ENAMELING DEFECTS DUE TO CAST IRON

A discussion of Malinovszky's paper on enameling defects due to cast iron. Problems are suggested for further investigation on the subject.     

ENAMELING DEFECTS DUE TO THE CAST IRON1

Troubles in manufacturing sanitary enameled ware are pinholing, breaking out, blistering, crazing, chipping, lifting, black specks, etc., due to the iron, the chemicals, methods of applying the slush or enamel, furnace temperatures, inexperience, lack of cleanliness, differences of the coefficients between iron and enamel and fitting of ground coat to the enamel, etc. Feldspar is most liable to cause trouble by variations of its constituents. Feldspar containing above 69% of silica should be looked upon with suspicion. Troubles arising from the cast iron are discussed. Results of chemical analyses and microscopic examinations and microphotos before and after enameling are shown.     

CHARACTERISTICS OF IRON AND STEEL FOR PORCELAIN ENAMELING *

The chemical and physical requirements of porcelain enameling sheets are presented. It is explained why the most satisfactory base metal for enameling purposes is one low in carbon, manganese, sulfur, phosphorus, and silicon. Thermal expansion curves and pertinent photomicrographs are shown.     

EFFECT OF ENAMEL ON THE STRENGTH OF ENAMELING IRON*

Ground-coat enamel was first applied on one side and then on each side of 26-, 20-, 18-, 10-, and 6-gauge enameling stock, and the strengths of the pieces were determined in cross bending. The results show a decided increase in strength for the enameled specimens over that of the stock before enameling. An increase in the thickness of the enamel also increases the strength within limits. The effect on the lighter gauges is greater than on the heavy gauges, and the strength is greater when the enamel is under compression than under tension.     

THE RELATION OF DEFECTS IN ENAMEL COATINGS TO HYDROGEN IN STEEL*

The effects of hydrogen as functions of time, temperature, and impurities in steel are described and experimentally demonstrated. Experimental evidence is presented to show that the blistering and boiling action over carbide areas in steel are due principally to hydrogen that associates with the carbon. Steel enameling stock contains quantities of hydrogen which may effuse during firing to cause or aggravate such phenomena as “boiling,”“primary boiling,”“rebelling,”“blistering,” and “bubbling.” Low-temperature effusion of hydrogen contributes to other defects, including “fishscaling,”“delayed fishscaling,” and possibly “pop-offs,”“jumping,”“shiners,” and some cases of “chipping” and, perhaps, “bursting” of enamel on cooking utensils. These defects have seldom been identified with hydrogen evolution. An indirect effect of hydrogen on “copperheads” and “black specks” is also identified. An exhaustive review of both English and German literature on enameling defects is included. Many observations recorded in the literature are shown to agree with the hydrogen theory. Certain types of inclusions in steel are shown to react with occluded hydrogen to form compounds that will not dissociate appreciably at some enamel-firing temperatures, and critical quantities of the hydrogen are therefore prevented from reaching the enamel coating during firing to cause blistering and related defects.     

THE RELATION OF METAL CLEANING TO PERFECT ENAMELING1

The importance of the cleaning phase of the enameling process as related to production of uniformly good work is not always appreciated. Many troubles encountered commonly in porcelain enameling plants can be traced to the pickling department turning out unclean work. In order to control the cleaning properly there are a number of points which should he considered, such as type of lubricant used, production schedule, evidence of water hardness, and selection of proper type of cleaner. A method has been suggested which applies to the water-break test for determining the exhaustion point of the cleaning tank. Rinsing, packing or racking, and electrolytic cleaning are also of importance and merit consideration.     

PROGRESS REPORT ON THE STUDY OF GASES IN ENAMELING IRON*

Since all ferrous materials contain within their structure a varied assortment and quantity of gases, a discussion is presented of the effect of such factors as time, temperature, atmosphere, and deliberate methods used in the manufacture of enameling iron. A study of the important gaseous impurities in ferrous materials is given. Melting investigations were made to deterrninc at high temperatures the absorptive capacity of specially prepared irons for air, nitrogen, hydrogen, and carbon monoxide, and the influence of carbon, manganese, silicon, and aluminum on the absorptive and retentive capacities. Manganese in increasing amounts increased the absorption of all gases. A critical percentage content of 0.25% of manganese was found to exist, at which maximum absorption of nitrogen occurred; below and above this percentage content the absorption of nitrogen decreased. Many grades of enameling iron were subjected to a similar investigation. Certain irregularities of various analyses of commercial irons were noted in that there were irregularities of absorption and retention of gases in a range of temperature which included the temperature at which porcelain enamels were fired. The enameling iron of low carbon and low manganese content, containing only the permissibly low phosphorus, sulfur, silicon, and alloying elements, should insure dependable performance during enameling operations.     

FACTORS AFFECTING SAGGING TESTS ON ENAMELING IRON*

A method is described by which sagging tests may be made and the determinations may be duplicated with considerable accuracy. The effect of different furnaces is noted as well as such factors as furnace size, temperature and its control, width of samples, enamel coating, time of testing, size of sample, and metal composition.     

RAPID METHOD FOR DETERMINING NICKEL ON THE SURFACE OF ENAMELING IRON*

A photometric method for determining nickel on nickel-coated steel is described. The method is very rapid, requiring less than 5 minutes. The standard deviation (σ) has been found to be 0.0028 gm. of nickel per sq. ft. when testing a sheet carrying 0.10 gm. of nickel per sq. ft. The sample is obtained by dissolving the nickel coating on a small area with dilute nitric acid. The nitric acid is confined by using a weighted rubber ring and is withdrawn by suction into a 200-ml. Erlenmeyer flask marked at 200 ml. A large excess of ammonium hydroxide is added, followed by ammonium persulfate and dimethylglyoxime. The solution is diluted to 200 ml. with water. After filtering off the ferric hydroxide, the intensity of the red color, which is proportional to the amount of nickel present, is measured by means of a photoelectric photometer.     

OBSTACLE IN CLEANING DRAWING COMPOUNDS FROM ENAMELING IRON*

Under certain conditions of storage, some drawing compounds (die lubricants) form or deposit a material on the surface of sheet enameling-iron parts which is insoluble in water and alkaline cleaners but is made soluble by organic solvents. Moisture and time seem to be necessary to the mechanism, and fatty acids, or the salts of fatty acids, may be the offending materials. A method of reproducing these conditions is described and some discussion of possible remedies is offered. The use of the test method described should aid in the development of improved drawing compounds and more effective cleaning methods.     

CAUSE OF PINHOLES AND SOME RELATED DEFECTS IN ENAMEL COATINGS ON CAST IRON*

The confusion in identifying hydrogen as the predominating cause of certain defects in enamel on cast iron has been due largely to the close association of carbon and hydrogen in cast iron and steel. The principal relation of carbide and graphite to enameling defects is the release of hydrogen from the carbon during enamel firing. The much-discussed “chill layer” therefore is important chiefly because this layer often contains hydrogen that is bound to the carbon in the cementite. Experiments show that when hydrogen is absent, regardless of the depth or nature of the surface chill, no pinholing or blistering results during firing at 725°C. Sources of the hydrogen that causes the defacement are found chiefly in melting and in casting. The low oxygen pressure of molten cast iron favors hydrogen absorption. Moisture in the atmosphere, in the charge, or chemically combined in the rust on scrap provides the greatest quantities of the gas, and moisture and organic materials in the mold are also prolific sources of hydrogen for absorption by the iron. At ordinary temperatures, rusting is often harmful. Flushing the melt with a dry, hydrogen-free gas, such as nitrogen, removes the dissolved hydrogen, and defects during subsequent enameling will not occur unless hydrogen is obtained later from other sources. Chipping phenomena probably are caused chiefly by hydrogen effusion, just as are analogous defects in sheet-steel enameling.     

THE THEORY OF CLEANING OF SHEET IRON AND STEEL FOR ENAMELING PURPOSES1

Cleaning may be done by sand blasting heating, use of solvents, or use of alkaline solutions. Of these methods, heating and use of alkaline solutions are important at present. In cleaning by heating the best results are obtained when the articles are first dipped in a solution of acid, heated uniformly to the lowest temperature that will cause the oils and greases to be burned off, and cooled slowly, both heating and cooling being done with the admission of as little air as possible. In cleaning by the use of alkaline solutions, saponifiable oils are removed by the formation of soaps and the subsequent solution of these; non-saponifiable oils are removed by emulsification. Efficient cleaning depends primarily on the choice of cutting and drawing oils so as to have present a proper ratio of saponifiable and non-saponifiable oils. The cleaning solution must vary in composition according to the kinds and amounts of oils to be removed.     

THE THEORY OF PICKLING OF SHEET IRON AND STEEL FOR ENAMELING PURPOSES1

The fundamental principal of efficient pickling is that efficient pickling does not depend on the solution of a large amount of metal but on the solution of a very thin layer of metal directly beneath the scale, rapidly and with copious evolution of hydrogen. The activity of an acid solution depends on the amount of acid present, the fraction of the acid that is ionized, and on the speed with which the ions move. Since the electrical conductivity of an acid solution is a measure of both the hydrogen-ion concentration and the speed at which the ions move, the electrical conductivities of a series of acid solutions indicate the relative activities of the various solutions. The most effective concentrations of acid lie between 15 and 20%. Heating acid solutions reduces their viscosity and renders the particles more mobile, thus increasing the activity of the solutions. Sulphuric acid is cheaper in first coat than hydrochloric acid, but the latter acts more rapidly and the pickling is completed with less solution of iron. Additions of small amounts of common salt (sodium chloride) have a marked effect on increasing the rate of pickling of sulphuric acid baths. This is due to the formation of a corresponding amount of hydrochloric acid. The chief action of addition agents is to reduce the amount of acid vapor in the fumes from pickling vats and thus permit the use of highly efficient concentrations of acid (between 15 and 20%). Hydrogen absorbed by metals during pickling may be largely removed by immersing the metal in boiling hot water for a few minutes.     

DEFECTS IN ENAMEL DUE TO CAST IRON

Microphotographic examination of iron on which enamel has pinholed, blistered, and come perfect are cited as evidence that carbon is practically the only agent in cast iron causing blistering and pinholing and that it is the condition and not the presence of carbon that causes these defects.     

FACTORS AFFECTING THE WARPAGE OF SHEET IRON AND STEEL IN ENAMELING1

The effects upon warpage of different methods of cleaning the metal, durations of firing periods, scaling practices, gages of metal, types of enamel, etc., are studied. Test pieces 16 × 16 inches are employed, several commercial enameling irons and steels being used and compared. Warpage is determined by obtaining the areas on five evenly spaced, vertical planes, parallel to one edge of the test piece,‘included between a flat, horizontal base plane and the contour of the test piece resting thereon. The average of the five areas in square centimeters is used to express numerically the degree of warpage. The conclusions are: (a) A wide divergence between the expansivities of the enamel and the metal base induces warpage, (b) there seems to be less tendency for the scaled metal to warp than for that cleaned chemically, (c) thin metal is more sensitive to the factors affecting warpage than thicker metal,(d) sudden, irregular cooling induces warpage, (e) tendency to warpage is reduced by properly supporting the ware during firing and cooling.     

STUDY OF METALLOGRAPHIC CHARACTERISTICS AND ENAMELING QUALITIES OF CERTAIN CAST IRONS *

The enameling properties of three different irons are described and their analyses are given. Photomicrographs are shown, depicting the structure of each iron near the surface before and after enameling. Structural changes during enameling are described and discussed as to their possible influence on enameling properties. It is shown that one iron anneals perfectly during enameling with no apparent surface alteration, another anneals perfectly but shows surface decarburization; the third sample does not anneal perfectly. Sample No. 1 enamels better than either No. 2 or No. 3.     

RELATION OF HYDROGEN TO ADHERENCE OF SHEET-STEEL ENAMELS*

The principal theories of adherence are reviewed, and a new theory involving hydrogen gas is discussed. A review of the literature and experimental data show that hydrogen is a factor in the adherence of sheet-steel enamels and that adherence, reboiling, and fish scaling are closely related.     

BLISTERING PHENOMENA IN THE ENAMELING OF CAST IRON1

Physical defects in the castings, especially “sponginess,” will cause blisters, as will also fanlty composition or application of enamels. There are, however, differences in the tendencies of different sound castings to give blisters when enameled under identical conditions. The gases forming these blisters are CO and CO₂. A gray iron casting acquires in freezing and cooling a very thin surface skin, or “microchill,” of varying thickness and hardness. The data indicate that removal of this skin from sound castings eliminates blistering. During the enameling process the combined carbon of this skin tends to break down to a nascent, readily oxidizable form of carbon, which evolves CO and CO₂. There are probably two kinds of nonblistering iron, one in which little combined carbon is present at the surface, and another in which it is stabilized. Some irons are more prone to give the “microchilled” layer than others. During the early stages of enameling both blistering and nonblistering irons evolve gas, which is attributed to quick oxidation of submicroscopic graphite and which escapes bcfore the enamel has fused to a retentive condition. Addition of graphitizing agents, such as silicon, may be beneficial, but the microchill is harder to avoid than the ordinary, or macrochill. Removal of the surface layer by deep sandblasting or “burning out” appears to be the most practical remedy for blistering of sound castings.