Suitability of cast iron for welding
- Previous service conditions
Cast iron that has been exposed to lubricants, superheated steam or aggressive
chemicals can become more difficult to weld:
There is tendency to form porosity in the weld metal or incomplete fusion
between the weld metal and the base metal.
- Brittleness of cast iron welds
In grey cast iron and spheroidal graphite cast iron, shrinkage stresses can
often exceed the tensile strength. In this case, cracks or breaks will
occur.
The risk of cracking increases with the presence of structural defects such as
casting porosity.
- Brittleness of grey cast iron
The brittleness of grey cast iron is due to the presence of lamellar graphite in
its structure:
Local stresses induced by preheating, welding and cooling are concentrated by
the graphite flakes.
Design of the weld
- The shape of the workpiece
The shape of the workpiece has an influence on the temperature distribution
during welding causing expansion / contraction forces.
The risk of cracking increases with the complexity of the shape and presence of
stress raisers.
The quenching effect on the weld and HAZ increases with the workpiece thickness
and raises the risk of cracking.
- Rigid workpiece
During welding, rigid workpiece cannot move freely. In grey cast iron and
spheroidal graphite cast iron, accumulated stresses can even exceed the tensile
strength.
In this case, cracks or breaks will occur.
- Free moving workpiece
Free moving workpieces have a positive influence on weldability.
Welding process and procedure: Arc welding processes
The heat-affected zone (HAZ) is hard and brittle because of the formation of a
mixture of iron carbide and martensite - in contrast to the relatively soft
structure of cast iron.
Contraction induced stresses can accumulate in the HAZ and, subsequently, lead
to microcracks.
Safety: Arc welding
see chapter safety
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