Patent Application
20070272231
The present disclosure is directed to glass, ceramic or porcelain coated metal products wherein the porcelain coating has a lubricious surface such that repeated sliding contact against another porcelain surface does not cause noticeable deterioration in the form of marring, chipping or flaking of the porcelain of either porcelain surface. In the preferred embodiment, these products are porcelain-enameled steel oven racks that are subjected to temperatures above 500° F., usually above 900° F., as in self-cleaning, pyrolytic ovens, and the metal is steel wire that has the composition disclosed in this assignee's U.S. Pat. Nos. 6,837,235 and 6,915,552, both hereby incorporated by reference. Alternately, the product can be formed of cast iron, such as a burner grate. The preferred combination of the steel wire together with the lubricious porcelain coating provides oven racks which do not discolor during cooking or during self-cleaning cycles when the oven racks remain in the oven, and the porcelain coating does not spall, fish-scale or chip, as a result of hydrogen out-gassing, which might otherwise occur from steel at the high temperatures of self-cleaning cycles. Further, the porcelain surface of the oven rack is not marred, chipped or otherwise deteriorated as a result of regular sliding contact of the porcelain oven-rack surface against either an oven wall porcelain rib liner surface or a porcelain coated so-called ladder rack during movement of the oven racks into and out of the oven, surprisingly even when the oven rack supports a heavy cooking load, at high cooking temperatures of 350-600° F., or during shipping of the oven and rack to the point-of-sale or to the ultimate consumer.
As described in this assignee's U.S. Patent Nos. 6,837,235 ('235) and 6,915,522 ('522), when a glass-coated steel wire oven rack is subjected to temperatures above 900° F., there is an emission of hydrogen gas from the steel upon cooling from that temperature, and absent a preventive expedient, the emitted hydrogen gas will attempt to escape from the steel through the glass coating causing the glass coating to chip, spall or crack.
There is no solution to preventing the chipping, spalling or cracking of glass-coated steel wire oven racks or of glass-coated drawn steel rod articles, with the exception of the solution described in this assignee's '235 and '522 patents and pending application Ser. No. 11/040,641, filed Jan. 21, 2005.
As described in this assignee's '235 and '522 patents, the drawn steel rod is subjected to at least 20% reduction in diameter during cold drawing; and the rod, at the time it undergoes drawing, is composed of steel comprising up to about 0.08% carbon and about 0.001 to about 0.2% of a carbon stabilizing transition metal selected from vanadium (V), titanium (Ti), niobium (Nb) and tantalum (Ta). This combination of features enables the glass-coated drawn steel rod article or wire oven rack to overcome the glass chipping or cracking problem as a result of hydrogen out-gassing.
In addition to the hydrogen out-gassing problem experienced at high temperatures with porcelain-encapsulated steel oven racks, another very significant problem has more recently been discovered during the manufacture, testing and use of the porcelain-coated oven racks. It has been found that the porcelain can deteriorate by marring, flaking or chipping off of the porcelain material from the oven racks as a result of the normal periodic sliding contact between the oven rack porcelain surface and a contacting porcelain wall surface of the oven cavity. That is, over the 13 to 15 year normal life expectancy of an oven, the repeated sliding porcelain-to-porcelain contact upon insertion and removal of the porcelain-coated oven racks, particularly when the oven racks are supporting a relatively heavy cooking load, can cause unwanted abrasion, chipping and squeaking of the sliding porcelain surface (of one type) against and across a porcelain surface (of the same or another type) on the oven wall. The identification of a suitable porcelain composition that solves this problem was found to be a daunting task since the porcelain composition must be strong enough to solve the chipping, spalling and fish-scaling problems that may result from the hydrogen out-gassing of the carbon steel as well as resist damage resulting from continued heating and cooling cycles experienced in cooking, and especially the high temperatures of self-cleaning oven cycles, while maintaining sufficient lubricity and hardness to pass enumerable quality tests typically required for a porcelain material to be suitable as an oven rack. For example, a suitable porcelain material for an oven rack must pass a lubrication test; gloss test; adherence test; thickness test; fish-scale test; must be resistant to acids; resistant to alkaline materials; be resistant to crazing; be resistant to abrasion; pass a rubbing test; blurring test; toxicity test; humidity test; specific gravity and corrosion test as well as others. Porcelain quality tests generally are specified in the Manual of Tests, Measurements and Process Controls PEI-1101, an enameling manual well known in the art. Even other such tests for porcelain quality are set by ASTM standards.
After-coating the oven rack with a liquid lubricant, such as the prior art method of using vegetable oil, requires repeated reapplication of vegetable oil since the oil dissipates, e.g., burns off, in both continuous-cleaning and self-cleaning oven cycles and also somewhat during other oven usage such as normal cooking cycles. Prior to this assignee's out-gassing solution, as described in the '235 and '522 patents, commercially satisfactory porcelain-coated oven racks to be used in self-cleaning pyrolytic ovens were non-existent so that assistance in attempting to solve the porcelain-to-porcelain abrasion and flaking problem in porcelain materials that are regularly subjected to temperatures above 900° F. was not forthcoming from the prior art.
Described herein is a lubricious porcelain-coated metal oven rack designed to be received within an oven cavity. In the preferred embodiment, the coated metal oven rack includes a plurality of elongated steel wire members formed of a special steel composition and joined together to form an oven rack having an outer surface; wherein the diameter of the steel rod material is reduced by at least about 20% when the steel rod material is drawn to form the steel wire; the outer surface of the oven rack being coated by a glass material having a lubricious, integral, dry outer surface, the glass material preferably being porcelain. The amount of carbon in the steel rod material, the amount of carbon stabilizing transition metal in the steel rod material and the degree to which the cross-sectional area of the steel rod material, is reduced, when the steel wire is drawn from the steel rod material is selected, i.e., balanced, so as to prevent chipping of the glass material away from the outer surface due to the release of hydrogen gas from the steel wire members when the steel wire is either heated or cooled.
In preferred embodiments, the glass material having a lubricious outer surface, preferably porcelain, is coated onto the steel wire in two distinct coating steps, wherein the lubricious (porcelain-to-porcelain friction-decreasing) additive may be homogenous throughout the two porcelain coatings; only in the outer coat (of the two porcelain coats); or may be provided only as a surface feature, such as by treating the porcelain outer surface using a process step that provides lubricity only to the outer surface of the porcelain.
In a preferred embodiment, the coated steel wire products described herein are oven racks designed to be received within an oven cavity. The coated steel wire oven rack includes a plurality of elongated steel wire members joined together to form an oven rack having an outer surface. The plurality of elongated steel wire members are made from a steel rod material containing from about 80 to about 99.9% by weight of iron; from up to about 0.08% by weight of carbon, e.g., 0.001% about 0.08% carbon, preferably from about 0.002% to about 0.05%, and more preferably from about 0.005% to less than about 0.05% by weight carbon, and most preferably from about 0.005% to about 0.03% by weight carbon; and from about 0.001 to about 0.2% by weight of a carbon stabilizing transition metal selected from the group consisting of Vanadium, Tantalum, Titanium, Niobium, and mixtures thereof. The plurality of elongated steel wire members are made from the steel rod material by drawing the steel rod material to form steel wire; wherein the cross-sectional area of the steel rod material is reduced by at least about 20% when the steel rod material is cold drawn to form the steel wire. The outer surface of the oven rack is coated by a glass material having a lubricious outer surface, preferably porcelain, wherein the amount of carbon in the steel rod material, the amount of carbon stabilizing transition metal in the steel rod material and the degree to which the cross-sectional area of the steel rod material is reduced when the steel wire is drawn from the steel rod material is selected, i.e., balanced, so as to prevent chipping of the porcelain away from the outer surface due to the release of hydrogen gas from the steel wire material when the steel wire material is either heated or cooled. In a preferred embodiment, the porcelain is coated onto the steel in two distinct coating steps preferably in two distinct electrostatic coating processes, followed by a single heating process in which the temperature is preferably raised to about 1550° F. or cured using infrared (IR) or other glass frit fusing techniques known in the porcelain coating or porcelain enameling art. In alternate embodiments, the heating process may be repeated and in yet other alternate embodiments, a wet coating, CVD, physical vapor deposition (PVD) or other processes can be used for applying the porcelain coat(s) to the steel wire oven rack.
The plurality of elongated steel wire members are made from steel rod material containing from about 80 to about 99.9% by weight of iron, up to about 0.08% by weight carbon, e.g., from about 0.001 to about 0.08% by weight of carbon, and from about 0.001 to about 0.2% by weight of a transition metal that will have a stabilizing effect on the carbon in the elongated steel wire members such that the carbon absorbs less hydrogen gas when the steel wire member is heated to temperatures above 500° F. than it would in the absence of the carbon stabilizing transition metal. In preferred embodiments, the transition metal is selected from the group consisting of Vanadium, Tantalum, Titanium and Niobium, and in the most preferred embodiment, the transition metal is Vanadium. The plurality of elongated steel wire members are preferably made from steel rod material by a cold drawing process to reduce the diameter of the steel wire. In the preferred process, the steel rod is pulled through a cold die that gradually reduces in diameter so that the rod is drawn repeatedly through the die and the cross-sectional area of the rod is reduced to form a steel wire having a cross-sectional area of diminished diameter. In preferred embodiments, the diameter of the steel wire is diminished at least about 20%, preferably at least about 30%, more preferably at least about 40%, even more preferably at least about 45%, and most preferably at least about 50%. It will be appreciated that the diameter reduction creates voids in the steel wire which are desirable to provide cavities into which hydrogen gas can be received and, perhaps, compress, without creating pressure to be released from the surface of the steel wire once the steel wire is coated with porcelain. It will be appreciated, that the diameter reduction, which creates cavities in the steel wire, and the inclusion of carbon stabilizing transition metal elements so that the steel absorbs hydrogen, will diminish the degree to which hydrogen gas out-gassing causes cracking, spalling and chipping of the porcelain surface of the elongated steel wire members of the oven rack which are coated by the glass material.
In other embodiments, the metal structure coated with a lubricious glass material may be cast iron; or other identified materials such as Type I, II or III porcelain enameling steels, (as described in Manual for Selection of Porcelain Enameling Steels PEI-201), hereby incorporated by reference; or any metal that will not cause chipping, flaking, spalling or fish-scaling of the glassy coating when subjected to temperatures of a self-cleaning cycle of an oven above 500° F., preferably above 900° F.
Ranges may be expressed herein as from “about” or “approximately” one particular value and/or to “about” or “approximately” another particular value. When such a range is expressed, another embodiment includes from the one particular value and/or to the other particular value. Similarly, when values are expressed as approximations, by use of the antecedent “about,” it will be understood that the particular value forms another embodiment.
The above-described features and advantages along with various advantages and features of novelty are pointed out with particularity in the claims of the present disclosure which are annexed hereto and form a further part hereof. However, for a better understanding of the disclosure, its advantages and objects attained by its use, reference should be made to the drawings which form a further part hereof and to the accompanying descriptive matter in which there is illustrated and described preferred embodiments of the preferred disclosure.
Referring to the drawings, where like numerals refer to like parts throughout the several views:
A lubricious surface on the oven rack porcelain coating can be achieved either by mixing a dry lubricant refractory powder homogeneously into the porcelain composition and then applying the porcelain composition to the steel oven rack; or the porcelain coating can be applied to the steel oven rack and sintered followed by coating the sintered porcelain with a lubricious, temperature-resistive coating composition. When a dry lubricant surface layer is applied over a sintered porcelain coating, the dry lubricant active material may form a portion of the uppermost coating layer of the porcelain material, dispersed homogeneously in additional fine powdered refractory materials or, the dry lubricant active material may be discontinuously or continuously embedded into the surface of the porcelain coating material as disclosed in U.S. published application 2006/0089270 A1, hereby incorporated by reference.
In accordance with a preferred embodiment, the lubricious porcelain material is coated over the steel oven rack in one or more coating steps, preferably multiple coating steps selected from wet spray, electro-static wet spray, wet flow coating, wet dip or electro-phoretic deposition (EPE-electro-phoretic enameling). Other suitable coating methods include chemical vapor deposition (CVD), physical vapor depositions (PVD), plasma deposition, and sputtering. At least this surface coating layer, as applied on at least the sidebars (i.e., edge framing wires of the oven rack) that contact the oven cavity side wall and its protruding rack supports, should include a dry lubricant-containing composition in an amount of about 0.1% to about 20% by weight, preferably 0.5% to about 10% by weight of a dry lubricant. The selected dry lubricant used cannot otherwise compromise the final porcelain coating on the oven rack, as such porcelain coating must still pass required quality control tests for porcelain-coated oven racks. Suitable dry lubricant porcelain additives include homogeneously distributed fine powdered particles, e.g., 5 μm to 200 μm, of carbon; graphite; boron nitride, preferably cubic boron nitride; molybdenum (IV) sulfide; molybdenum disulfide; molybdenum sulfide; molybdenum (IV) selenide; molybdenum selenide, tungsten (IV) sulfide, tungsten disulfide, tungsten sulfide, silicon nitride (Si3N4); TiN; TiC; TiCN; TiO2; TiAlN; CrN; SiC; diamond-like carbon; tungsten carbide (WC); zirconium oxide (ZrO2); zirconium oxide and 0.1 to 40 weight % aluminum oxide; alumina-zirconia; and/or antimony or its oxides or trioxides. The dry lubricant is conveniently distributed throughout the porcelain or glass frit composition by adding the dry lubricant to the glass frit (porcelain composition) and then milling the entire porcelain composition containing the dry lubricant, to the final particle size distribution so that the dry lubricant has approximately the same particle size as the other glass components. The particle size of the glass frit or porcelain compositions described herein is not critical and should be the common particle size distribution used by those skilled in the art of porcelain enameling of steel, e.g., 5 μm to about 200 μm. The lubricious porcelain composition can be adhered to the metal oven rack in any manner known in the art, e.g., electrostatically as in electro-porcelain enameling. If the porcelain powdered material is difficult to adhere, a nickel-based or cobalt-based pretreating composition may be coated on the steel prior to the porcelain coating for better adherence of the porcelain to the metal oven rack, as well known in the art.
In another embodiment, the porcelain-coated steel is over-coated (i.e., over the base porcelain coat) with a ceramic wear-resistant powdered refractory composition, generally in a thin layer, e.g., 1 to 10 mils, of wear-resistant ceramic material having, for example, a particle size in the range of about 5 to about 200 microns, preferably about 10 to about 45 microns, followed by sintering.
In one embodiment, the lubricious wear material is a ceramic wear-resistant powder such as a carbide, particularly a chrome carbide. The chrome carbide is typically a material such as Cr23C6, Cr7C3, Cr3C2, and combinations thereof. The chrome carbide is generally in the form of a pre-alloyed carbide powder, wherein the particles of the powder are homogeneous and uniform throughout their cross sections. Alternatively, the chrome carbide, such as Cr3C2, is blended with another material, such as NiCr which functions as a metallic binder. The carbide may be subsequently treated with a halogen etchant gas at high temperature to provide additional lubricity in the integral surface thus-formed, as described in U.S. Pat. No. 6,579,833, hereby incorporated by reference.
In another embodiment, the particulate material for the lubricious coating is comprised of an alloy wear material. In this case, it is advantageous to utilize an alloy that forms a lubricious oxide film over its surface during actual use, which oxide functions to lubricate the interface between the treated porcelain surfaces of the oven racks and the porcelain surfaces of the oven cavity walls at high temperatures (e.g., at least about 900° F. during oven cleaning) to reduce wear. For example, wear is reduced due to presence of the oxide forming alloy during the self-cleaning oven cycle. One particular group of materials that forms a lubricating or lubricious oxide film includes cobalt alloys. Suitable cobalt-based lubricious alloys include the following:
The lubricious, wear resistant outer coating is fused to the underlying porcelain by heating to the fusing temperature, e.g., 1550-2000° F. followed by cooling. Alternatively, this lubricious wear-resistant cobalt or chrome carbide material or cobalt-based alloys can be applied directly to the metal oven rack and fused thereon to provide the lubricious, wear-resistant surface.
Other useful methods of applying the initial porcelain coating over the steel oven rack or for applying a final lubricious coating layer over the base porcelain layer, include chemical vapor deposition and plasma deposition, as well as sputtering. It should be noted that sputtering is a momentum transfer process wherein atoms of the coating material are bombarded onto an underlying porcelain layer by energetic particles. The bombarding species are generally ions of a heavy inert gas, such as argon. The sputtered dry lubricant atoms collide repeatedly with the heavy inert gas atoms before reaching the porcelain layer where they condense to form a coating of the lubricious, wear resistant outer layer. As well known in the art, the underlying porcelain layer may be given a pretreatment, e.g., a plasma treatment to help the outer lubricious, wear-resistant layer adhere to the outer surface of an underlying porcelain layer. Plasma ion bombardment of the outer surface of an underlying porcelain layer may be useful to modify the outer layer of the porcelain by plasma etching in order to achieve better adherence of an outermost layer of lubricious, wear-resistant refractory powder material in order to achieve excellent bonding of the final lubricious coating layer.
Another excellent final finishing lubricious surface coating material includes the self-lubricating material PS-200 developed by NASA, which is a chromium carbide matrix having particles of silver and calcium fluoride-barium fluoride eutectic dispersed therein. In accordance with this embodiment, the chromium carbide matrix may be applied directly over an underlying porcelain material or, as described in U.S. Pat. No. 5,413,877, hereby incorporated by reference, the underlying material may be a zirconia thermo barrier material and the outer chromium carbide layer may be nickel alloy-bonded thereto.
In accordance with still another embodiment of providing an outer lubricious, wear-resistant temperature-resistant outer surface on the oven rack and/or interior surface of the oven cavity, the glassy or porcelain material can be formed from a metal carbide, such as silicon carbide, and treated in a halogen-containing gaseous etchant at high temperature, e.g., about 100° C. to about 4000° C., preferably about 800° C. to about 1200° C. in order to form an integral carbon or diamond surface on the metal carbide, as disclosed in U.S. Pat. No. 6,579,833, hereby incorporated by reference. Another method for forming a diamond surface on the outside of the oven rack or exterior of the oven cavity is disclosed in U.S. Pat. No. 5,108,813 and published U.S. Application No. 2006/0059688 A1, both of which are hereby incorporated by reference.
Referring now to the drawings, and in particular
Referring now also to
Referring now also to
The preferred oven rack 10 is coated with a lubricious glass material 20, preferably porcelain, which is coated onto the outer surface 22 of welded steel wire parts 15 of the coated oven rack 10, in a process which generally follows these steps. Steel rod material (not shown) is preferably purchased, which is made primarily of iron but includes the elemental composition shown below.
PEMCO POWDER-1st Coat: GP2025 (CAS# 65997-18-4), 2nd Coat: GP1124 (CAS# 65997-18-4, plus 0.1-20% dry lubricant)
Furnace Line Speed: 22 ft/min (494 hangers/hour), 988 parts/hour
Washer Line Speed: 22 ft/min (494 hangers/hour), 988 parts/hour
4-10 mil thickness
1585° F. Zone 1 Temp.
1543° F. Zone 2 Temp.
25 minutes in furnace
10,000 lbs/hr maximum line capacity
Specific Gravity: 2.59
The preferred steel rod is then drawn in an area reduction process, preferably through a cold (e.g., room temperature) die, to reduce the diameter of the cross-sectional area, preferably at least about 20%, more preferably at least about 30%, more preferably at least about 35%, even more preferably about 40%, even more preferably about 45%, and most preferably about 50%, in order to incorporate cavities within the steel wire which allow hydrogen to be released into the cavities and also to reduce the diameter of the wire to that which is desired. The table above gives the general specifications for non-iron elements and other aspects of the steel wire and the steel rod used to make the steel wire.
Once the preferred steel rod is converted into wire in the wire drawing process, the steel wire is straight cut to predetermined lengths according to need. The various cut steel wire members are then formed, e.g., bent, as needed to provide the various parts of the coated oven rack. These parts are then welded together to form an oven rack substrate (not shown), for subsequent coating, in a standard welding operation. The oven racks are then cleaned in a washing process and then power acid washed with an electrically charged acid wash material to remove any remaining weld scale. The rack is then dried in an oven at about 500° F. and then air cooled. The clean oven rack is then sprayed with powdered glass preferably in an electrostatic charged paint (porcelain enameling) process in which the oven rack substrate is charged negatively and the glass powder is charged positively. Other metal rack-cleaning methods may be used e.g., blasting (glass beads, steel balls or sand) ultrasonic cleaning, high temperature or low temperature alkaline cleaning or acid cleaning; or the like.
The preferred spraying process is divided into a first coating process in which a first or base coat is placed upon the oven rack substrate. In preferred embodiments the first coat is a Pemco powder, GP2025 (CAS# 65997-18-4) from Pemco International Corp. It will be appreciated that other similar or equivalent porcelain powders may also be used in alternate embodiments. After the first coat is applied a second or top coat is applied. In preferred embodiments, this coat is a Pemco powder, GP1 124, from PEMCO (CAS# 65997-18-4) containing 0.1% to about 20%, preferably 0.5% to about 10% of a dry lubricant refractory material. The final coating also may include a coloring refractory material, such as TiO2, generally of a larger particle size, e.g., >200 μm, added to the milled porcelain composition and homogeneously distributed, in an amount of about 0.1 to 10% by weight, more preferably about 1% to about 5%, to provide white surface fleck coloring in the black composition, for aesthetic purposes. Again, it will be appreciated that other similar or equivalent powders containing the active dry lubricant powder, distributed homogeneously throughout, may also be used in alternate embodiments. The coated oven rack substrate is then heated in an oven to about 1500-1600° F., e.g., about 1550° F. for about 25 minutes and then cooled. This coating and baking process is generally referred to as a double coat, single fire coating process. The coated oven racks are then cooled and then packaged for shipping to the customer. It is to be noted that, in view of the lubricious outer coating, and contrary to the prior art, no additional step of then after-coating the finished porcelain-coated steel wire oven rack with a suitable vegetable oil, e.g., Wesson oil, is needed.
In an alternate process to provide a lubricious outer coating, the oven rack substrate is coated using a wet spray process, wherein the porcelain is coated onto the steel wire, in number of steps selected from each of five distinct wet coating processes including wet spray, electrostatic wet spray, wet flow coating, wet dip or electro-phoretic deposition, or, more specific, as applied to porcelain, “EPE-Electro-phoretic enameling.” This later process involves the use of a dip system where electric power is used to deposit porcelain enamel material on a metal surface. The wet coating processes can be single step, double step or multiple step processes followed by at least single or double heating process steps in which the temperature is preferably raised to a temperature in the range of about 1 500° F. to about 1600° F., preferably about 1550° F. In these processes, porcelain can be coated to steel by any well known basic methods of wet spraying by air atomization, including hand spraying, automatic spraying and electrostatic spraying. When the steel oven rack is processed through a dipping operation, the part is immersed in the “slip”, removed, and the slip is allowed to drain off. In flow coating, the slip is flowed over the part and the excess is allowed to drain off. Carefully controlled density of the porcelain enamel slip and proper positioning of the part is necessary to produce a uniform coating by dip or flow coat methods. The dry lubricant-containing porcelain composition can be coated on the steel oven racks by immersion or flow coating, as well, by five basic methods: hand dipping, tong dipping, automatic dip machines or systems, electro-phoretic deposition systems and flow coating. It will be appreciated that any number of these various methods may be adapted for use in providing a final porcelain layer or surface that is sufficiently lubricious for porcelain-to-porcelain sliding contact without the need for a subsequently-added liquid or oil lubricant for wear-resistance or any periodic re-applications of the same to the oven rack by the ultimate consumer.
Other potential metal substrates to receive a lubricious porcelain coating can include Type I, II, and III porcelain enamel coated steels, as described in PEI-201 Manual for Selection of Porcelain Enameling Steels. Examples of other porcelain coated wire, cast iron or other metal products to receive a lubricious porcelain coating in addition to porcelain coated oven racks includes ladder racks, barbeque grill racks and stove burner grates.
It is to be understood, however, that even though numerous characteristics and advantages of the various embodiments of the present invention have been set forth in the foregoing description, together with details of the structure and function of the various embodiments of the present invention as shown in the attached drawings, this disclosure is illusrtative only and changes may be made in detail, especially in manners of shape, size and arrangement of the parts, within the principles of the present invention, to the full extent indicated by the broad general meaning of the terms in which the appended claims are expressed.
