can be used without a large difference in the value.
The ram velocity is preferably about 2 m/s or more. Because, in order to obtain a sufficient compaction energy density while maintaining the ram velocity at a low level, the weight of the ram must be high. This is a large load for facilities.
The number of times of striking the upper punch with a ram (the number of times of high-velocity compaction) is once, which is sufficient. However, the upper punch may be struck twice or more.
Then, the compact is re-sintered to produce a sintered body.
The re-sintering is preferably performed in an inert atmosphere, in a reducing atmosphere, or in a vacuum for preventing oxidation of a product. The temperature for the re-sintering is preferably within the range of about 1050° C. to about 1300° C. When the temperature is lower than about 1050° C., the progress of sintering between particles and the diffusion of C contained in the compact are insufficient and thereby a desired strength cannot be provided to the product. On the other hand, when the temperature is higher than about 1300° C., grains become coarse and thereby the product strength is decreased.
The thus resulting sintered body is subjected to heat treatment if necessary.
The heat treatment may be selected from carburizing, quenching, tempering or the like according to need.
In and any heat treatment such as gas carburizing and quenching, vacuum carburizing and quenching, bright quenching, induction hardening or the like, the condition for heat treatment are not specifically limited. For example, in the gas carburizing and quenching, a sintered body is preferably heated to a temperature of about 800° C. to about 900° C. in an atmosphere with a carbon potential of about 0.6 to 1 mass % and then is quenched in oil. The term carbon potential means a carburizing ability of a carburizing atmosphere. That is to say, the carbon potential is the C content (mass %) in the surface of a steel when the C content is equilibrium to the gas atmosphere used for carburizing at the temperature for the carburization.
In the bright quenching, a sintered body is preferably heated to a temperature of about 800° C. to about 950° C. in a protective atmosphere such as an inert atmosphere like Ar gas or the like, or a nitrogen atmosphere containing hydrogen or the like, in order to inhibit a high temperature oxidation or decarburization of the sintered body surface, and then is quenched in oil. In addition, in the vacuum carburizing and quenching or the induction hardening, it is also preferably that a sintered body is heated to the above-mentioned temperature range and then quenched. By these heat treatments, the strength of a product can be further improved.
Furthermore, after the quenching, tempering may be performed if necessary. The tempering temperature is preferably in the range of about 130° C. to about 250° C. as in usually known tempering temperature range.
In addition, before or after such heat treatment, a sintered body may be subjected to machining for adjusting the size and the shape. The machining may be conducted if necessary even if heat treatment is not applied.
A compact without subjected to re-sintering can be a product (end member). The above-mentioned heat treatment and machining may be conducted according to need. Properties such as strength and density of a product prepared by such a process do not have any problems.
Advantages with respect to known technologies will be additionally described.
(Advantage with Respect to Known High-Velocity Compaction Technology)
According to our technology, occurrence of breakage and cracking when a compact is taken out from a die is low. It is thought that the occurrence of breakage and cracking is low because that the binding force between particles is strong and the plastic deformation property is high in a sintered preform according to the present invention so that cold forging can be applied and thereby the preform can overcome a stress generated due to spring back when a load is removed after compaction. On the other hand, in high-velocity compaction of a powder according to a known technology, it is thought that since the binding force between particles in a compact is low, breakage and cracking occur frequently when a load is removed.
Further, with our methods, a complicated-shaped product may be once compacted by a usual powder metallurgy and then subjected to preliminary sintering and a high-velocity compaction. Therefore, a product with a high density and a complex shape can be readily manufactured. On the other hand, in a high-velocity compaction of a powder, only a simply-shaped product that can be compacted by uniaxial press (single level compaction) can be manufactured as described above.
It is further noteworthy that the compaction energy density necessary for providing a density to a product can be significantly decreased compared with that when the same density level is obtained by high-velocity compaction of a powder. That is to say, this is unexpected effects, from the viewpoint of an increase in the density to near a theoretical value, because our methods might seem to have disadvantageous factors, such as sintering between metal powders in preliminary sintering and diffusion of carbon, compared with high-velocity compaction of a powder. Those effects work well in the viewpoints of manufacturing costs and facility capability.
(Advantage with Respect to Known Sinter Cold Forging Technology)
In known sinter cold forging, a high density level near the true theoretical density is achieved by cold forging which causes deformation of a preform to several tens percent. A high density level can be realized by only applying uniaxial compaction to a preform. The process by the uniaxial compaction can be performed at a speed several times higher than that by the cold forging. This is significantly advantageous in the view of productivity. In addition, the cold forging is required to determine a specification of a die by performing a trial-and-error process several times. However, in a die used in the uniaxial compaction process, the resulting shape is correctly calculated. Therefore, the uniaxial compaction is much more simple and easy.
As described above, the compaction energy necessary for increasing the density is lower than a predicted value, which is a significant advantage contrary to the expectation
In addition, when a member is formed into a complicated shape so that only cold forging can be applied, there may be a case that is not necessary to apply the present invention. However, in such a case, a cold forging process in addition to the process may be additionally performed.
Iron-based metal powders shown in Table 1 were each mixed with a graphite powder and a lubricant in a content and a type shown in Table 1 using a V-type mixer to prepare an iron-based mixed powder.
As an iron-based metal powder, a pure-iron powder A, a partially alloyed steel powder B, and a hybrid alloyed steel powder C were used. The pure-iron powder A was composed of an iron powder (JIP301A manufactured by JFE Steel Corporation) containing C: 0.006 mass %, Mn: 0.08 mass %, O: 0.15 mass %, and N: 0.0020 mass % (and the balance of Fe and unavoidable impurities). The partially alloyed steel powder B was prepared by mixing 0.9 mass % of a molybdenum oxide powder with the pure-iron powder A and placing the resulting mixed powder in a hydrogen atmosphere at 875° C. for 3600 seconds so that Mo was partially diffused and adhered to the particle surfaces. The composition of the partially alloyed steel powder B was C: 0.006 mass %, Mn: 0.08 mass %, O: 0.11 mass %, N: 0.0023 mass %, and Mo: 0.58 mass % (and the balance of Fe and unavoidable impurities). The hybrid alloyed steel powder C was prepared by partially alloying 0.4 mass % of Mo to particle surfaces of a pre-alloyed steel powder containing C: 0.007 mass %, Mn: 0.14 mass %, O: 0.15 mass %, N: 0.0020 mass %, and Mo: 0.4 mass % (and the balance of Fe and unavoidable impurities) as in above. In addition, a mixture (powder D) of the hybrid alloyed steel powder and a metal powder was prepared by partially alloying Mo to particle surfaces of a pre-alloyed steel powder containing a predetermined amounts of Mn and Mo as in above and further mixing a Ni powder with the resulting partially alloyed mixed powder. The composition of the powder D was C: 0.006 mass %, Mn: 0.05 mass %, O: 0.080 mass %, N: 0.0020 mass %, Mo: 0.6 mass % (pre-alloyed amount: 0.45 mass %, partially alloyed amount: 0.15%), and Ni: 1 mass % (and the balance of Fe and unavoidable impurities).
The graphite was natural graphite and the lubricant was zinc stearate.
The lubricant contents in the iron-based mixed powders shown in Table 1 are expressed in terms of part by mass with respect to 100 parts by mass of the total amount of the iron-based metal powder and the graphite powder.
These iron-based mixed powders were each put into a die and pre-compacted using a hydraulic compressor into a tablet-shaped preliminary compact of 25 mm in diameter and 15 mm in height. The density of each preliminary compact was 7.2 Mg/m3 or more. The density of a sample (No. 13) was adjusted to 7.1 Mg/m3 by controlling the compaction pressure.
The resulting preliminary compacts were subjected to preliminary sintering under conditions shown in Table 1 to produce sintered preforms. Some samples (Nos. 15 to 21) were subjected to annealing as a continuing subsequent process after the preliminary sintering. Here, the sintered preforms prepared by using the pure-iron powder A were pure-iron based compositions, and the sintered preforms prepared by using the partially alloyed steel powder B or hybrid alloyed steel powder C or D were alloy-based compositions.
Some of the resulting sintered preforms were investigated for composition, surface hardness HRB (Rockwell hardness according to JIS Z 2245), and free graphite content.
Table 2 shows the results.
The compositions of the sintered preforms were determined by sampling a test piece from sintered preform and measuring total C amount, N amount, O amount, and free graphite amount. The total C amount and the O amount were measured by a combustion-infrared absorption method. The N amount was measure by a combustion-inert gas fusion thermal conductivity method. Further, the residue after dissolving a test piece sampled from sintered preform with nitric acid was measured for C amount by the combustion-infrared absorption method as free graphite amount. The solute C amount was calculated from {(total C amount)−(free graphite amount)}.
Then, the resulting sintered preforms were subjected to high-velocity compaction in conformance with a method disclosed in US Patent Publication No. 2002/0106298. Specifically, the high-velocity compaction was performed using a machine shown in
The compaction energy density in each high-velocity compaction is shown in Table 2. The density of each of the resulting compacts is also shown in Table 2.
Then, the resulting compacts were subjected to re-sintering to produce sintered bodies. The re-sintering was conducted by placing the compacts in a gas atmosphere of 80 vol % of nitrogen and 20 vol % of hydrogen at 1140° C. for 1800 seconds.
The density of each resulting sintered body was measured by an Archimedes method. A part of each sintered body was cut and polished, and without conducting etching the across section was photographed using an optical microscope at magnification of 400. An average pore length of 100 pores was determined by image analysis of the photographed cross section.
Then, these sintered bodies were subjected to carburization. The carburization was conducted by placing the sintered bodies in a carburizing atmosphere having a carbon potential of 1.0 mass % at 870° C. for 3600 seconds. Then, the sintered bodies were subjected to heat treatment for quenching in oil at 90° C. followed by tempering at 150° C. After the heat treatment, each sintered body was measured for the hardness HRC (Roackwell hardness according to JIS Z 2245) and the density by the Archimedes method.
The results are also shown in Table 2.
As shown in Table 2, our compacts had a high density of 7.74 Mg/m3 or more. Such a high density was retained not to be decreased by sintering and heat treatment for producing a sintered body. Further, in our sintered bodies, the number of elongated pores was small and the average pore length was less than 10 μm. Further, the sintered bodies after heat treatments had a high hardness of 32 HRC or more. Particularly, in the invention examples (Nos. 7, 8, 14, 20, and 21) containing Mo, the hardness values after heat treatments were further high such as 60 HRC or more.
In addition, the N contents of sintered preforms (Nos. 16, 17, 20, and 21) that were annealed at a temperature range according to the present invention after preliminary sintering were each 0.010 mass % or less even if the nitrogen partial pressure of an atmosphere for the preliminary sintering was from 30 to 95 kPa.
On the other hand, in sintered preforms (Nos. 1 and 2) that were pre-sintered at a temperature lower than the appropriate temperature range, the free graphite contents were high, i.e., 0.17 mass % (No. 1) and 0.13 mass % (No. 2), and a large number of elongated pores extending along the direction of forging were observed. The average pore lengths were 50 μm (No. 1) and 35 μm (No. 2).
In sintered preforms (Nos. 10 and 11) of which N contents were higher than the appropriate range, compacts each having a low density were obtained.
In the sintered preform (No. 12) of which C content was higher than our range, a compact having a low density was obtained.
When the density of a sintered preform was lower than 7.2 Mg/m3 (No. 13), the density of the compact was also low, and the average pore length of the sintered body was long, i.e., 53 μm.
In comparative examples (Nos. 15 and 18) that were subjected to annealing, after the preliminary sintering, at a temperature outside the appropriate range, the N contents were more than 0.010 mass % and the densities of the compacts were low, even if the nitrogen partial pressure of the atmosphere for the preliminary sintering was 95 kPa or less.
When the nitrogen partial pressure of the atmosphere for the preliminary sintering was higher than 95 kPa (No. 19), the N content was more than 0.010 mass % and the density of the compact was low, even if annealing was conducted after the preliminary sintering.
When the compaction energy density did not satisfy the appropriate range (No. 22), the density of the compact was low.
A partially alloyed steel powder was prepared by diffusing and adhering 1.5% of Mo to the pure-iron powder A in EXAMPLE 1. The partially alloyed steel powder was mixed with 0.2% of a natural graphite powder and 0.2 parts by mass of zinc stearate as a lubricant to produce an iron-based mixed powder (the base of blending quantity was the same as that in EXAMPLE 1). The iron-based mixed powder was pre-compacted into a cylindrical-column-shaped preliminary compact of 25 mm in diameter and 15 mm in height. The density of the preliminary compact was 7.35 Mg/m3. The preliminary compact was pre-sintered under the same conditions as those for sample No. 5 in EXAMPLE 1, and then was subjected to high-velocity compaction.
The results shows that when the compaction energy was about 1000 J (compaction energy density: 2.0 MJ/m2), the density of the resulting sintered preform was 7.56 Mg/m3, and that when the compaction energy was about 1260 J (compaction energy density: 2.6 MJ/m2), the density of the resulting sintered preform was 7.7 Mg/m3.
The above-mentioned Skagerstrand's document discloses data obtained when a mixed powder prepared by mixing a powder pre-alloyed with 1.5% of Mo to 0.2% of a natural graphite powder was subjected to high-velocity compaction while striking an upper punch with a ram having a weight of 12 to 30 kg. In this document, the compact is cylindrical-column-shaped with a diameter of 25 mm. The experiment conditions are estimated to be almost the same as those of the present invention. According to the document, the achieved density when the compaction energy is 3000 J (compaction energy density: 6.1 MJ/m2) is about 97% (7.56 Mg/m3) of the true theoretical density.
The compositions of the sintered preform and the experiment of the above-mentioned document are similar to each other, and it is likely that a load necessary for obtaining a certain density level is almost the same as each other except the hardening by preliminary sintering. However, as shown in this example, by our method, a high compaction density can be achieved at a compaction energy (i.e., compaction energy density) which is significantly lower than that in the high-velocity compaction of a powder disclosed in the document.
Iron-based mixed powders were prepared using the iron-based metal powders A and B described in EXAMPLE 1 by the same method as that in EXAMPLE 1. Raw materials of the mixed powders and the blending quantities are shown in Table 3.
These iron-based mixed powders were each put into a die and pre-compacted using a hydraulic compressor into a tablet-shaped preliminary compact of about 30 mm in diameter and 15 mm in height. The density of each preliminary compact was 7.4 Mg/m3. The density of a sample (No. 8) was adjusted to 7.1 Mg/m3 by controlling the compaction pressure.
The resulting preliminary compacts were subjected to preliminary sintering under conditions shown in Table 3 to produce sintered preforms. Some samples (Nos. 10 to 16) were subjected to annealing as a continuing subsequent process after the preliminary sintering.
The composition, surface hardness HRB (Rockwell hardness according to JIS Z 2245), and free graphite content of each resulting sintered preform were investigated by the same methods as those in EXAMPLE 1.
Table 4 shows the results.
Then, the resulting sintered preforms were subjected to high-velocity compaction by a method similar to that employed in EXAMPLE 1. However, in samples other than No. 17, the compaction energy was adjusted to 1.8 MJ/m2 or more by appropriately controlling not only the ram stroke distance 7 but also the accelerate force 8 applied to the ram and the weight of the ram 4. The compaction energy of the sample No. 17 was lower than 1.0 MJ/m2.
The ram velocities and the number of times that the ram struck the upper punch are shown in Table 5. The densities of the resulting compacts are also shown in Table 5.
Then, the resulting compacts were subjected to re-sintering under the same conditions as those in EXAMPLE 1 to produce sintered bodies. Densities and average pore lengths were similarly measured.
Then, the resulting sintered bodies were subjected to heat treatment by carburization, and quenching and tempering under the same conditions as those in EXAMPLE 1. After the heat treatment, hardness HRC and density of each sintered body were measured as in EXAMPLE 1. Table 5 shows the results.
As shown in Table 5, our compacts had a high density of 7.8 Mg/m3 or more. Such a high density was retained not to be decreased by sintering and heat treatment for producing a sintered body. Further, in our sintered bodies, the number of elongated pores was small and the average pore length was less than 10 μm. Further, the sintered bodies after heating had a high hardness of 32 HRC or more. Particularly, in examples (Nos. 15 and 16) containing Mo, the hardness values after heating were further high, i.e., 58 HRC or more.
In addition, the N contents of sintered preforms (Nos. 11, 12, 15, and 16) that were annealed at a temperature range according to the present invention after preliminary sintering were each 0.010 mass % or less even if the nitrogen partial pressure of an atmosphere for the preliminary sintering was from 30 to 95 kPa.
On the other hand, in sintered preform (No. 1) that was pre-sintered at a temperature lower than the appropriate temperature range, the free graphite content was high, i.e., 0.13 mass %, and a large number of elongated pores extending along the direction of forging were observed. The average pore length was 35 μm (No. 1).
In sintered preforms (Nos. 5 and 6) of which N contents were higher than the appropriate range, compacts each having a low density were obtained.
In the sintered preform (No. 7) of which C content was higher than our range, a compact having a low density was obtained.
When the density of the sintered preform was lower than 7.3 Mg/m3 (No. 8), the density of the compact was also low, and the average pore length of the sintered body was long, i.e., 53 μm.
In comparative examples (Nos. 10 and 13) that were subjected to annealing, after the preliminary sintering, at a temperature outside the appropriate range, the N contents were more than 0.010 mass % and the densities of the compacts were low, even if the nitrogen partial pressure of the atmosphere for the preliminary sintering was 95 kPa or less.
When the nitrogen partial pressure of the atmosphere for the preliminary sintering was higher than 95 kPa (No. 14), the N content was more than 0.010 mass % and the density of the compact was low, even if annealing was conducted after the preliminary sintering.
When the high-velocity compaction did not satisfy the appropriate range (No. 17), the density of the compact was low.
Compacts, sintered bodies, and sintered bodies subjected to heat treatments were prepared using the sintered preforms of Nos. 5 and 21 in EXAMPLE 1 (see Tables 1 and 2) under the same conditions as those in EXAMPLE 1 except that the compaction energy densities were modified as shown in Tables 6 and 7. Observation was conducted as in EXAMPLE 1. However, the ram stroke distance 7 was up to 90 mm.
The results of No. 5 are shown in Table 6, and the results of No. 6 are shown in Table 7. In the pure-iron based composition (No. 5), when the composition energy density was 1.4 MJ/m or more, the compact can be provided with a density of 7.70 Mg/m3 or more, and when the composition energy density was in the range of 2.2 MJ/m2 or more, the effect on an increase in the density was saturated. In addition, in the alloy-based composition (No. 6), when the composition energy density was 1.8 MJ/m2 or more, the compact can be provided with a density of 7.65 Mg/m3 or more, but when the composition energy density was in the range of 3 MJ/m2 or more, the effect on an increase in the density was saturated.
A high-density iron-based compact with a complex shape, which cannot be realized by high-velocity compaction of a powder, can be more stably manufactured at a lower compaction energy level with productivity higher than that in sinter cold forging.
An iron-based sintered body having a high strength and a high density can be obtained by subjecting this high-density iron-based compact to re-sintering and/or heat treatment according to need.
| Number | Date | Country | Kind |
|---|---|---|---|
| 2004-340492 | Nov 2004 | JP | national |
| 2005-234145 | Aug 2005 | JP | national |
This is a §371 of International Application No. PCT/JP2005/022041, with an international filing date of Nov. 24, 2005 (WO 2006/057434 A1, published Jun. 1, 2006), which is based on Japanese Patent Application Nos. 2004-340492, filed Nov. 25, 2004, and 2005-234145, filed Aug. 12, 2004.
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/JP2005/022041 | 11/24/2005 | WO | 00 | 12/3/2007 |
