WELDWELL WELDING ELECTRODES ELECTRODES FOR WELDING CAST IRON
Cast iron contains 3-5% carbon and smaller quantities of silicon and manganese; phosphorus and especially sulphur are undesirable impurities. Alloyed cast irons may also have additions of nickel, molybdenum, chromium or copper. In normal grey cast iron the carbon is present in the form of flakes of free graphite which is responsible for the comparatively low tensile strength. With special metallurgical techniques cast irons with improved properties can be produced, the effect being that the strength impairing flakes of graphite are replaced by nodules of graphite. These ductile irons (nodular cast irons; spheroidal cast irons; SG irons) are more ductile and have tensile strengths two to three times that of grey iron. Both grey cast iron and ductile iron are weldable. As may be expected, the latter is easier to weld than grey iron, which is more likely to develop fusion-line and base-metal cracks. However relatively little is demanded of joints in this material in terms of mechanical properties, since service stresses are mainly compressive. Cast iron must be joined with adapted welding-techniques and suitable weld metal compositions. The major part of cast-iron welding is carried out with nickel base electrodes designed for low energy input, ie "Low Heat". Low Heat input restricts expansion and contraction stresses, which, due to the low degree of plastic deformation of the base metal, easily result in cracks. Moreover, the zone adjacent to the weld develops, under the influence of the welding-heat and the inherently fast cooling-rates, undesirable hard and brittle structures. The depth of this zone is relative to the heat input. Nickel-based electrodes have emerged as the most satisfactory filler metal for the welding of cast irons, generally because of their good ductility and bond strength and their ability to precipitate the carbon picked up from the base metal in the form of free graphite. The commonly used nickel-based electrodes are represented in the Weldwell range by the Supercast Ni containing over 96% nickel and the Supercast Ni Fe, a 60/40 nickel-iron type. These electrodes operate on a low current intensity and a low arc voltage, thus ensuring minimum heat input. The Austarc 16TC electrode is ideally suitable for the welding of dirty and heat affected cast iron, where machine ability is not necessary. Also suited for buttering and sealing runs prior to using Supercast Ni.
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