Patent Grant
10704125
Field of the Invention
This invention relates generally to a free-machining steel article and to a method for making same. More particularly, the invention relates to elongated product forms such as wire, rod, bar, band, and strip made from a substantially lead-free, free-machining, powder metallurgy steel.
Description of Related Art
Small, precision-machined parts for the watch, automotive, and other industries are made from steel wire that has been cold drawn and then straightened. Good machinability is required for manufacturing such parts and is usually obtained by including in the structure of the steel one or more additives to enhance the machinability property. Pb, S, and Se are among the most common additions to steel for enhancing machinability. However, the addition of Pb has some safety issues. Therefore, a lead-free machining steel with the same or better machinability than leaded steel is desired.
U.S. Pat. No. 8,282,701 B2 and U.S. Pat. No. 8,795,584 B2, describe a lead-free, free-machining steel article for use in the production of high quality, precision parts and a process for making such an article. The microstructure of the steel described in those patents is fine grained and has a fine and uniform distribution of manganese sulfides (MnS). As described in those patents, the microstructure is obtained by using a controlled alloy chemistry, gas atomization to produce alloy powder, followed by hot consolidation of the alloy powder to form a powder compact. A billet is prepared from the powder compact which is then hot worked and cold finished to make wire and bar products for machining into small precision parts.
The alloy and method described in U.S. Pat. No. 8,282,701 B2 and U.S. Pat. No. 8,795,584 B2 has been used to provide small diameter wire and bar with acceptable machining characteristics for making small, precision parts. However, it has been determined in practice that better machinability is needed for the machining of very small precision parts. Accordingly, it is an object of the current invention to provide small diameter wire and bar products that have a finer and more homogenous distribution of carbides to thereby enhance machinability and processability of the wire or bar beyond what was previously achievable with the material made in accordance with U.S. Pat. No. 8,282,701 B2 and U.S. Pat. No. 8,795,584 B2. An additional object of the present invention is to provide a process for making very small diameter wire and bar, including a heat treatment that, in combination with the modified chemistry, will readily provide the desired microstructure in order to obtain the enhanced combination of properties described above. The entire disclosures of U.S. Pat. No. 8,282,701 B2 and U.S. Pat. No. 8,795,584 B2 are hereby incorporated herein by reference.
In accordance with one aspect of this invention, there is provided an elongated article having a small cross-sectional area such as wire, rod, bar, and strip. The elongated article is made from a prealloyed metal powder having the following broad and preferred weight percent compositions.
The balance of the alloy is iron and usual impurities. The wire and bars according to this aspect of the invention are further characterized by a microstructure that includes i) a ferritic matrix having uniform, fine grain size, preferably defined by an ASTM E-112 grain size number of 8 or greater, ii) a uniform distribution of manganese sulfides that are not larger than about 2 μm in major dimension uniformly distributed in said matrix, and iii) a uniform dispersion of fine, spheroidal carbides that are not larger than about 4 μm in major dimension uniformly distributed in said matrix.
In accordance with another aspect of the present invention, there is provided a method of making a small diameter, elongated article, such a wire, bar, or strip, having machinability that is superior to the known materials. In a first step of the method of this invention, a steel alloy having the following weight percent composition is melted in a melting furnace:
The method comprises the further steps of atomizing the steel alloy with an inert gas to form a prealloyed steel powder and consolidating the steel powder to substantially full density to form a powder compact. The powder compact is hot worked to form an elongated intermediate article. The method also includes heat treating the intermediate article by performing the following steps: a) heating the intermediate article at a first temperature of about 40 C.° below to about 25 C.° above the alloy's Acm temperature for about 45-90 minutes per inch of thickness of the intermediate article; b) cooling the intermediate article from the first temperature at a rate sufficient to transform the alloy to one or more of martensite, upper bainite, lower bainite, and combinations thereof in said intermediate article; c) heating the intermediate article at a second temperature of about 150 C.° below the alloy's A1 temperature to the A1 temperature for a time sufficient to precipitate a plurality of fine carbides in the matrix material of the alloy; d) cooling the reheated intermediate article in air from the second temperature to room temperature; e) heating the intermediate article at a third temperature of about 10-50 C.° above the alloy's A1 temperature for about 1.5-6 hours per inch of thickness; f) cooling the intermediate article from the third temperature at a rate of about 5-80 C.°/hour to an intermediate temperature of about 100-400 C.° below the A1 temperature; and then g) air cooling the intermediate article from the intermediate temperature to room temperature. The heat treated article is then further processed to reduce its cross-sectional area to provide an elongated article having a small cross section or diameter such as wire, rod, strip or bar for precision machining of parts. The further processing may include cold drawing and/or cold rolling. The cold drawing and cold rolling steps may be accomplished in one or more steps to reach the final dimensions.
In accordance with another embodiment of the process according to this invention the powder compact may be hot worked as by hot rolling to provide an elongated article such as wire, rod, or band having final or near-final dimensions. The hot rolled article is then heat treated as described in steps a) to g) above.
Here and throughout this application the following terms are defined as follows. The term “percent” and the symbol “%” mean percent by weight or mass percent unless otherwise indicated. Upper bainite is defined in accordance with its known definition as an aggregate of ferrite and cementite that contains substantially parallel lath-shaped elements of ferrite. Lower bainite is defined in accordance with its known definition as an aggregate of ferrite and cementite that has an acicular appearance. Ferrite and cementite are known phases of steel. The Acm temperature is defined in accordance with its known definition as the temperature below which cementite starts forming in steel during cooling. The A1 temperature is defined in accordance with its known definition as the temperature at which the austenite phase of steel transforms to a eutectoid comprising pearlite. The term “small diameter” refers to a product form having a round cross section and is defined to mean a diameter of not more than about 1.725 inches (43.81 mm). The terms “thin” and “small thickness” are defined to mean a thickness of not more than about 3 mm.
The foregoing summary and the following detailed description will be better understood when read with reference to the drawings, wherein:
For the purposes of this application the elemental weight percent ranges described above are balanced to provide a microstructure comprising a ferritic matrix containing very fine and uniformly distributed, spheroidized carbides that provide better machinability during low speed and high speed cutting of the metal than the known materials. Based on the desired properties the following elemental ranges are selected for the alloy composition according to this invention.
Carbon is an austenite stabilizer and an element that forms carbides with other elements present in the alloy of this invention. About 0.88-1.00% carbon should be present in the alloy for the broad aspect of this invention and preferably about 0.92-0.98% carbon is be present.
Manganese is also an austenite stabilizer and may cause modification of the A1 temperature of the alloy when combined with other elements. Manganese combines with available sulfur to form manganese sulfides that are beneficial to the excellent machinability provided by this alloy. For these reasons the alloy contains about 0.20-0.80% manganese.
Silicon is a ferrite stabilizer and may also be present in this alloy as a residual from deoxidizing additions during melting of the alloy. The alloy contains up to about 0.50% and preferably contains about 0.12-0.22% silicon.
Sulfur combines with manganese to form manganese sulfides that are needed for good machinability. The alloy is processed in a way that results in fine, dispersed sulfides. For these reasons the alloy contains about 0.010-0.100% and preferably about 0.010-0.090% sulfur.
Chromium is a strong carbide former and also imparts some corrosion resistance to the alloy. Chromium is considered necessary in the alloy of this invention to form the fine carbides that provide sites that function as nuclei for the precipitation and growth of substantially spherical carbides instead of lamellar-shaped carbides during the spheroidization process. However, too much chromium will cause the Acm temperature to increase which leads to stabilization of large and coarse primary carbides that are difficult to dissolve during heat treatment. Also, chromium increases the hardenability of the steel which raises the likelihood of cracking when the alloy is subsequently heat treated as described below. In view of the foregoing, the alloy contains about 0.15-0.90% chromium and preferably the alloy contains about 0.30-0.60% chromium.
Nickel is an austenite stabilizer and also benefits the hardenability of the steel without significantly raising the Acm temperature. For these reasons the alloy contains about 0.10-0.50% nickel and preferably contains about 0.10-0.25% nickel.
A small amount of molybdenum may be present in this alloy as a residual from charge materials used during melting. Molybdenum is also a strong carbide former and would help with the production of primary fine carbides as a substitute for at least some of the vanadium. Accordingly, up to about 0.25% molybdenum may be present in this alloy for either of the foregoing reasons.
Copper is also an austenite stabilizer. Copper combines with sulfur to form sulfides that are beneficial for the machinability provided by the alloy. Copper may also provide some corrosion resistance in the alloy. However, the use of copper is restricted to levels that do not result in incipient melting in the alloy during processing and heat treatment. Accordingly, the alloy contains about 0.08-0.23% and preferably about 0.10-0.23% copper.
A small amount of vanadium is present in the alloy to aid in the production of primary, fine, and stable MC-type carbides that contribute to breaking up of carbide lamella to form more spherical carbides. For these reasons the alloy of the present invention contains about 0.025-0.15% vanadium and preferably contains about 0.035-0.060% vanadium.
Nitrogen may be present in the alloy primarily as a result of adsorption when the alloy is atomized with nitrogen gas. Nitrogen is preferably restricted to a maximum of about 0.060% (600 ppm) in the alloy powder according to the present invention.
The balance of the alloy is iron and the usual impurities found in alloys used for the same or similar purposes. In particular, phosphorus is considered an impurity in this alloy and should be limited to not more than about 0.050% and preferably to not more than about 0.030%. Oxygen is also considered an impurity in the alloy powder of this invention and is preferably limited to not more than about 0.040%.
The method of making a small diameter elongated steel article with good machinability and stability includes the following process steps. The alloy is melted in a melting furnace preferably by vacuum melting. The molten alloy is atomized with an inert gas to form a pre-alloyed powder having the weight percent composition described above. The inert gas may be nitrogen, argon, or a combination thereof. Preferably, the metal powder is produced by atomization with nitrogen gas in an induction melting and gas atomization unit. The atomized powder is preferably screened to about −100 mesh and may be blended with one or more other heats having essentially the same alloy composition to produce a blended metal powder. The alloy powder is vibration filled into a low carbon steel canister. The powder-filled canister is then vacuum hot outgassed and sealed. Hot outgassing is described, for example, in U.S. Pat. No. 4,891,080, the entirety of which is incorporated herein by reference. The sealed canister is then hot isostatically pressed (HIP) preferably at about 1121° C. and 15 ksi for a time sufficient to fully densify the metal powder. Argon gas is preferred as the pressurizing fluid. After HIP the fully dense metal powder is hot rolled from a temperature of about 1149° C. to form an elongated intermediate form such as a billet that includes the consolidated metal powder and a cladding consisting of the low carbon steel alloy of the canister.
The elongated intermediate form is heat treated using a three-step process which includes one or more operations comprising time and temperature conditions selected to produce a general microstructure containing fine, dispersed, spheroidized carbides and fine sulfides in a ferrite matrix. In another embodiment of the process of this invention, the billet or other intermediate form is hot worked, such as by hot rolling, to provide bar, wire, rod, strip, or band having final or near-final dimensions and then given the three-step heat treatment to provide the desired microstructure.
The heat treatment used in the method according to this invention is further described as follows. In a first heating step, the intermediate article is heated at a temperature of about 40 C.° below the Acm temperature of the alloy to about 25 C.° above the Acm temperature. The Acm temperature boundary for this alloy is shown by the upper arrow in
The first heating step is followed by rapid cooling in an appropriate media such as inert gas or a liquid quench (e.g., oil quenching) to cause transformation of the stabilized austenite to a lower temperature microstructure, like martensite, lower bainite, upper bainite, or a combination thereof. The cooling rate is selected to be high enough to avoid the formation of pearlite in the alloy, but low enough to avoid cracking because the alloy contains relatively high carbon and a relatively small number of alloying elements that benefit the hardenability of the alloy. Accordingly, after the first heating step the intermediate article is cooled at a rate of about 20-60 C.°/sec. from the Acm temperature to room temperature.
An alternative method to transform the austenite to martensite, lower bainite, and/or upper bainite as an intermediate microstructure includes isothermal transformation of austenite at temperatures above the martensite transformation start temperature (Ms) until complete, followed by cooling in a gas or liquid medium. The Ms temperature for the preferred chemistry of the alloy according to the present invention was calculated to be 140° C.±15 C.°.
The heat treatment is continued with a second heating step in which the elongated article is heated at a second temperature of about 150 C.° below the A1 temperature up to the A1 temperature. Preferably, the elongated article is heated at a temperature that is about 120 to 80 C.° below the A1 temperature. The A1 temperature calculated for a preferred chemistry of this alloy is about 720-730° C. The A1 temperature boundary is identified by the lower arrow shown in
In a further heating step the article is heated at a third temperature that is about 10 to 50 C.° above the A1 temperature, preferably at about 15 to 35 C.° above A1 for about 90 to 360 minutes (1.5-6 h) per inch of thickness and preferably for about 90 to 120 min (1.5-2 h) per inch of thickness. The article is then cooled at a cooling rate of about 5 to 80 C.°/hr and preferably at a cooling rate of about 15 to 35 C.°/hr to a temperature of about 100 to 400 C.° below the A1 temperature. The article is then air cooled to room temperature.
After heat treatment, the elongated intermediate article may undergo one or more cycles of cold drawing, each cold reduction step followed by stress relief annealing, until the desired diameter is obtained. The cold-drawn material is typically provided as wire with a diameter of about 1.75 mm, 3 mm, 4.5 mm, or 6.5 mm. Larger diameter wire can also be produced to provide small diameter finished bar up to about 15 mm in diameter. As an alternative process, the elongated intermediate article may be cold rolled after the heat treatment to provide strip.
In order to demonstrate the improved microstructure provided in the article and by the method according to the present invention three experimental heats were melted and atomized to produce prealloyed metal powder. The weight percent compositions of the experimental heats are shown in the table below.
The balance of each composition was iron and usual impurities, including <0.030% P.
Heats 098 and 223 have weight percent compositions that embody the alloy according to the present invention. Heat 560 embodies the alloy described in U.S. Pat. No. 8,282,701 B2 and U.S. Pat. No. 8,795,584 B2. The experimental heats were vacuum induction melted and then atomized with nitrogen gas to form pre-alloyed metal powder. The metal powder from each heat was screened to −100 mesh and then filled into a low carbon steel canister. The powder filled canisters were vacuum hot outgassed and then sealed.
The powder filled canisters were then hot isostatically pressed (HIP'd) at 1121° C. at a pressure of 15 ksi for a time sufficient to provide substantially fully dense powder compacts. The powder compacts were then hot rolled to form billets consisting of the consolidated metal powder and a cladding formed from the canister. The billets were further hot rolled to an elongated intermediate form and then air cooled to room temperature.
The intermediate elongated forms of Heats 098 and 223 were heat treated as follows. The elongated forms were austenitized by heating at 850° C. for one hour and then quenched in oil. After quenching, the elongated intermediate forms were tempered by heating at 620° C. for 4 hours and then air cooled. The elongated forms were then re-austenitized by heating at 750° C. for 2 hours, furnace cooled at 20 C.° per hour down to 580° C., and then air cooled to room temperature. After the heat treatment, the elongated forms of Heats 098 and 223 were shaved to remove the carbon steel cladding and then cold drawn to a final diameter of 0.3208 inches.
The elongated intermediate form of Heat 560 was heat treated as follows. The elongated intermediate form was austenitized by heating at 738° C. for 8 hours, furnace cooled at 10 C.°/hour to 600° C., and then air cooled. After cooling, the elongated form was shaved to remove the carbon steel cladding layer and then cold drawn to a final diameter of 0.2055 inches. The wire was then re-austenitized by heating at 738° C. for 8 hours, furnace cooled from the austenitizing temperature to 600° C. at 10 C.°/hour, and cooled in air to room temperature.
Longitudinal metallographic specimens were prepared from the wire produced for each of Heats 098, 223, and 560 and photomicrographed in accordance with ASTM A 892. Representative micrographs for Heats 098, 223, and 560 are shown in
The terms and expressions which are employed herein are used as terms of description and not of limitation. There is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof. It is recognized that various modifications are possible within the invention described and claimed herein. Moreover, it is contemplated that the steps of the process described herein may be carried out by more than one entity. For example, the steps of melting and atomizing the alloy powder may be carried out by a first entity, the step of consolidating the alloy powder may be carried out by a second entity, and the hot-working, heat treating, and cold-working steps may be carried out by one or more other entities.
This application claims the benefit of U.S. Provisional Patent Application No. 62/252,671, filed Nov. 9, 2015, the entirety of which is incorporated herein by reference.
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