Page 126 - CITS - Welder - Trade Theory

WELDER - CITS

LESSON 44 & 45 : Cast iron - Types - Properties and uses,
welding methods of cast iron

Objectives

At the end of this lesson you shall be able to
• describe the types of cast iron
• explain the welding method of cast iron
• explain the cast iron filler welding.

Introduction
Cast iron is widely used in the manufacture of machine parts, since it has a good compressive strength and easy
to make the castings. There are different problems in the welding of cast iron in comparison to mild steel, even
though this is also in the group of ferrous metals.
Types of cast iron
There are four basic types of cast iron available.
a Grey cast iron
b White cast iron

c Malleable cast iron
d Nodular cast iron (or) spheroidal graphite iron
a Grey cast iron: Grey cast iron is soft and tougher than the white cast iron which is hard and brittle. The
good mechanical properties of grey cast iron are due to the presence of particles of free state carbon or
graphite, which separate out during slow cooling. Grey cast iron is of a weldable type. It contains 3 to 4%
of carbon.
b White cast iron: White cast iron is produced from pig iron by causing the casting to cool very rapidly. The
rate of cooling is too rapid and this does not allow the carbon to separate from the iron carbide compound.
Consequently the carbon found in white cast iron exists in the combined form. This type of cast iron is very
hard and brittle and is not weldable and also not easily machinable.

c Malleable cast iron: Malleable cast iron is obtained by annealing white cast iron over a prolonged period
of time, and then allowing it to cool slowly. This heat treatment results in greater resistance to impact and
shock.
d Nodular cast iron: It is also known as spheroidal graphite iron (SG iron). It is obtained by adding magnesium
to the molten grey cast iron. The tensile strength and elongation of nodular iron is similar to that of steels
which makes this iron a ductile material.
Properties of grey cast iron: Grey cast iron is mostly used in the manufacture of machine components. It has
got good mechanical properties due to the free state carbon/graphite. The other constituents are silicon, sulphur,
manganese and phosphorous. The grey cast iron has a much higher compressive strength than steel but has low
ductility and tensile strength.
Since the carbon is in free graphite form it gives a grey colour to the fractured structure.
The edges of grey cast iron can be prepared by different methods such as chipping, grinding, machine and filing.
The above methods are used according to the condition and type of the job. Usually it is required to weld, a
cracked casting or a butt joint. Also the thickness of the casting to be welded or repaired will be 6 mm and above.
So usually a single V butt joint is prepared as shown in Fig 1.

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CITS : CG & M - Welder - Lesson 31 & 32