I also agree with the the the wrong choice of filler metal for these dissimilar welds especially when one considers the various differences between the two metals as well as the weldability issues involved... the kobelco reference.pdf is a nice one to use in understanding the various weldability issues which could potentially be the root cause of the transverse cracking... Still I think we need a whole lot more information to get a better understanding of the entire manufacturing processes as well as the designing of whatever automobile component this OP is referring to... My gut instinct is that more will eventually be revealed that should've been brought to our attention and included in the original post, but as usual this is wishful thinking thee days...
"Arc Welding of Cast Iron
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2. Weldability of cast irons in fusion welding Arc welding is used particularly for repairing foundry defects contained in iron castings and machinery parts damaged or worn out in service. All types of cast irons except white cast irons are considered to be weldable but to a lesser degree than steels; however, the weldability varies depending on the type of cast iron. Some types of cast irons are readily welded while others require special welding procedures.
2.1 Common problems in weldability
All cast irons have common problems affecting their weldability; namely, (1) much carbon content, (2) lower ductility, (3) high content of phosphorus, sulfur and oxygen, (4)
casting defects, and (5) impregnated oil. (1) Under a welding thermal cycle, the cast iron base metal immediately adjacent to the weld metal is locally heated to an extremely high temperature, and the cooling rates of the entire heat-affected zone are quite high. Consequently, iron carbide tends to precipitate in the heat-affected zone adjacent to the weld metal (referred to as the white-cast-iron zone), and the remainder of the heat-affected zone tends to form high-carbon martensite (the martensite zone). Both the white-cast-iron and martensite zones are characterized by the hard and brittle nature. In addition, the white-cast-iron zone is apt to contract much more than does the
unaffected base metal. Therefore, with the brittle structure and high contraction stresses, the heat-affected zone tends to generate cracks either spontaneously or under load during services. The degree of brittleness and propensity to cracking depend, to some extent, upon the type of cast iron and the welding procedure. In addition, high carbon can be oxidized to become a CO gas, which tends to cause blowholes in the weld metal.
(2) Fusion welding involves localized heating and cooling and thus causes thermal stresses
in the weld area being accompanied, as shown in Fig. 2.3, by expansion on heating, and by contraction on cooling. The base metal should be capable of local plastic deformation to accommodate the welding stresses, thereby preventing the occurrence of crack. Cast irons are generally liable to produce cracks because their low ductility may not be able to withstand the contraction stresses arisen by the cooling weld during fusion welding. 3) High phosphorous content of cast irons tends to form hard metallic compounds with
ferrous and carbon, which make the castings brittle. High oxygen and sulfur are apt to accelerate the precipitation of carbides, thereby causing a hard and brittle white-cast-iron
microstructure. Phosphorous, sulfur and oxygen dissolved in weld metals can cause cracking of the weld metal. (4) Cast irons often contain casting defects such as sand inclusions and shrinkage cavities, which prevent complete fusion or better wetting of molten weld metal onto the base metal. (5) Cast irons after service are often contaminated with impregnated oil which causes welding defects such as blowholes and cracks.
2.2 Effects of graphite form on weldability
To minimize the formation of massive carbides and high-carbon martensite, it is most helpful to have graphite present as spheroids, which have a low surface-to-volume ratio.
As the surface area of the graphite in contact with the austenitic matrix decreases, the amount of carbon in the microstructure can decrease at room temperature. Graphite flakes in gray cast irons display the greatest tendency to dissolve in austenite because of their relatively large surface area. Gray cast irons are inherently brittle and often cannot withstand the contraction stresses arisen by welding. Lack of ductility is caused by graphite flakes, and those cast irons containing long graphite flakes are more brittle and less weldable than those with short flakes or spheroids. Ductile cast iron has graphite in spheroidal form and thus better ductility; therefore, this type of cast iron has superior weldability to any other types.
3.2 Gas metal arc welding
Gas metal arc welding (GMAW) is used in the repair, reclamation and fabrication of iron castings. The low heat input modes such as short-circuiting transfer and pulsed-current arcs can restricts the heat affects on the base metal, thereby minimizing the formation of brittle iron carbides in the heat-affected zone with marginally better crack resistance. In addition, minimized dilution of the weld metal by the base metal will reduce the tendency of weld cracking. The spray transfer mode is used only for the production of simple, low-stress welds in low-strength high-ductility grades of cast irons. Solid wires and flux-cored wires are available. 98%Ar-2%O2, 75%Ar-25%CO2, or 100%CO2 is used for the shielding gas. AWS ERNi-CI and ERNiFeMn-CI are common grades of solid wires. Flux-cored wires for iron castings include nickel-iron type and nickel-iron-manganese type. ENiFeT3-CI is a flux-cored wire that uses no shielding gas, the typical application of which is overlaying crane hoist drums (gray cast iron), overlaying ingot molds (gray cast iron), and welding end caps of ductile cast iron to cylinders of low-carbon steel pipe."
Sure does make better sense after reading the above paragraphs from the Kobelco .pdf Electrode and I thank you for sharing it with us... I'm guessing that more will be revealed.
Respectfully,
Henry
