Residual Stress Evaluation in Vitreous Enameled Steel Sheets by Digital Images Analysis of Microstructures - Visual Computing: Scientific Visualization and Imaging Systems - page 166

Fig. 9.1 Scheme of the frits
for vitreous enamel raw
material production
grains that can be applied to the metal substrate by an electrostatic deposition
technology.
The wet enameling process consists of four phases shown in Fig. 9.2 : (i) wet
milling, (ii) wet spraying (or flow coating deposition), (iii) drying, and (iv) firing at
high temperature (about 550 C for low melting alloys, about 750 C for cast iron
and about 850 C for steel). The dry process is based on three phases: (i) dry
milling with silicone oil addition, (ii) electrostatic deposition, and (iii) firing at
high temperature (about 550 C for low melting alloys, about 750 C in case of
cast iron and about 850 C for steel).
Figure 9.3 gives an example of the firing curve. This figure shows the typical
firing diagram where the heating, the maturing, and the cooling phases are high-
lighted together with some photos of the enamel pellets over a metal substrate
taken during the firing process. The photos of the pellets, taken with a heated
microspore, show the enamel pellet transition from A up to B temperature during
the firing phase, and the complete fusion of the enamel pellet during the maturing
phase (C). It is interesting to note that during the cooling phase the enamel lens
(D and E) does not change its shape and it consolidates over the metal surface.
During the firing process, the enamel raw material melts and interacts with the
metal substrate, thus enabling the formation of a continuous varying structure.
As shown in the micrograph of the transversal section in Fig. 9.4 , the interface
zone between the substrate and the external layer is made of a complex material
system where the vitreous enamel and the metal constituents are mixed. In par-
ticular, three main regions can be identified, starting from the bottom of Fig. 9.4 a:
a first region is made of metal, the second region is the interphase where both
metal and enamel constituents are mixed, and the third region is composed by the
vitreous enamel material. Referring to Fig. 9.4 b, the presence of metallic dendrites
that hinder the substrate and the external layer passing through the interphase
region can also be noted.
As already mentioned in the introduction, the mechanical behavior of the vit-
reous enamel-metal composite at room temperature is greatly influenced by