JP2016516135A - Powder metal composition for wear and temperature resistant applications and method for producing the same - Google Patents

Abstract

Powder metal compositions for high wear and high temperature applications include 3.0-7.0 wt% carbon, 10.0-25.0 wt% chromium, 1.0-5.0 wt% Tungsten, 3.5-7.0 wt% vanadium, 1.0-5.0 wt% molybdenum, 0.5 wt% or less oxygen, and at least 40.0 wt% iron Made by spraying a molten iron-based alloy. High carbon content reduces the solubility of oxygen in the melt, and therefore lowers the oxygen content to a level below which it would oxidize carbide-forming elements during spraying. The powder metal composition comprises metal carbide in an amount of at least 15% by volume. As the amount of carbon increases, the microhardness of the powder metal composition increases, typically about 800-1500 Hv50.

Description

  This application claims priority from US Application No. 13 / 837,549, filed March 15, 2013, the contents of US Application No. 13 / 837,549 are hereby incorporated by reference in their entirety. Incorporated into the book.

TECHNICAL FIELD The present invention relates generally to powder metal compositions and methods for producing such powder metal compositions from iron-based alloys.

BACKGROUND OF THE INVENTION High hardness pre-alloyed iron-based powders such as tool steel grade powders are used alone or mixed with other powder metal compositions in powder metallurgy manufacturing of various products. It can be either. Tool steel, _M 6 _C bound to the carbon, _MC, to form various carbides such as _{M 3 C, M 7 C 3} , M 23 C 6, including chromium, vanadium, elements such as molybdenum and tungsten. These carbides are very hard and contribute to the wear resistance of the tool steel.

  The use of powder metal processing makes it possible to form particles from a fully alloyed molten metal, so that each particle has a fully alloyed chemical composition of a molten batch of metal. The powder metal process also allows the molten metal to rapidly solidify into small particles, eliminating macro segregation normally associated with ingot casting. In the case of high alloy steels such as tool steel, a uniform distribution of carbides can occur within each particle, resulting in a very hard and wear resistant powder material.

  It is common to produce powders by spraying. For tool steels and other alloys containing high levels of chromium and / or vanadium that are highly oxidizable, gas atomization is often used and a stream of molten alloy is injected through a nozzle into a protective chamber, resulting in a stream of molten metal Is affected by the flow of a high-pressure inert gas such as nitrogen that disperses the liquid in the droplets. The inert gas protects the alloying elements from oxidation during spraying, and the gas sprayed powder has a characteristic smooth and rounded shape.

  Water spray is also commonly used to produce powder metal. Water spraying is similar to gas spraying except that high pressure water is used instead of nitrogen gas as the spray fluid. Water can be a more effective quenching medium and therefore will have a faster solidification rate compared to conventional gas sprays. Water sprayed particles typically have a more irregular shape, which may be more desirable during subsequent powder compaction to achieve greater green strength of the powder metal compact . However, in the case of tool steels and other steels containing high levels of chromium and / or vanadium, the use of water as a spray fluid causes the alloy elements to oxidize during spraying and bind these alloy elements to form carbides. Making it unavailable for reaction with carbon to form. As a result, if water spray is utilized, it may be necessary to perform another redox and / or annealing cycle following the water spray, and the powder is heated and the reducing agent such as powdered graphite or carbon or It is held at an elevated temperature for a very long period (about several hours or days) in the presence of other sources of other reducing agents or by another reduction process. Graphite carbon combines with oxygen to free the alloying elements so that they can be used in carbide formation during subsequent sintering and tempering steps after solidification of the powder into a green compact Will. It will be appreciated that the need for additional annealing / reduction steps and the addition of graphite powder increases the cost and complexity of forming high alloy powders by a water spray process.

SUMMARY OF THE INVENTION One aspect of the present invention provides a method for producing a powder metal composition, the method comprising providing a molten iron-based alloy, wherein the molten iron-based alloy is 3.0-7. 0.0 wt% carbon, 10.0-25.0 wt% chromium, 1.0-5.0 wt% tungsten, 3.5-7.0 wt% vanadium, 1.0 -5.0 wt% molybdenum, 0.5 wt% or less oxygen, and at least 40.0 wt% iron, the method further providing spray droplets of the iron-based alloy Spraying the molten iron-based alloy.

  Another aspect of the present invention provides a method of manufacturing a sintered article, the method comprising providing a molten iron-based alloy, wherein the molten iron-based alloy is 3.0 to 7.0 weight. % Carbon, 10.0-25.0% chromium, 1.0-5.0% tungsten, 3.5-7.0% vanadium, and 1.0-5. The method further comprises 0 wt.% Molybdenum, 0.5 wt.% Or less oxygen, and at least 40.0 wt.% Iron, and the method further includes spraying a small amount of the iron-based alloy, referred to as a powder metal composition. Spraying the molten iron-based alloy to provide droplets. The method then includes mixing the powder metal composition with another powder metal, compressing the mixture to form a preform, and sintering the preform.

  Another aspect of the present invention provides a powder metal composition, wherein the powder metal composition is 3.0 to 7.0 wt% carbon, and 10. 0-25.0 wt% chromium, 1.0-5.0 wt% tungsten, 3.5-7.0 wt% vanadium, 1.0-5.0 wt% molybdenum, 0.5 wt% or less oxygen and at least 40.0 wt% iron.

  Another aspect of the invention provides a sintered powder metal composition, wherein the sintered powder metal composition is 3.0 based on the total weight of the sintered powder metal composition. -7.0 wt% carbon, 10.0-25.0 wt% chromium, 1.0-5.0 wt% tungsten, 3.5-7.0 wt% vanadium, 0.0 to 5.0 weight percent molybdenum, 0.5 weight percent oxygen or less, and at least 40.0 weight percent iron.

  These and other features and advantages of the present invention will become apparent to those skilled in the art from the detailed description and accompanying drawings that schematically illustrate the process used to produce the powder.

1 is a schematic diagram of a process for producing a powder metal composition. FIG. It is a graph which shows hardness with respect to carbon content.

DETAILED DESCRIPTION A process for producing an iron-based alloy high carbon powder, also referred to as a pre-sintered powder metal composition, is schematically illustrated in FIG. The powder metal composition is inexpensively manufactured and has a high hardness, which exceeds the hardness commonly achieved with equivalent alloy compositions having lower carbon levels by gas or conventional water spray processes. it is conceivable that.

  The process first involves preparing a batch 10 of an iron-based alloy. The iron-based alloy is fully alloyed with carbide forming elements including chromium (Cr), molybdenum (Mo), tungsten (W) and vanadium (V). The iron-based alloy is melted and then sent to the atomizer 12. In the embodiment of FIG. 1, the sprayer is a water sprayer 12, but may alternatively be a gas sprayer. Using gas spraying can improve some properties over water spraying, for example, better flow, improved apparent density, and lower oxygen content. Gas atomization also provides droplets having a generally circular shape.

  In the water spray step of FIG. 1, the stream of molten batch 10 is affected by a flow of high pressure water that rapidly solidifies by dispersing the molten stream into irregularly shaped, fully alloyed metal droplets or particles. . The outer surface of the metal particles can be oxidized by exposure to water and an unprotected atmosphere. The spray particles are sent to the grinder 16 via the dryer 14 where the particles are mechanically ground or crushed. A ball mill or other mechanical reduction device may be utilized. If an oxide film is formed on the spray droplets, the mechanical pulverization of the particles crushes the outer oxide film and separates it from the particles, and then the pulverized particles are separated from the oxide. A spray powder metal composition 18 and oxide particles 20 are produced. Powdered metal particles and / or oxide particles can also be crushed and consequently reduced in size. The powder metal composition 18 can be further categorized by size, shape, and other properties typically associated with powder metal.

  The batch 10 of iron-based alloy provided for spraying has a high carbon content. In one embodiment, the iron-based alloy is at least 3.0 wt% carbon, or 3.0-7.0 wt% carbon, or 3.5- wt%, based on the total weight of the iron-based alloy. It contains 4.0 wt% carbon, preferably about 3.8 wt% carbon. The amount of carbon present in the iron-based alloy depends on the amount and composition of the carbide-forming elements. However, the carbon is preferably present in an amount sufficient to form an amount of metal carbide greater than 15% by volume based on the total volume of the powder metal composition 18 during the spraying process.

  Another reason for adding excess carbon to the iron-based alloy is to protect the iron-based alloy from oxidation during the melting and spraying steps. As the amount of carbon increases, the solubility of oxygen in the molten iron-based alloy decreases. The amount of carbon also ensures that the matrix to which the carbide deposits belong is essentially one of austenite and / or martensite, especially when the level of Cr and / or V is high. .

  A “low” oxygen content is an amount of 0.5% by weight or less based on the total weight of the iron-based alloy. In one embodiment, the oxygen content is not more than 0.3% by weight, for example 0.2% by weight. Decreasing the oxygen level in the melt can cause carbide-forming alloy elements such as chromium (Cr), molybdenum (Mo), tungsten (W) and vanadium (V) to oxidize during the melting or spraying step, resulting in It has the merit of protecting from free bonding with carbon to form carbides.

  Also, the iron-based alloys chromium (Cr), molybdenum (Mo), tungsten (W), and vanadium (V) are metal carbides in an amount of at least 15.0 vol% based on the total volume of the powder metal composition 18. Is present in an amount sufficient to form For cost reasons, there is also a need to increase the amount of some of the carbide forming alloy elements over other elements. Thus, Mo is an excellent option for forming very hard carbides with high carbide density, but is currently very expensive compared to, for example, Cr. To develop low-cost tool grade quality steel that is at least equivalent in performance to the more expensive conventional M2 grade tool steel, iron-based alloys have a relatively high level of Cr, a lower level of Mo and increased amounts of C can be included. The amount of W and V can vary depending on the desired amount of carbide to be formed. By increasing the amount of carbide-forming alloying elements in the iron-based alloy, the amount of carbide formed in the matrix during the spraying step can also be increased. Also, Cr, Mo, W and V are preferably present in an amount sufficient to provide very good wear resistance at low cost compared to other powder metal compositions.

  In one embodiment, the iron-based alloy comprises 10.0-25.0 wt% chromium, preferably 11.0-15.0 wt% chromium, most preferably 13.0 wt% chromium, 1.0-5.0 wt% tungsten, preferably 1.5-3.5 wt% tungsten, most preferably 2.5 wt% tungsten and 3.5-7.0 wt% vanadium, Preferably 4.0-6.5 wt% vanadium, most preferably 6.0 wt% vanadium and 1.0-5.0 wt% molybdenum, preferably 1.0-3.0 wt%. Molybdenum, most preferably 1.5% by weight molybdenum.

  The iron-based alloy may optionally contain other elements, which may contribute to improved wear resistance or other material properties. For example, iron-based alloys include cobalt (Co), niobium (Nb), titanium (Ti), manganese (Mn), sulfur (S), silicon (Si), phosphorus (P), zirconium (Zr) and tantalum ( Ta) may be included. In one embodiment, the iron-based alloy comprises 4.0-15.0 wt% cobalt, up to 7.0 wt% niobium, up to 7.0 wt% titanium, up to 2.0 wt% manganese. At least one of up to 1.15% sulfur, up to 2.0% silicon, up to 2.0% phosphorus, up to 2.0% zirconium, and up to 2.0% tantalum. Contains one. In one embodiment, the iron-based alloy includes pre-alloyed sulfur to form a sulfide or sulfur-containing compound in the powder. Sulfides such as MnS and CrS are known to improve machinability and may be beneficial for wear resistance.

  The balance of the iron-based alloy provided for spraying is iron. In one embodiment, the iron-based alloy comprises at least 40.0% iron, or 50.0-81.5% iron, preferably 70.0-80.0% iron. Yes.

  When the spraying process is a water spraying process, the molten iron-based alloy stream can be rapidly solidified by dispersing the molten iron-based alloy stream into irregularly shaped fully alloyed metal droplets. Affected by water flow. Preferably, each atomized droplet comprises 3.0-7.0 wt% carbon, 10.0-25.0 wt% chromium, 1.0-5.0 wt% tungsten, 3 Having a complete iron-based alloy composition comprising 5-7.0 wt% vanadium, 1.0-5.0 wt% molybdenum, and at least 40.0 wt% iron. The outer surface of the droplet can be oxidized by exposure to water and an unprotected atmosphere. However, the high carbon content and low oxygen content greatly limit oxidation during the spraying step.

In the as-sprayed state, the carbide-forming alloy can exist in a supersaturated state (eg, vanadium) due to the rapid solidification that occurs during spraying. Combining the unoxidized supersaturated state of the alloying elements with a high carbon content, carbides (eg carbides rich in M ₈ C ₇ V) can then be used without the need for extending the previous annealing cycle (hours or days). It is possible to deposit very quickly (within a few minutes) during the sintering phase of this and grow well. Nanometer carbides present in the as-sprayed powder grow to micrometer size after sintering. However, the powder metal composition 18 may be annealed as needed, for example, at a temperature of about 900-1100 ° C., for example 1 to 48 hours, or according to other annealing cycles as needed. Annealing can be performed before or after grinding the powder metal composition 18. It is understood that annealing is optional and not essential.

  The spray droplets are then sent via a dryer to a pulverizer where the spray droplets are mechanically pulverized or crushed to remove the oxide film and then screened. Even if little or no oxide film is present, the mechanical grinding step can also be used to break up powder metal droplets to reduce their size. The hard, very fine nanostructure of the droplets improves the ease of grinding. A ball mill or other mechanical size reduction device may be utilized. If an outer oxide film is formed on the spray droplets during the spraying step, as typically occurs during water spraying, mechanical grinding breaks up the outer oxide film and separates it from the droplet bulk. Let The crushed droplets are separated from the oxide film to produce a powder metal composition 18 and oxide particles 20. However, the droplets of carbide forming elements are protected from oxidation by the high carbon content during the melting and spraying steps. The pre-sintered powder metal composition 18 can be further classified by size, shape, and other properties typically associated with powder metal. In certain cases, such as when gas spraying is used, the outer oxide film is minimal and can be part of the powder metal composition and is acceptable without removal, and therefore In some cases, grinding for the purpose of breaking at least the outer oxide layer is optional. However, milling can still be used to reduce the size of the powder metal composition.

  The composition in weight percent of the pre-sintered powder metal composition 18 is the same as that of the iron-based alloy described above prior to melting and spraying. The powder metal composition 18 is typically 10.0-25.0 wt% chromium, preferably 11.0-15.0 wt% chromium, most preferably 13.0 wt% chromium, 1.0-5.0 wt% tungsten, preferably 1.5-3.5 wt% tungsten, most preferably 2.5 wt% tungsten and 3.5-7.0 wt% vanadium, Preferably 4.0-6.5 wt% vanadium, most preferably 6.0 wt% vanadium and 1.0-5.0 wt% molybdenum, preferably 1.0-3.0 wt%. Molybdenum, most preferably 1.5% by weight molybdenum.

  Also, the powder metal composition 18 is at least 3.0 wt% carbon, or 3.0 to 7.0 wt% carbon, or 3.5 to 4.0 wt% carbon, preferably about 3.8. Contains carbon by weight. The carbon is present in an amount sufficient to provide a metal carbide in an amount of at least 15% by volume based on the total volume of the powder metal composition 18.

As the amount of carbon in the powder metal composition 18 increases, the hardness of the powder metal composition 18 also increases. This is because a greater amount of carbon forms a greater amount of carbide during the spraying step, resulting in increased hardness. The amount of carbon in the powder metal composition 18 is referred to as total carbon (C _tot ).

The powder metal composition 18 also includes a stoichiometric amount of carbon (C _stoich ), which represents the total amount of carbon bonded in the alloy carbide in an equilibrium state. The type and composition of carbide varies depending on the carbon content and the alloying element content.

The C _stoich required to form the desired amount of metal carbide during spraying depends on the amount of carbide forming elements present in the powder metal composition 18. The C _stoich for a particular composition is obtained by multiplying the amount of each carbide-forming element by a factor specific to each element. For a particular carbide forming element, the magnification is equal to the amount of carbon required to precipitate 1% by weight of that particular carbide forming element. The magnification varies based on the type of precipitate formed, the amount of carbon, and the amount of each alloying element. Also, the specific carbide magnification will vary with the amount of carbon and the amount of alloying elements.

For example, in order to form a precipitate of ₈ C ₇ (also referred to as M ₈ C ₇ (Cr _23.5 Fe _7.3 V _63.1 Mo _3.2 W _2.9 ) ₈ C ₇ into the powder metal composition 18, The magnification of the carbide forming element is calculated as follows. First, the atomic ratio of M ₈ C ₇ carbide is determined, Cr is 1.88 atoms, Fe is 0.58 atoms, V is 5.05 atoms, Mo is 0.26 atoms, W is 0.23 atoms, and C is 7 atoms. Next, the mass of each element per mole of M ₈ C ₇ carbide was determined, V = 257.15 grams, Cr = 97.76 grams, Fe = 32.62 grams, Mo = 24.56 grams, W = 42.65 grams, and C = 84.07 grams. Next, the weight ratio of each carbide forming element was determined, V = 47.73 wt%, Cr = 18.14 wt%, Fe = 6.05 wt%, Mo = 4.56 wt%, W = 7. 92% by weight and C = 15.60% by weight. The weight ratio indicates that 47.73 grams of V reacts with 15.60 grams of C, which means that 1 gram of V reacts with 0.33 grams of C. In order to deposit 1.0 wt% V in M ₈ C ₇ carbide, 0.33 wt% carbon is required, and therefore the V magnification is 0.33. A similar calculation is made to determine the magnification: Cr = 0.29, Mo = 0.06, and W = 0.03.

Next, the C _stoich in the powder metal composition 18 is determined by multiplying the amount of each carbide-forming element by the associated magnification and then adding each of those values. For example, if the powder metal composition 18 contains 4.0 wt% V, 13.0 wt% Cr, 1.5 wt% Mo and 2.5 wt% W, C _stoich = (4.0 × 0.33) + (13.0 × 0.29) + (1.5 × 0.06) + (2.5 × 0.03) = 5.26% by weight.

In addition, the powder metal composition 18 includes an amount of C _tot / C _stoich less than 1.1. Thus, if the powder metal composition 18 contains an upper limit of 7.0 wt% carbon, C _stoich will be equal to or less than 6.36 wt% carbon. I will. The C _tot / C _stoich ratio will vary depending on the amount of alloying elements at a fixed carbon content, but the C _tot / C _stoich ratio will remain below 1.1.

  Table 1 below shows examples of other carbide types found in the powder metal composition 18 as well as Cr, V, Mo and W magnifications for common carbide stoichiometry. However, the metal atoms in each carbide listed in the table may be partially replaced by other atoms that will affect the magnification.

  The metal carbide is formed during the spraying process and is in an amount of at least 15.0% by volume, but preferably 40.0-60.0% by volume, or 47.0-52.0% by volume, typically It is present in an amount of about 50.0% by volume. In one embodiment, the powder metal composition 18 includes a chromium rich carbide, a molybdenum rich carbide, a tungsten rich carbide, and a vanadium rich carbide in a total amount of about 50.0 volume percent. Yes.

Metal carbide has a nanoscale microstructure. In one embodiment, the metal carbide has a diameter of 1 to 400 nanometers. As suggested above, the carbides can be of various types including M ₈ C ₇ , M ₇ C ₃ , MC, M ₆ C, M ₂₃ C ₆ and M ₃ C, where M is Fe, Cr , V, Mo and / or W, and the like, and C is carbon. In one embodiment, the metal carbide is selected from the group consisting of M ₈ C ₇ , M ₇ C ₃ , M ₆ C, where M ₈ C ₇ is (V ₆₃ Fe ₃₇ ) ₈ C ₇ , and M ₇ C ₃ is selected from the group consisting of (Cr ₃₄ Fe ₆₆ ) ₇ C ₃ , Cr _3.5 Fe _3.5 C ₃ and Cr ₄ Fe ₃ C ₃ , and M ₆ C is Mo ₃ Fe ₃ C, Mo Selected from the group consisting of ₂ Fe ₄ C, W ₃ Fe ₃ C and W ₂ Fe ₄ C. Moreover, the fine structure of the powder metal composition 18 includes nanoscale austenite, and may include nanoscale martensite together with the nanoscale carbide.

  In one embodiment, the powder metal composition 18 consists essentially of 3.0-7.0 wt% carbon, 10.0-25.0 wt% chromium, 1.0-5.0 wt%. % Of tungsten, 3.5-7.0% by weight of vanadium, 1.0-5.0% by weight of molybdenum, 0.5% by weight or less of oxygen, the balance being made of iron, 5 It consists of incidental impurities in an amount of not more than 0.0% by weight, preferably not more than 2.0% by weight. However, the powder metal composition 18 may optionally contain other elements that can improve the material properties. In one embodiment, the powder metal composition includes at least one of cobalt, niobium, titanium, manganese, sulfur, silicon, phosphorus, zirconium and tantalum. For example, iron-based alloys include 4.0-15.0 wt% cobalt, up to 7.0 wt% niobium, up to 7.0 wt% titanium, up to 2.0 wt% manganese, 1.15. At least one of up to 2.0% by weight sulfur, up to 2.0% by weight silicon, up to 2.0% by weight phosphorus, up to 2.0% by weight zirconium, and up to 2.0% by weight tantalum You may go out. In one embodiment, the powder metal composition 18 includes pre-alloyed sulfur to form a sulfide or sulfur-containing compound within the powder. Sulfides such as MnS and CrS are known to improve machinability and may be beneficial for wear resistance.

  The balance of the powder metal composition 18 is iron. In one embodiment, the powder metal composition comprises at least 40.0 wt% iron, or 50.0-81.5 wt% iron, preferably 70.0-80.0 wt% iron. Yes. The powder metal composition has a melting point of about 1235 ° C. (2255 ° F.). The powder metal composition is completely melted at about 1235 ° C. (2255 ° F.), but may contain a slight liquid phase at temperatures as low as 1150 ° C. The melting point of the powder metal composition 18 will vary depending on the carbon content and the alloying element content.

  The powder metal composition 18 typically has a microhardness of 800-1500 Hv50. FIG. 2 shows the hardness of the powder metal composition without annealing compared to the carbon content, showing that the hardness increases as the amount of carbon increases. Table 2 below also shows that the powder metal composition was 13.0 wt% chromium, 2.5 wt% tungsten, 6.0 wt% vanadium, 1.5 wt% molybdenum, Hardness values for various amounts of carbon before and after annealing when containing 2 wt% oxygen, 70.0-80.0 wt% iron, and impurities up to 2.0 wt% Is shown. The data shows that the hardness of the powder metal composition increases with increasing carbon content, whether without annealing or after annealing. The carbon content is the amount before annealing. The carbon content can decrease slightly during annealing, for example to 0.15% by weight. However, the hardness value still increases as the amount of carbon increases.

  Hardness can essentially be maintained through sintering and tempering, but some of the excess carbon contained in the powder metal composition exceeds the amount required to grow carbides. When mixed with another iron powder composition having a lower carbon content, it can diffuse out of the hard powder metal composition. This excess carbon diffusion eliminates or eliminates the need for carbon-rich powder (eg, powdered graphite) additives that are sometimes added during compression and sintering to control microstructure and improve properties. Has the added benefit of reducing. Also, prealloyed carbon will reduce the tendency of graphite separation that can be caused by other graphite additives.

  The powder metal composition 18 is typically compressed and sintered to form articles that can be used in a variety of applications, particularly automotive parts. Prior to sintering, the powder metal composition 18 is preferably mixed with another powder metal or a mixture of other powder metals. Other powder metals may include unalloyed steel powder, low alloyed steel powder, or alloyed steel powder, and non-ferrous powder. Small amounts of other metals or components may also be present in the mixture.

  In one embodiment, the mixture comprises 10.0 to 40.0% by weight of the powder metal composition 18, preferably at least 20.0% by weight of the powder metal composition 18. The mixture also contains 30.0-90.0% by weight of other powdered metals, but typically contains about 60.0-80.0% by weight of other powdered metals. The mixture is then compressed and then sintered.

  Prior to sintering, the high carbon powder metal composition can be annealed. Annealing increases the compressibility of the powder metal composition 18, thereby allowing more powder metal composition 18 to be used in the mixture or allowing it to be pressed to a higher green density. Become. As the powder metal composition 18 is annealed, the amount of the powder metal composition 18 in the mixture can be increased to an amount greater than 40.0 wt%, for example up to 60.0 wt%. However, heat treatments such as long-term annealing of powder metal materials or oxide reduction, as is necessary with other powder metal compositions with low carbon levels to reduce oxygen and produce a suitable microstructure, It is not necessary before sintering.

  The sintered powder metal composition preferably includes metal carbide finely and uniformly dispersed throughout the powder metal composition. When 100% of the sintered composition is formed with the powder metal composition 18, the metal carbide is at least 15% by volume, preferably 40.0-60.0% by volume, or 47.0-52. Present in the sintered metal composition in an amount of 0.0% by volume, typically about 50.0% by volume. In one embodiment, the sintered powder metal composition has a total amount of about 50.0% by volume of chromium rich carbides, molybdenum rich carbides, tungsten rich carbides, and vanadium rich carbides. Is included. In another embodiment, the sintered powder metal composition is a carbide rich in vanadium in an amount of about 5.0 to 10.0 volume percent, based on the total volume of the sintered powder metal composition. And chromium-rich carbides in an amount of about 40.0-45.0% by volume.

The metal carbide of the sintered powder metal composition has a microscale microstructure. In one embodiment, vanadium-rich MC carbide has a diameter of about 1 μm, and chromium-rich M ₇ C ₃ carbide has a diameter of about 1-2 μm. Also, a fine carbide structure can exhibit a more uniform microstructure. The carbides can be of various types including M ₇ C ₃ , M ₈ C ₇ , MC, M ₄ C ₃ , M ₆ C, M ₂₃ C ₆ , M ₆ C ₅ and M ₃ C, where M is It is a metal atom and C is carbon. For example, carbides are high in Nb such as carbides containing a lot of V such as M ₈ C ₇ , M ₄ C ₃ , M ₆ C ₅ , MC _x (x varies from 0.75 to 0.97) Carbide containing, or carbides rich in Ti and Ta, such as MC. Further, the microstructure of the sintered powder metal composition includes microscale austenite, and may include microscale martensite together with the microscale carbide.

  Table 3 contains examples of powder metal compositions prepared according to the method of the invention and commercial grade M2 tool steel for comparison.

  The powder metal composition 18 was mixed with another powder metal and sintered. Based on the total weight of the mixture, the powder metal composition was present in an amount of 20.0% by weight and the other powder metal was present in an amount of 80.0% by weight. The powder metal composition 18 of the sintered mixture includes, based on the total volume of the powder metal composition 18, chromium-rich carbides in an amount of about 40-45% by volume and vanadium in an amount of about 7% by volume. It contained much carbide. The carbide rich in chromium had a size of about 1-2 μm, and the carbide rich in V had a size of about 1 μm. The matrix around the particles on which the carbides were deposited was essentially austenitic and had several regions of martensite and ferrite.

The microhardness of the mixture after sintering was in the range of about 800-1500 Hv ₅₀ . The powder metal composition of the present invention was mixed with primary low carbon low alloy powder composition at 15 and 30% by volume. The hardness of the high carbon particles remained above 1000 Hv ₅₀ after compression, sintering and tempering. Some of the carbon from the composition of the present invention diffused into the adjacent low carbon content primary powder matrix material of the mixture.

  By controlling the sintering and tempering cycles, the properties of the primary matrix containing varying amounts of ferrite, pearlite, bainite and / or martensite can be controlled. Additives such as MnS and / or other compounds may be added to the mixture to change the properties of the mixture, for example to improve machinability. The powder metal composition of the present invention remained essentially stable and the properties were not essentially constrained by the subsequent heat treatment utilized to improve the properties of the primary matrix material.

  Since the present invention has been described in connection with the presently preferred embodiments, the description is illustrative rather than limiting in nature. Modifications and variations to the disclosed embodiments will be apparent to those skilled in the art and may be within the scope of the invention. Accordingly, the scope of the invention should not be limited to these specific examples.

Claims (24)

A powder metal composition comprising:
3.0-7.0 wt% carbon, 10.0-25.0 wt% chromium, 1.0-5.0 wt% tungsten, based on the total weight of the powder metal composition A powder comprising 3.5 to 7.0 wt% vanadium, 1.0 to 5.0 wt% molybdenum, 0.5 wt% or less oxygen, and at least 40.0 wt% iron. Metal composition.   3.5-4.0 wt% carbon, 11.0-15.0 wt% chromium, 1.5-3.5 wt% tungsten, 4.0-6.5 wt% vanadium The powder metal composition of claim 1, comprising 1.0 to 3.0 wt% molybdenum, 0.3 wt% oxygen or less, and 50.0 to 81.5 wt% iron. .   3.8 wt% carbon, 13.0 wt% chromium, 2.5 wt% tungsten, 6.0 wt% vanadium, 1.5 wt% molybdenum, 0.2 wt% oxygen, 70. 3. A powder metal composition according to claim 2, consisting of 0 to 80.0% by weight of iron and impurities in an amount of up to 2.0% by weight.   The powder metal composition according to claim 1, comprising at least one of cobalt, niobium, titanium, manganese, sulfur, silicon, phosphorus, zirconium and tantalum.   The powder metal material according to claim 1, comprising metal carbide in an amount of at least 15.0% by volume, based on the total volume of the powder metal material. Said metal carbide _is selected from the group consisting of _{M 8 C 7, M 7 C} 3, M 6 C, M is at least one metal atom, C is a carbon of claim 5 powder Metal material. M ₈ C ₇ is (V ₆₃ Fe ₃₇ ) ₈ C ₇ and M ₇ C ₃ is (Cr ₃₄ Fe ₆₆ ) ₇ C ₃ , Cr _3.5 Fe _3.5 C ₃ and Cr ₄ Fe ₃ C. _The M ₆ C selected from the group consisting of ₃ and selected from the group consisting of Mo ₃ Fe ₃ C, Mo ₂ Fe ₄ C, W ₃ Fe ₃ C and W ₂ Fe ₄ C. Powder metal material.   A sintered material comprising a powder metal composition, the powder metal composition comprising 3.0 to 7.0 wt% carbon and 10.0 to 25 based on the total weight of the powder metal composition. 0.0 wt% chromium, 1.0-5.0 wt% tungsten, 3.5-7.0 wt% vanadium, 1.0-5.0 wt% molybdenum, 0.5 A sintered material comprising no more than wt% oxygen and at least 40.0 wt% iron.   The powder metal composition comprises 3.5 to 4.0 wt% carbon, 11.0 to 15.0 wt% chromium, 1.5 to 3.5 wt% tungsten, 4.0 to 4.0 wt%. 9. 6.5% by weight vanadium, 1.0-3.0% by weight molybdenum, 0.3% by weight oxygen or less, and 50.0-81.5% by weight iron. The sintered material described in 1.   The powder metal composition comprises 3.8 wt% carbon, 13.0 wt% chromium, 2.5 wt% tungsten, 6.0 wt% vanadium, 1.5 wt% molybdenum, 0.2 wt% 10. Sintered material according to claim 9, consisting of wt% oxygen, 70.0-80.0 wt% iron, and impurities up to 2.0 wt%.   9. A sintered material according to claim 8, comprising metal carbide in an amount of at least 15.0% by volume, based on the total volume of the powder metal composition. The firing of claim 11, wherein the metal carbide is selected from the group consisting of M ₈ C ₇ , M ₇ C ₃ , M ₆ C, M is at least one metal atom, and C is carbon. Bonding material. M ₈ C ₇ is (V ₆₃ Fe ₃₇ ) ₈ C ₇ and M ₇ C ₃ is (Cr ₃₄ Fe ₆₆ ) ₇ C ₃ , Cr _3.5 Fe _3.5 C ₃ and Cr ₄ Fe ₃ C. is selected from the group consisting of _{3, M 6} C _{is, Mo 3 Fe 3 C, Mo} 2 Fe 4 C, is selected from the group consisting of W 3 Fe ₃ C and _W 2 Fe ₄ C, according to claim 12 Sintered material.   The metal carbide includes a vanadium-rich carbide in an amount of about 5.0-10.0% by volume, and an amount of about 40.0-45.0% by volume, based on the total volume of the powder metal composition. The sintered material according to claim 11, comprising a carbide rich in chromium.   The sintered material of claim 11, wherein the metal carbide has a diameter of 1 to 2 micrometers.   The sintered material of claim 8 having a microhardness of 800-1500 Hv50 and a melting point of about 1235 ° C (2255 ° F).   The sintered material according to claim 8, further comprising at least 30.0% by weight of another powder metal in addition to the powder metal composition. A method of forming a powder metal composition comprising:
Providing a molten iron-based alloy, the molten iron-based alloy comprising 3.0-7.0 wt% carbon and 10.0-25 based on the total weight of the molten iron-based alloy. 0.0 wt% chromium, 1.0-5.0 wt% tungsten, 3.5-7.0 wt% vanadium, 1.0-5.0 wt% molybdenum, 0.5 Containing no more than wt% oxygen and at least 40.0 wt% iron, the method further comprising:
Spraying the molten iron-based alloy to provide spray droplets of the iron-based alloy.   The method of claim 18, comprising pulverizing the spray droplets to remove oxide film from the spray droplets. A method of forming a sintered product,
Providing a molten iron-based alloy, the molten iron-based alloy comprising 3.0-7.0 wt% carbon and 10.0-25 based on the total weight of the molten iron-based alloy. 0.0 wt% chromium, 1.0-5.0 wt% tungsten, 3.5-7.0 wt% vanadium, 1.0-5.0 wt% molybdenum, 0.5 Containing no more than wt% oxygen and at least 40.0 wt% iron, the method further comprising:
Spraying the molten iron-based alloy to provide spray droplets of the iron-based alloy;
Optionally crushing the spray droplets;
Compressing the droplets to form a preform;
Sintering the preform.   24. The method of claim 21, comprising annealing the droplets prior to the sintering step.   The method of claim 21, wherein the spraying includes forming a metal carbide in an amount of at least 15% by volume.   The method of claim 21, comprising mixing at least 30.0% by weight of another powder metal with the spray droplets.   The powder metal composition of claim 3, having a melting point of about 1235 ° C. (2255 ° F.).

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