Oxyacetylene Welding of Cast Iron

Oxyacetylene welding of cast iron is relatively easy if attention is paid to some simple aspects of it. Not just the aspects specific to cast iron, but aspects in general about oxyacetylene welding as well. This article contains some hands-on advice on the subject.

The commonly used variety of cast iron is the grey cast iron. If has two types of carbon in it: combined carbon and graphitic carbon. Appearance wise, it has a soft texture, is easily machinable, has a melting point of around 2060°F – 2200°F (1127°C – 1204°C), which is well below than that of steel.

If graphitic carbon is changed to combined carbon, we get white cast iron. This variety has a hard microstructure, and is brittle, and due to this reason – has low weldability. Due to high hardness, it is difficult to machine as well.

Proper attention must be paid to the following aspects when welding cast iron with oxyacetylene welding:

  1. Slow heating and slow cooling.
    1. Use of a proper welding filler rod.
    1. Use of a good flux
    1. Skilful manipulation of the torch

Preheat is important in the welding of cast iron to prevent the ill effects of expansion and contraction. A careful cooling and heating is necessary to prevent hardening of cast iron. Preheating should be given to the structure evenly, for this – heating slowly is the best way.

A bevelling of the plates and correct fit-up is necessary.

The size of the tip used for cast iron is generally same as that used for welding steel of same thickness. Light up the torch, bring it to the proper flame type, hold the filler rod in other hand, and we are ready for welding. If the preheat of the metal is an inconvenience, heat resistant material can be used to cover the metal surface to protect the welder.

During the welding, the acetylene cone should be positioned just over the metal surface, but not touching it. Bring the filler rod near the flame a little bit to heat it. Then dip the filler rod in a flux such that it picks up some flux. The flux picked up by the heated rod is then placed in the spot on the metal where the flame is hovering. This is generally sufficient to break the film of oxide and enable welding.

Sometimes it is necessary to break through the oxide film by stirring the molten metal. If other contaminants are present on the surface, it needs to be removed out of the way physically by using the filler rod.

Direct the flame at the sides of the root, and make the melted metal flow towards the bottom such that a well formed puddle of molten metal is formed. Keep the filler rod in touch with the flame constantly to avoid it from getting cooled. Slowly dip the filler rod in the puddle such that its’ tip melts, and a drop gets deposited at the root. Keep up this melting and adding of metal steadily and evenly distributed along the length of the root.

Flux should not be used in excess. It should not be thrown in with the hand. Instead, the small amount picked up by the rod is sufficient to serve the purpose. When it is required to stop, the torch should not be pulled up suddenly. Use circular motions and gradually withdraw it.

Depositing the molten metal exactly at the desired place requires some skill. Due to the gust of the flame, the metal drop end up a little farther than where we aim it. Through skilful handling of the filler rod, this problem can be overcome.

Unlike steel, cast iron does not solidify immediately after depositing the weld. It remains fluid for some time before it becomes solid. One problem arising out of this is that the liquid metal may flow over, and not stick at the desired spot. The way to overcome this is to watch the molten carefully till it fuses with the surrounding base metal and becomes one with it.

Another problem is collapsing of the weld metal. The force of the flame on the liquid metal is enough to cause it to collapse and create a hole. Efforts to fill the hole end up making it larger. The way to resolve the problem is to understand it first. It is the force of the flame that is sufficient to cause a collapse.

In order to fill the hole thus created, one should work down from the sides, like one does when starting the weld at the root. Be careful that the metal deposited remains in fusion with the side walls and does not have a chance to flow down. The skill lies in to direct the flame appropriately, by careful manipulation of the oxy-acetylene torch.

If properly done, it is possible to obtain a weld metal that is as strong as the original metal. The welding of cast iron is usually in the form of repair work. So welder does not have a choice of welding position, joint design, etc. He must work with the conditions that he is presented with. He must inspect the repair needing to be done, make a plan keeping in mind the effects of expansion and contraction, and his plan to overcome them.

The choice of the filler rod must be such that it does not result in a hard structure. The flux must be such that it males the metal fluid and enable removal of oxide film. Judgement is necessary in the welding of cast iron.

This was in brief about welding of cast iron with oxy-acetylene welding process. Please share your thoughts in the comments section below.

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