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How to Weld Cast Iron

Cast iron is a most common metal in industry because of its simplicity of manufacture.  It can be cast with only a gas furnace whereas steel, having a higher melting point, requires an electric furnace for casting.  Cast iron can be machined easier and at higher speeds than steel.  This metal alloy is readily and economically manufactured into useful machinery because of its low melting point, fluidity, and simplicity of melting.

Cast iron is manufactured from an endless number of formulae.  A great deal of scrap iron of unknown analysis is used in manufacturing cast iron.  Most cast iron contains in addition to iron and carbon, silicon, manganese, sulphur, and phosphorous.

The main difference between steel and cast iron is its carbon content.  Mild steel contains less than 0.30% carbon, and most high carbon steel contain less than 1.0% carbon.  The maximum carbon that can be put into steel is 1.7% as this is the maximum carbon that can be absorbed in solution with iron.  When larger amounts of carbon are combined with iron, the carbon not absorbed by the iron is present in the form of small flakes of graphite.  Grey iron contains up to 4.5% carbon, usually between 3.0% and 4.0%.

When cast iron is heated, at a temperature near its melting point, practically all of the carbon goes into solution with the iron in a combined form of iron carbide.  If the cast iron is allowed to cool very slowly nearly all of the carbon will pass out of the combined state and segregate as free flakes of graphite.  If the iron is cooled rapidly a large portion of the carbon will remain combined with the iron as iron carbide.

It is this high carbon content that makes cast iron so different from steel.  If we could remove the graphite flakes from cast iron and squeeze what is left together, we would have steel.

The factor of the two forms in which the carbon can exist in cast iron requires major attention in welding.  If the cast iron (or parts of it) is melted and then cools slowly, the weld and the base metal will be soft and machinable.  If cast iron is melted during welding and cooled rapidly, the cast iron, or at least areas of it, will be hard and difficult if not impossible to machine.  This is what causes the condition of "hard-spots" in cast iron welds.

Because cast iron has the flake-graphite structure which prevents it from bending and causes it to have no elongation, it breaks readily.  It is a common event in factories, construction companies, farms, and all other industries for cast iron machinery to fracture.  Often a costly casting breaks simply from vibration.  Costly downtime from mishap with cast iron machinery is common in industry.  Also, because cast iron is soft, it often wears. For example in threaded holes, the threads wear or strip easily.  No one can estimate the loss to industry by breakage of automobile and truck motor blocks, exhaust manifolds, transmission housings, and in factories of such indispensable machines such as pump housings, punch presses, electric motor housings and the myriad of other cast iron machinery components.

When a cast iron part breaks, the cost is enormous to almost any industry.  It is impossible for an industry to carry spare castings in their store room.  Often the machinery is old and obsolete and the manufacturer cannot provide a spare.  To make a new casting usually involves making a pattern first.  This can take up to four weeks just to make a pattern and often the pattern can cost thousands of dollars.

It is for these reasons that industry must be well prepared with Magna Maintenance Welding Electrodes and Alloys, to enable quick restoration of the broken machinery to useful service.

Many engineers who have encountered repeated failures in attempts to repair cast iron with ordinary cast iron production welding rods.

Some engineers state that they have been able to weld cast iron, in some cases using brazing rods or gas welding rods, which require a long complicated procedure.  Usually brazing or gas welding cast iron involves:  Dismantling; building a fireplace around the casting; preheating, often for as much as 24 hours; gas welding; burying the casting in lime or other insulating material; and slow cooling for up to one week.

The answer to successful welding of cast iron is the development of Magna 770 which has brought industry a practical solution.

Maintenance-designed cast iron electrode

There are a number of companies that market production welding cast iron electrodes. They usually offer from 3 to 7 different cast iron electrodes, since they readily admit that each electrode has only a limited range of applications on which it can be used on.

Obviously welding electrode manufacturers that offer several different electrodes for cast iron are not capable of serving the needs of maintenance.  Such a variety of cast iron electrodes, each with a limited scope of usage, is generally all right for production welding where only a limited number of applications exist.  A production factory manufacturing, for example, pumps and has only one analysis and one thickness of cast iron to weld under perhaps only one condition, can select one of these production cast iron electrodes for the one application.

In maintenance the conditions are completely different. In maintenance they never know what type of cast iron will break, what thickness it will be or whether or not the weld will have to be machined or not.  Generally they do not know what the analysis of the casting that may break will be.

MAGNA has solved this old industrial problem of cast iron failures with Magna 770, which welds all types of cast iron, thick or thin, including grey, malleable, meehanite and nodular iron.  It welds in all positions, including overhead or vertical. It makes porosity-free welds without undercut.  The welds are fully machinable and crack-free. Magna 770 even welds cast iron to steel.

Magna 770 is the one practical solution that can help you prevent costly downtime and loss of profit due to cast iron failure.