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Home > Tips and Facts > Repairing a Crack in Casting Repairing a Crack in CastingThe following section summarizes the information needed to repair by welding a crack found in a Cast Iron body. It is the result of recurring requests on the part of our readers who asked for detailed answers to their problems. PreparationIt is understood that a cracked item to be welded should be stripped of any accessories and parts assembled to it, and freed of any external loads. Assuming that the crack is limited in size, and the part is not broken in two or more separate pieces, one needs to drill a small "stop hole" (about 3 mm or 1/8") at each end of the crack and its branches. To find the end one should grind the area with sandpaper and look with a magnifying glass. Then the crack has to be removed by mechanical means like milling or hand grinding with a proper grinding wheel, to provide a V shape channel, with an opening of 60 degrees reaching the bottom of the crack, usually leaving a thin "land" to be completely melted. In thin sections, backing may be required to get a full penetration root pass. Thicker walls may need special shape channels. Completes the preparation a thorough cleaning of the area, especially if the casting is soaked with oil, grease, paint or other contaminants. A burnout should be done in a furnace for 15 minutes at 480°C (900°F), followed by energetic brushing to remove residues.
PreheatingReasons for preheating were already pointed out in this page. Preheating should be performed in a furnace at 200 to 300°C (400 to 600°F) for at least an hour. For a small casting torch heating may be acceptable.
WeldingShielded Metal Arc Welding (SMAW or stick) with covered electrodes is the most applied process. Filler metal available ranges from electrodes for cast iron (designated ECI) to Nickel containing electrodes (ENi-CI, ENiFe-CI [preferred for general purposes], ENiCu-A, ENiCu-B) and other types, including bronzes. The selection should be based essentially on experience, preferring the most economic material that permits welding without producing other cracks. For one occasional repair it is suggested that the Nickel electrodes be used, although more expensive, because they are more ductile and as such capable of absorbing larger stresses without cracking. The technique should concentrate in melting the minimum of base metal, by introducing the least amount of heat, with the smallest electrode at the least current, with thin weld beads. The weld is built up with additional beads (after slag removal) until finished, without cooling down.
Post heatingThe welded part should then be allowed to cool down slowly under covering of insulating material or, preferably, be stress relieved immediately in a furnace at 600 to 650°C (1100 to 1200 °F) for one hour and then furnace cooled.
Other processesOxyacetilene Welding is also possible, with the same preparation, but then, with proper filler metal, Braze-Welding can be performed as per above reference.
DisclaimerPlease be advised that this presentation hereafter deals only with the most common class of Gray Cast iron where no special requirements are demanded from the material. In case of doubt readers are urged to investigate if their application is more specialized and then they should seek specific advice for action. Copyright ©2003.
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