US3679467A - Burner grate - Google Patents

Abstract

A BURNER GRATE FOR GAS BURNING RANES IS DISCLOSED. THE INTENSE HEAT AREAS OF THE GRATE COMPRISE A STEEL SUBSTRATE INHILIBITED WITH RESPECT TO CARBON MIGRATION COATED HAVE A REMLT TEMPERATURE IN EXCES OF ABOUT 1600*F. AND INCLUDE, IN ADDITION TO SIO2, OXIDES WHICH FUNCTION TO PROMOTE CRYSTALLIZATION, DISSOLVE METAL SCALE, CONTROL THERMAL EXPANSION, AND INCREASE COATING ADHESION.

Description

July 25 1972 c. l.. BE-rzl-:R 3,679,467

BURNER GRATE F'ld Sept. 4, 1970 Invenvon C/ ovo L. Bfrzf@ United States Patent O 3,679,467 BURNER GRATE Cloyd L. Betzer, Kankakee, Ill., assignor to Roper Corporation, Kankakee, lll. Filed Sept. 4, 1970, Ser. No. 69,673 Int. Cl. B32b 15/04; F24c 15/10 U.S. Cl. 117-129 3 Claims ABSTRACT F THE DISCLOSURE DESCRIPTION OF THE INVENTION The present invention relates to -grates for gas yburning ranges and in particular to top burner grates which have unexpectedly long periods of usefulness.

Conventional grates for gas burning ranges and the like are generally comprised of a metal substrate coated with an amorphous ceramic coating such as the Well known porcelain grate coatings. The coatings principally serve to prevent oxidative degradation of the basic metal substrate, to provide resistance to food soil acids which are encountered in the use of the grate, and to give an overall pleasing appearance to the grate. Quite obviously, in order to serve its intended function, the ceramic coating should be resistant to defects such as cracking, crazing, blistering, pin holing, and spalling. Unfortunately, with present grate manufacturing technology, obtaining satisfactory overall performance for extended periods of time has not been achievable, particularly when low cost is also a necessary requirement.

For example, in order to achieve craze and crack resistance with conventional amorphous ceramic coatings in combination with good substrate adhesion, it is generally necessary to employ very thin coatings even when customary adhesion promoting operations such as mechanical cleaning, sand blasting or pickeling are employed. The necessity for a thin coating undesirably restricts the manner in which the coating can be applied (spraying being necessary as opposed to dipping) and also increases the probability of defects such as porcelain pin holes. An even more serious problem with respect to conventional amorphous ceramic coatings is their tendency to spall at the ordinary temperatures of operation of a gas range. In order to diminish spalling, it has been conventional practice to eliminate or minimize the coating in the area of the grate which is subjected to the direct impingement of ame. Thus, the lower edges of conventional grates are normally only given a dust coating of the `amorphous ceramic. Consequently, this area of the grate is Very susceptible to oxidation and rusting, a feature which severely limits the grates useful life. Moreover, even when cracking, crazing, and spalling are minimized as above discussed, conventional grates still exhibit significant blistering in actual use. Blistering is thought to be due to oxide scaling on the metal substrate during processing of the ceramic coating or to gas formation at the substratecoating interface during use in a high temperature environment.

Accordingly, it is a principal object of the present invention to provide a burner grate which can be used for a long period of time without exhibiting those defects which are customarily encountered with conventional ICC grates. A related object is to provide a burner grate having the characteristics specified in the principal object, and which has only Ia single, continuous ceramic coating and can -be economically prepared.

Other objects and advantages of the present invention will become apparent as the following description proceeds, taken in conjunction with the accompanying drawing in which: Y

FIG. l is a top plan view of a burner grate illustrating an embodiment of the present invention;

FIG. 2 is a sectional view of the burner grate of FIG. 1 looking along line 2 2 in FIG. 1; and

FIG. 3 is -a fragmentary section looking along line 3 3 of FIG. 2 illustrating the metal substrate and the ceramic coating with the thickness of the coating being greatly exagerated for the purpose of illustration.

While the invention is susceptible of various modifications and alternative constructions, there is shown in the drawing and will herein be described in detail the preferred embodiments. It is to be understood, however, that it is not intended to limit the invention to the specific forms disclosed. On the contrary, it is intended to cover all modilications and alternative constructions falling within the spirit and scope of the invention as expressed in the appended claims. For example, while FIG. l illustrates a particular design for a top burner grate, the invention is not limited to the particular design illustrated, and it should be understood that a wide variety of grate designs can be used.

Turning now to the drawing, there is shown in FIGS. l and 2 a burner grate 10 suitable for use in combination with a gas range. In use, the grate which is comprised of the ring 12 and the V-shaped support members 14 is positioned over the gas burner of the range. In operation, the item being heated is placed directly on the V-shaped grate support members 14 which are evenly spaced around the ring 12 and welded or otherwise firmly attached thereto. Those portions of the V-shaped support members which extend beyond the ring 12 rest on the range when the grate is in position over the burner.

The material of construction for the support members 14, and particularly those portions of the support members which are subjected to intense heat such as the direct impingement of the burner tiame, constitutes an important aspect of the present invention. As particularly illustrated in FIG. 3, the support members 14 structuraly comprise a steel substrate 16 having a conventional size .and conguration containing a continuous coating of a semi-crystallized ceramic 18. Typically, the ceramic coating 18 has a thickness of about 3-8 mils and, as illustrated, extends continuosly around the substrate 16. By, as hereinafter described, using a particular ceramic coating in combination with a particular metal substrate, a burner grate having the advantages indicated previously with respect to longer useful life can be obtained.

Referring rst to the semi-crystallized ceramic coating, it has been found that the composition described in U.S. Pats. 3,397,076; 3,463,646; and 3,458,344 issued to John R. Little and Elbert A. Sanford are particularly suitable for use herein. Basically, these compositions contain Si02 as the principal ceramic forming ingredient, and contain additional quantities of other oxides in order to promote crystallization of the ceramic, dissolve scale which may form on the metal substrate during processing, and to accomplish various other effects such as adjusting the fusion temperature and thermal expansion of the coating and enhancing the smeltability of the coating and its adhesion to the metal substrate.

lFor example, oxides such as BaO, CaO, and MgO, are particularly useful in promoting crystallization of the ceramic coating with TiO, also being used principally as a nucleating agent. With respect to scale solution, the inclusion of B20, and P205 have been found to be particularly effective though, as explained in U.S. Pat. 3,463,646, their amounts must be controlled in order to avoid inhibiting crystallization or other undesirable effects. With respect to substrate adhesion promotion, the following oxides are particularly effective; CeOg; ZnO; C0304; NiO, Mn02; and Fe304.

While a variety of compositions disclosed in the aforementioned patents to Little and Sanford can be used in formulating the semi-crystallized ceramic coatings useful herein, it is particularly important, in order to obtain good substrate adhesion and resistance to spalling, that the coating contain a scale dissolving oxide and have a remelt temperature in excess of about 1600 F. and, preferably, in excess of about 1700 F. The remelt temperature 4refers to that temperature at which the coating will again revert to a fluid after it has been semicrystallized. Due to crystallization, the remelt temperature is ordinarily about several hundred degrees greater than the initial firing temperature. By using a coating with the indicated high remelt temperature, it has been found that a continuous coating on the grate support members can be used without danger of spalling occurring in the areas of direct llame impingement. The ability to employ a continuous coating not only leads to increased grate life but has economic advantages since the coating operation is simplified.

Coating compositions particularly useful herein will generally contain about 45-65 weight percent SiOz, about 4-20 weight percent of a crystallization promoting oxide, 5-20 .weight percent TiOg, 5-20 weight percent of a scale dissolving oxide with P305, when included, being present such that the ratio of P105' to B203 is between 1:3 and 2:1, and up to 3 weight percent of adhesion promoting oxides. In addition, other oxides such as NazO and KzO can be included in varying amounts, e.g., -20 weight percent, in order to control thermal expansion and fusion temperature. In this respect, a coating having a thermal expansion characteristic of about 4-7Xl0 in./in./ F. over a 80-1000 F. range is preferably employed herein.

In addition to using a ceramic coating having the characteristics described above, it is a further important aspect of the present invention that the metal substrate have certain characteristics. In order to avoid detrimental crazing or cracking of the ceramic coating, it is important that the metal substrate have thermal expansion characteristics which are compatible with those of the ceramic coating. To this end, the thermal expansion of the metal should be slightly greater than that of the ceramic coating and generally be in the range of about 548x104 in./in. F. over a 80-1000 F. range. Preferably, the thermal expansion characteristic of the substrate is iabout 5-10X l0/in./in./ EF. A further impory tant characteristic of the steel substrate is that carbon migration to the surface of the substrate is effectively inhibited under the temperature encountered in burner grate By inhibiting carbon migration, it has been found that 'carbonmigratiom Additionally, useful stainless steels can also contain a small quantity of nickel in order to md in inhibiting surface oxide formation. Particularly useful stainless steels contain, on a weight basis, about ,0.040.08% carbon, about 913% chromium, about 0.5% nickel, about 0.5% titanium, and about 1%` 3 manganese. Such steels, for example, are sold under the l Other than those of the stainless variety, titanium stabilized low carbon steels can also be used herein. An example of such a steel is Ti-Namel sold by Inland Steel Corporation and characterized by the following composition by weight: 0.06% carbon; .3% Mn: 0.01% silicon; 0.12% copper; 0.05% aluminium and 0.3% titanium. From an economic view point the titanium stabilized'low carbon steels have advantages over the stainless varieties.

Processing of the ceramic coating onto the metal substrate can be accomplished by known techniques though the firingtemperatures are ordinarily higher. A useful processing procedure includes mixing the ingredients of the ceramic coating composition together and thereafter fusing the composition at a temperature of about 2400- 2800 F. A frit is then produced by quenching the molten ceramic directly into cold water. Thereafter, the frit is milled with water and other appropriateadditives such as clay, sodium nitrite, potassium chloride, etc. to produce a slip which is then applied to the metal substrate by any well known technique such as spraying or dipping. Prior to application, the surface of the substrate is ordinarily Y mechanically cleaned, sand blasted, or chemically etched in order to increase coating adhesion. After application of the coating to the substrate, the coating is red at a temperature generally in excess of about 1600 F. for 5-10 minutes depending on thickness and design of the grate and thereafter cooled. The resulting ceramic coating on the metal substrate is semi-crystallized.

In order to illustrate the superior performance characteristics of a grate prepared in accordance with the present invention, a grate was prepared as follows: A structure similar in design to that illustrated in FIG. l with the V-shaped support members made out of the previously described titanium stabilized low carbon Ti- Namel steel was prepared and the surface of the support members sand blasted with silica. Thereafter, the support members of the grate were sprayed with a continuous coating prepared from Nucerite R-l16 coatingfrit. Coating thickness was about 4-6 mils. Nucerite R-116 coating was supplied by the Pfaudler division of Sybron Corporation and is a semi-crystallizable ceramic composition within the scope of those described in the aforementioned U.S. patents to Little and Sanford. It is characterized by a density of about 2.93, a fusion button of about 139 (vrs. 53), and a ring gap of about -16. After application of the coating the grate was heated at about 1600 F. for about 5 minutes and then cooled.

The grate prepared as above-described was subjected to accelerated testing over a conventional burner flame. It was observed that the semi-crystallized ceramic coating on the grate maintained good integrity even after 7000y hours of exposure. In contrast, conventional grates contammg amorphous ceramic coatings show a noticeable destruction of coating integrity after only about a 30-300 hours of exposure. Thus, it is clear that the burner grates prepared in accordance with the present invention have a substantially greater useful life than do conventional grates. f

I claim as my invention:

1. In a burner grate for use in combination with a gas burning range or the like, the improvement wherein those portions of the grate which are subjected to intense heat f comprise a stainless steel or low carbon steel containing a carbon migration inhibiting quantity of titanium substrate having a firmly adhered continuous coating of fused semi-crystallized ceramic having a remelt temperature in excess of about 1600 F. and prepared from a composition which comprises, on a weight basis, 45-60% SiOg, 4-20% of crystallization promoting oxides selected from BaO, CaO, and rMgO, 5-20% TiOa, 5-20 of scale dissolving oxides selected from B30, and P305 such that,

when P205 is present, B203 is also present in an amount such that the ratio of P203 to B203 is between 1:3 and 2:1, and up to 3% of adhesion promoting oxides selected from CeOa; ZnO, C0304, NiO, MnO'2, and Fe304.

2. The burner grate of claim 1 wherein the coating of semi-crystallized ceramic is prepared from a composition which also contains NazO or KZO.

3. The burner grate of claim 2 wherein the steel substrate is a low carbon steel containing a carbon migration inhibiting quantity of titanium.

References Cited UNITED STATES PATENTS 2,861,563 11/1958 Jensen 126-214 C OTHER REFERENCES Tensile and Hardness Properties of Carbon and Alloy Steel, Pittsburgh, Heppenstall Co., 1948, p. 23.

ALFRED L. LEAVI'IT, Primary Examiner lo J. A. BELL, Assistant Examiner U.S. C1. X.R. 126-214 C

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