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Vitreous Enamel Sheet

Typical Applications Application Considerations Tolerances Bulletins

U. S. Steel Vitrenamel Steel Sheet products are designed to optimize the performance during the application of a porcelain (or vitreous) enamel coating. Porcelain enamel coatings are hard, fused, glassy coatings that are applied to the surface of steel sheet to improve the surface appearance of the sheet and to provide a hard, protective coating. The coatings are applied using specially designed, finely ground, glass powder coatings (called a "frit") to the surface of the steel sheet, and then heating ("firing") the ground glass film to develop a hard, fused coating. Prior to baking, the frit is applied to the steel sheet surface either as a water-based slurry or as a dry, electrostatically-applied powder.

Porcelain enamel coatings are unlike paint in that the coatings are inorganic, and form coatings that are much harder than paint coatings. As a result, porcelain-enameled coated steel sheet products are ideal for applications such as stove tops and oven cavities; applications that require both hardness to prevent scratching and elevated temperature stability. Other applications include home-laundry appliances, hot-water heater tanks (glass-lined), cookware, and chemical processing containers.

Steels that are used for enameling need to have specialized properties in order to be coated successfully and obtain good adhesion between the enamel and steel surface. In addition, the steel sheet must possess good formability characteristics to allow the formation of intricate parts prior to enameling. Another important characteristic for the steel is to possess good resistance to sagging and high strength during the firing process. Firing to fuse the coating involves temperatures as high as 1600^°F. Steels developed to meet all these criteria are a special class of cold rolled sheet; they are manufactured to possess the special set of properties that the specific application requires to achieve both good formability and good enamelability.

Porcelain Enamel Coatings
There are two basic types of porcelain-enamel coatings. One is applied as a two-coat system, a ground coat plus a cover coat. The ground coat is applied to the steel sheet to promote adhesion of the cover coat. Ground-coat frits contain oxides that help to promote adhesion between the steel surface and the enamel coating. The ground coat is applied first and fired to develop a hard, fused coating; then the cover coat is applied over the ground coat. It is also applied as a powder coating over the ground coat, and then fired at high temperature. The cover coat provides the desired color and surface appearance. This product is called a two-coat, two-fire system. There are some electrostatically-applied, two-coat systems that can be applied using only one firing step.

The other type of porcelain-enameling is called a one-coat system, and as the name implies, this is a single-coat, single-fire coating. This method of enameling requires the use of more selectively processed steels.

Steels for Porcelain Enameling
U.S. Steel produces three grades of steel for porcelain enameling. They are called USS  Vitrenamel 1 Steel, Vitrenamel 2 Steel, and Flexnamel Steel. These three grades are designed to be suitable for enameling in that they have properties that (1) resist fishscaling during the enameling process, (2) resist carbon boil during enameling, and (3) provide the sag resistance needed during the firing process.

• Vitrenamel 1 Steel
  Vitrenamel 1Steel is a Porcelain-Enameling Sheet in which the carbon content has been reduced to a very low level by means of a decarburizing anneal during manufacture in the plant. This steel grade can be porcelain-enameled either by the one-coat process or by a multiple-coat process. The steel may exhibit some loss in strength after firing. The degree of loss in strength is influenced by the severity of the formation and the actual time/temperature cycle used during the firing process.
   
  
• Vitrenamel 2 Steel
  Vitrenamel 2 Steel is a Porcelain-Enameling Sheet in which a low carbon content and the addition of important alloying elements are achieved during steelmaking. The steel is not given a special decarburizing anneal following the cold rolling operation. This grade of porcelain-enameling steel is suitable for the application of either a single ground coat or multiple coats. It does not perform as well as Vitrenamel 1 for the application of a single, decorative cover coat.
   
  
• Flexnamel Steel
  Flexnamel Steel is an extra deep-drawing, fully stabilized steel that is made by utilizing vacuum degassing during steelmaking to reduce the carbon level to a very low amount, and then adding stabilizing elements that combine with the interstitial elements to make an "interstitial-free" product. This product is suitable for applications in which ground coat and cover coat enameling operations are used. Flexnamel Steel has good sag resistance, excellent formability, and is intended for parts that involve very severe drawing during manufacture.

The typical steel chemistries for these three grades of Vitrenamel Steel Sheet are presented in the following table. Also included in this table are the general characteristics of each type of steel sheet.
 

Code: E = Excellent; G = Good; F = Fair; P = Poor; Y = Yes; N = No

Definitions of Terms

• Warpage
  Warpage is a term that refers to the distortions that occur in the formed part during the firing cycle. To minimize warpage, the part design should provide as much symmetry as possible and part strength to minimize dimensional changes during the firing cycle.
   
  
• Sag
  Sag refers to the dimensional changes that occur during the firing cycle related to the creep of the steel under its own weight and is related to the steel's gauge, incoming strength and transformation temperature. At carbon levels less than 0.025 percent, the sag performance depends on the precise carbon content of the steel. Up to 0.025 percent, a higher carbon content will increase the tendency to sag.
   
  
• Reboil
  Reboil refers to a phenomenon that relates to the the effect of the carbide-phase morphology present in the steel during firing. When subsurface carbides are close to the surface, they can react with the oxygen in the frit to form carbon monoxide and carbon dioxide gases. As these gases move upward to the surface of the frit as it is being heated and fused, they can cause bubble-like defects in the fused enamel coating.
   
  
• After-Fire Strength
  After-fire strength is primarily determined by the carbon content and the amount of strain related grain growth that occurs during the high temperature exposure during the firing stage.
   
  
• Fishscale
  This term relates to a type of surface defect that can occur in the enamel coating. Its occurrence relates to the formation of hydrogen in the firing stage.

  When steel is heated, the solubility of hydrogen increases substantially. As a result, the breakdown of moisture into hydrogen and oxygen that occurs during the firing cycle may lead to an increased amount of hydrogen in the steel while it is at an elevated temperature. On cooling at the end of the firing cycle, the solubility of hydrogen in the steel decreases. It cannot escape, however, because it can not permeate the hard enamel coating. The pressure of the hydrogen gas entrapped at the steel/enamel interface becomes locally very high; so high that it can cause the enamel coating to develop fissures and actually "pop" off the steel. The small area of flaked-off enamel has the appearance of a fish scale; hence, the name of the defect.

  Fiscale is a hazard when sheet steel in enamel coated on both surfaces, thus preventing the escape of hydrogen during cooling.  To overcome this hazard, a good enameling steel will contain numerous micro-voids and/or discontinuities in the steel microstructure which act as accumulation sites to trap and hold the hydrogen within the steel.

ASTM Specifications
ASTM Specification A424 is the specification that covers Steel Sheet for Porcelain Enameling. This is the product specification. The ASTM specification that covers general issues such as dimensional tolerances is the same as the general requirements specification for cold rolled sheets, A568.

Surface Finish
All the Vitrenamel grades are typically furnished with a rough matte finish to improve adhesion of the enamel coating. The approximate surface roughness is 55 to 90 microinches. A rougher surface finish, 90 to 150 microinches, can be provided if required for a specific application.

Sheet Dimensional Tolerances
The dimensional tolerances for USS Vitrenamel Steel Sheet products are the same as those for USS Cold Rolled Steel Sheet.

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