Patent ID: 12251759

DETAILED DESCRIPTION OF THE INVENTION

The highly machinable parts of this invention can be made using conventional procedures for manufacturing powder metal parts. However, the powder metal composition utilized in making the part will include 0.05 weight percent to 5 weight percent of calcium aluminoferrite powder. Such a procedure normally includes the steps of (1) placing a metal powder composition into a mold having the desired shape of the part, (2) compressing the metal powder in the mold into the shape of the part under a pressure of 20 tons per square inch to 70 tons per square inch to produce a green metal part, (3) removing the green metal part from the mold, and (4) sintering the green metal part at an elevated temperature which is typically within the range of about 60% to about 90% of the melting point of the metal composition to produce the sintered metal part.

In manufacturing the powder metal parts of this invention a mold of the desired shape is filled with a powder metal composition. After the metal powder formulation is introduced into the mold the powder is compressed under high pressure, typically from 20 to 70 tons per inch2(tsi) and more typically 40 to 65 tons per inch2(tsi). This compressed part or preform is then considered to be green or uncured. The green part is then cured or sintered by heating in a sintering furnace, such as an electric or gas-fired belt or batch sintering furnace, for a predetermined time at high temperature in an inert environment or reducing atmosphere. Nitrogen, vacuum and Noble gases, such as helium or argon, are examples of such inert protective environments. Metal powders can be sintered in the solid state with bonding by diffusion rather than melting and re-solidification. Also, sintering may result in a decrease in density depending on the composition and sintering temperature. For instance, chromium containing compositions typically maintain or decrease in density while nickel containing compositions generally increase in density.

Typically, the sintering temperature utilized will be about 60% to about 90% of the melting point of the metal composition being employed. The sintering temperature will normally be in the range of 1830° F. (1000° C.) to 2450° F. (1343° C.). The sintering temperature will more typically be within the range of 2000° F. (1093° C.) to about 2400° F. (1316° C.). In any case, the appropriate sintering temperature and time-at-temperature will depend on several factors, including the exact chemistry of the metallurgical powder, the size and geometry of the compact, and the heating equipment used. Those of ordinary skill in the art may readily determine appropriate parameters for the molding steps to provide a green preform of suitable density and geometry which is then placed into a furnace at temperature which is within the range of 2000° F. (1093° C.) to 2450° F. (1343° C.) for approximately 30 minutes in a protective atmosphere to sinter the metal.

The final density of the part will vary widely depending on its composition and the particular pressing and sintering parameters employed. The density of the final part will normally be within the range of 6.6 g/cc to 7.5 g/cc. The final part will typically have a density which is within the range of 6.7 g/cc to 7.4 g/cc and will commonly have a density which is within the range of 6.9 g/cc to 7.3 g/cc.

The metal powders that can be utilized in manufacturing powder metal parts are typically a substantially homogenous powder including a single prealloyed alloyed or unalloyed metal powder or a blend of one or more such powders and, optionally, other metallurgical and non-metallurgical additives such as, for example, lubricants. In any case, the metal powder composition used in the practice of this invention will contain 0.05 weight percent to 5 weight percent of calcium aluminoferrite powder.

The powder metal composition will generally contain from 0.08 weight percent to 3 weight percent of the calcium aluminoferrite powder and will more generally contain from 0.1 weight percent to 2 weight percent of the calcium aluminoferrite powder. It is normally preferred for the calcium aluminoferrite powder to be present in the metal composition at a level which is within the range of 0.15 weight percent to 1 weight percent with it being more preferred for the coarse graphite to be present in the metal composition at a level which is within the range of 0.2 weight percent to 0.5 weight percent.

The calcium aluminoferrite powder can be naturally occurring brownmillerite powder of the formula: Ca2(Al,Fe)2O5, or it can be synthetic calcium aluminoferrite powder. The calcium aluminoferrite powder used in the practice of this invention typically has an average particle size of less than 75 microns and preferably less than 63 microns. Powder of the desired particle size can be made by any appropriate grinding means, such as by using a ball mill. In any case, the calcium aluminoferrite is a composite oxide powder which includes 30 weight percent to 50 weight percent Al2O3, 30 weight percent to 50 weight percent CaO, and 10 weight percent to 30 weight percent Fe2O3. The calcium aluminoferrite will generally include 35 weight percent to 45 weight percent Al2O3, 32 weight percent to 45 weight percent CaO, and 12 weight percent to 28 weight percent Fe2O3. The calcium aluminoferrite will typically include 36 weight percent to 44 weight percent Al2O3, 34 weight percent to 44 weight percent CaO, and 12 weight percent to 25 weight percent Fe2O3. The calcium aluminoferrite will more typically include 36 weight percent to 44 weight percent Al2O3, 34 weight percent to 44 weight percent CaO, and 12 weight percent to 20 weight percent Fe2O3. The calcium aluminoferrite will frequently include 38 weight percent to 42 weight percent Al2O3, 36 weight percent to 42 weight percent CaO, and 13 weight percent to 17 weight percent Fe2O3. The calcium aluminoferrite compositions that are useful in the practice of this invention will normally have a maximum SiO2contain of 7 weight percent, a maximum MgO content of 1.5 weight percent, and a maximum SO3content of 0.5 weight percent. The Fe2O3can beneficially be included at a level which is within the range of 15 weight percent to 20 weight percent, 20 weight percent to 25 weight percent, or 25 weight percent to 30 weight percent, basis upon the total weight of the machinability enhancing agent.

The base metal powders to which the calcium aluminoferrite powder is added in manufacturing powder metal parts in accordance with this invention are typically a substantially homogenous powder including a single alloyed or unalloyed metal powder or a blend of one or more such powders and, optionally, other metallurgical and non-metallurgical additives such as, for example, lubricants. Thus, “metallurgical powder” may refer to a single powder or to a powder blend. There are three conventional types of base metal powders used to make powder metal mixes and parts. The most common base metal powders are homogeneous elemental powders such as iron, copper, nickel and molybdenum. These are blended together, along with additives such as lubricants and the coarse graphite, and molded as a mixture. A second possibility is to use pre-alloyed powders, such as an iron-nickel-molybdenum steel. In this case, the alloy is formed in the melt prior to atomization and each powder particle is a small ingot having the same composition as the melt. Again, additives of the coarse graphite, lubricant and elemental powders may be added to make the mix. A third type is known as “diffusion bonded” powders. In this case, an elemental powder, such as iron, is mixed with a second elemental powder or oxide of a powder, and is subsequently sintered at low temperatures so partial diffusion of the powders occurs. This yields a powder with fairly good compressibility which shows little tendency to separate during processing. While iron is the most common metal powder, powders of other metals such as aluminum, copper, tungsten, molybdenum and the like may also be used. Also, as used herein, an “iron metal powder” is a powder in which the total weight of iron and iron alloy powder is at least 50 percent of the powder's total weight. While more than 50% of the part's composition is iron, the powder may include other elements such as carbon, sulfur, phosphorus, manganese, molybdenum, silicon, and chromium. Copper and nickel can also optionally be present in pre-alloyed base metal powder compositions. Typically, the base metal powder will contain at least 95 weight percent iron and will preferably contain at least 97 weight percent iron.

At least four types of metallic iron powders are available. Electrolytic iron, sponge iron, carbonyl iron and nanoparticle sized iron are made by a number of processes. Electrolytic iron is made via the electrolysis of iron oxide, and is available in annealed and unannealed form from, for example, OM Group, Inc., which is now owned by North American Hoganas, Inc. Sponge iron is also available from North American Hoganas, Inc. There are at least two types of sponge iron: hydrogen-reduced sponge iron and carbon monoxide-reduced sponge iron. Carbonyl iron powder is commercially available from Reade Advanced Materials. It is manufactured using a carbonyl decomposition process.

Depending upon the type of iron selected, the particles may vary widely in purity, surface area, and particle shape. The following non-limiting examples of typical characteristics are included herein to exemplify the variation that may be encountered. Electrolytic iron is known for its high purity and high surface area. The particles are dendritic. Carbonyl iron particles are substantially uniform spheres, and may have a purity of up to about 99.5 percent. Carbon monoxide-reduced sponge iron typically has a surface area of about 95 square meters per kilogram (m2/kg), while hydrogen-reduced sponge iron typically has a surface area of about 200 m2/kg. Sponge iron may contain small amounts of other elements, for example, carbon, sulfur, phosphorus, silicon, magnesium, aluminum, titanium, vanadium, manganese, calcium, zinc, nickel, cobalt, chromium, and copper. Other additives in addition to the coarse graphite may also be used in molding the green part.

After being sintered, the part made has improved machinability by virtue of containing the calcium aluminoferrite powder which facilitates the ease of drilling, grinding, cutting, and other machining operations. The part accordingly can be machined as needed with reduced requirements for labor, reduced energy consumption, and less wear on machining tools, such as drill bits, grinders, and cutting blades. All of these benefits result in a greatly reduced manufacturing cost and also frequently lead to a higher quality part having enhanced corrosion and stain resistance.

After being machined the sintered part can optionally be further processed by (1) densifying the surface of the sintered metal part by shot-peening to produce a densified metal part, (2) compacting the surface of the part with a diamond coated arbor to further densify the surface of the part, (3) slurry finishing the powder metal part to remove surface burrs, (4) carburizing the sintered metal part to produce a carburized metal part, (5) tempering the metal part at an elevated temperature which is sufficient to stress relieve the part to produce a tempered metal part, (6) tape polishing the surface of the part to further improve the surface finish of the part, (7) washing to clean the surface of the metal part, and/or (8) rinsing the metal part with a rust inhibitor.

This invention is illustrated by the following examples that are merely for the purpose of illustration and are not to be regarded as limiting the scope of the invention or the manner in which it can be practiced. Unless specifically indicated otherwise, parts and percentages are given by weight and the Mesh size given were determined using U.S. Standard test sieves.

Example 1 and Comparative Examples 2-4

In this series of experiments a series of iron-copper-carbon test bars where made using conventional powder metal technology. These test bars were made using machinability enhancing agents at the levels and having the compositions shown in Table 1 with the exception of the test bar made as a control (Comparative Example 2) which did not include any type of machinability enhancing agent. The density, RB hardness, and total carbon content of the test samples made are also reported in Table 1.

TABLE 1Example1234% Machinability Agent*0.30.00.50.3Composition of Machinability Enhancing Agent% Al2O3~420.00.0~52% CaO~380.00.0≤40% Fe2O3~150.00.0≤3% SiO2≤60.00.0≤6% MgO≤1.50.00.0≤1.5% SO3≤0.40.00.0≤0.4% MnS0.00.0100.00.0Powder Metal Test Bar PropertiesDensity (g/cc)6.926.966.926.93RB Hardness77.276.976.378.5% Total Carbon0.7550.7270.7820.747*The machinability enhancing agent used in Example 1 was calcium aluminoferrite powder. The calcium aluminoferrite powder is reported to have a bulk density of ~1.15 g/cm3, a specific gravity of 3.0-3.1 g/cm3, a melting point of ~1350° C. (2500° F.), and a particle size wherein <25% is retained on a 325 mesh screen.

The machinability enhancing agent used in Comparative Example 4 was a second form of calcium aluminoferrite powdered material with a bulk density of ˜1.0 g/cm3, a specific gravity of 3.0-3.1 g/cm3, a melting point of ˜1440° C. (2624° F.), and a particle size wherein <30% is retained on a 325 mesh screen.

The mechanical properties, machinability, and corrosion resistance of the test bars made were determined and the results of this testing is reported in Table 2. The test samples were cut to determine machinability with the machining parameters used being as follows:Feed Rate: 0.08 inch/rev (2 mm/rev)Surface Speed: 900 SFMDepth of Cut: 0.025 inch (0.635 mm)Cycle Time: ˜4 minites per sample (four cuts)Surface Area of K-2004 (prior to first cut): 15.17 in2Surface Area of 4 inch Carrier Face: ˜12.57 in2Insert: 2NUCCGA 32.52 HS BN7500 (supplier by REBCO)Insert Holder: Steel 0.625 inch×6 inch Swiss Style Square Shank Tool Holder to hold a Positive CCMT 32.51 at −5° Lead (Purchased from Sumitomo)

TABLE 2Example1234Mechanical PropertiesApparent (RB)86848586Transverse Rupture Strength (MPa)1,0211,0289711,021Size Change (mm/mm)0.068330.068580.074680.07061Sinter Braze TestPassedPassedPassedPassedMachinability and Corrosion Resistance# of Turns per Tool3,200+5603,200+3,200+Days to Rust on Machined Surface**>2177**14Machined Surface Finish After 25 Cuts (Ra)17.58.019.511.5Machined Surface Finish After 400 Cuts (Ra)17.510.527.515.5Machined Surface Finish After 800 Cuts (Ra)18.59.029.016.5**The test samples were placed in a humidity chamber with the number of days until rust could be visually detected.

It should be noted that the test sample of Comparative Example 3 which utilized MnS as a machinability enhancing agent was very badly rusted after being in the humidity chamber for only 7 days. This is in contrast to Example 1 which utilized calcium aluminoferrite powder as a machinability enhancing agent and wherein no rusting could be detected after being aged in the humidity chamber for over 21 days.

As can be seen from Table 2, the test samples of Example 1 showed excellent machinability and exhibited superior rust resistance to all of the other comparative examples. Comparative Example 2, which did not include a machinability enhancing agent proved to be extremely difficult to process. On the other hand, Comparative Examples 3 and 4 naturally showed improved machinability by virtue of including a machinability enhancing agent, but exhibited poor corrosion resistance. The test samples made in accordance with this invention utilizing calcium aluminoferrite powder as a machinability enhancing agent (Example 1) were the only ones that proved to exhibit both good machinability and excellent corrosion resistance. In fact, the samples made in accordance with this invention proved to have better corrosion resistance than the test specimens made without including any machinability enhancing agent (Comparative Example 2). In other words, by manufacturing powder metal parts in accordance with this invention it is possible to make high quality corrosion resistant parts that could not conventionally be made without encountering difficult machinability.

While certain representative embodiments and details have been shown for the purpose of illustrating the subject invention, it will be apparent to those skilled in this art that various changes and modifications can be made therein without departing from the scope of the subject invention.