SMAW, stick welding, is not the best method for cast iron welded repair. Stick welded repairs have some success in static situations, but in a dynamic or pressure containing application it could in fact can cause serious equipment or personnel injury.
Cast iron, nominally with 2% - 4% carbon, will produce a martensitic structure during the cooling portion of a SMAW repair – 100% of the time. Martensite is an extremely hard, brittle, microstructure immediately adjacent to the weld. This zone is particularly susceptible to cracking, no matter what the filler metal, preheat, or intermediate action such as peening, this hardened zone will never be annealed.
Welders have come up with practical means such as preheating, backstepping, peening, and slow cooling to minimize the martensitic hardened zone, but it will always be present and is extremely crack sensitive.
This doesn’t mean cast iron can’t be successfully welded where life and limb are not jeopardized, but in all cases where there is dynamic loading or safety of personnel is at stake, cast iron should never be welded.
Many years ago, we evaluated a number of “specialty” cast iron stick electrodes such as Eutectic, Cor-in-Nor and others. Basically, these electrodes are just an expensive version of commercially available austenitic welding rod such as E310-15, E309-16 or similar to Inconel with slightly altered Ni, Mo, and Mn and extra low carbon. A welder can usually do just as well with standard 309 stainless rod, AWS CI-55 (55% Ni), or the more expensive CI-99 (99% Ni).
Incidentally, peening does not relieve stresses. Peening, when it works, introduces compressive surface stresses to the weld. It is simply a means to counteract shrinkage tensile stresses introduced by weld solidification, a natural consequence of any welding process. Peening, when used, should always be performed with a blunted round nosed tool. Sharp edges introduce severe stress risers, which also are crack sensitive
Mechanical metal linking works in many situations for cast iron repair and there are companies specializing in this method. The average mechanic can also make mechanical repairs with kits available from outfits such as Goodson Supply.
If cast iron must be welded for things like cylinder head modification there are welders specializing in oxy/fuel gas welding with a high percentage of success. I have a welder that makes small groove welds, filling the water jacket opening next to the combustion chamber in Vortec heads, with no leaks or cracks. The only noticeable discontinuity is porosity near the ends, but the machined finish and hardness are not discernable. Drag racers routinely have ports brazed or welded to correct the profile and maximize port efficiency. The drawback…it ain’t cheap.
Infrared temperature monitoring instruments are not at all accurate for welding. Infrared devices depend on surface emissivity, continually changing during the welding cycle, which render the readings off-by-a-country-mile. The only accurate means for monitoring weld surface temperature are temperature indicating crayons (Tempil sticks) or contact pyrometers.
This note is not written to castigate or minimize the success of stick welded cast iron repairs. However, as a welding engineer for the past 40 years, I wouldn’t want anyone to have false expectations or have someone’s personal safety endangered with unrealistic expectations for SMAW repairs on cast iron.
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