US2004632A - Method of enameling - Google Patents

Description

9 1y suited for use as enameling stock. It is well depend .upon the purpose for which the article 40 Patented June 11,1935 I 4 2,004,632

UNITED STTES PATENT orrlce METHOD OF ENAMELING Wesley G. Martin, Milwaukee, Wis., assignor to A. 0. Smith Corporation, Milwaukee, Wis., a corporation of New York No Drawing. Application June 11, 1934,

Serial No. 730,063

7 Claims. (01. 91-73) This invention relates to the art of vitreous The method of this invention makes it possienameling. ble to do so, and to effect important economies Anobject of the invention is to provide amethby the use of less expensive material than the d of enameling to produce a hard, non-porous, enameling grades of iron and steel now used by glossy enamel. the enameling art. The method of the invention 5 A further object of the invention is to provide also provides for the application of enamel coata method of enameling which avoids difficulties ings to the higher carbon steels and the producthat have heretofore been encountered in enameltion of enameled articles having a base of greater ing operations, and one which also permits the strength than the low carbon steels now used in 10 application of durable enamel coatings to kinds the enameling industry. It further facilitates the 10 of metal which have previously been used but application of enamels to cast iron. little, if at all, as a base for the application of In accordance with the invention, the enamel enamel. covering is fused to the metal base in an atmos- This application describing and claiming my phere which is substantially inert or non-reacimproved method of enameling is a continuation tive with respect to the enamel and metal, in- 15 in part of my two copending applications, Sestead of in air, the atmosphere in which the firrial No. 655,215, filed February 4, 1933, and Seing or burning of enamels is now done. As a rerial No. 656,567, filed February 13, 1933. sult of fusing the enamel in a, substantially inert While coatings of vitreous enamel have been, or non-reactive gas, there is substantially no blisand are being extensively used to cover the surtering, reboiling, or copperheading. 20

faces of articles made from ferrous metal, it is Nitrogen and carbon dioxide are examples of to be observed that the ferrous metals used as gases which are suitable for the practice of this a base for the application of an enamel coating invention. Neither of these gases reacts appreare in general of two kinds, either a cast iron, or ciably with the enamel or the metal under the 5 a low carbon iron 'or' steel. In the making of conditions encountered in the practice of this household utensils, panels, signs, and innumerprocess, a d oth may b rme in rt gases able other articles, steel or iron containing less for the purpose of the invention. Other inert than 0.20% carbon is generally utilized. While gases may a so be us d if d, gaseous 0.20% carbon steel is in reality a low carbon mixtures which are substantially inert or nonsteel, it is considered a high carbon steel for reactive with the enamel or the metal base. 'A 30 enameling purposes. Difiiculties in applyingasatpractical advantage of a gaseous mixture is that isfactory coating of enamel to higher carbon a suitable mixture of gases can often be obtained steel have militated against the use of such steels more readily and cheaply than a substantially for enameling stock, and so far as applicant is pure gas.

aware, such steels are used only in limited quan- This invention has been practiced successfully 35 tities, if at all, as a metallic base to which enamel wth cast irons and with irons ranging from the is applied. lowest carbon irons ordinarily utilized in making Nor are all irons and steels of low carbon conenameled articles to steels containing about one tent, even though well below 0.20% carbon, equalper cent carbon. The steel or iron selected will known to the enameling art that many steels i emp y which have a low carbon content and which also The enamel mixture used to form the covering possess the mechanical properties required of of enamel upon the article may be one of theenameling stock, give rise to difficu'lties in the commercial mixtures available on the market.

burning of enamel coverings and are therefore The selection of the enamel will depend upon 45 not used as a base for the application of enamel, the results which it is desired to obtain. The preference being given to special enameling stock choice of the enamel forms no part of this invenwhich more readily receives an enamel coating tion since theinvention can be practiced witha even though it is more expensive than the other wide range of enamels. Excellent results have materials. The use of the higher priced enamelbeen obtained with enamels producing blue 50 ing stock naturally results in a greater expense ground coats. The invention also facilitates the in the production of enameled articles than application of white ground coat, enamels which would be necessitated if it were feasible to apply contain substantially no cobalt oxide. The folsatisfactory enamel coverings to a less expensive lowing formula is one example of an enamel which base. 4 has been usedin the practice of this invention 55 but it is to be understood that the invention is not limited thereto:

Parts by weight Silicon dioxide 20 Feldspar 25 Borax 36 Soda ash 6 Fluorspar 6 Sodium nitrate Manganese dioxide 1.5 Cobalt oxide 0.5

pickling, sand blasting, or any suitable cleaning process. A coating of enamel mixture is then applied in any suitable way, for example, by dipping or spraying.

After the articles have been coated they are dried, preferably in a drying chamber or oven. This oven may be any well known construction and heated by any of the standard methods, such as by means of'burning coal, oil, or gas, or by means of electricity.

After being dried, the articles are preferably heated in air or some other oxidizing atmosphere to a temperature which is too low to fuse the enamel, but which is sufficiently high to cause oxidation'of the metal surface, and are held at this temperature until an oxide film has formed on the metal surface below the coating of enamel mixture. The formation of an oxide film by this preliminary heating in 'air or other oxidizing atmosphere may be substituted by different modifications of the process which are described hereinafter, or may, in some cases, be dispensed with entirely. In general, however, it has been found that the most satisfactory results are obtained by heating the article in air to form an oxide film after the enamel mixture has been applied to the article, and hence this procedure is usually preferable to the other modifications.

The article which has been oxidized on the surface is then introduced into a fusing chamber in which the enamel is fused in a substantially inert or non-reactive atmosphere. The fusing chamber may be of any suitable construction and is maintained at a temperature which is suitable for the fusion of the enamel being used. Many enamels fuse satisfactorily at a temperature of about 1600 F.

The atmosphere in the fusing chamber is nitrogenfcarbon dioxide, or other suitable inert gas -or mixture of gases. A suitable and inexpensive mixture of gases consisting primarily of nitrogen and carbon dioxide can be simply and cheaply produced by the reaction between air and coke. The producer gas produced by blowing air through an incandescent coke bed contains principally nitrogen and carbon monoxide, some carbon dioxide, and small quantities of other gases.

When secondaryair is introduced into the gas producer above the coke bed it combines with the producer gas formed by blowing the primary air through the gas producer, and oxidizes the carbon monoxide to carbon dioxide to an extent which depends upon the amount of secondary air. By proper regulation of the secondary air, gas mixtures are easily produced which consist primarily of nitrogen plus carbon monoxide and carbon dioxide in any. desired proportions. Mixtures containing about 70% nitrogen, 20% carbon monoxide, and 10% carbon dioxide have been used successfully in burning enameled articles in a leaky furnace, the carbon monoxide in the mixture combining with the oxygen, in the air leaking into the furnace and consequently insuring that the atmosphere in the furnace is substantially devoid of free oxygen. For use with a tight furnace in which there is no leakage of air into the furnace, a higher proportion of carbon dioxide is preferable in the gas mixture, excellent results being obtained with a gas which is substantially all nitrogen and carbon dioxide.

- From the standpoint of heat economy it is desirable that the gases produced by the reaction between air and coke should be immediately introduced into the fusing chamber in order that they may assist in maintaining it at the temperature required to fuse the enamel. These gases, on the other hand, should be free from tarry matter and any solid particles which might deposit upon the enamel ware and injure the surface. This may require, in some cases, that the gas be cooled in order that it may be sufficiently cleaned before it is introduced into the fusing chamber.-

should also be freed from the water vapor produced by combustion of the hydrogen. The simplest way of removing the water vapor is to cool the gas to condense out the water vapor. Cooling the gas to room temperature results in the removal of all but a proportion of water vapor which is too small to be injurious. A gas mixture containing 84.0% nitrogen, 2.0% carbon monoxide, 13.0% carbon dioxide, and 1.0% hydrogen, was produced in this way and was found to be satisfactory for the practice of this invention, the proportions of hydrogen and water vapor being too low to do .any damage.

When gases of this nature are forced into the fusing chamber, they force out the air and establish an atmosphere which is substantially free from oxygen. Although oxygen is present in air to the extent of about 20% and enameled articles possessing excellent coatings of enamel can be produced by burning the enamel in air, it seems, nevertheless, that oxygen is responsible for certain of the difliculties that are encountered in enameling practice. Tests that have been conducted in connection with the practice of this invention lead to the conclusion that uncontrolled oxidation of the steel during the fusion of the enamel is intimately connected with such defects the controlled. atmosphere of non-reactive gas and the burning or firing of the enamel completed in the same atmosphere. It is sufiicient, however, and the process is somewhat more economical if the enamel is fused in the controlled atmosphere and the firing is completed in air.

The fusing in an inert atmosphere should be continued until the enamel mixture has been melted into .a continuous layer. This requires about four or five minutes of the firing cycle under the conditions specified in the preceding paragraph. By breaking the firing cycle into two parts, the first being carried on in a non-reactive atmosphere and the second in air, a saving can be effected in the amount of inert gas required. Thefiring of the enamel may be completed'in the non-reactive atmosphere, however, and will still be of low cost-because of the cheapness of the gas which is satisfactory for the purpose of the invention.

One of the most pronounced difficulties encountered in attempts to enamel steels containing more than about 0.20%or 0.30% carbon by firing the enamel in air, is the tendency of the enamel to blister. The enameling of cast iron also presents diflicultiesdue to the formation of blisters. While cast iron can be enameled by firing in air, this is done by the use of softer enamels having a lower firing temperature than those used on sheet iron, and it has heretofore not been feasible to use the wet enameling process to apply to cast iron the same enamels as those applied to .sheet iron, or enamels having approximately the same properties. With the method of the present invention it is possible to apply sheet iron enamels to cast iron and thereby obtain higher quality-enameling of cast iron.

By using a controlled atmosphere of a nonreactive or inert gas, however, steels ranging from 0.20% carbon up and cast iron as well as low carbon iron and steel can be enameled without blistering or copperheading, and with substantially no fishscaling. When the proper formula for the enamel is selected; there is no fishscaling. An enamel having a coefficient of expansion corresponding to the coefiicient of expansion of the metal should be selected to combat fishcaling. When this is done by the methods of choosing an enamel having certain characteristics which are well known to enamelers, and-the enamel is fused in a non-reactive gas, fishscaling, copperheading, and blistering, are avoided.

Furthermore, when the controlled inert at mosphere-is utilized in firing a ground coat, another coat may be applied without any danger of reboiling. This is a very vital factor in enameling articles which require more than one coat of enamel.

Consequently by the practice of my improved method of enameling, it is practicable to apply satisfactory coatings of enamel to steels of any carbon content, and to apply to cast iron enamels having the hardness and high fusing tem-v peratures of the enamels which have been used on low carbon irons and steels. By avoiding difliculties formerly encountered in enameling, the improved method not only facilitates the application of an enamel coating to metals of the kind previously used in enameling, but also extends the applicability of enameling processes to metals other than those which it has been feasible to enamel successfully in the past.

The result of the formation of an oxide film on the surface of the metal article before the enamel is fused is to produce a better bond between the metal and the enamel than is obtained' whenthe formation of the oxide film is omitted. In oxidizing the metal, it first takes on astraw color, then changes to blue, then to brown, and finally to black. While it is usually preferable to oxidize the metal until it becomes brown. in some instances'the bonding of the enamel to the metal is improved by a thinner film of oxide, one which is bluish in color, and

in other cases good results are obtained by permitting the process of oxidation to continue beyond the brown.- been employed successfully with low carbon stock. The degree of oxidation which gives the most satisfactory results depends upon a number of factors, among which may be mentioned the roughness of the surface to which the enamel is applied and the carbon content of the metal base. Usually the degree of oxidation which gives the best results decreases as the roughness of the surface or the carbon content of the steel increases. However, when enameling the higher carbonsteels it is sometimes found that a degree of oxidation which is satisfactory for a roughened surface is too much for a very smooth surface of the same steel. The most satisfactory results have been obtained in enameling the higher carbon materials when a rough surface is used. For such materials sand blasting is preferable to pickling as a means for cleaning th surface. The degree of oxidation suitable for any particular set of conditions isreadily ascertained by applying the enamel mixture to several specimens of the metal, heating these for different periods of time to produce different degrees of oxidation, burning in the controlled atmosphere, and comparing the results.

. The following example is illustrative of suitable conditions for one application. Articles made from 20 gauge iron and steel containing about 0.05% carbon were covered with a coating of enamel mixture, dried, heated in air to 1000 F. for ten minutes to form an oxide film on the metal, and then fired at 1600 F. for five minutes in a substantially inert atmosphere to fuse the enamel. An execellent enamel coating was produced. Attempts to fuse the same enamel to the same steel in air were uniformly unsuccessful, the enamel coatings produced being in all cases badly biistered and copperheaded. The steel used in these tests,"

The thick film of oxide has improved process. This process therefore permits the use of less expensive materials as a base for enamels, and consequently makes possible a considerable saving in the production of enameled articles.

While it is usually preferable to follow the procedure described above and form an oxide film on the surface of the metal before the enamel is fused in an inert atmosphere, in some cases this may be omitted. The bond between the enamel and the metal appears to depend upon the character of the metal surface and to be better when the surface is rough as a consequence of the way in which it has been'manufactured or prepared; In enameling roughened surfaces, such as sand blasted surfaces particularly of articles which are relatively thick and which consequently require a longer burning time than thinner articles, a bond which is satisfactory for some purposes is obtained by simply applying the enamel mixture to the article, drying, and fusing the enamel in an inert atmosphere without the preliminary heating in air to form an oxide film. When the bond produced by this simplified procedure is insufllcient to be entirely satisfactory, the previously described procedure is employed, or is substituted by one of the following modifications.

Instead of oxidizing the metal surface after the enamel has been applied, the metal article may be heated in air to cause the formation of an oxide film, after which the enamel mixture is applied to the oxidized surface, dried, and fused in an inert atmosphere. If this is done, the metal has to be heated to a high temperature to cause oxidation and then cooled sufliciently to permit the application of the enamel. In some cases this may be desirable, but it is usually preferable to oxidize the metal surface after the enamel mixture has been applied. When the oxidation is effected by heating the article after the enamel mixture has been applied and dried, the heated article can be immediately introduced into the fusing chamber in which the enamel is fused in an inert atmosphere. By thus utilizing the heat which the article retains as a result of the heating to produce oxidation, there is a saving in heat expenditure over the modification in which the article is first heated to effect oxidation of the surface, then cooled to permit the application of the enamel mixture, and then reheated to fuse the enamel.

Other methods of applying a coating of oxide to the surface may also be used. Iron oxide, nickel, manganese or cobalt oxide, or other oxide of similar characteristics may be mixed with a vehicle and applied by painting or spraying over the surface of the metal. In applying oxide in this way. great care should be exercised in getting the coating even.

Satisfactory results have also been obtained by mixing iron oxide or other similar oxide in the enamel mixture. The oxide may be one, like iron oxide, which is not ordinarily found in ground coat enamels, or it may be an additional quantity of such oxides as cobalt, manganese, ornickel oxide, which are found in many of the ground coat enamels designed to be fused in air. This additional oxide may be smelted in with the other ingredients of the enamel or added when the enamel frit is milled, and serves as a bonding agent. When iron oxide is used in this way it tends to color the enamel, and this may or may not be a disadvantage, depending upon circumstances. 1

' All of the above described modifications possess the application of the enamel, by applying a coating of iron or other oxide to the metal before applying the enamel, or by adding iron oxide or a similar oxide to the enamel composition customarily used for fusion in air. By the use of suitable amounts of combined oxygen and the practical exclusion of freeoxygen from the fusing chamber, uncontrolled oxidation of the metal surface giving rise to copperheads and blisters is avoided and a hard, dense, glossy enamel coating is produced. Furthermore, the fusion of the enamel in an atmosphere from which free oxygen is excluded prevents burning off the enamel and permits the use of thinner coats of enamel than when the fusion is done in air.

It is to be understood that in each of the modi-, fications, the burning of the enamel can be begun and completed in the controlled .atmosphere, or begun in the controlled atmosphere and completed in air.

I claim:

i 1. In the art of vitreous enameling ferrous metals by a wet process, the step of improving the physical properties of such an enamel when mosphere substantially devoid of free oxygen,

water vapor and hydrogen and containing a mixture consisting principally of nitrogen, carbon dioxide and carbon monoxide.

2. In the art of vitreous enameling ferrous metals by a wet process, the step of improving the physical properties .of such an enamel when applied to non-enameling iron and steel stock, which consists in fusing the enamel in an atmosphere substantially devoidof free oxygen, water vapor and hydrogen and which contains principally one or more of the gases from the group consisting of nitrogen, carbon monoxide and carbon dioxide.

3. The method of wet process vitreous enameling of ferrous metal articles in which the metal of the article is of such characteristics as to give rise to one or more of the group of defects in the enamel known as reboiling, copper heading, blistering and fishscaling when the enamel is fused in air, comprising applyingto the metal surface to be enameled an even coat of enameling material, drying the enameling material, and fusing the enameling material in an atmosphere substantially devoid of free oxygen, water vapor, hydrogen and other deleterious substances, and substantially non-reactive with the metal base and the enameling material, to produce a dense, glossy coat'of vitreous enamel substantially free from defects.

4. The method of wet process vitreous enameling of ferrous metal articles in which the metal of the article is of such characteristics as to give rise to one or more of the group of defects in the enamel known as reboiling, copper heading, blistering and fishscaling when the. enamel is fused in air, comprising applying to the metal surface to be enameled'an even coat of enameling material, drying the enameling material, heating the article and its coat of enameling material to a temperature less than fusing temperature of the material in the presence of oxygen to obtain a predetermined coat of iron oxide beneath the coat of enameling material, and fusing the en- 5. The method of wet process vitreous enameling of cast iron comprising, applying to the metal surface to be enameled an even coat of enameling material of the kind ordinarily applied to sheet iron and steel and having a high fusing temperature, drying the enameling material, and fusing the enameling material in an atmophere substantially devoid of free oxygen, water vapor, hydrogen and other deleterious substances, and substantially non-reactive with the metal base and the enameling material, to produce a dense, glossy coat of vitreous enamel substantially free from defects.

6. The method of wet process vitreous enameling of ferrous metal containing in excess of 20% carbon comprising, applying to the metal surface to be enameled an even coat of enameling material, drying the enameling material, and fusing the enameling material in an atmosphere substantially devoid of free oxygen, water vapor, hydrogen and otherdeleterious substances, and

substantially non-reactive with the metal base and the enameling material to produce a dense,

glossy coat of vitreous enamel substantially free from defects. 1

7: The method of wet process vitreous enamel-Q ing ferrous metal articles and eliminating reboiling of ground coat enamels in the application of 3 wet process vitreous enamels to ferrous metal articles the metal ofwhich, after firing of a ground coat in air, tends 'to liberate gases beneath the enamel coating during subsequent firing operations causing reboiling of the enamel, w

comprising applying an even coat of ground coat enameling material to the metal surface to be enameled, drying the enameling material, fusing theenameling material in an atmosphere substantially devoid of free oxygen, water vapor, 35

hydrogen and other deleterious substances, and which is substantially non-reactive with the enameling material and metal base, applying a cover coat of enameling material to said article, and subsequently firing said cover coat enm amel.

WESLEY G. TIN.