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SOLUTIONS: Cast Iron

Introduction

To approach welding of cast iron, the special properties of this material must be understood, and the techniques for joining it together followed.

Welders who attempt to repair cast iron based upon their experience from welding steel, and on the assumption that " iron is iron" often end up with a poor result without understanding why.

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Hence, cast iron repairs has been a feared operation to welders. However, many grades of cast iron can indeed be welded successfully. Cast iron is essentially an alloy of ferrum (iron), carbon and silicon. The carbon content ranges from 2.4% up to 4.5%, which means that it is present in excess of the amount that can be contained in solid solution. Although up to 6% of carbon can be dissolved in iron when molten, actually less than 1.75% can remain in solution when the metal solidifies. The excess carbon separates out during solidification and remains present and dispersed throughout the cast iron in the form of free carbon (graphite). Two of the factors, which influence the amount of carbon, that separate out when the cast iron solidifies are the length of time it takes to solidify and the amount of silicon that is present. Silicon tends to drive out any carbon in solution as graphite if it is given enough time and temperature. Thus the percentage of free graphite is a function of the cooling rate. This excess carbon is the reason for many of the desirable properties of cast iron, such as high fluidity, low

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shrinkage, high damping capacity on vibrations and ready machinability. But the carbon is also source of problems. Due to its high content the cast iron is brittle and has very little elongation when subjected to strain. Because of this it cracks easily when subjected to local heating and cooling, as is the case when welding. Thus the welding properties of cast iron differ from those of steel. While steel melts at approximately 1450°C, the high carbon content of cast iron lowers the melting point to 1100–1300°C (depending on type), making it easier to melt and allowing it to run freely into a mold to assume the shape intended for the casting. On boars ships cast iron has found wide uses as for instant engine blocks, heads, liners, water jackets, transmissions, pump and valve housings, manifolds, pipe fittings, cargo lines, etc.

There are five common types of cast iron: Grey White Malleable Ductile

High alloy

These different cast irons cannot be identified by chemical analysis alone. It is the form of the excess carbon that determines the kind of iron that the melt becomes. With regard to the fifth type, high alloy cast iron, this is obtained by adding quantities of alloying elements to gray, white or ductile irons. In the following we shall take a look at the characteristics of the various types.