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Preparing cast iron for welding

 

Frequently Asked Questions

Whether cast iron is being joined or repair welded, the vicinity of the joint must be cleaned of any foreign material that could contaminate or affect the integrity of the weld. This includes casting skin, sand, paint, oil, grease, dirt and rust. Grinding, wire brushing and cleaning solvents can be used. Any impregnated oil can be removed by swabbing with paraffin and heating to ~450°C for approximately 15 minutes. However, this treatment will not remove impregnated sand and silt from some castings, e.g. pump parts, which have been in service underwater for long periods. If, after welding, imperfections such as porosity are present in the deposit, the affected area should be ground back to sound metal, before further deposition. This process should be repeated after depositing each bead, until a repair free from porosity is achieved.

In braze welding, the cleanliness of the surface being welded is of vital importance because no impurities are removed by melting of the casting surface, since the filler metal has a lower melting point. The ideal cleaning method in this case is salt bath treatment. When grinding is used, graphite smeared on the ground surface is detrimental to braze quality, so quickly heating to dull red with an oxidising torch flame followed by wire brushing is recommended for its removal.

When repair welding casting imperfections, such as blowholes, porosity or cracks, it is essential that all the defective area is removed, either by cold chiselling, grinding or gouging. If using an air-carbon arc or a covered electrode for gouging, a heat affected zone will form around the gouged area. The casting should be preheated to 300°C before gouging to reduce the risk of cracking in this region. The groove should also be lightly ground to remove hardened material before depositing the repair, since graphite in this region may dissolve during gouging, increasing its sensitivity to cracking during subsequent welding. When removing cracks or linear defects, the ends of the crack should be blunted by drilling before gouging, to prevent further propagation during the preparation for repair. The true ends of the crack, which may be very fine, should be located by dye penetrant or magnetic particle methods before drilling.

Preheating a casting before weld repair can be very useful in controlling the cooling rate after welding. This is particularly important when repairing complex shapes since different thicknesses of material respond differently to the heat from the weld pool, which can result in damaging thermal stresses and distortion.

See also Welding of cast irons - a guide to best practice , from which this FAQ is extracted.

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