SINTER-HARDENING POWDER AND THEIR SINTERED COMPACTS
A sinter-hardening raw powder can yield a press-and-sinter compact with high hardness. The raw powder for sintering includes Fe as its primary component and also includes 0.3-0.8 wt % C, 5.0-12.0 wt % Ni, 1.0-5.0 wt % Cr, and 0.1-2.0 wt % Mo, wherein the mean particle size of the raw powder for sintering is between 50 and 100 μm. The sintered and tempered compact, without any quenching treatment, has high hardness.
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This application is a continuation-in-part of a prior application Ser. No. 11/308,824, filed on May 11, 2006. The prior application is a continuation-in-part application of application Ser. No. 10/907,155, filed on Mar. 23, 2005, now abandoned, which claims the priority benefit of Taiwan application serial no. 93116634, filed on Jun. 10, 2004 and Taiwan application serial no. 93126297, filed on Sep. 1, 2004. The entirety of each of the above-mentioned patent applications is hereby incorporated by reference herein and made a part of this specification.
BACKGROUND OF THE INVENTION1. Field of the Invention
The present invention generally relates to sinter-hardening powders, in particular, to compositions useful for yielding high hardness in sintered parts by the powder metallurgy process.
2. Description of Related Art
To attain high hardness, most sintered components require a heat treatment such as quenching in order to form martensite. However, while the quenching is performed, several problems such as deformation, size inconsistency, or cracking may occur due to the volume expansion when the part transforms from austenite to martensite, or due to the thermal stress caused by the fast cooling of the quenching treatment. In addition, performing heat treatment on the components incurs additional costs. Therefore, sinter-hardening powders have been developed in the press-and-sinter process by adding carbon and high hardenability alloying elements such as molybdenum, nickel, manganese, and chromium to iron powders, pressing out the green compact and sintering the green compact into a finished product with a hardness above HRC30. However, the minimum cooling rate required is usually 30° C./min in order to generate martensite. An example of the sinter-hardening powder is disclosed in U.S. Pat. No. 5,682,588, which relates to a powder mixture created by adding 1 to 3 wt % of Ni, 1 to 2 wt % of Cu, and graphite powders to a prealloyed powder with a composition consisting of 3 to 5 wt % Ni, 0.4 to 0.7 wt % Mo, and the remainder Fe. The claimed powders are compacted, sintered between 1130-1230° C., and then cooled at a rate of 5-20° C./min to attain the sinter-hardening effect. This may improve the process by lowering the minimum cooling rate of 30° C./min, as described in the previously mentioned processes. However, the strength and other properties, in particular the ductility and toughness, of these sinter-hardening powders are still unsatisfactory.
There are also several material standards for sinter-hardened alloys set forth by the Metal Powder Industries Federation (MPIF), examples being FLNC-4408 (1.0-3.0 wt % Ni, 0.65-0.95 wt % Mo, 1.0-3.0 wt % Cu, 0.05-0.3 wt % Mn, 0.6-0.9 wt % C, and the remaining portion is Fe) and FLC2-4808 (1.2-1.6 wt % Ni, 1.1-1.4 wt % Mo, 1.0-3.0 wt % Cu, 0.3-0.5 wt % Mn, 0.6-0.9 wt % C, and the remaining portion is Fe) low-alloy steel powders. After sintering and tempering, the sinter-hardening FLC2-4808 steel can reach a tensile strength of 1070 MPa at a density of 7.2 g/cm3, and a hardness of HRC40, but the ductility is less than 1.0%. Another standard of sinter-hardenable prealloyed steel is the FL-5305 (Fe-3Cr-0.5Mo-0.5C), which can attain a tensile strength of 1100 MPa and a hardness of HRC 33 at a density of 7.3 g/cm3, but the ductility is less than 1.0%. Although these alloys are of the sinter-hardening type, the mechanical properties are not satisfactory, and the required cooling rate is still very fast, at a minimum of 30° C./min. Thus, an additional high cooling rate system has to be installed in the sintering furnace. In addition, these high cooling rates, while slower than those of quenching in oil or water, are still fast enough to cause problems such as deformation, inconsistency of the dimensions, and quenching cracks. Thus, a new sinter-hardening alloy that can yield high hardness, high strength, and good ductility and allows the use of a low cooling rate is still very much desired.
There are some known powder metal alloys with compositions similar to that of this invention. To attain improved tensile strength without undue loss of ductility, Marshall et al. disclosed in GB Patent 1009425 a metal powder mixture from which sintered steel articles may be made with a composition comprised of 1-4.9 wt % Ni, 0.1-2 wt % Mn, 0.1-5 wt % Mo, 0.1-1 wt % C, and the remaining being iron plus the usual impurities. It further discloses that up to 5 wt % of the iron can be replaced by one or more other elements which do not adversely affect the tensile strength and ductility of the sintered parts. The list of the elements and the upper limits include 1 wt % Al, 0.3 wt % B, 5 wt % Cr, 5 wt % Cu, 1 wt % Mg, 4 wt % Nb and/or Ta, 0.3 wt % P, 1 wt % Si, 2 wt % Ti, 4 wt % W, 0.3 wt % V, 0.6 wt % Zr, 0.6 wt % Se, and 0.5 wt % Pb.
The other alloy similar to the powder disclosed in this invention is described in U.S. Pat. No. 7,163,569, which presents a prealloyed powder for sintering that can yield a sintered compact with high density, precise dimensions, and uniform properties. The mean particle size is 8 microns or less. The powder can be used directly as the raw powder for the metal injection molding process. It can also be granulated, such as by spray drying, to improve the flow properties of the fine powder, and then compacted, debinded, and sintered.
The above background emphasizes that a powder metal product with high hardness requires a combination of carefully selected base powder, particle size, form of the alloying powder, i.e., elemental powder or ferro-alloy powder, and a well designed combination of high hardenability alloying elements. Such a combination is not easy to design or select even by people who are familiar with the skill and practice of the powder metallurgy process. This is why very few conventional sinter-hardening alloys can obtain high hardness. Novel technology to form the sinter-hardening raw powder is still under development.
SUMMARY OF THE INVENTIONIn view of the background described above, the present invention provides a raw powder mixture by which sintered compacts with high hardness can be attained immediately after sintering without the need for quenching. Only tempering is required after sintering.
In an aspect of the present invention, a sinter-hardening raw powder includes iron (Fe) as its primary composition and also includes 0.3 to 0.8 wt % of Carbon (C), 5.0 to 12.0 wt % Nickel (Ni), 1.0 to 5.0 wt % chromium (Cr), 0.1 to 2.0 wt % molybdenum (Mo), and usual impurities, wherein the mean particle size of the raw powder for sintering is between 50 and 100 μm.
It is to be understood that the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention, as claimed.
The accompanying drawings are included to provide a further understanding of the invention, and they are incorporated in and constitute a part of this specification. The drawings illustrate embodiments of the invention and, together with the description, serve to explain the principles of the invention.
To produce economically sinter-hardened compacts with high strength and high hardness, elements that have high hardenability must be added in the optimum amount and ratio. Carbon, manganese, silicon, chromium, molybdenum, and nickel are the most widely used elements used in the wrought alloy steels. However, theses elements are not all suitable for press-and-sinter parts unless some preventive measures are taken. For instance, chromium is quite reactive and is prone to react with the residual oxygen or water vapor in the sintering atmosphere and form chromium oxides. Manganese is an even more reactive element and will react easily with the sintering atmosphere to form manganese oxides. Moreover, manganese and chromium have a very high vapor pressure and thus, when green compacts are sintered in vacuum, the amounts of manganese and chromium retained in the sintered parts are usually significantly reduced unless argon is backfilled to a certain level.
Silicon is another reactive element and can form oxide more easily than Cr and Mn during sintering. In spite of these problems, these alloying elements could still provide good hardening effect if these elements are added in the form of ferro-alloy powder and use a well-controlled sintering atmosphere, such as with a low dew point, so that oxide formation can be prevented. One of the methods used in this invention is to use ferro-alloy powders, such as Fe—Cr, Fe—Mn, Fe—Si, and their derivatives, such as Fe—Cr—Mo and Fe—Cr—Mo—Mn. These ferro-alloy powders can be used as the base powder or used as the alloying powder for the base iron powder.
The active Cr, Si, and Mn are prealloyed in the powder so that their activities are reduced as compared to using pure elements. Thus, these active elements can be solutioned in the iron matrix in a regular sintering furnace without forming oxides and provide the hardenability needed. In addition, the sintering atmosphere should have a high reduction power, such as with high hydrogen content and low water vapor content. When a vacuum furnace is used, the degree of vacuum must be high enough to prevent the formation of the oxides. Meanwhile, some backfilled inert gases, such as argon, should be added to reduce the loss of the elements, such as Cr and Mn, which have high vapor pressures.
The sinter-hardening powder disclosed in this invention comprises 0.3-0.8 wt % of C, 5.0-12.0 wt % Ni, 1.0-5.0 wt % Cr, 0.1-2.0 wt % Mo, wherein the mean particle size of the raw powder for sintering is between 50 and 100 μm. The powder may further contain at least one element from among 2 wt % or less Cu, 1 wt % or less Mn, 1 wt % or less Si, and 1 wt % or less of P, wherein up to 3 wt % of the iron content can be replaced by one or more of these elements. The compact produced using this powder can be sinter-hardened to a hardness of HRC30 or higher, even with a slow cooling rate of 6° C./min.
Ni is an element that could yield high hardenability and could also give high toughness and elongation for sintered compacts. Moreover, Ni is a very effective additive in improving the sintered density and toughness of steel compacts compared to most other alloying elements, such as Cu, Mo, Cr, Mn, and Si. Thus, it is advantageous to add Ni in the sinter-hardening powder. In the present invention, the Ni content preferred is between 5 wt % and 12.0 wt %; since the sinter-hardening characteristics are not obvious when the Ni content is lower than 5 wt % or when the Ni content is higher than 12 wt %, the sinter-hardening effect diminishes because too much austenite will be retained after sintering. In another aspect, when the Ni content is greater than 12 wt %, the benefits obtained are limited, and the cost of the sintered parts increases. Since Ni-containing prealloyed powder yield poor compressibility, it is preferred to add most Ni, if not all, to the base powder in the form of elemental Ni powder.
It has been found that the distribution of Ni in sintered steels is usually not uniform due to its slow diffusion rate in iron. Another reason is that carbon has a fast diffusion rate into iron and can thus quickly penetrate into the core of the iron powder. When Ni diffuses toward the iron powder core, the carbon tends to repel Ni, since Ni increases the chemical potential of carbon. Thus, it is difficult for Ni to be homogenized in the Fe matrix, and as a result, its sinter-hardening benefits are lost. The Ni-rich areas thus formed are low in strength and hardness and become vulnerable sites under stresses. However, this invention has found that when Cr is present, the repelling effect between Ni and C is alleviated and the distribution of Ni becomes more uniform as has been demonstrated using X-ray mapping. With a more uniform Ni distribution and the elimination of the soft Ni-rich areas, the overall hardness of the sintered compacts increases.
The chromium content used in this invention is between 1 and 5 wt %. With less than 1 wt % Cr, the hardenability effect is insignificant. When the Cr content is over 5 wt %, the amount of martensite will be reduced. The Cr may be included in the base powder such as Fe—Cr prealloyed powder and its derivatives, such as Fe—Cr—Mo powder. The Cr may also be added to the base iron powder in the form of Fe—Cr powder and its derivatives. As described above, this will reduce the activity of Cr and ensure its effectiveness in improving the alloying of Ni into the iron matrix during sintering without forming chromium oxides. The amount of Cr required, however, depends on the particle size of the iron powder and the Ni content used.
Table 1 and
Table 1 shows the effects of Fe particle size and Ni content on the optimum Cr content of Fe-8Ni-0.8Mo-0.5C and Fe-4Ni-0.5Mo-0.5C.
When cast or wrought bulk alloys or prealloyed powders are designed, the optimum contents of the alloying elements will be lower than in mixed powders since homogenization of the various alloying elements have been achieved during melting. Another reason of the lower alloying content of the cast and wrought alloys is that the raw material must be soft enough so that plastic deformation and other secondary operations, such as machining, can be performed to meet the dimensional specifications of the part. The optimum mechanical properties are then attained with quenching and tempering treatment. In contrast, the powder metallurgy process is a net shaping process. Thus, it will be an advantage if the part, which has attained the dimensions after sintering, can also be sinter-hardened without further quenching treatment. This eliminates the defects that are usually developed during quenching. The elimination of the quenching process also makes the sinter-hardening materials more economical and more competitive. Thus, the optimum composition designed in this invention for the sinter-hardening powder for press-and-sinter products is carefully designed and is different from the AISI and MPIF standard alloy steels. The narrow composition range disclosed in this invention is effective in producing new and unexpected results.
Molybdenum is another effective alloying element and its amount used in this invention is between 0.1 and 2 wt %. When the Mo content is over 2 wt %, the hardness will be reduced due to the insufficient amount of martensite produced in the sintered parts. Moreover, molybdenum is also an expensive element. With too much Mo addition, the cost of the sintered products will become too high and thus make it less competitive compared to those counterparts made by other manufacturing processes. The molybdenum can be added in the form of elemental Mo powder, ferro-molybdenum powder, or a complex ferro-molybdenum powder that contains other alloying elements, or it can be included in the base powder in the prealloyed form.
Manganese has a very high hardenability and only a small amount is needed. The amount of Mn used in this invention is less than 1.0 wt %. When the amount of Mn is over 1.0 wt %, the ferrite matrix is strengthened and the compressibility is significantly reduced. To decrease its activity and prevent oxide formation during sintering, manganese is added in the form of ferro-manganese powder, which may further contain other alloying elements, or it can be included in the base powder in the prealloyed form.
Silicon also has a high hardenability and the amount used in this invention is less than 1 wt %. When Si content is over 1 wt %, the sintered compact becomes too brittle and is not suitable for structural parts. To reduce the activity of Si and prevent oxide formation, the silicon may be added in the form of ferro-silicon powder, which may further contain other alloying elements, or it can be included in the base powder in the prealloyed form.
The most economic and effective hardening element is carbon. Carbon can be supplied from graphite powders, such as in most powder metallurgy parts. It can also be supplied from carbon black powders.
The above alloying elements may be added in the elemental powder form, or in the ferro-alloy powder form. Alternatively, two or more of the alloying elements may be added to the base iron powder in the form of prealloyed or master alloy powder.
To obtain high hardness, the selection of the base powder is critical. For example, high compressibility powder, such as elemental iron powder or soft ferro-alloy powder, such as Fe—Mo, Fe—Cr, or Fe—Cr—Mo powder, is preferred so that high green density and thus high sintered density compact can be attained. In contrast, prealloyed powders that contain carbon and/or nickel are difficult to compact to high densities. The particle size is also critical. Fine powders can provide high driving force for sintering due to the large surface area. However, the flow properties are poor and the apparent density is low, which cause problems in die filling during pressing. In contrast, these problems are not apparent for coarse powders. In addition, press-and-sinter parts usually require accurate dimensions after sintering. This means that the dimensional change should be kept minimal during sintering and thus fine powders are not preferred. The particle size of the base powder used in the present invention is between 50 and 100 μm.
With a selected combination of the base powder and composition of the alloying elements, this invention provides a sinter-hardening powder that yields sinter-hardened compact with high hardness without the need for quenching.
Example 1Following the compositions of Sample 1 and 2 listed in Table 2, the water atomized iron powder is mixed with fine elemental Ni, Mo, and graphite powders, 0.8% zinc stearate, which serves as a lubricant, and fine Fe—Cr prealloyed powder. The mixture is compacted, debound at 550° C. for 15 minutes, and then sintered at 1200-1250° C. Sample No. 3, 4, and 5 use Fe-3Cr-0.5Mo as the base powder to which lubricant, elemental Ni powder, and graphite powder are added. Sample No. 6 is prepared by mixing lubricant, elemental Ni and graphite powders and Fe-1.5Cr-0.2Mo prealloyed powder. To prepare Sample No. 7, 8, and 9, 50% Fe-3Cr-0.5Mo powder and 50% Fe-1.5Cr-0.2Mo powder are mixed and then used as the base powder. Different amounts of Ni and graphite powders are then added to the base powder. All these samples in Table 2 are sintered to a density of about 7.2 g/cm3. The mean particle sizes of the base powders are between 50 and 100 μm. Two cooling rates are applied, 6 and 48° C./minute, which are measured between 600° C. and 300° C. After sintering, the specimen is tempered at 180° C. for 2 hours. Sample No. 1 to 6, which are cooled at a rate of 6° C./minute, and Sample No. 7, 8, and 9, which are cooled at a rate of 48° C./minute, all attained a hardness greater than HRC30. Another advantage of the sinter-hardened compact with the composition of this invention is that reasonable ductility is attained. For example, the elongations of the samples in Table 2 are all greater than 1%. Sample 9, which contains 12% Ni, can even yield about 3.0% elongation. This ductility makes these sinter-hardened compacts suitable for structural parts. In comparison, most current sinter-hardened specimens have an elongation less than 1%.
Table 2 shows the hardness of some sintered-and-tempered compacts with the compositions (wt %) disclosed in this invention.
Sample No. 10, 11, and 12 use mixed Fe-3Cr-0.5Mo and Fe-1.5Cr-0.2Mo powder as the base powder. Different amounts of Ni and graphite powder are then added to the base powder. As shown in Table 3, Sample No. 10 has only 2% Ni and thus a low sintered hardness, as compared to Sample No. 7, which has a similar composition of C, Cr, and Mo but with 6% Ni. This example indicates the importance of the Ni in the composition disclosed in this invention. Sample No. 11 and 12 also show low hardness due to their low carbon contents. Sample No. 13 to 18 use Fe—Cr—Mo prealloyed powder as the base powder, which is mixed with graphite powder. For Sample 15 and 16, additional Ni is added. When sintered to a density of 7.3 g/cm3, the hardness of samples 13 to 16 is lower than HRC30 when cooled at a slow rate of 6° C./min. When cooled at 48° C./min, the Fe-1.5Cr-0.2Mo, Sample No. 17 does not sinter-harden. However, sample No. 18 can attain a hardness of HRC33 due to its higher amounts of Cr and Mo. For comparison, sample 9 in Table 2, which is cooled at the same fast cooling rate, attains a hardness of HRC38 even it has low amounts of C, Cr, and Mo. This hardness is much higher than that of sample 18. These examples show the importance of including Ni in the sinter-hardening powder, as demonstrated by the Examples in Table 2, which contain high amount of Ni. However, there is a need for a good combination of all alloying elements. For example, Samples No. 15 and 16 have high Ni content but with low C content. The hardness of these two specimens is still lower than HRC30. This demonstrates the importance of the combination of C and Ni.
Table 3 shows the hardness of some sintered-and-tempered comparative examples.
In conclusion of the above description, the sinter-hardening composition of the present invention can attain easily a hardness greater than HRC30 for sintered compacts that are furnace-cooled at a rate of about 6° C./min. When cooled at a fast cooling rate, the hardness further increases. In contrast, the current sinter-hardening alloys need to use a cooling rate of a minimum of 30° C./min to achieve the hardness of HRC30 and greater. Since a low cooling rate is acceptable for the compositions of this invention, the sintered body provides advantages in the areas of better dimensional control, less defects, and lower costs.
This invention has solved the above-mentioned problems as a result of the carefully selected combination of the base powder and the optimum amounts and types of the alloying elements. The powder mixture uses elemental iron powder, such as atomized or reduced iron powder, or the ferro-alloy powder, which has high compressibility, as the base powder. The mean particle size is between 50 and 100 μm. The alloying elements consist of 0.3-0.8 wt % of C, 5.0-12.0 wt % Ni, 1.0-5.0 wt % Cr, 0.1-2.0 wt % of Mo. The above composition can further contain at least one other minor strengthening elements at the amount of 5.0 wt % or less. The strengthening elements can be selected from the group consisting of Cu, Mn, Si, and P. Furthermore, the present invention provides a sintered compact that can be sinter-hardened at a normal furnace cooling rate of 3-30° C./min in the sintering furnace without the need of using fast cooling, which is required for other sinter-hardening powders. The sintered compact does not require any quenching treatment. Only low temperature tempering is needed to obtain optimum mechanical properties. A higher production yield is attained due to the elimination of defects, such as cracks and distortions, which occur during fast cooling or quenching.
It will be apparent to those skilled in the art that various modifications and variations can be made to the structure of the present invention without departing from the scope or spirit of the invention. In view of the foregoing, it is intended that the present invention cover modifications and variations of this invention provided they fall within the scope of the following claims and their equivalents.
Claims
1. A raw sinter-hardening powder, comprising iron (Fe) as a primary composition and further comprising 0.3 to 0.8 wt % of Carbon (C), 5.0 to 12.0 wt % Nickel (Ni), 1.0 to 5.0 wt % chromium (Cr), and 0.1 to 2.0 wt % molybdenum (Mo), wherein a mean particle size of the raw powder for sintering is between 50 and 100 μm.
2. The raw sinter-hardening powder according to claim 1, further comprising at least one element from among 2 wt % or less Cu, 1 wt % or less Mn, 1 wt % or less Si, and 1 wt % or less of P.
3. The raw sinter-hardening powder according to claim 2, containing 0.5-1.5 wt % of Copper (Cu), 0.1-0.8 wt % of Manganese (Mn), and 0.1-0.8 wt % of Silicon (Si).
4. The raw sinter-hardening powder according to claim 1, containing 0.4-0.7 wt % of Carbon (C), 6.0-10.0 wt % Nickel (Ni), 1.5-4 wt % chromium (Cr), 0.2-1.5 wt % of molybdenum (Mo).
5. The raw sinter-hardening powder as cited in claim 1, wherein the raw powder is elemental powder or ferroalloy powder or a mixture of the two.
6. A sintered compact comprising compositions of the sinter-hardening powder as cited in claim 1.
Type: Application
Filed: Feb 5, 2009
Publication Date: Jun 4, 2009
Applicant: TAIWAN POWDER TECHNOLOGIES CO., LTD. (Taoyuan County)
Inventors: Kuen-Shyang Hwang (Taoyuan County), Yung-Chung Lu (Taoyuan County)
Application Number: 12/366,627
International Classification: C22C 38/44 (20060101);
