EFFECTS OF COMPOSITION ON THE PROPERTIES OF GROUND COAT ENAMELS FOR SHEET STEEL1

A series of ground coats was developed about each of three refractory bases, one of which consisted of flint and feldspar in a ratio frequently used commercially, another consisting of feldspar only, and the third of flint only. In each case the refractory portion was 58.5 parts of the whole. Besides the refractory, a “fixed addition” of 11.5 parts, comprised of 0.5 cobalt oxide, 1.0 manganese oxide, 5.0 boric oxide and 5.0 sodium oxide, was used in every enamel, bringing the basic constant portion for each series to 70 parts of the whole. The variable portion in each series was made up of equal parts of three constituents commonly used in enameling, chosen from a total of six in such a way that every possible combination was employed, making a total of twenty enamels in each series. All were given two cover coats of a standard white and tested for resistance to mechanical and thermal shock. In general the ingredients which were most conducive to resistance to the test treatments were flint in the refractory portion, and sodium oxide in the variable portion, constituted of fluxes. Also, boric oxide favorably influenced resistance to mechanical shock in the series containing both flint and feldspar. Resistance to mechanical and thermal shock in ground coats was considerably less affected by variations in expansivity than is the case in cover coats, the influence of that factor appearing to be partially obscured by other factors.     

WET PROCESS ENAMELS FOR CAST IRON1

Wet process enameling of cast iron is becoming of importance because of its application to the enameling of stove parts, sanitary fittings and hardware. This paper is a report of an extended investigation of wet process enamels for cast iron, in connection with the use of a ground coat. Compositions used in dry process enameling have served as a basis for the work, which includes a study of ground coats and white cover enamels. The effect of varying methods of preparing the frits, mill additions, and the relation of composition of the enamels to such properties as adherence, texture and opacity have been carefully studied. Ground Coats. —Sintering of most ground coat frits has been found desirable in order to develop best adherence on the castings. Clay gave best results as a mill addition for the groundvt and feldspar were found to be less satisfactory. Excessive additions of clay or flint produced flaking of the ground coat while feldspar tended to develop blistering. About 15 per cent of clay or 10 per cent each of clay and flint are recommended as mill additions for the ground coats. Excessive additions of any one flux were not desirable, and best results were obtained when sodium, lead and boric oxides were combined in more or less definite pro-portions. Sodium oxide in excess of about 10 per cent gave rise to blistering. Boric oxide increased the firing range of the ground coats and was preferable to lead oxide on this account, although it tended to promote crawling of ground coats high in boric oxide if they were applied somewhat heavily. The best ground coats developed are Rg-26, 17, 1, 25 and 18. Cover Enamels. —Cover enamels were smelted in the usual manner. The mill additions consisted of 5 per cent of clay, 8 per cent of tin oxide, and 45 per cent of water, all based on the weight of the dry frit. For best results, the cover enamel must he adapted to the ground coat in refractoriness. Boric acid increased the firing range of the enamels but tended to promote crawling, although this was less pronounced in the case of the more fusible compositions. Boric oxide improved the opacity when suhsituted for such fluxes as sodium and lead oxides. Cryolite increased opacity but additions above 10 per cent, based on the melted weight, tended to promote crawling. Considering all factors, cover enamels R-14 11, 28, 1 and 18 gave best results.     

RELATION OF COEFFICIENT OF EXPANSION TO IMPACT RESISTANCE OF PORCELAIN ENAMELS*

The tentative standard impact test procedure, adopted by the Porcelain Enamel Institute, was used to test and compare cylinders coated with several experimental and commercial enamels. The important facts noted were (1) the effect of cubic thermal expansion on the impact resistance, (2) the importance of the ground coat to the resistance of any one cover enamel, and (3) the wide variation between cover enamels in resistance to impact.     

RELATION OF COEFFICIENT OF EXPANSION TO CROSS-BENDING STRENGTH OF SHEET-STEEL ENAMELS1

Two series of enamels were made with calculated cubical coefficients of expansion varying from 361 to 306. In both series of enamels, the cross-bending values were inversely proportional to the coefficient of expansion. This is due to the fact that all enamels on sheet steel have lower coefficients of thermal expansion than steel. In cooling, after the enamel is fused to the metal, strains are set up in the enamel; the lower the expansion of the enamel the greater the strains. When the enameled piece is bent, the greater the stresses present, the farther the pieces can be bent before there is any real tensile strain on the enamel. High bending ability is dependent primarily on high compressive strains induced by lower coefficient of expansion of the enamel than of the metal base. Compressive strains cause warping, fishscaling (in ground coat), and chipping; therefore the bending strength should not be too high. In all control work both the minimum and the maximum bending strength should be specified.     

THE CAUSES AND CONTROL OF FISH SCALING OF ENAMELS FOR SHEET IRON AND STEEL1

Data obtained.—This paper presents measurements of the following factors and properties and their influences upon fish scaling in the case of typical single and three-coat enamels: time and temp. during melting of the frit; fusibility of the enamel; coeff. of expansion of the enamel, and of a variety of representative irons and steels up to 500°C (accuracy about 0.1%); annealing the enamel coat after firing; chem. comp. of the steels and irons; mechanical treatment (rolling, spinning, drawing, and so forth of the stock; microscopic surface structure of the stock; effect of chem. comp. upon thermal expansion of enamel. Incidentally it was found that the formulas of Mayer and Havas for computing the thermal expansion of enamel from its chem. comp. can not be relied upon: Conclusions.—On the basis of these exhaustive tests, the following conclusions have been drawn as to the causes and the control of fish scaling. (A) Causes.—1. The cause of fish scaling lies in the difference of the coefficients of expansion of the enamel and the stock, that for steel being higher than for enamels so that the latter are under a compressive stress. The factors influencing this phenomenon are as follows: (a) Composition of the enamel as affecting the coefficient of expansion of the enamel; (b) Overfiring—thus volatilizing those substances which tend to keep the coefficient high; (c) Lack of annealing enameled ware, the enamel coating of which is a glass and should logically be treated as such. 2. Fish scaling is due secondarily to a number of factors which may affect the strength of the enamel or its adherence to the metal. (a) The physical condition of the surface of the metal as influenced by drawing, spinning, cold rolling or other mechanical treatment; (b) Composition of the glass as affecting its elastic strength; (c) Underfiring, enamel not fused to metals; (d) Cleanliness of the surface as regards removal of drawing compounds, grease, etc. (B) Remedies.—1. (a) Adjusting the composition of the enamel so as to increase its coefficient, e. g., by decreasing the boric acid content; (b) Correct firing of the enamel to avoid unnecessary volatilization of such fluxes as soda and cryolite; 2. (a) Adjusting enamel composition to increase its strength; (b) Correct firing to give proper adherence. 3. Treatment of metal to give best adhesion by (a) cold rolling or other suitable mechanical treatment of the untreated steel; (b) thorough cleaning. (C) Summary.—Broadly stated, then, fish scaling can be absolutely controlled by one or all of the following methods: (1) By the development of an enamel with an expansion fitting the metal; (2) by annealing the enameled ware; (3) by treatment of the metal as cold rolling or some other method of working: (4) by developing an enamel with a wide range of compressional elasticity and applying this enamel to metal treated as indicated above.     

EFFECT OF COEFFICIENT OF EXPANSION OF GROUND- AND COVER-COAT ENAMELS ON THERMAL-SHOCK AND IMPACT RESISTANCE *

Four antimony cover-coat enamels, four fluoride cover-coat enamels, and four ground-coat enamels were formulated to give varying coefficients of expansion. The calculated values were 250, 275, 300, and 325 × 10^–7. The observed coefficients of expansion are included in the paper. The enamels were applied to standard 2-quart pudding pans and were tested for thermal shock and impact resistance using the standard methods of the Enameled Utensil Manufacturers’Council. The results show that thermal-shock resistance is controlled chiefly by the coefficient of expansion of the ground-coat and cover-coat enamels. The best results were obtained by using a cover-coat enamel with a low coefficient of expansion and a ground coat with a high coefficient of expansion. No conclusive evidence was found to indicate any effect of the coefficient of expansion of the ground- and cover-coat enamels on impact resistance. The solubility resistance and reflectance curves for the enamels are also included.     

SUGGESTIONS FOR DEVELOPMENT IN ENAMELING TECHNOLOGY1

This paper comprises the address of the Chairman of the Enamel Division of the American Ceramic Society at the Atlantic City Meeting, February, 1924. Among the suggestions for development in enameling technology are: The more general use of melted weights in representing enamel compositions; the use of coefficient of thermal expansion factors; improvement in the mechanical properties of enamel glasses by increasing the percentages of zinc oxide, boric oxide and titanium oxide, and by the use of thinner coatings; the widening of the heat range of ground coat enamel compositions by the use of more complex compositions than are now employed; and, finally, the more careful control of variation in the composition of the feldspars used in making enamels.     

A STUDY OF DRY-PROCESS CAST-IRON GROUND-COAT ENAMELS*

This investigation covers a study of varying firing treatment of ground coats for cast iron and the resulting effect on the quality of adherence and the tendency to blister of dry-process enamels. Five ground coats have been used, composed of one and two frits and with varying clay and flint additions at the mill. Data on the impact resistance of the enameled specimens obtained with a pendulum-type impact tester are included. Conclusions are noted on some of the factors affecting firing range, and plans for further studies are outlined.     

EFFECT OF ZIRCONIA IN ENAMELS FOR SHEET STEEL1

This paper deals with the results obtained from a study of the effect of additions of zirconium oxide to enamels for sheet iron and steel. The work includes additions of the zirconia to the raw smelter batch of both the ground and cover enamels, also the substitution of the zirconia for tin oxide in the mill batch. Several tests were used to determine its effect on some of the properties of the enamels, such as resistance to impact, thermal shock and acid.     

DEVELOPMENT OF LOW-TEMPERATURE ENAMELS ON A FLUORIDE EUTECTIC BASIS1

Three fluoride eutectics were substituted for cryolite and fluorspar in three standard enamels: sheet-steel ground coat, sheet-steel cover coat, and cast-iron single coat, with a view to lowering their maturing temperature. Substitution in ground coats produced inferior enamels, but no apparent change was evident in cast-iron enamels. In the case of sheet-steel cover coats satisfactory enamels were obtained with a maturing temperature 150°F lower than the original enamel which matured at 1450°F.     

Effect of Firing Schedules on Stress-Temperature Relations in Enamel-Metal Systems

The effect of box and continuous enameling furnace firing schedules on the thermal deflection of enameled iron strips was studied. Effective coefficient of thermal expansion values were calculated from coefficient of thermal deflection data. Results indicate that the effective thermal expansion values for annealed and unannealed enameled iron agree with the expansion data obtained by an interferometer study of the same enamels. Variation in the cooling rate of the enameling furnaces studied is sufficient to produce a marked change in the development of thermal stress in the enameled iron. Residual compressive stresses in the enamel are increased by rapid cooling from firing temperatures. Tensile stress developed in the enamel during reheating is reduced by previous annealing.     

OBSERVATIONS ON “HAIRLINES” IN SHEET IRON ENAMELS1

Hairlines in sheet iron enamels may be caused either by under-firing of the ground coat enamel, or by “cold spots” on the ware when the cover enamel is fired. The defect may be prevented by properly firing the ground coat enamel, and by proper arrangement of the ware on the firing supports in the furnace.     

SOME EFFECTS OF VARYING SODA AND BORIC OXIDES IN GROUND COAT ENAMELS1

Variations were made from 5 to 25 % in soda oxide and boric oxide content of a given ground coat enamel, and results observed on trial pieces. It was found that an increase of soda oxide at the expense of boric oxide decreased the tendency of the enamels to blister and fishscale, and increased the tendency toward the formation of copperheads. Conversely, the increase of boric oxide at the expense of soda oxide increased the tendency of the enamels to blister and fishscale, but decreased the tendency to copperhead.     

Standardizing Shop Practices Relative to Making and Application of Steel Ground Coat Enamels. I

I The effect of manganese dioxide on the physical properties of ground coat enamels is: (1) to give a good gloss; (2) to give a harder enamel; (3) to reduce the coefficient of expansion of the glass in which it is used; (4) when used with cobalt a uniform color desirable in stove work is obtained; (5) no difference in the adhesion of the ground coat between enamels using manganese, cobalt, or combinations of dioxides is apparent; (6) no effect on chipping is apparent; (7) while not absolutely essential in ground coats it is very desirable for use from an economy stand point. II. A theory of ground coat adherence based on the fact that part of the steel is oxidized to iron oxide which is partly absorbed by the fused enamel and into which some of the enamel penetrates is set forth, and an explanation given of various effects obtained. III. Most of the things which should be done to maintain control in making and firing of ground coats for sheet steel are emphasized. IV. The method of control for application of ground coat enamel by weighing the amount of dry enamel on a sheet of metal of known area is described. This is used to maintain a uniform consistency of the enamel. Other essential features which have in the past caused trouble to many enamelers are mentioned.     

PROGRESS REPORT ON THE EFFECT OF FURNACE GASES ON THE QUALITY OF ENAMELS1

The appearance of sheet-steel enamels fired in atmopheres of nitrogen, carbon dioxide, sulphur dioxide, and Urbana city gas has been determined. A study of the effect of variations in the composition of sheet-steel ground coats and sheet-steel cover coats on their resistance to attack by sulphur gases has been made. Plans for future work on the problem are outlined.     

A METHOD FOR TESTING THE CROSS-BENDING STRENGTH OF ENAMELS1

Methods which have been proposed for measuring the strength of enamels, by means of impact tests, are not entirely applicable to enameled flat ware, where the ordinary strains are due to bending of the sheets. An apparatus has been devised by means of which bending stresses can be applied in gradually increasing increments so that the normal behavior of enamels under such stresses can be observed and measured.     

RESISTANCE OF ENAMELS TO THERMAL SHOCK*

Thermal-shock resistance tests were made on enamels of varying thermal expansions applied on test pans. Increased weight of application and high expansion of the cover coat decreased the thermal-shock resistance, whereas high expansion of the ground coat increased the resistance.     

THE EFFECT OF FURNACE ATMOSPHERE ON THE FIRING OF ENAMEL1

It is shown that in firing ground coat and other one coat enamels, the atmosphere of the furnace plays an important part. Oxygen in the muffle penetrates the enamel during the early stages of the firing forming a film of iron oxide on the surface of the steel. This film, whether applied before enameling or formed incidentally to firing, partially dissolves in the fused enamel giving the intimate bond between metal and enamel. Ground coat enamel fired in an atmosphere of nitrogen was found not to adhere to the steel. Approximate values are given for the proportion of furnace space to enameled surface to insure sufficient oxidation when residual air is depended upon to supply the oxygen.     

Comparative Testing of One-Coat Enamels for Pipes

Comparative testing of industrially produced enamels and synthesized fluorine-free one-coat enamels was carried out. It was established that fluorine-free enamels of new compositions in certain properties (firing interval, chemical resistance, impact strength, etc.) exceed the existent analogs. The enamels are environmentally safe, have been subjected to extensive industrial testing, and have been accepted for production at various companies manufacturing steel pipes with enameled inner surfaces.     

SOME OBSERVATIONS ON THE AGING OF ENAMELS1

The tendency toward more rapid production of enameled ware has reduced to minimum the time allowed in most plants for aging of enamels. Aging of the enamel slip allows the clay to become thoroughly slaked, adsorb the larger enamel particles, and thus prepare it for its cohesion to the metal base. Aging or soaking of the clay before use is an excellent substitute for part of the time consumed in aging the enamel slip. Observations of results obtained in commercial application of enamels has lead to the following conclusions: (1) Enamels should be aged before use in order to gain the full benefit of the adsorption properties of the clay and enamel colloids. (2) If the proper time of aging of enamels cannot be allowed, at least the clay should be brought to fine state of subdivision by aging or soaking in water. (3) If aging is not allowed in some form, the enamel usually is not as easily applied with the ordinary handling encountered in average production. (4) If aging of the clay is allowed there is quicker turn over of the stock enamel in the mill room.