Patent Application
20130177469
The present disclosure generally relates to steel production and, more particularly, to methods and compositions of improved high melting point element iron alloys for steel production.
High strength, high performance steels have various applications in both the commercial and military industries. For example, commercial applications of high strength, high performance steels include the following: pressure vessels; hydraulic and mechanical press components; commercial aircraft frame and landing gear components; locomotive, automotive, and truck components, gas and oil drilling platforms, including die block steels for manufacturing of components; and bridge structural members. Example military applications of high strength, high performance steels include hard target penetrator warhead cases, missile components including frames, motors, and ordnance components including gun components, armor plating, military aircraft frame and landing gear components.
Many high performance steels, however, suffer from inconsistent mechanical properties. For example these steels may have inconsistent hardness, ultimate tensile strengths, yield strengths, and notch toughness.
The present disclosure generally relates to steel production and, more particularly, to methods and compositions of improved high melting point element iron alloys for steel production.
The present disclosure provides, in certain embodiments, methods comprising providing a composition comprising iron and a high melting point element; heating the composition to an elevated temperature up to about 3,500° F.; holding the composition at the elevated temperature for a time sufficient for the heat's temperature to stabilize; and allowing the composition to cool or solidify.
The present disclosure provides, in certain embodiments, methods comprising providing a master alloy comprising iron and up to about 30% by weight of a high melting point element; and adding the master alloy to a heat of steel.
The present disclosure provides, in certain embodiments, compositions comprising an alloy of iron and high melting point element in which the alloy is up to about 30% by weight of the high melting point element.
The present disclosure provides, in certain embodiments, compositions comprising an alloy of iron and high melting point element having a substantially uniform microstructure.
The present disclosure provides, in certain embodiments, compositions comprising an alloy of iron and high melting point element having the high melting point element uniformly distributed throughout the microstructure of the composition.
The present disclosure provides, in certain embodiments, compositions comprising an alloy of iron and high melting point element having a near or complete absence of an elemental form of the high melting point element.
The features and advantages of the present disclosure will be readily apparent to those skilled in the art upon a reading of the description of the embodiments that follows. While numerous changes may be made by those skilled in the art, such changes are within the spirit of the invention.
The patent or application file contains at least one drawing executed in color. Copies of this patent or patent application publication with color drawing(s) will be provided by the Office upon request and payment of the necessary fee.
These drawings illustrate certain aspects of some of the embodiments of the present disclosure, and should not be used to limit or define the invention.
Some high strength and/or high ductility steels require a high melting point element (HME) to achieve desired mechanical properties. For example, Eglin Steel requires a tungsten range of 0.90% to 1.10% tungsten. The use of a ferro-HME master alloy (e.g., ferrotungsten) has been used to introduce HMEs into heats of steel.
The addition of a master alloy in the production of steel appears to work for materials that are evaluated on a macro basis particularly where discreet carbide particles are required, such as in some types of tool steel. When this alloy is added to materials that are evaluated on a micro basis, however, there are varying results that include a high rejection rate of production items. In addition, the inventors have observed that some heats of steel made in this manner fail to achieve consistent mechanical properties
The inventors believe that these inconsistent mechanical properties stem from difficulties in getting the HME component into solution during the steelmaking process. The present disclosure is based, at least in part, on the observation that ferro-HME alloys do not completely dissolve into the heat of steel resulting in material that may contain areas or phases of high HME content, or with HME bearing secondary phase particles, resulting in materials with inconsistent mechanical properties. The inventors believe this may be due, at least in part, to HME phases that are not sufficiently alloyed to reach the eutectic point. That is, the master alloy and/or resulting steel may include three phases: substantially pure HME, iron (e.g., ferrite), and Fe—HME alloy. Because steels are normally prepared at temperatures below the melting point of high melting point elements (HMEs), the melting point of the HME is never reached. This results in master alloys and steels with substantially pure HME phases, or non-eutectic phases, which likely result in the inconsistent mechanical properties observed in steels made without the benefit of the present disclosure. Accordingly, the present disclosure provides methods for making master alloys and steels, as well as master alloys and steels, with improved mechanical properties (e.g., toughness and impact resistance).
The present disclosure provides, in certain embodiments, a method for making a master alloy comprising providing a composition comprising iron and a high melting point element (HME); heating the composition to an elevated temperature up to about 3,500° F.; holding the composition at the elevated temperature for a time sufficient for the compositions temperature to stabilize; and allowing the composition to cool or solidify. As used herein, the term “high melting point element” (HME) refers to an element having a melting point above 3,100° F. Examples of HME's include tungsten (W), Rhenium (Re), Osmium (Os), Tantalum (Ta), Niobium (Nb), Iridium (Ir), Boron (B), Ruthenium (Ru), Hafnium (Hf), Technetium (Tc), Rhodium (Rh), Zirconium (Zr), Platinum (Pt), and Thorium (Th). In some embodiments, the HME in the composition may be present up to about 30%, by weight. While the composition contains iron and HME, it also may include other elements known in the art to be useful in steel, such as, for example, carbon (c), manganese (Mn), silicon (Si), chromium (Cr), Nickel (Ni), Copper (Cu), phosphorous (P), sulfur (S), calcium (Ca), nitrogen (N), and aluminum (Al).
The composition is heated to an elevated temperature so that it will diffuse to form a molten composition. This molten composition is then held at the elevated temperature to allow the HME to diffuse throughout the molten composition. In certain embodiments, the molten composition may be mixed while held at the elevated temperature. For example, the composition may be mixed through induction stirring along with argon bubbling. By way of explanation, and not of limitation, it is believed that holding the molten composition at an elevated temperature allows the HME to diffuse throughout the molten composition providing opportunity for the Fe and HME to form a Fe—HME alloy phase of the intended composition such as a eutectic or peritectic point. The particular hold time chosen can vary depending on the temperature of the molten composition until that temperature is substantially stable. The particular hold time chosen may vary depending on the exact amount and type of elements present in the composition. Examples of suitable hold times include from about 1 hour to about 10 hours. In general, suitable holding temperatures may be fixed close to or above the target eutectic or peritectic temperature (e.g., approximately 50° F. to 100° F. above the eutectic or peritectic temperature).
The composition may be processed further to form a master alloy, as is known in the art. For example, the composition may be allowed to solidify or actively cooled and formed into suitable structures, such as, for example, splatter metal, wire/rod forms, and notch bars.
In certain embodiments, a master alloy made as described above may be used to produce high strength and/or high ductility steels. For example, a master alloy of the present disclosure may be added to a heat of steel and allowed to melt. The master alloys of the present disclosure, according to certain embodiments, are able to provide the HME such that it is capable of melting at useful furnace operating temperatures up to about 3,500° F. Once the master alloy melts into the heat of steel, it may be held for a time sufficient to allow diffusion of the master alloy throughout the heat of steel in a timeframe which allows commercial production. Subsequent processing steps known in the art are also contemplated, such as, for example, cooling, casting, forging, rolling into bar stock or tube production, ESR and VAR remelting, and the like.
As noted above, the present disclosure provides methods that may be used to make HME master alloys and HME alloy steels. Accordingly, the present disclosure provides, in certain embodiments, a master alloy comprising iron (Fe) and a HME in which the HME is present up to about 30% by weight. For example, in certain embodiments, the HME may be tungsten present at about 30% by weight.
The present disclosure also provides HME alloy steels with improved mechanical properties with the HME present from about 0.5% to about 5% by weight. For example, in certain embodiments, the HME in the HME alloy steel may be tungsten present at approximately 30% by weight.
In general, in particular embodiments, the master alloys and steels of the present disclosure should comprise sufficient Fe—HME phases to achieve consistent mechanical properties. Such master alloys and steels of the present disclosure may show a more uniform distribution of phases under SEM and a more uniform distribution of Fe to HME as shown by EDS. In certain embodiments, the HME alloy steels may have a substantially uniform microstructure as measured by scanning electron microscopy. In other embodiments, the HME alloy steels may have uniform mechanical properties, such as hardness and notch toughness (e.g., hardness in the high 40S to low 50S RC). In other embodiments, improved elevated temperature strength and ductility also may be present.
In certain embodiments, the master alloys and steels of the present disclosure are characterized by a near or total absence of the elemental form of the HME present in the microstructure. See, for example, Example 5 below.
In some embodiments, the HME master alloys and HME alloy steels of the present disclosure can be manufactured by the following processes: (i) Electric Arc, Ladle Refined and Vacuum Treated; (ii) Vacuum Induction Melting; Argon Oxygen Decarbization, Vacuum Oxygen Decarburization, Plasma Re-Melting (iii) Vacuum Arc Re-Melting; and/or (iv) Electro Slag Re-Melting. The use of the end item will dictate the manufacturing process that should be applied. End products made from the compositions of the present disclosure can be produced using open die forging, close die forging, solid or hollow extrusion methods, static or centrifugal castings, sand casting, investment casting, permanent mold casting, “V”-process molding, lost foam processes, continuous casting, plate rolling, bar rolling, tube production, or other methods known in the art. Additionally, various heat treatments may be employed, normalizing, homogenization, austenitizing, quenching including air, oil, polymer, water, and/or cryo quenching/treatment, followed by single or multiple tempering processes.
The HME master alloys and HME alloy steels of the present disclosure have utility wherever high strength, high performance steel is desired. In certain embodiments, the HME master alloys and HME alloy steels of the present disclosure may be useful in industrial applications, such as mining (e.g., surface mining ground engagement tooling, subsurface mining parts, such as conveyor flight bars, racks, rack gears), railroad components (e.g., knuckles, couplers, car components, and draft system components). In other embodiments, the HME master alloys and HME alloy steels of the present disclosure may be useful in military applications, such as military hardware and armaments. For example, the HME master alloys and HME alloy steels of the present disclosure may be useful for penetrating war heads, bombs, canon tubes, breach blocks, and armor. In certain embodiments, the compositions of the present disclosure may be particularly useful in projectile penetrator applications wherein high impact velocities, such as those greater than 1,000 feet per second, are imparted to the projectile to cause deep penetration of rock and concrete barriers. The strength, toughness and wear resistance of the steel produced according to the present invention provides enhanced penetrator performance.
To facilitate a better understanding of the present disclosure, the following examples of certain aspects of some embodiments are given. In no way should the following examples be read to limit, or define, the entire scope of the invention.
In the case of ferrotungsten, ASTM specification A 144 provides four grades: A, B, C, and D. Grade A has a tungsten range of 85% to 95%, while the remaining grades B through D each have a tungsten range of 75% to 85% and differ from one another in terms of other elements including carbon, phosphorous, sulfur, silicon, molybdenum and aluminum. A supplementary requirement includes limitations on several other additional elements. These alloys can include approximately 5%-25% iron.
Iron melts at approximately 2795° F., and tungsten melts at approximately 6170° F. A combination of Fe 70% and W 30% melts at approximately 2800° F., and a combination of Fe 20% and W 80% melts at approximately 4300° F.
Upon analyzing an existing Fe—W alloy addition material at Fe 20% and W 80% on a scanning electron microscope (SEM), one may find that the tungsten alloy existed as a mixed structure of Fe—W, Fe, CaWO4 and W. The problem with this alloying element is that the pure W will never melt—it will remain as W in the final chemistry. This is because the melting and refining process used to produce high strength high ductility steels is ≦3000° F.
In particular embodiments, using an alloy addition material that includes a tungsten range of up to about 30% may be optimal to meet the requirements of high strength and/or high ductility materials. It may also reduce the current tungsten separation problems found when using commercially available ferro-alloys with 75% and higher tungsten composition. This ferro-alloy may include an iron range of approximately 70%-90%. The use of a tungsten ore such as Scheelite (calcium tungstate CaWO4) may be one approach to producing this alloy.
Eglin Steel (ES) is a high-strength, high-performance, low-alloy, low-cost steel, developed in collaboration between the US Air Force and Private Industry. The development of Eglin Steel was commissioned in order to find a low-cost replacement for strong and tough but expensive superalloy steels such as AF-1410, Aermet-100, HY-180, and HP9-4-20/30. The material can be less expensive because it can be electric-arc melted, ladle-refined & vacuum de-gassed. It does not require vacuum re-melting or electro-slag re-melting processing. Unlike some other high-performance alloys, Eglin Steel can be welded easily, broadening the range of its application. Also, it uses about 1% nickel where as superalloys can use up to about 14% nickel and/or cobalt, substituting silicon to help with toughness and particles of vanadium carbide and tungsten carbide for additional hardness and high-temperature strength. The material also involves chromium, tungsten, and low to medium amounts of carbon, which contribute to the material's strength and hardness.
At room temperature, ES's yield (tensile strength before deformation) is 224,500 PSI, ultimate yield (breaking point) is 263,700 PSI. At 900° C., yield is 193,900 PSI, and ultimate yield is 246,700. Rockwell hardness is 45.6. For toughness, the Charpy notch impact is 56.2 foot-pounds at room temperature, and 42.7 ft-lbs at −40 F.
Eglin steel samples were analyzed: V297 (Original Item); V298 (Large Ingot); V299 (Small Ingot). Table 1 provides hardness data as HV-10 (HRC). Table 2 provides Charpy impact test results for V299 and V298. Table 3 provides test results for V297.
Five samples of potential alloying compounds of the present disclosure were evaluated to show the different phases present in each with a corresponding EDS comparison of the elemental composition of each phase. Scanning electron microscopy (SEM) was used in combination with energy dispersive x-ray spectroscopy (EDS) to document the appearance of different phases and to verify their elemental composition in the various samples provided. EDS analysis was performed on 30% FeW and 1% FeW references for comparison with the three “Melt” samples: 1st Melt (hold time 2 hours); 2nd Melt (hold time 4 hours); 3rd Melt (hold time 6 hours).
Each EDS spectrum was acquired using EDS system operating at an acceleration energy of 15 kV. Each SEM micrograph was taken in the backscattered electron image (BEl) mode using an instrument operating at acceleration energy of 15 kV. Prior to SEM/EDS analysis, each sample was sputter-coated with a thin film of carbon to enhance SEM image quality and/or EDS data collection. Each optical light microscopy photograph was taken with a stereo light microscope, equipped with a digital camera, using a high angle light source.
1% FeW and 30% FeW.
The EDS spectra of
1st Melt.
The micrographs of
2nd Melt.
The micrographs of
3rd Melt.
The micrographs of
After holding for 4 hours (2nd Melt) the sample showed a more uniform distribution of phases under SEM, a more uniform distribution of ferrotungsten, and a near or complete absence of elemental W as shown by EDS.
A 17% FeW sample as-cast was polished and etched with nital. The sample showed a distinct surface layer and columnar grains extending into the section. The surface layer contained two separate phases with an equiaxed grain orientation, transitioning to the more direction grains in the interior. The interior showed large oriented grains with two phases. (Data not shown.)
Scanning electron microscopy (SEM) using an SEM beam energy setting of 25 kV and energy dispersive x-ray spectroscopy (EDS) was used to study the surface and inclusions/phase structures and corresponding elemental composition of 17% FeW samples. All SEM micrographs were taken in either the secondary electron image (SEI) or BEl modes using a Tescall Vega II instrument operating at an acceleration energy of 15 kV. EDS spectra were collected with an Oxford INCA EDS system using an electron beam operating at an acceleration energy of 15 kV. Prior to SEM/EDS analysis, each polished sample was sputter-coated with a thin film of carbon to enhance SEM image quality. All optical light microscope photographs were taken with an Olympus SZX12 stereo light microscope, equipped with a digital camera using a high angle light source.
The 17% FeW sample showed a distinct surface layer when etched, but EDS scans of the surface layers showed no difference from the interior. Two distinct phases were present in the alloy with the second phase appearing as acicular grains about 1 to 4 microns wide by 10-20 microns long. The second phase had significantly higher levels of tungsten, but still was an intermetallic compound of tungsten and iron rather than pure tungsten. The iron-tungsten phase diagram suggests that the matrix is an alpha phase with about 8% tungsten while the second is a delta phase with upwards of 60% tungsten
The optical light microscope photograph of
The SEM micrograph of
In these samples, elemental W was detected indicating that the W did not go into solution. These samples also did not show a uniform distribution of phases under SEM and did not show a uniform distribution of ferrotungsten.
SEM was used in combination with EDS to document the appearance of different phases and to verify their elemental composition of 20% FeW X0900200 and M722575 ES1.
Each EDS spectrum was acquired using a system operating at acceleration energies of either 15 kV or 25 kV. SEM micrographs were taken in the secondary electron image (SEI) and backscattered electron image (BEl) modes using a instrument operating at acceleration energies of 15 kV (20% FeW X0900200) or 25 kV (M722575 ESI). Prior to SEM/EDS analysis, the 20% FeW X0900200 sample was sputter-coated with a thin film of carbon to enhance SEM image quality and/or EDS data collection. The M722575 ES1 sample was not carbon-coated. Each optical light microscopy photograph was taken with a stereo light microscope, equipped with a digital camera, using a high angle light source.
Both the 20% FeW and ES1 samples were sectioned and portions from both the face and section were mounted, ground and polished for metallographic examination.
20% FeW X0900200 (Surface Orientation).
A 20% FeW (sample X0900200) under a low magnification optical light microscope is shown in
20% FeW X0900200 (Cross-sectional Orientation).
A 20% FeW (sample X0900200) under a low magnification optical light microscope is shown in
M722575 ES1 (Surface Orientation).
A ES1 steel (sample M722575) under a low magnification optical light microscope is shown in
M722575 ES1 (Cross-sectional Orientation).
A ES1 steel (sample M722575) under a low magnification optical light microscope is shown in
In these samples, a near or complete absence of elemental W indicating that the W went into solution. These samples also showed a more uniform distribution of phases under SEM and a more uniform distribution of ferrotungsten.
A 30% FeW master allow of the present disclosure was used to prepare an ES1 alloy of the present disclosure in a VIM furnace. This ES1 alloy was tested for impact and tensile strength. The data, shown in the tables below, shows that the ES1 (an example of an HME alloy steel of the present disclosure) had improved mechanical properties.
Impact testing was performed using Charpy V-notch according to ASTM E23-07a at −40° F. (Table 4), −65° F. (Table 5), and 74° F. (Table 6). Tensile testing was performed at room temperature using a seed of 0.005 in./in./min., 0.05 in./min./in. (Table 7) according to ASTM E8-08.
Therefore, particular embodiments are well adapted to attain the ends and advantages mentioned as well as those that are inherent therein. Numerous other changes, substitutions, variations, alterations and modifications may be ascertained by those skilled in the art and it is intended that various embodiments encompass all such changes, substitutions, variations, alterations and modifications as falling within the spirit and scope of the appended claims.
This application is a continuation of International Application Number PCT/US2011/041740 filed Jun. 24, 2011, entitled “IMPROVED FERRO-ALLOYS”; which claims the benefit of U.S. Provisional Application No. 61/359,284 Filed Jun. 28, 2010, entitled “IMPROVED FERRO-ALLOYS,” the contents of which are incorporated herewith by reference.
| Number | Date | Country | |
|---|---|---|---|
| 61359284 | Jun 2010 | US |
| Number | Date | Country | |
|---|---|---|---|
| Parent | PCT/US2011/041740 | Jun 2011 | US |
| Child | 13724880 | US |
