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Taking this into account care must be taken when putting '"a new--=
kettle into service on which the protectiVe zinc-iron alloy layer
is not yet present. The heating up phase is very critical, because high stresses can develop especially when high differential temperatures are present.
A new kettle should be filled with zinc without lead so. that the protective zinc-iron alloy layer can be formed. Lead should be added afterwards. The critical temperature whereby LMC comes into being is raised without lead from 328 deg .c (the melting point of lead) to 420 deg C.LMC can only be formed when cracks are present in the steel, which are soon closed by the rapid formation of the zinc-iron alloy layer by the liquid zinc at 420 deg C, so that no intercrystalline cracks can be formed.
6.2 Blue brittleness
In soft unalloyed steels a brittleness caused by stretch aging (elongation at break and diminuation of constriction) comes into being in warm tensile tests in the temperature range between 200 and 300 deg C. This effect is called blue brittleness. With the steels used today for the construction of zinc kettles the susceptibility to blue brittleness is almost completely eliminated, so that blue brittlen ss during the heating up phase is hardly of any significance.
6.3 Brittle fractures
Brittle fractures in kettles which can be formed by the affect of low temperatures (around -15 deg C), are in fact traceable to bath materials which are prone to the formation of brittle cracks in combination with stresses resulting from the manufacturing process. In general it can be assumed that with the kettle materials used today lower welding temperatures are possible than was the case with steels used in the past. However, transport at low temperatures should be avoided, unless absolutely necessary.
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6.4 Stresses
The stresses in the inside kettle wall in combination with liquid metal corrosion are created by:
- impeded expansion by heat as a result of differences in temperatures (thermal stresses) between the heated and un heated parts of the kettle
- Hydrostatic pressure of the zinc
Stresses originating from the construction
When a kettle filled with close packed zinc blocks is heated a load on the walls is created because the thermal, expansion of zinc is greater than that of steel {between 20 deg C and 419.6 deg C it is 12.8 mm/m greater). This can 'be avoided by leaving sufficiently large air gaps in the middle of the kettle. The stresses in the kettle, which manifest themselves mostly when
heating up older installations and theoretically can lead to
damage of the kettle, are created by the temperature differences
between the warmer upper part and the colder bottom of the
kettle. Moreover stresses are created by temperature differences
between the interior and exterior of the kettle walls. Also by differential expansion along the heated long sides of the kettle
often leading to distortion (figure 8). The zinc presses on the bottom so that the transition between bottom and side wall is subject to strain:•
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Figure 8• ScheMntlc of n 'bow In the botton of' the kettle cnused by teMpera. ture difference t2) 1n the ket:tle