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Welding
of
Cast
Iron -Cast irons include a number of iron base materials that contain
Carbon 1.7 to 4.5%
Silicon 0.5 to 3%
Manganese 0.2 to 1.3%
Phosphorus0.8% Max.
Sulphur 0.2% Max.
Alloy cast irons contain the following elements in addition to the ones mentioned above
Molybdenum
Nickel
Chromium
Copper
Cast Irons can be classified as
(i) White Cast Iron
(ii) Gray Cast Iron
(iii) Malleable Cast Iron
(iv) Nodular Cast Iron.
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(i) White Cast Iron
Welding or brazing of white Cast Iron is rarely required or done.
(ii) Gray Cast Iron
It shows a grayish surface when fractured.
It contains
C 2.5-3.8%
Si 1.1-2.8%
Mn 0.4-1.0%
P 0.15%
S 0.10%
Fe Balance
It is marked by the presence of flakes of graphite in a matrix of ferrite or pearlite. It contains a portion of carbon in free form as graphite flakes and the rest in the combined form as cementite, pearlite, etc.
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Gray cast iron possesses lowest melting point of all the ferrous alloys. Gray cast iron is easy to machine. Cast iron is brittle in nature. Gray cast iron is probably the most difficult of all metals to weld economically. Nevertheless, the number of gray iron castings made and the inevitably high percentage of breakages that occur in service make welding necessary.
The three major areas of application of welding to cast iron are:
(i) Repair of casting defects in the foundry.
(ii) Repair of castings that have become damaged or worn in service.
(iii) To join together separately cast sections.
(iii) Malleable Cast Iron
Whereas in gray cast iron, the graphite has a flake appearance, it possesses a quasi-spheroidal (temper carbon) appearance in malleable iron. Malleable iron is obtained when white cast iron is heated to 760°C for 24 hours per each 25 mm of thickness and then cooled slowly. This heat treatment converts graphite from flake form to quasispheroidal shape. Welding malleable iron destroys this heat treatment and turns the metal into white or gray cast iron.
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Malleable iron contains
C 2-3%
Si 0.6-1.3%
Mn 0.2-0.6%
P 0.15%
S 0.10%
Malleable iron has a solidification range of 1400-1130°C.
(iv) Nodular Cast Iron
Whereas in gray cast iron, the graphite has a flake appearance, it possesses a spheroidal appearance in nodular cast iron. The spheroidal graphite, which is the result of small additions of magnesium, cerium or cadmium to the cast iron, gives to it superior ductility and toughness.
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Nodular, Ductile or Spheroidal Cast Iron contains
C 3.2-4.2%
Si 1.1-3.5%
Mn 0.3-0.8%
P 0.08%
S 0.02%
Most of the welding is done on gray cast iron. Malleable iron welding and ductile iron welding are comparatively less common.
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