THE RELATIONS BETWEEN COMPOSITION AND PROPERTIES OF ENAMELS FOR SHEET STEEL1

Very little has been reported on the relations existing between enamel composition and the properties except the resistance to acids. In this investigation a study was made of the relation between enamel composition and such properties of the enamel as expansion, strength and acid resistance, as well as the effect of the relative “fit” of the ground and cover coats on resistance of the enameled ware to impact and to thermal shock. The work was done on fourteen ground coats and white cover enamels applied to eight-inch dinner plates. The latter were tested for resistance to impact, to thermal shock, and to acetic acid. Determinations were made of the expansion and compression strength of the enamels before application to the steel. Conclusions.—1. The replacement of boric oxide by sodium oxide in the enamels increased the coefficient of expansion. Fish-scaling decreased with increase of ex- pansion, due to increases of sodium oxide. 2. The compressive strength of the ground coats appeared to decrease with re- placement of boric oxide by sodium oxide, although the reverse conditions occurred in the case of the cover enamels. 3. Impact on the parts of enameled ware not free to deflect, such as the curved corners at the bottom, gives a measure of the toughness of the enamel as applied on the steel. The resistance of the enamel on corners to impact apparently is a function of the inherent strength of the enamel rather than its “fit” on the ware. 4. For greatest resistance to thermal shock, the ware should be coated with a ground coat having a coefficient of expansion equal to or preferably greater than that for the cover enamel. 5. Acid resistance, while dependent on the enamel composition, is not affected in the same way for all enamels. For these particular cover enamels, resistance to acids decreased with replacements of boric oxide by sodium oxide.     

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.     

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.     

RELATION OF METAL THICKNESS,ENAMEL THICKNESS,AND BOTTOM RADIUS TO IMPACT RESISTANCE OF PORCELAIN ENAMELED UTENSILS*

Three series of standard two-quart pudding pans of different metal thicknesses, coated with one, two, and three coats, respectively, of enamel were tested for impact resistance, using the Enameled Utensil Manufacturers' Council standard impact test. The impact resistance of the pans was increased with increase (1) in metal thickness of the pans, (2) of the enamel thickness, and (3) in the size of the bottom radius. The size of the chip formed on impact also increased with increase in enamel thickness. The literature dealing with the fracture of glass and enamels is reviewed, and the factors influencing spontaneous chipping and chipping caused by bending and impact are discussed.     

Low-Melting Chemically Resistant Enamel for Steel Kitchenware

Technological conditions for obtaining titanium-borosilicate glasses and properties of enamel coating, spreadability of enamels, and luster of coatings are investigated. The effect of boron and alkali metal oxide on the physicochemical properties of titanium-borosilicate enamels is studied. A chemically resistant enamel for steel kitchenware is developed, with improved properties compared to currently used industrial enamels.     

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.     

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.     

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.     

MECHANICS OF ENAMEL ADHERENCE, III

A study of the use of sheet-steel enamels on copper showed that cobalt oxide decreased the adherence of the enamel. Several sheet-steel enamels were fired on copper and in general the adherence was less than on sheet steel. A white cover coat was found to be most satisfactory for copper. Several color photographs are given showing contact lines between enamel and copper, enamel and gold, and enamel and silver.     

A PECULIARITY OF SHEET-IRON ACID RESISTING COVER COAT ENAMELS*

It has been demonstrated that the small bare spots evident in the first coat of acid-resisting enamel applied directly upon the ground coat are not necessarily the result of a process fishscaling in the ground coat during the firing operation, but are more often the result of reboiling of the ground-coat enamel and lack of film strength in the yet unfused layer of acid-resisting enamel. Reboiling, or other mechanical movements of the ground coat, may promote defects in acid-resisting enamels which would not be visible in those of the nonacid-resisting type, particularly when these enamels are applied directly upon the ground coat. The electrolytes used with acid-resisting enamels are directly connected with the popping condition noted. A choice of the proper kinds and amounts of electrolytes for “setting-up” the enamel can, in some cases, eliminate or minimize this trouble. The use of a nonacid-resisting enamel, properly set up as an intermediate layer between the ground coat and the finish coat of acid-resisting enamel, will also prevent troubles of this kind. The tendency for this peculiar defect to occur depends also upon the type of enameling base metal used and its tendency to reboil.     

CRAZING OF ENAMEL ON STOVE TOPS DUE TO HEATING IN SERVICE*

Examination of stove tops that had crazed in service show two types of crazing. One is in the form of a network of cracks which may be attributed to thermal shock, while the other is in the form of parallel lines and occurs in the normal use of the stove without any particular abuse. In studying the latter type of failure, localized heating of the stove top causes a cross-bend failure of the enamel due to flexing from the heat, and the higher the temperature the less is the amount of deflection required to cause failure. The temperature necessary for this condition is lower than 550°F. Sample plates subjected to various tests afforded a means of studying the different variables that produce crazing in stove tops. From these tests, the following observations are made: (1) Stove tops that have tensile strains in the enamel due to fabrication or assembly are likely to craze in the areas of these stresses if they are heated; (2) the lower the coefficient of expansion of the cover-coat enamel, the less likely is the enamel to craze; (3) the lighter the weight of application of cover coat, the greater is the resistance to crazing; (4) the heavier the gauge of metal the greater is the resistance to crazing; and (5) heating the enamel over a long period of time tends to induce crazing.     

Galvanic Corrosion Theory for Adherence of Porcelain Enamel Ground Coats to Steel

The galvanic corrosion theory of adherence between ground-coat enamels and steel was investigated as a part of a broad study of the bonding mechanism between ceramics and metals. The theory, which is outlined in this report, is based on the mechanical anchoring of the enamel into the pits formed by the galvanic attack of the enamel on the steel surface. The theory was first examined from the standpoint of the data on adherence obtained in earlier studies at the National Bureau of Standards. In addition, several experiments were performed which demonstrated that galvanic corrosion of the metal base could occur during the short firing times encountered in enamel processing. On the other hand, certain inconsistencies were observed in the data which indicated that the mechanism of galvanic attack followed by mechanical anchoring was not the only important factor affecting the bond strength.     

On the Adherence of Porcelain Enamel to Sheet Steel

A study of the reactions at the interface of enamels and steel during firing has been made using a reflecting microscope, an electron microscope, an X-ray spectrometer, and a light spectrograph. Much attention has been given to the tendency of a number of oxides, e.g., cobalt oxide and nickel oxide, to develop a typical roughening of the base metal during firing. This roughening appears to be caused by two successive electrochemical reactions at the interface of the enamel and steel. With the aid of the results, a short explanation is given of the various ways in which enamels may adhere to steel.     

Improving the mechanical impact and bending resistances of enamel via B2O3 addition

Enameled metal equipment possesses the advantages of the material strength of metal and the corrosion resistance of glass; therefore, such equipment is used extensively in industries involving chemical processes. However, the mechanical properties of enamel affect its service life. Therefore, the enhancement of the mechanical properties of enamel to extend its service life is important. To improve the mechanical impact and bending resistances of enamel, we used enameled samples with Q345R steel as a metal matrix for controlled-temperature sintering. The enameled samples were prepared using varying amounts of B₂O₃ in the formulation of the enamel glaze, and the mechanical impact and bending resistances of the enamel were studied via the ball impact and three-point bending methods. The results show that B₂O₃ addition improves the mechanical impact and bending resistances of enamel significantly, although these resistances decrease when the B₂O₃ content is too high. The enamel exhibits an optimal mechanical impact resistance with a B₂O₃ content of 2%, and an optimal bending resistance with a B₂O₃ content of 3%. These results provide important reference values that can be used to optimize the formulation of enamel and extend the service life of enamel equipment.     

金属-氮化物结合刚玉质滑板的结构与性能

由80%～90%板状刚玉及20%～10%金属铝组成的坯料经氮化处理(温度1100℃)后,再进行表面氧化处理(温度800℃),可制得显气孔率为2%的Al-AlN-Al2O3滑板材料,其1400℃高温抗折强度高达48.7 MPa. 该滑板材料浇钢的使用寿命是Al2O3-C滑板的2倍. 显微结构分析表明,部分金属铝氮化形成AlN的体积膨胀效应及其对刚玉晶粒的结合作用,提高了材料的结构致密度和强度,赋予材料优良的抗钢液侵蚀性能. 部分金属铝的高温塑性状态、金属铝及氮化铝的高导热性、刚玉与氮化铝的复相热失配等,是材料具有高抗热震性能的主要原因.     

Colorimetric characteristics of colored single-coat enamels

Compositions for single-coat glass enamels that allow obtaining a wide color range of coatings with sufficiently good surface quality for steel articles were developed. Different dyes were tested. Properties of the coatings such as the chemical stability, adhesive strength, gloss, and thermal stability were investigated and the firing temperature range, 670–740°C on average, was determined. The colorimetric characteristics of single-coat enamels were calculated. The enamel coatings obtained can be used in mass production.     

ZrO2-AlN复相材料的制备与性能研究

试样以部分稳定氧化锆（PSZ）和金属Al粉为原料。在N5气氛下经1600℃×4h烧结可制备出以AIN为结合相的ZrO2-AlN复相材料（ZAN）。试验研究了金属Al的氮化反应过程。以及金属Al的加入量对复相材料性能的影响。结果显示：Al粉反应生成AIN的全过程,主要由界面反应和内扩散反应交替完成．且整个反应过程应为扩散机制控制。随金属Al粉引入量的增加、ZAN试样的热膨胀系数减小,抗折强度增大,抗热震性及抗氧化性提高。ZAN-3试样的综合性能优良,其常温抗折强度为73．61MPa,热膨胀系数为6．06×10^-6/℃。室温～1100℃水冷残余抗折强度为4．64MPa。     

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.     

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.