US2755210A - Method of treating iron or mild steel to promote the adherence of porcelain enamel, and stock so produced - Google Patents

Description

July

F. G. SUTPHEN ET AL METHOD OF TREATING IRON OR MILD STEEL TO PROMOTE THE ADHERENCE OF PORCELAIN ENAMEL, AND STOCK SO PRODUCED Fi led Sept. 7, 1955 INVENTORS. /7?Ea G. 31111-7015, Jzsss J CmvF/zLa,

Foes" L. Mrfes, ties-5P E. 34016 QZIQMM ATTORNEYS.

United States Patent METHOD oF TREATING moN OR- MILD STEEL TO PROMOTE THE ADHERENCE or ro 'cE- LAIN ENAMEL, AND sTocK so PRODUCED Fred G. Sutphen, Jesse J. Canficld, Robert L. Myers, Joseph E. Sams, and Robert S. Burns, Middietown, Ohio, assignors to Armco Steel Corporation, Middletown, Ohio, a corporation of Ohio Application September 7, 1955, Serial No. 532,986

22 Claims. (Cl. 148-16) This application is a continuation-in-part of our 00- pending application of the same title, Serial No. 101,464, filed June 25, 1949, since abandoned.

A primary object of our invention is the provision of a process of treatment for promoting the adherence of porcelain enamels to ferrous metals. Certain of the teachings herein are applicable to ferrous metal articles of any kind suitable for porcelain enameling; and in this connection it is an object of our invention to provide an adhesion promoting treatment which is simple, corivenient, and inexpensive.

It is a fundamental object of our invention to provide an improved enameling. stock made of iron or mild steel in sheet form, and having the property of accepting single, fired-on porcelain enamel coatings of white or light color without surface defects and with great improvement in adhesion.

There has in recent years been considerable activity in endeavors to manufacture iron or mild steel sheet stocks capable of being enameled with a single, light colored or white coat; but these efforts have been concerned principally with the prevention of surface defects. It is essential with such stocks that the metal to be enameled be treated in order to prevent boiling in the initial coat of porcelain enamel applied and fired on the surfaces of the metal. Various suggestions have been made for such treatments, including the fixing of the carbon in the iron or mild steel sheet stocks by means of titanium or other metallic elements. Iron or mild steel stock may be rendered capable of accepting a one white coat enamel directly on the base metal with the surface of the enamel substantially free from pits and black specks by the inclusion of a surface decarburizing step in the routing-s for the production of the stock, and by the correlation of this step with other processing steps whereby to reduce greatly the carbon and other gas-forming substances in and on the sheet surfaces, which substances have been found responsible for the surface imperfections appearing in an initial coating of porcelain enamel. The adhesion promoting aspect of this application is of importance in the preparation of any of these stocks inasmuch as, while. the treatments mentioned in varying degrees alleviate the boiling of initial enamel coatings, the problem of adherence in one coat enameling is of great importance because the use of an undercoat of adhesion promoting enamel is limited or eliminated. As a consequence, another object of this invention is the provision of a process especially adaptable to the production of improved one coat White enameling stocks, as well as the provision of improved stocks of this character. In particular, it is anobject of the invention to provide an enameling stock not only capable of receiving a single-fired coat of'white or light colored enamel with substantial or complete elimi- Whenever we refer to white or light colored enamel H in this specification, it is to be understood that we include taillight colored enamels which are the most sensitive to 5 2,755,210 Patented July 17, 1956 Ind pits and black speck defects, and which must, therefore, be free from such defects for the sake of appearance, and which are too light to permit the inclusion of sufiicient cobalt compounds to result in good adherence without unduly affecting their color.

These and other objects of our invention, which will be set forth hereinafter or will be apparent to one skilled in the art upon reading these specifications, we accomplish by that procedure and in that enameling stock of which we shall now describe an exemplary embodiment.

Reference is made to the accompanying drawings wherein:

Fig. l is a cross-sectional photograph at 1000 magnification of ordinary enameling stock which in its processing included cold reduction on a Pangborn or shot blasted rel-l, normalizing, and pickling.

Fig. 2 is a similar view in cross-section of material made in accordance with this invention.

Fig. 3 is a photomicrograph of the surface of the enameling stock of Fig. 1 under dark field illumination at 500 magnification.

Fig. 4 is a similar view of the surface of a material treated in accordance with this invention.

T he need for adhesion promoting treatments has hitherto been recognized, in view of the experience that the bond of the enamel to the base metal appeared generally to be the poorest when the enamel surface was most perfeet. The deposition of some nickel by chemical displacement upon the surface of an enameling stock has been recognized as a treatment to promote the adhesion of porcelainenamels chemically. A roughening of the surfaces of enameling stock by various means has also been recognized as promoting adhesion; but the processes hitherto suggested for the obtaining of a useful type of surface roughening have been expensive, inconvenient, ineffective, or otherwise undesirable.

We have now discovered a process for the production of channeling stock which will have an excellent enamel bond but which will still have all of the surface characteristics of the original material inclusive of those materials which have been treated to prevent boiling upon initial firing.

in the practice of our invention, the routings for the enameling stock include, among others, the following steps which must be practiced in the order mentioned:

1. The surface of the ferrous metal sheet stock or article is coated with a thin layer of cobalt or nickel, as will presently be described.

2. The treated stock is scaled, i. e. oxidized, by heating it in an atmosphere oxiding and non-carburizing to iron. This appears to alloy at least a part of the coating metal with the iron, and produces other efiects hereinafter discussed.

3. The scaling is followed by an acid pickle. ordinary pickling solutions may be employed.

We do not desire to be bound by theory, but we believe the enhanced adhesion which is the result of our process is due in part at least to the production on the surfaces of the iron or steel stocks or articles of a particular type of roughening which is especially suitable for the retention of enamel. We believe that the coating of imposed metal affects the nature of the surface oxidation during the scaling operation in an unusual manner, such that when the oxide is pickled from the surfaces of the stock an especially useful type of roughening occurs. We believe that during the oxiding heat treatment, which is step 2 indicated above, some alloying of the imposed metal with the base metal occurs, resulting in unequal distribution of the oxide and the imposed metal. Upon subsequent pickling, as the scale is removed, the pickling solution not only readily attacks the surfaces of the metal sheets or articles, but attacks these surfaces unequally The and preferentially in certain areas, resulting in the particular type of roughening to which reference has been made.

We shall now discuss the requirements respecting the three steps set forth above before taking up general routings, including these steps, for enameling stocks.

The metal for imposition upon the ferrous enameling stock should be cobalt, nickel, mixtures or alloys of the two, or alloys rich in cobalt, nickel or both. Our investigations have shown that cobalt and nickel act, under the conditions of the procedure herein set forth, 111 a unique way toward iron in the surfaces of ferrous enameling stock, in that minute, adjacent surface areas are formed which differ in their ability to become oxidized, so that, when the oxide is ultimately removed by pickling, a wholly novel type of surface is produced. Other metals differ as to their ability to alloy with iron, or their ability to oxidize under processing conditions, or form oxides which sublime under operating conditions, or as to their cost and availability. Nickel and cobalt act similarly; they may be used alone or in admixture; and inasmuch as the bare presence of other metallic elements is not detrimental per se, metallic combinations rich in nickel or cobalt or both may be used.

As to the quantity of nickel or cobalt applied in the first step above, in general, we apply from substantially .02 to .2 gram per square foot of surface of the sheet stock or articles, our preference being for .05 to .10 gram per square foot, as producing the most consistent and dependable results.

Nor are we limited to the manner in which the imposed metal is applied. It may be applied by electrochemical displacement, as for example when a cleaned ferrous sheet is immersed in a nickel or cobalt salt solution for a period of time to permit nickel or cobalt from the bath to displace iron in the surfaces of the stock or articles. Thus we may place a ferrous sheet in a water solution of nickel sulphate containing from one-quarter to two ounces per gallon of the salt, or even more. The bath is preferably maintained at a temperature between 130 and 190 F.; and the sheet is allowed to remain immersed in the salt solution until the desired quantity of nickel is deposited upon its surfaces.

Again, the metal may be deposited upon the surfaces of the iron or mild steel stock or articles by reduction of a salt or salt mixture thereon. For example, as generally set forth in United States Patent 2,101,950 to McGohan, we may pass ferrous sheets through a substantially 30% to 40% solution of nickel sulphate and permit the nickel sulphate solution to dry on the surfaces of the sheet. Thereafter the sheet is subjected to a heat treatment, which results in the decomposition of the nickel salt, and the deposition of nickel on or in the surfaces of the metal. If this heat treatment is carried out in a neutral or reducing atmosphere as the McGohan patent prefers, a subsequent scaling anneal must be added. Or the heat treatment required for the decomposition of the salt may be combined with the scale anneal in our process. A cobalt salt may be similarly imposed and decomposed.

We prefer to coat the surfaces of the iron or steel stock or articles with the imposed metal or metals by electroplating, since this is faster, and the nature of the coating is more easily controlled. For this purpose any commercial electroplating bath will serve, for example, the well-known nickel-cyanide or nickel-ammonium-cyanide baths. Our preference is for a so-called Watts bath, slightly modified by weakening the solution, when working with nickel. The electrolyte in this instance comprises nickel sulphate and nickel chloride in water solution. Nickel electrodes are employed, and plating is done at commercial speed with a current density of substantially 20 to 30 amperes per square foot. The plating is, of course, continued only long enough to deposit the quantity set forth above, and it may be noted that physical properties of the deposit are not of importanoe in view of the subsequent treatments to which the stock or article will be put. Hence, other baths, including those operating at much higher current densities, may be employed. Similar procedures are known for the deposition of cobalt.

As to the scale anneal, we have discovered as indicated above that when enameling stock is coated with the imposed metal or metals within the ranges set forth above and then is properly scaled, the result of pickling in ordinary pickling solutions will be the production of a desirable type of roughness leading, among other things, to greatly enhanced adherence.

A properly scaled material usually has a characteristic appearance under the high power microscope after the metal deposition and scale anneal, but before pickling. Beneath the layer of scale there lies a thin, broken layer of iron-nickel alloy, iron-cobalt alloy, or both. The unusual feature is the appearance of fingers of alloy or oxide which extend for a considerable distance down into the iron, in some instances at the grain boundaries, but more often at other points within the grain. Our inves tigations have indicated that there tends to be more grain boundary penetration at higher scaling temperatures. We are not certain whether these fingers are of oxide or alloy of the imposed metal or metals. They do not form in a neutral or reducing atmosphere. One explanation which has been advanced is that the fingers are composed of oxide which has penetrated the thin imposed metal layer at breaks which are observed to occur frequently. Subsequently pickling penetrates these crevices rapidly, providing much undercutting and irregularity.

In any event, a new type of surface is formed after the pickling. Fig. 1 is a cross-sectional view at 1000 magnification of ordinary iron enameling stock showing the nature of its surface. The upper, lighter area is a sectional view of a copper body lying next the iron. This copper body is in two parts, a mass of copper plated directly against the iron surface and an overlying sheet of copper clamped thereagainst and polished with the sample. Certain black specks in the lighter area indicate the line of demarcation between the copper plating and the overlying copper sheet. The sample is polished but not etched. The darker gray area indicates the iron; and it will be noted that its surface is characterized by some roughening but substantially no under-cutting. The sample illustrated in Fig. 1 is characteristic of ordinary enameling stocks, i. e. those which have been cold reduced with the aid of a Pangborn or shot blasted roll, which in itself produces some surface roughening. The sample has been subjected to a normalizing treatment and to a pickle. Various routings are common for ordinary enameling stocks. All such stocks are regularly normalized, but some may be box annealed following the normalizing treatment, while others may be temper rolled and shipped in that condition. However, the surface illustrated in Fig. 1 is characteristic of all of these stocks.

The product illustrated in Fig. 1 has not been subjected to a nickel flash or treatment. While a nickel flash is ordinarily used with the regular enameling stocks, it is a treatment applied in most instances by the enameler subsequent to forming and pickling and just before the stock or article is enameled.

Fig. 3 is a surface view of the material of Fig. 1. The surface exhibits some roughening, and will be recalled that it is a pickled surface. There are a few etch pits, presumably along the crystallographic boundaries. But it will be noted that substantially all parts of the surface are in focus; there is substantially no undercutting; and the surface might be described as existing in a series of irregular plateaus at very slightly different levels.

By way of contrast, reference is made to Fig. 4 wherein a very high and irregular degree of roughening is apparent. In this instance the microscope was focused at the top levels of the protuberances. The dark areas are indicative of depressions. To bring these into sharp focus, the microscope must be racked considerably, indicating that there is a great difference in the levels of the tops of the protuberances and the bottoms of the depressions. Furthermore, the photomicrograph of the surface of materials treated in accordance with this invention exhibits a characteristic Woolly appearance indicative of the fact that the projections appear to be spongy, and the protu-berances exist in the form of needles, spicules or tentacles.

This will be further confirmed by reference to Fig. 2 which is a cross-sectional view of a typical surface of the material of our invention at 1000 times magnification. Again, the light area at the top of. the view is. a crosssection of copper, while the darker material is the iron. But it will be noted that not only is the main surface of the iron very rough, but that above this roughness there are what might be termed floatingv islands of iron which are portions of needles or tentacles having their bases elsewhere than at the specific cross-section. In some instances it will be noted that needle-like portions of the base metal extend upwardly. Fig. 2 is illustrative of the extraordinary degree of undercutting and a spongy type of surface which provides the unsual adherence for vitreous enamel attained in our process.

In our reference to floating islands of iron above, we do not wish to be limited to the exact substances involved, since the tentacles or needles may be formed of nickel-iron alloy, cobalt-iron alloy, nickel, cobalt, or combinations of iron and the imposed metal or metals with various oxides.

The formation of the unusual surface condition shown in Figs. 2 and 4 is dependent upon the presence of nickel, cobalt, or both, and the nature of the scaling treatment to which the material is subjected. As indicated, the atmosphere throughout the scaling. furnace, to which the enameling stock is subjected, must be oxidizing to iron. We have attained successful results with a variety of at mospheres including air, DX gas, burned natural gas with variable ratios of gas to air, and such atmospheres in conjunction with varying amounts of moisture. It is known that moisture affects the oxidizing characteristics of atmospheres, and it is quite possible toproduce asatisfactory scaling in an atmosphere of steam alone, or in a moisture-containing atmosphere of a gas otherwise having no. oxidizing characteristics, e, g. nitrogen. Entirely satisfactory results have been secured indry air atmospheres, on the other hand, such as those in the ordinary enameling furnace, although incidental advantages may sometimes be obtained with a high moisture content, e. g. from 20% to 80% by volume, or by producing. the oxidation by or in the presence of carbon dioxide.

The exact amount of scale produced is not a necessary limitation on the invention. Further, different chemical compositions of scale are produced under different conditions. In an endeavor to set forth what we believe to be about the minimum scaling conditions which will produce our result in combination with the other steps set forth above, we may say that heating the enamel stock in an atmosphere of burned natural gas containing 11% combustibles based on dry gas analysis and having a: dew point of 150 F. for 2 min. in afurnace at 1 550 P. will give a scale which is operative in our process. A more oxidizingv atmosphere or higher ternperatures may be employed with advantage. For example, an equal or better enamel bond results if the scaling is carried on at a somewhat higher temperature or for a longer time, or both; or if a lower temperature is used with a lengthened time. Thus, good results are obtained in- 5- min. in a furnace at 1400" F. in the same type of burned gas atmosphere. A temperature range of l500 to 1-800" F, is preferred by us, although higher temperatures may be employed such: as those upto2000 F.,- the top limit being that at which undue grain growth for satisfactory fabricating properties is encountered.

There is some indication that somuch scale can be put on the stock that our surface elfect is impaired, althoughv in general the upper limit of scale is usually determined by pickling time and metal loss. Scaling occurs very rapidly in air atmosphere, for which reason we usually prefer to use a less oxidizing atmosphere, since with it the scale can be more readily controlled, while the times are not unduly long. Improved results can be attained when scaling in an air atmosphere by applying the nickel or cobalt in the form of sulphate in such fashion that there is sulphate remaining on the sheet surfaces at the time of scaling in air. With atmospheres less oxidizing than air, there is little likelihood that too much scale will be produced within the commercial time limitations of an open, i. e. continuous, anneal.

For most purposes a uniform condition of all surface areas of the stock is desired. Hence, the imposition of metal, the scaling treatment, and the pickle should generally be uniform as respects all surface areas. An open or continuous scaling treatment or scale anneal, as contemplated by us, is thus preferably one in which the stock being treated is sent continuously through a furnace containing the desired atmosphere with both sides of the stock open to the atmosphere of the furnace.

The amount and kind of scale preferred are best considered in connection with a specific set of conditions which may be used as a reference standard. If ZO-g'auge enameling sheet (.0359 in. in thickness) is heated to l700 F. in 45 seconds, is held between 1700 and 1800 F. for 40 seconds, and is cooled to 900 F. in 2 minutes, the complete cycle being in an oxidizing atmosphere of burned natural gas, analyzing at the furnace exit: 9% of carbon dioxide, 5% of carbon monoxide, 12% of hydrogen, the balance being substantially nitrogen, based on dry gas analysis and including Water vapor to yield a dew point of to 180 F., a very' desirable scale will be formed. The desirability of scale formed at other temperatures, for different lengths of time, and in other atmospheres may be gauged by reference to this as a standard.

It will be seen that the attainment of the proper scale is consistent with commercially economical continuous furnace operation, and that permissible temperatures are consistent with normalizing practice if normalizing be desired at the stage of the scale anneal ina routing for enameling stock.

The pickling step can be carried on with ordinary acid pickling solutions, and these we prefer to use for reasons of economy and convenience. For example, but without limitation, we ordinarily employ a solution of sulphuric acid in Water of a strength ranging substantially between 6% and 25%. Accelerators may be added as. is known in the art, but are not necessary. Such a pickling, solution may be used at a temperature lying substantially between 160 and F. or higher. This is not limiting, and is indicative of the fact that ordinary pickling solutions may be employed in Ways entirely consonant with commercial practice.

Our preference is for sulphuric acid as a pickling agent. Other acids, such as hydrochloric, nitric or phosphoric, may be either too slow in operation or have varying rates of attack upon the nickel, productive of less desirable surfaces. However, it is to be noted that once our desirable surface is formed, as illustrated in Figs. 2 and 4, an enameling shop picklingv may later by practiced by the enameler in his plant, Without destroying the desirable surface already formed in accordance with our invention.

Within the ranges set forth above, the amount of I nickel or cobalt deposited upon the enameling stock proheavy that it has a tendency to peel away from the surface during pickling. The nature of the scale is important in obtaining our results; and differences in the thickness or quantity of the scale will, of course, to some extent influence the pickling time required to remove it. But we have ascertained that where an enameling stock has been coated with sufiicient nickel or cobalt and properly scaled as set forth above, a pickling treatment suitable for removing the scale will at the same time produce the desirable type of roughening of the metal surface. The pickling time may be increased within very substantial limits without detriment to the product, and in some in stances to its advantage; although it will be obvious that if the pickling after the removal of the scale is so prolonged as to destroy entirely the effects produced by the combination of the imposed metal and the scale anneal, the adhesion produced by our procedure will not be obtained. In general, the removal of the scale is a reliable guide to the extent of the pickling treatment required. In our commercial work, pickling times with sulphuric acid pickling baths range generally from about 1 to about minutes, the preferred range being about 2 to about 7 minutes, with materials subjected to the exemplary treatment above, depending upon the temperature of the bath and the nature of the available pickling equipment.

The steps which we have thus far described produce adherence through the particular type of roughening action referred to without producing enamel defects. Actually, the process results in the loss of some of the imposed metal which is apparently pickled away during the roughening treament. Hence, the function of the imposed metal at this point in the process is primarily that of a material which, deposited upon or alloyed with the surface of the iron or steel stock or articles, the stock or articles being thereafter suitably scale-annealed, will enable ordinary pickling baths to roughen that surface in the way set forth. In normal practice, the enameler receiving the sheet stock from the steel will will ordinarily form the desired finished articles from it by cutting drawing, spinning, stamping, forming and the like, after which he will clean and pickle the stock and give it a so-called nickel flash (i. c. a nickel coating by chemical displacement). If the utmost effect of nickel as an agent in itself for the chemical promotion of adhesion of porcelain enamels is desired, these steps should not be omitted. Thus, if the three steps outlined above are practiced on already formed articles, they are preferably followed by a nickel flash to increase the quantity of nickel at the surfaces of the stock to that desired for chemical adhesion. The amount of nickel so applied is preferably within the range of .02 to .30 gram per square foot of surface. The solutions and procedure for applying this final nickel flash are well understood in the art and are the same as those previously described for making the first application of nickel by chemical displacement. Thus, the imposed metal treatment coacts with the scale anneal and pickle in our process to control the roughening of the surface, while a final nickel flash effects the adherence of the enamel to the surface in a chemical manner.

We will consider now exemplary general routings for the production of enameling stock suitable for receiving single fired coats of white or light colored enamels.

Ferrous enameling stocks, particularly those which have been reduced to gauge by cold rolling, will ordinarily require normalizing to produce a desired grain size and physical characteristics, especially where the 'stock is intended for drawing. Normalizing, as is well known, comprises heating the stock to a temperature above A3 temperature followed by a relatively rapid cooling. It is ordinarily carried on in a continuous-type furnace.

The decarburizing treatment mentioned hereinafter is a brief continuous heat treatment at a temperature from substantially l250 to 1650 F. in an atmosphere oxidizing to carbon but not oxidizing to iron, and containing from about 2% to about 30% of water vapor by volume. Various atmospheres may be employed, including hydrogen, mixtures of hydrogen and other gases which are either inert or in the particular admixture are non-oxidizing and non-carburizing toward iron at the temperatures involved. Cracked ammonia gas is a source of a hydrogen-bearing atmosphere which may be employed. The decarburization treatment results in a lowcring or elimination of carbon from the surface layers of the stock, the carbon being reduced in the surfaces to a value such that it will not produce boiling upon initial firing of porcelain enamel. It has been found that such treatments result in a decarburization of the surface layers of the stock to the extent that a subsequent pickling of normal character will not leave on the said surfaces a carbonaceous pickling residue. Also, it has been found that the surfaces of the iron or steel enameling stock are sutficiently decarburized so that migration of carbon to the surfaces does not occur in a detrimental degree upon box annealing. For some uses a box annealing is desired following the normalizing treatment. Once the special surface is formed on the enameling stock in accordance with our invention, it is not destroyed or seriously impaired by subsequent forming or drawing operations to which the stock may be subjected. Hence, our process is excellently adapted for the production of enameling stocks as such, and does not need to be practiced by the enameler. This represents a substantial economy.

An exemplary routing for the production of white onecoat enameling stock in accordance with our invention may comprise, as steps subsequent to the reduction of the metal to gauge by cold rolling, the following:

Normalize Pickle Metal deposition Scale anneal Pickle Decarb urize If a box anneal is desired, it may be added to the above routing as a final treatment.

A large number of variations of this routing may be made as long as the steps of metal deposition, scale annealing, and pickling remain in this order. The nickel or cobalt may be applied at any stage preceding the scale anneal. Thus, it may be applied, if desired, prior to the normalizing step, and will not be destroyed thereby. The decarburizing step cannot serve as the scale anneal; but the scale anneal can follow the decarburizing step, if desired. Thus, another routing would be:

Normalize Pickle Metal deposition Decarburize Scale anneal Pickle In the processes of this invention, where a scale anneal follows the metal deposition, the scale anneal will perform the function of destroying undesirable chemical residues which would tend to produce gas upon the initial firing of porcelain enamel. Thus, the deposition of the nickel or cobalt can follow the decarburization if desired. On the other hand, the presence of the imposed metal during decarburization does not prevent that action occurring, so that it is possible to deposit nickel or cobalt on the surfaces of the stock prior to decarburization, and follow the decarburization with a scale anneal as set forth above.

The scale anneal is not inconsistent with normalizing, and the two steps may be combined with substantial economy where this is desired. Another exemplary routarse-mo ingin accordance with the present invention may be set forth thus:

Pickle Metal deposition Decarburization Normalize in oxidizing atmosphere Pickle In a variation of this routing, the metal deposition may be carried out after the decarburization and before the normalizing, and scaling treatment. It may also be pointed out that if in any routing the normalizing follows the decarburizing treatment, the grain size may be somewhat larger than when these steps are in the opposite sequence, and the resulting stock may suffer in drawability. This is because of the fact that if the carbon. in an. ironor steel sheet is carried to a low value, the result of a subsequent normalizing treatment will be grain growth. This can be combated by decarburizing the stock superficially, i. e. by so carrying on the decarburizing treatment that while the carbon in the surface layers is carried to a sufiiciently low value for single-fired white coat enamel usage, the entire thickness of. the stock is not thoroughly decarburized.

Perhaps the simplest routing to produce a one white coat enameling stock in accordance Witlrthis invention Metal deposition Normalize in an-- oxidizing atmosphere Pickle Decarburize In this routing the first heat treatment does double duty both as a normalizing treatment and as. the scale anneal.

Other routings wil suggest themselves. In all such routings the essential requirement must be met that the steps of metal deposition, anenaling toformscale, and acid pickling must appear in. this order. Other processing steps may precede, follow, or be interspersed between between these steps in indifferent order so far asop'erability is concerned. It the essential steps of our process afiect such: other steps, the worker will take this into account in arriving at his preferred procedure. It is, for

example, ordinarily desirable to decarburize a clean sheet,

so that a pickling prior to decarburizing is usually preferred.

Our invention, when practiced as a process for producing sheet stock designed for a single white coat enameling work is not limted by the subsequent use of any particular enamel. Various enamels differ in their adherence to the base stock, and in their sensitivity to the various causes of pits and black spotsand some of them have good adherence although they may have certain disadvantages making them less desirable for the purpose. Titania base enamels are preferred although they are rather expensive. The fact that they can be applied in a very thin coat and still produce good opacity tends to offset the extra cost and yield additional advantages. In addition, they are less sensitive to the presence of minor amounts of pickling residues and other causes of pits and black spots. Zirconium enamels are cheaper, but must be used in a somewhat heavier coat. Enamels opacified with antimony are cheaper still, and adhere very well, but have to be put on in a very thick coat to get good opacity. In addition, they do not cover iron oxide specks as well as do certain other enamels. Molybdenum enamels exhibit good adherence, but have poor opacity and are expensive. Our procedure, however, will increase the adherence of all of these enamels in a highly useful degree; and frequently enables us to use, with excellent adherence, a more desirable form of enamel which may itself be deficient in its normal adherence to the iron base.

Modifications may be made in our invention without 10 departing from the spirit of it. Having thus described our invention in an exemplary embodiment, what We claim as new and desire to secure by Letters Patent is:

1. A process of increasing the adherence characteristics of anxiron or mild steel stock or article intended for coating with porcelain enamel, which process comprises in the order named the steps of depositing, upon the surface of the stock or article a material chosen from a class consisting of nickel, cobalt, mixtures of the two, and alloys rich in nickel, cobalt, or both, subjecting the stock or article to a scale anneal comprising subjecting the stock or article. to heat in an atmosphere noless oxidizing' than an atmosphere of burned natural gas. containing 11% combustibles based on dry gas analysis and having a dew point of F., in afurnace at temperatures of substantially 14-00 to 2000 F. for a time of substantially 2 to 5 minutes, whereby to produce a dis.- tinct scale, and then acid pickling the article at least to the extent of removing the scale from the surfaces thereof.

2. The process claimed in claim 1 in which the said material is deposited upon the surfaces of the stock or article in an amount lying substantially between .02 and .2 gram per square foot of surface.

3 The process claimed in claim 1 in: which the said material is deposited upon the surfaces of the stock or article in an amount lying substantially between .05 and .10 gram per square foot of surface;

4. The process claimed in claim 1 in which the said material is deposited upon the surface of the stock or article in an. amount lying substantially between .05 and .10 gram per square foot of. surface, and which the picklingt-reatment is carried on in a solution of sulphuric acid of a strength substantially between 6% and 25% at temperatures between substantially and F.

5. The process claimed in claim 1 in which the said material is deposited upon. the surfaces of the stock or article in an amount lying substantially between .02 and .2 gram per square foot of surface and inwhich the said material is applied to the surface of the stock or article by electrodeposition.

6. The process claimed in claim 1 in which the said material is nickel.

7. The process claimed in claim. l in. which the said material is cobalt.

8. The process claimed in claim 2 in which the said material is nickel.

9. The process claimed in claim 2 in which the said material is cobalt.

10. The process claimed in claim 5 followed by a subsequent deposition of nickel on the surfaces of the treated stock or article preparatory to enameling.

11. The process claimed in 5 including a becarburizing step for reducing the carbon in and near the surfaces of said stock or article to a degree non-productive of boiling upon the initial firing of a coating of porcelain enamel thereon.

12. A ferrous enameling stock having a deeply undercut surface produced by the process of claim 1.

13. A ferrous enameling stock having a deeply undercut surface produced by the process of claim 6.

14. A ferrous enameling stock having a deeply undercut surface produced by the process of claim 7.

15. A ferrous enameling stock having a deeply undercut surface produced by the process of claim 11.

16. In a process of producing an iron or mild steel porcelain enameling stock capable of accepting a single coating of white or light colored porcelain enamel substantially free from pits and specks and having a high degree of adherence, the steps of depositing upon the stock substantially .02 to .2 gram per square foot of surface of a material chosen from a class consisting of nickel, cobalt, mixtures of the two, and alloys rich in nickel, cobalt, or both, heating the stock to 1700" F. in 45 secfonds, holding it between 1700 and 1800 F. for 40 seconds, and cooling it to 900 F. in 2 minutes, the complete cycle being in an atmosphere of burned natural gas containing 9% of carbon dioxide, carbon monoxide, 12% hydrogen, the balance being substantially nitrogen, based 011 dry gas analysis, and including water vapor to yield a dew point of substantially to 180 F., acid pickling the oxidized stock with sulphuric acid at least until all the oxide has been removed, the above steps being practiced in the order named and in combination with a decarburizing step in indifferent order with respect to the foregoing steps for reducing the carbon in and near the surfaces of said stock.

17. A process of producing a ferrous porcelain enameling stock which includes the following steps in the order named as practiced upon the sheet gauge stock: normalize, pickle, deposit upon the surfaces of the stock a material chosen from a class consisting of nickel, cobalt, mixtures of the two, and alloys rich in cobalt, nickel, or both, decarburize the stock, subject the stock to a scale anneal by heating it in an atmosphere substantially no less oxidizing than an atmosphere of burned natural gas containing 11% combustibles based on dry gas analysis and having a dew point of F., in a furnace, at temperatures substantially between 1500 and 1800 F. for a time of substantially 2 to 5 minutes, whereby to form upon the surfaces of said stock a distinct scale, and pickling the stock for a length of time at least sufiicient to remove the scale from the surfaces thereof.

18. The process claimed in claim 17 in which a box anneal follows the scale anneal and pickle.

19. A ferrous enameling stock having a deeply undercut surface produced by the process of claim 18.

20. A process of increasing the adherence characteristics of an iron or mild steel stock or article intended for coating with porcelain enamel, which comprises in the order named the steps of depositing nickel upon the surfaces of the stock or article by treating such surfaces with nickel sulphate solution and drying some of said solution on the surfaces of said stock or article so as to leave nickel sulphate as such thereon, subjecting the stock or article to a scale anneal in air at an elevated temperature to produce a coating of scale thereon which may be removed in a period of time substantially between one and ten minutes in a solution of sulphuric acid of substantially 6% to 25% strength at a temperature of substantially 12 to 190 F., and subjecting the stock or article to a pickling treatment at least to the extent of removing'said scale, whereby to produce a roughening of the surfaces of said stock or article.

21. A process of increasing the adherence characteristics of an iron or mild steel stock or article intended for coating with porcelain enamel, which comprises applying to the previously decarburized stock or article a nickel sulphate solution, then normalizing at 1800 F. for two minutes in a controlled atmosphere consisting of 9% carbon dioxide, 5% carbon monoxide, 12% hydrogen, the balance being substantially nitrogen determined on a dry basis with water vapor present to give a dew point of 130 F. to F., and finally removing the scale formed in such normalizing treatment by means of a sulphuric acid pickle.

22. A process of increasing the adherence characteristics of an iron or mild steel stock or article intended for coating with porcelain enamel, which comprises applying to the previously decarburized stock or article a solution of a nickel salt, and with the nickel salt thereon subjecting the stock or article to a scale anneal producing upon its surfaces scale at least equivalent substantially to that produced by heating the stock or article in an atmosphere of burned natural gas containing 11% combustibles based on dry gas analysis and having a dew point of 150 F. in a furnace at 1550 F. for five minutes, and then acid pickling the article at least to the extent of removing the scale from the surfaces thereof, whereby to produce a roughening of the said surfaces.

References Cited in the file of this patent UNITED STATES PATENTS 979,931 Cowper-Coles Dec. 27, 1910 1,360,318 Reiser Nov. 30, 1920 2,032,256 Canfield et al Feb. 25, 1936 2,101,950 McGohan Dec. 14, 1937 2,127,388 Canfield Aug. 16, 1938 2,187,844 Scharschu Jan. 23, 1940 2,199,804 Natthes May 7, 1940 2,206,597 Canfield July 2, 1940 2,449,023 Thornton Sept. 7, 1948 2,455,331 Eckel Nov. 30, 1948

Claims (1)

1. A PROCESS OF INCREASING THE ADHERENCE CHARACTERISTICS OF AN IRON OR MILD STEEL STOCK OR ARTICLE INTENDED FOR COATING WITH PORCELAIN ENAMEL, WHICH PROCESS COMPRISES IN THE ORDER NAMED THE STEPS OF DEPOSITING UPON THE SURFACE OF THE STOCK OR ARTICLE A MATERIAL CHOSEN FROM A CLASS CONSISTING OF NICKEL, COBALT, MIXTURES OF THE TWO, AND ALLOYS RICH IN NICKEL, COBALT, OR BOTH, SUBJECTING THE STOCK OR ARTICLE TO A SCALE ANNEAL COMPRISING SUBJECTING THE STOCK OR ARTICLE TO HEAT IN AN ATMOSPHERE NO LESS OXIDIZING THAN AN ATMOSPHERE OF BURNED NATURAL GAS CONTAINING 11% COMBUSTIBLES BASED ON DRY GAS ANALYSIS AND HAVING A DEW POINT OF 150* F., IN A FURNACE AT TEMPERATURES OF SUBSTANTIALLY 1400* TO 2000* F. FOR A TIME OF SUBSTANTIALLY 2 TO 5 MINUTES, WHEREBY TO PRODUCE A DISTINCT SCALE, AND THEN ACID PICKLING THE ARTICLE AT LEAST TO THE EXTENT OF REMOVING THE SCALE FROM THE SURFACES THEREOF.

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