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.     

FURTHER CONSIDERATIONS OF HYDROGEN AS A CAUSE OF ENAMEL DEFECTS, V

The argument that reboiling is primarily a function of hydrogen is expanded with additional evidence obtained from metallurgical research during the past year. The behavior of hydrogen in rimming-steel enameling stock is discussed, and an explanation is given for the varying behavior of the edge and center sections of rimming-steel sheet in the development of certain hydrogen-caused defects.     

Influence of Cold Reduction on Hydrogen Behavior in High-Purity Iron

Experiments to determine the effect of cold reduction on the behavior of hydrogen in high-purity sheet iron were conducted using two laboratory-melted heats. One heat was air-melted and poured and contained 0.00470 carbon and 0.17% oxygen; the other was vacuum-melted and poured and contained 0.003% carbon and 0.006% oxygen. An increase in hydrogen solubility (acid-charged) with increasing cold reduction was observed for both lots. The hydrogen solubility increase occurring from 0 to 80% cold reduction was much more pronounced, however, for the lot containing the higher amount of oxygen. Ground-coat reboiling tests were made on both lots of high-purity iron. The material containing high oxygen exhibited light reboiling for all levels of cold reduction, whereas the material containing low oxygen exhibited light reboiling for intermediate levels of cold reduction and heavier reboiling for low and for high levels of cold reduction. For the cold-reduced material containing high oxygen, the frequency of delayed defects decreased with increasing cold reduction. For the purer material, delayed defects were observed only for the specimens having the highest level of cold reduction.     

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%。     

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 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.     

Relationship Between Hydrogen Solubility and Reboiling Tendency in Enameling Steels

Enameling steels that had exhibited various degrees of reboiling behavior after ground-coat enameling were charged with hydrogen to saturation by immersion in 2 N sulfuric acid at 38° C. Hydrogen contents were then determined by warm extraction at 160° C. under mercury. It is shown that for a given grade of steel the specimens which had been rated as heavy reboilers in the enameling tests occluded at 38° C. considerably less hydrogen than those rated as trace or light reboilers. A given level of hydrogen solubility, however, does not necessarily indicate the same reboiling tendency for different grades of steel. The determination of hydrogen solubility by the techniques described provides a method of indicating the reboiling tendency of a particular steel that is independent of the effects introduced by ground-coat composition.     

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.     

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。     

Quantitative High-Temperature Oxidation of Porcelain Enameled Iron

A continuous gain-in-weight method was used to measure quantitatively the oxidation taking place at elevated temperatures for long periods of time on enameled and unenameled iron. The oxidation was studied as a function of time and temperature on both bare and enameled iron. Specimens were subjected to variations in (1) enamel thickness, (2) thickness of electroplated-nickel deposit, and (3) metal surface preparation of the iron. The rate of oxidation of enameled iron was lower than that of bare specimens at all temperatures. This effect was most significant at lower temperatures; it decreased in extent as the test temperature was increased to 1850°F. An increase in enamel thickness was very effective in reducing the rate of oxidation. To a lesser extent, an increase in nickel-plate thickness on both enameled and unenameled specimens, and the use of sandblasting instead of mechanical abrasion and chemical cleaning as a method of metal preparation before enameling, also reduced the rate of oxidation. After the initial oxidation had proceeded at a linear rate, the oxidation rate of enameled and unenameled specimens followed a parabolic relation.     

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.     

NICKEL FLASHING AND ITS RELATION TO ENAMEL ADHERENCE*

Nickel flashing as applied to enameling stock is shown to be primarily metallic nickel. The role of nickel in developing enamel adherence is apparently bound up with the retardation of oxidation which the nickel flash imparts in the enameling cycle. The problem of adherence may be considered to be a corrosion phenomenon of the base iron developed by the action of gases and other agents which may be present at enameling temperatures. As such, the degree of adherence is a function of oxygen pressure at the interface when the glass is fused. The equilibria developed are strongly affected by the presence of nickel. Using cover coats directly on iron, the amount of nickel required will vary with each particular enamel composition. Şome evidence as to the nature of the adherence-promoting oxide is presented.     

SOLUBILITY OF A ZIRCONIUM OXIDE OPACIFIER IN ENAMEL GLASSES*

The solubility of a zirconium oxide opacifier was determined by fusion in enamel glasses and then by heat-treating at enameling temperatures and noting the amount of crystallization. An increase in the alumina, zinc oxide, and calcium oxide content in the glasses caused a decided decrease in the solubility of the zirconium oxide opacifier. The fusion temperatures of the enamels were raised by solution of zirconium oxide.     

Effect of Oxygen Content of Furnace Atmosphere on Adherence of Vitreous Coatings to Iron

A series of porcelain enamel ground coats of the same basic composition but with various amounts of cobalt oxide was fired on enameling iron in atmospheres consisting of various oxygen-nitrogen mixtures. The effect of the oxygen content of the atmosphere on adherence was determined and the effect on interface roughness was estimated from an examination of metallographic cross sections. A decrease in the amount of oxygen in the firing atmosphere necessitated an increase in the amount of cobalt oxide in the enamel if optimum adherence was to be secured. Ground coats containing 3.2% cobalt oxide or more developed a weak but definite bond in oxygen concentrations as low as 0.02 mole %. A qualitative correlation was found between adherence and surface roughness regardless of the oxygen content of the firing atmosphere.     

PHOSPHORUS STEELS FOR VITREOUS ENAMELING*

A summary is given of the tests which were conducted to appraise the utility of phosphorus steels in vitreous enameling. Enameling tests were conducted on both killed and rimmed steels of low-carbon iron and steel with a phosphorus content up to 0.60%. Phosphorus was found to reduce the sag of laboratory-prepared enameling stock to a marked degree. The most effective range of phosphorus was from 0.10 to 0.15%, and killed steels were superior to rimmed steels in their sag resistance. Phosphorus appeared to have no significant influence on common enameling defects. In amounts of less than 0.40%, it had no marked effect on the adherence of the enamel. The deep drawing qualities of the sheet, based on Olsen ductility tests, were somewhat impaired by phosphorus. This is in line with the strong tendency for phosphorus to raise the strength of the sheet.     

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.     

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.     

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.     

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.     

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.