To successfully weld cast iron it is necessary to
consider both welding techniques and the characteristics of the
material. It is also necessary to investigate the reason for the damage
and to take into account the requirements of the finished piece.
Types of Cast Iron
Gray cast iron is usually identified according to its tensile strength.
30 grade cast iron has a tensile strength of 30,000 psi; 40 grade cast
iron has a tensile strength of 40,000 psi; and 60 grade cast iron has a
tensile strength of 60,000 psi. The most common grades of cast irons are
the 20 and 40 grades. Gray cast iron is a ferrous material containing
more than 1.75% of carbon, the greater part of which is present in the
structure in the form of graphite. This graphite gives to the fracture a
color varying from light to dark gray. Compared with steel, gray cast
iron has low malleability or ductility caused by the separation of the
major portion of the elementary carbon in the form of graphite, which
form a network throughout the metallic matrix and separate from it.
The chemical composition of a common machine casting is as follows:
Carbon 3.0% to 4.2%
Silicon 1.5% to 2.5%
Manganese 0.6% to 1.2%
Phosphorus 0.4% to 0.6%
Sulphur 0.05% to 0.18%
Basically, cast iron is steel with a lot more carbon. Steel contains
carbon up to 1.75%. If steel has more than 1.75% carbon, it is called
cast iron. Steel can dissolve up to 1.75% carbon. If more carbon is
added, it wont dissolve and remains a free carbon. When you look at
cast iron under a microscope the free carbon appears as black flakes,
hence the name gray cast iron. With the use of proper techniques and
the correct filler alloy, most gray cast irons can be successfully
welded.Be aware that there is another type of cast iron, called white
or chilled cast iron. It is generally considered not weldable. White
cast iron does not have the distinctive gray color of most cast iron and
is generally found in equipment where extreme hardness is needed (e.g.
rock crushers, steel mill rollers, etc.).
