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SOLUTIONS: Cast Iron

2.07

If the electrode is connected to the minus pole of the machine (straight polarity) we get high, concentrated heat input to the base material. This will cause excessive melting and digging into the material. In addition to contaminants such as phosphorous and sulphur, cast iron contains quantities of the gassed nitrogen, oxygen, carbon dioxide and carbon monoxide. Excessive melting will bring unwanted quantities of these impurities into the weld. The more impurities contained in the base material the lower the quality of the weld. The high heat input will also cause the formation of iron carbides, with hard zones in the weld and the heat affected area.

If we connect the electrode to the plus pole (reverse polarity) we get a wide, shallow weld zone with minimal amounts of graphite, phosphorous, sulphur and gasses. The low heat input till reduce the formation of iron carbides.

When welding with DC, reverse polarity to the electrode should be the first choice. However, in cases where the cast iron is heavily contaminated and has poor weldability, straight polarity may be tried in the first run in order to use the higher heat input and melting to achieve bonding between the base material and the weld deposit.

Length of arc

To reduce the voltage across the arc, and to help minimize heat input into the base material, the shortest practicable arc should be maintained. By experience it may be found that the first pass should in some cases be done with a somewhat longer arc than the following runs. This would be

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in cases where the composition of the base material makes good bonding more difficult, and the firs bead has to be "painted" on to it.

Correct size electrode

Always use the largest size diameter electrode that the groove can accept (but not at the expense of not getting down into the groove!). Using a large size diameter means that you reduce the heat input in relation to the amount of filler metal deposited. For instant a 4,0 mm electrode deposits four times as much weld metal at only two times as much amperage compared to a 2,5 mm electrode. However, if the first bead shows porosity, a small diameter electrode, low amperage setting and high welding speed should be used for this run to reduce heat input.

The welding

Remember that cast iron is very brittle, with only 1–2 % elongation. Should the shrink forces exceed the tensile strength of the cast iron it will crack. Hence, when welding it is crucial to the success of the operation that the heat input to the base material is kept at a minimum to avoid cracking.

The way to achieve this is to avoid putting down long, continuous beads (as when welding mild steel), and instead to weld short, straight stringer