Hi,
Rods in the ESt group (covered electrode with steel core wire) are generally used where lowest cost or maximum strength are desirable, and a non-machineable bead is acceptable. Rods in the ENi group (covered electrode with nickel base core wire) provide a soft, ductile, machinable bead and fusion zone with a slight sacrifice in strength, an increase in cost, and in cases of larger castings can be used without preheat, but only if light loads are applied to the casting. Rods in the Eci group (covered electrode with cast iron core wire) provides a weld metal most similar to that of the base metal of the casting, but shouldn’t be used unless the casting can be moved to a furnace and/or subjected to an extremely high (red hot) preheat.
Surface preparation should consist of removing casting skin and foreign material from joint surfaces and adjacent areas, and if the casting has become impregnated with oil or grease, remove it with a solvent or by steam cleaning. If the casting has become impregnated with a plastic or glass sealer, it probably can’t be repaired because the sealer will produce gross porosity or inhibit bonding.
Amp ranges for 1/8" diameter rods in the ESt group are around 80-100, with DCRP or AC. Amp ranges for 1/8" diameter rods in the ENi group are around 60-110 with DCRP, and 65-120 with AC. Amp ranges for 1/8” diameter rods in the ECi group are around 150-175 with DCRP or AC. Practically the same welding procedures are used for both gray iron and malleable iron. The prime difference is the need to heat treat malleable iron to restore its properties after welding.
When welding gray and malleable irons, the area adjacent to the weld is raised above the critical temperature, about 1450 degrees F. The mass of the casting around the weld tends to draw heat from the weld area rapidly. If this cooling of the heat affected zone and the weld is more rapid than it was when the casting was originally made, a highly brittle, crack sensitive zone develops. Preheating the casting to slow the cooling rate will help prevent the iron from rapidly absorbing heat from the weld area. Some castings can't be preheated, so when selecting welding techniques and electrodes, be sure take into account whether or not your casting can be preheated. If your casting is small, it can be welded satisfactorily without preheat, although preheat usually improves the toughness of the weld. Slow cooling is always essential. If you choose not to preheat, a two-pass weld can be made if a soft fusion zone is what you're looking to achieve. The first pass serves as a preheating for the second pass, which tends to slow the cooling and produce an annealing that makes a softer, more machinable fusion zone.
This link may be helpful to you:
http://www.twi.co.uk/j32k/protected/band_3/jk25.html
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