Factors in Selecting Steel

The most important factors in the selection of steel for porcelain enameling are enamelability, freedom from surface defects, formability, sag characteristics, strength, and weldability. The relative importance of each depends on the requirements of the finished product.

Enamelability. Oxidation of carbon in the steel during enamel firing is the source of the carbon monoxide that causes primary boiling of the enamel. A limited amount of such gas evolution can e tolerated in two-coat/two-fire porcelain enameling, because the ground-coat enamel has high fluidity and tends to heal over defects. Because primary boiling does not reoccur on subsequent firing, a defect-free cover coat can be obtained.

In direct-on cover-coat enameling, however, very little gas evolution can be tolerated. Therefore, it is important to select a steel that is relatively free of carbon or one that has the carbon stabilized by the addition of titanium or a combination of titanium and/or niobium. When the higher-carbon steels are being selected for porcelain enameling, surfaces must be free of large angular cementite (iron carbide) particles.

Surface Defects. It is imperative that the surface of the steel be free of defects such as deep scratches, pits, slivers, scale, and open laminations. These defects are particularly troublesome in steels that are used for direct-on cover-coat applications.

Jagged metal burrs along scratches can protrude through the ground-coat enamel. Oxidation of these burrs during firing can cause copperheads in the ground coat and blistering in the cover coat. Folded-over burrs, deep scratches, and surface laminations can entrap dirt, drawing compounds, or cleaning and pickling solutions, all of which promote the formation of blisters in the porcelain enamel during firing.

Formability. Sheet steels for porcelain enameling are available in conventional commercial-quality, drawing-quality, and drawing-quality special-killed grades for varying severity of draw. The steels are usually supplied with a light application of oil to minimize rusting. This is particularly important in systems where acid etching is not employed during metal preparation.

Sag Characteristics. Sag is the permanent deformation or creep of steel due to its own weight during enamel firing. The sag resistance of steel is related to its strength at elevated temperatures and to the temperature at which the steel starts to transform from ferrite to austenite on heating. In general, the higher the strength and the higher the transformation temperature, the better the sag resistance of the steel. Figure 1 is a plot of typical metal sag of different enameling steels as a function of temperature. Thickness of these steels was 0.89 mm (0.035 in.).

Fig. 1 Comparison of sag resistance of selected enameling steels. A, low-carbon enameling steels; B, decarburized steels; C, interstitial-free steels

Strength Retention. Some steels will exhibit critical grain growth and resulting loss of strength during the firing of the enamel coating 790 to 840 °C (1450 to 1550 °F) temperature range. Because the fired enamel coating is normally fractured when the base metal is strained beyond the elastic limit of 0.002 mm/mm (0.002 in./in.), the ability of a steel substrate to retain its strength after firing becomes an important factor when selecting steels for many service applications of the enameled component. In addition, the contour and thickness of the metal are other factors for consideration. Figure 2 shows the relative strength-after-firing characteristics of some commonly used enameling steels.

Strain, %

Fig. 2 Comparison of yield strength of four enameling steels after firing at 870 °C (1600 °F). A, low-carbon enameling steels (drawing quality); B, low-carbon enameling steels; C, interstitial-free steels; D, decarburized

Weldability. Good welding characteristics are required for fabricated shapes. Welds must be free of inclusions, blowholes, and laminations.

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