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.     

RELATION OF SURFACE OXIDATION TO CERTAIN DEFECTS IN ENAMEL COATINGS ON STEEL, III

Oxide on the surface of enameling stock is found to play an important part in controlling the occurrence of hydrogen-caused defects. Actually, steel with a preformed oxide coating of certain characteristics may be fired directly with a white cover coat without incurring fish scaling, copperheading, blistering, or reboiling. The oxidizing treatment probably removes some of the inherent hydrogen from the stock and then prevents subsequent absorption during firing from the reaction of iron with moisture which remains in the dried enamel coating. The advantages of preoxidation are discussed. All other gases that may evolve from the enamels pass through the enamel without causing defacement. The primary boil simultaneously oxidizes and hydrogenizes the steel; and the pickup of hydrogen, which occurs during firing, is often the portion responsible for subsequent fish scaling and reboiling.     

CHIPS, FISH SCALES, AND SHINERS, IV

The effusion of hydrogen from enameled steel to cause fractures in the enamel was investigated in an attempt to show that this gas may cause defects that range from plate-like fractures, or “chips,” through the known types of fish scales to the most minute shiner. The difference in the appearance of these defects was found to depend principally on differences in (1) the physical properties of the enamel, (2) the metal-enamel bond, and (3) the rate, quantity, and localization of the hydrogen effusion. Reboiling is shown to be related to fish scaling and to the other fracture-type defects because all are primarily hydrogen functions. The principal source of the critical quantities of hydrogen necessary to cause the defects is often found in the water of hydration in the dried slip, and the injection of hydrogen by that water during firing is demonstrated by experiment just as was shown for blister-type defects (see this issue, pp. 180-90). The prevention of fracture-type defects is discussed, and some novel effects of protective surface oxidation are illustrated.     

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.     

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.     

SCRATCH BLISTERS, II

The enamel defect known as “scratch blisters” is consistently found to be caused by hydrogen that is introduced in large quantities at scratch marks. Several independent phenomena may contribute this hydrogen. An electrochemical reaction between iron and its oxide scale during pickling increases the hydrogen absorption at the scratch sufficiently to lead regularly to scratch blisters when steel with a certain type of oxide coating is used. Galvanic nickel coatings aggravate scratch blistering in a manner that is not well understood. Surface impurities and mechanical effects may produce scratch blisters but only circumstantially. The rolling direction of the sheet is found to be related to the occurrence of scratch blisters.     

Formation mechanism and adhesive strength of white single-layer low-melting enamels on steel

The formation mechanism of white single-layer glass enamel coating on steel surface in low-temperature firing is considered. The basic laws governing strong adhesion are identified. The effect of various methods of pre-treatment of steel before single-layer low-temperature enameling on the structure and composition of the transitional adhesive contact layer determining the adhesion strength of steel — enamel composite is demonstrated.     

PRACTICAL AIDS TO WET-PROCESS CAST-IRON ENAMELING1

The importance of design of castings and proper foundry practice is discussed, to- gether with the advantages of the proper type of “filler” for holes in porous castings. The functions and advantages of a ground-coat enamel are discussed, and the effect of the time-temperature comeback of the enameling furnace is shown to be important toward getting best results. Multiple-deck firing aids in obtaining a slower comeback and increases production.     

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.     

ELIMINATION OF BLISTERING OF ENAMELED-IRON NUMBER PLATES *

In the manufacture of enameled-iron number plates for dials on pay-station telephones, blisters, formed in the enamel during the low-temperature firing of the printed characters and background, resulted in an average process loss of 14%. Proposed remedies tried without success were (1) the use of different enamels and enameling irons, (2) firing the characters at other temperatures than the standard temperature of 1310°F, and (3) special treatment of the iron surface before enameling, such as deep steelblasting, heating at 1310°F prior to enameling, or copper plating. The blistering was finally eliminated by increasing the length of the heating-up period in the character firing operation.     

Influence of Cold-Reduction Practice on Hydrogen Behavior in Enameling Steels

As part of a continuing study intended to relate enameling characteristics to measurable physicochemical properties, experiments were performed in which the behavior of acid-charged hydrogen in enameling steel and the enamel-ability of enameling steel were compared for as-cold-reduced and normalized lots of steel produced from hot-rolled specimens given various degrees of cold reduction in the laboratory. Concurrent with increased cold reduction of the steel in the range 0 to about 75% was an increase in hydrogen solubility in the steel and a general decrease both in the intensity of hydrogen-caused ground-coat reboiling and in the tendency of the steel to develop delayed defects in direct-white enamel coats. The influence of cold reduction on hydrogen-caused defects in enamel coats was diminished by previous normalizing of the cold-reduced sheet and by the heat-treatment given the steel during the enamel-firing process. The influence of cold reduction on hydrogen solubility in steel was only partly overcome by heat-treatment. For as-cold-reduced specimens, increased cold work increased the hydrogen-desorption rate to a maximum below 20% reduction and a further increase in reduction decreased the de-sorption rate. For normalized specimens there was a trend for the desorption rate to decrease with an increase in the amount of previous cold reduction of the steel.     

Role of Nickel Dip in Enameling of Sheet Steel

An investigation was made of the effects of (a) the firing time and (b) the weight of nickel deposited from the nickel-dip solution on the adherence developed by a cobalt-free and a cobalt-bearing ground-coat enamel on both enameling iron and a titanium-bearing low-carbon steel. At 1550°F. the nickel improved the measured adherence index the most during the 2-minute firing. The effect of the nickel, however, decreased markedly with longer firing times. There was an optimum nickel deposit for maximum adherence, an optimum that varied from 50 to about 120 mg. per sq. ft., depending on the type of cleaning used and on the type of enamel applied. The nickel dip reduced the tendency to fish-scale. Furthermore, there was an apparent relation between fish-scaling resistance and good adherence; no specimen with an adherence index of 70 or above showed fish-scaling tendencies. Metallographic studies of the interface of coated specimens showed that nickel dipping of the steel before enameling brought about a surface roughening during the firing operation. A relationship was noted between the degree of roughening and the measured adherence index. The roughening of the steel surface was ascribed to galvanic corrosion.     

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.     

Characterization of vitreous enamel–steel interface by using hot stage ESEM and nano-indentation techniques

In this study, phase transformation of the vitreous enamel and the interface steel–enamel during firing was analyzed. The thermal transformation of vitreous coating on steel was observed “in situ” with an environmental scanning electron microscopy and the mechanical properties of the steel–enamel interface were studied by using a nano-indenter. The interface reactions and the resulting structure can strongly influence the adhesion mechanism between glass coatings and the metal substrate. An in-depth investigation and structural characterization was therefore performed to define the correlation between interface morphology and the final chemical and mechanical properties of the enamel–steel interface.     

METHODS FOR THE DETERMINATION OF THE FIRING RANGE OF VITREOUS ENAMELS USED IN WET-PROCESS ENAMELING1

Several methods for making firing range studies in case of wet-process enamels are described. The methods used in producing enamel deposits of predetermined weights are given. The use of these methods has shown that enamels used in wet-process enameling of metal vary widely in action under various conditions of heat and a general discussion of the subject is included.     

Nature of Adherence of Porcelain Enamels to Metals

An investigation of the fundamentals of the adherence of porcelain enamels to metals indicated that good adherence is the result of metal-to-metal bonds between the atoms in the base metal and the proper metallic ions in the enamel. To accomplish this type of bond, the enamel must meet certain chemical and thermodynamic requirements: (1) The enamel at the interface must be saturated with an oxide of the metal and (2) this oxide must be one which, when in solution in the glass, will not be reduced by the metal. In the case of iron, the oxide is FeO. Many of the phenomena observed in commercial enameling were investigated and found to be related to adherence, but not essential for its development. An example is the precipitation of metallic particles in the enamel. Much of the complexity in commercial enameling arises from the limitations imposed by practical considerations. For example, because enamels usually are fired in air, the heavy scale developed during the early stages of firing must be removed before adherence can be developed. Likewise, as the conditions of the enamel-metal interface change rapidly during firing, "adherence-promoting oxides" are used to help maintain the necessary conditions for the time required in commercial enameling. Surface roughness, although not necessary for excellent adherence, was found to improve the apparent adherence when the bond between the enamel and the metal was relatively weak.     

3种测试溶液对搪瓷用钢抗鳞爆性能评价指标的影响

采用3种常见的测试溶液检测了10组搪瓷用钢板的氢渗透性能,并同时采用涂搪实验进行验证。结果显示,测试溶液A适用于试验选定的各种厚度的搪瓷用钢,其检测结果准确率可达100%;而溶液B和溶液C在测试厚度1.5 mm以上的搪瓷用钢板料出现部分误判现象。溶液B若基于有效氢扩散系数D_eff,其准确率为70%,若基于TH值,其准确率仅为50%;而溶液C误判率达30%。     

Catalytic Activity of Nickel at the Glass/Metal Interface in the Porcelain Enameling System

It is suggested that the function of the nickel flash in the one-coat enameling system is to atomize catalytic-ally the hydrogen which forms at the interface and thus facilitate its diffusion into the steel. In the absence of nickel, the bubble which forms between the enamel and the metal has been shown to contain hydrogen. Of the metals tested, only platinum and palladium were effective replacements for the nickel. These two metals are known to catalyze the atomization of hydrogen.     

纳米二氧化硅改善钢板搪瓷浴缸耐酸化学稳定性

通过磨加法将纳米二氧化硅引入到钢板浴缸搪瓷釉中,在不改变钢板浴缸搪瓷釉工艺性能的条件下,提高钢板浴缸搪瓷的耐酸化学稳定性。研究结果表明,磨加一定量纳米二氧化硅到钢板浴缸搪瓷釉中,不仅可以显著提高钢板浴缸搪瓷的耐酸化学稳定性,而且工艺性能优良。     

ABSRACTERS

Steel containing sufficient titanium to combine with all of the carbon does not react with vitreous enamels like ordinary low-carbon steel or ingot iron so that white cover-coat enamels, without a ground coat, can be fused on it without blistering or black specking. The titanium content must be more than 4.5 times the carbon content. This steel has been made commercially by the basic open-hearth process, and the requirements for successful manufacture are explained. It is of excellent quality for deep-drawing and does not have a definite yield point even when normalized or annealed so that stretcher strains cannot occur in it. It is not subject to strain aging of any kind, even when strained as much as 17% and aged at 450°F. It is resistant to caustic embrittlement, and to attack by hydrogen at high temperatures and pressures, and it also resists sagging at enameling temperatures better than regular enameling steel or iron. The yield strength of this steel at room temperature as now manufactured commercially is low, but titanium steels of higher strength containing manganese, nickel, and copper are described, which have the same favorable enameling quality. These stronger steels have excellent ductility and toughness, fair weldability, good resistance to strain-aging embrittlement, and better resistance to grain growth at high temperature than the regular titanium enameling steel. Their properties, however, still remain to be checked on commercial heats.