Welding allows the carbon to go back into solution and revert to a cast iron upon cooling. While the preheat is advisable, it is doubtful in my opinion that your post weld heat treatment will be adequate to allow the excess carbon to form nodules rather than platelets upon cooling to ambient temperature.The use of high nickel filler metal will prevent the weld from cracking upon cooling, but the use of nickel filler metal is not going to benefit the heat affected zone. Underbead cracks would not be unexpected and can be predicted with a high degree of certainty.I would consult with the valve manufacturer. They may be able to provide some insight on the proper way to facilitate a welded repair. I have had excellent results repairing large castings with oxy-acetylene welding using cast iron rods, but that is a little different than welding ductile iron. Even with torch welding the final weld is cast iron, not ductile iron.Below is a recommendation from a DI producer on the welding of same using AL Bronze Electrode. I have use this method many times with excellant results. One thing to remember is that when welding DI or CI you paint the metal on instead of "burning it in"["AWS A5.15 Class ENiFeT3-CI, or AWS A5.6 (ASME SFA 5.6) Class Cu A1-A2. These electrodes shall be capable of producing suitable welds without preheating or postheating of the pipe and ring. A recommended semi-automatic welding wire, with appropriate weld specifications, is as follows:"}AMPCO-TRODE? 10A versatile aluminum bronze alloy available in covered electrode, spooled wire and 36" bare filler rod for joining like and dissimilar metals and for overlay of bearing, wear-and corrosion-resistant surfaces. Conforms to Class CuAl-A2 of Specifications ASME-SFA-5.6; ASME-SFA-5.7*; AWS-A5.6; AWS-A5.7*, AWS-A5.13*;.

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