Residual Stress Evaluation in Vitreous Enameled Steel Sheets by Digital Images Analysis of Microstructures - Visual Computing: Scientific Visualization and Imaging Systems - page 169

Fig. 9.5 Thermal expansion diagram of the low carbon steel and of the vitreous enamel material
used in the present study
temperature) but this is a failure in the structural strength as the enamel actually
continues to expand as indicated by the dotted line. Because of the high mobility of
the enamel above this temperature, the stress and strain in enamel low-carbon-steel
composite system become very low, even though they both continue to expand at
different rates with increase in temperature. The flow of the enamel glass sub-
stantially relieves the stresses during the heating and cooling of the composite at
temperatures above the BRT. The amount of tension in low-carbon-steel and
compression in enamel would actually be a function of the contraction differences
in cooling from BRT to room temperatures during the cooling phase. For a fixed
component geometry, the intensity of residual stresses acting on the enameled steel
composite can be pre-determined by means enamel design (compositions of frits
and additions) aimed to match proper values of both BRT and coefficient of thermal
expansion.
9.3 Materials and Characterization
Sheets of 0.8 mm thick of a very low carbon steel, as reported in Table 9.1 , were
coated by two blue enamels: one is a wet enamel prepared for the wet-spray
application and the other one is a dry enamel for electrostatic deposition. Steel
specimens have a rectangular shape: 250 mm in length and 40 mm in width.
Both enamel raw materials were prepared by the same types of frits whose
compositions, given by the frits producer, are summarized in Table 9.2 .