RELATION OF COEFFICIENT OF EXPANSION TO CROSS-BENDING STRENGTH OF SHEET-STEEL ENAMELS1

Abstract

Two series of enamels were made with calculated cubical coefficients of expansion varying from 361 to 306. In both series of enamels, the cross-bending values were inversely proportional to the coefficient of expansion. This is due to the fact that all enamels on sheet steel have lower coefficients of thermal expansion than steel. In cooling, after the enamel is fused to the metal, strains are set up in the enamel; the lower the expansion of the enamel the greater the strains. When the enameled piece is bent, the greater the stresses present, the farther the pieces can be bent before there is any real tensile strain on the enamel. High bending ability is dependent primarily on high compressive strains induced by lower coefficient of expansion of the enamel than of the metal base. Compressive strains cause warping, fishscaling (in ground coat), and chipping; therefore the bending strength should not be too high. In all control work both the minimum and the maximum bending strength should be specified.