I have a cast iron part (Old Delta chop saw belt gaurd) that looks to be grey cast iron (looks like light grey). This doesn't have much stress and was broke while moving. I don't want to promote more cracking, and it does look to be very brittle metal. I have both a Torch, Tig and Mig so I have choices.Won't need the mig here. Thank you
It could be cast iron, but it could also be pot metal. Pot metal is light grey and its fractures look kinda like fractured cast iron. If you're not sure, test it with a magnet. Bob
Its cast iron some rust, its very heavy and magnets stick to it. The weld is in a no see area so things don't have to look perfect. I have a good amount of welding experience with mild steel and aluminum but I have never had to weld cast iron.
Remove the paint as much as possible. V-grind out the crack. Only leave enough for alignment puroses. Preheat the parts to about 500degF with a torch or stove. You can use either a Nickle TIG rod or silicon bronze. Your choice. Both are strong enough for what you are doing. Run the bead in short stitches allowing some time in between for the heat to disperse. When done, allow to cool slowly, preferably buried in a pile of vermiculite or powdered lime. Both are available at Garden stores.
be sure to grind the surface scale off in the weld area also...it doesn't like to accept the flux. Don't get it TOO hot when brazing or you will pull out alot of the carbon and it won't braze for crap. tensile strength of braze is 30,000 so it is plenty strong....keep areas that are far away from the weld area warm too or the heat will want to move from the hot weld area to the cold area too fast and cause cracking...patience is a virtue with brazing!!!
Good Luck!!
Doobie
Wayne wrote: (clip)I have never had to weld cast iron. ^^^^^^^^^^^ I once tried repairing an ornate grape-vine pattern garden bench by brazing. Each time I completed one of the fixes, it would crack in a new place. I learned through experience that cast-iron will crack if the piece is connected in such a way that tension can develop as it cools. This happens if the part is shaped like a flywheel, or otherwise has "loops" in it. If it is a piece that does not connect back on itself, it will just shrink, but no tensile stresses will develop, so no cracks.
In the latter case, either brazing or welding will work. If you are a "little" worried about stresses, I think you are better off arc welding, and then peening the weld as it cools. It's better yet to weld a little, and then peen a lot, and then weld some more, and so on. Arc welding does not heat the surrounding metal as much as gas welding, so you will have less cooling stress.
If the piece is really complex, and interconnected, the only way is to preheat and post-cool slowly.
With all due respect (which is quite a bit) , one advantage of cold welding with nickel is not needing to do the often impractical and always annoying preheat. Weld 1/2 inch to an inch, peen the crap out of it RIGHT AWAY, weld 1/2 inch to an inch somewhere else on the crack, peen the crap out of it, repeat until done, never get the casting hot. Take a break if you're in danger of getting the casting hot. Very localised application of heat, lots of beating, very quickly, on the nickle to let the nickle move as the weld bead cooled, rather than let the cooling weld bead crack the iron. Keep peening until the bead has cooled.
I personally have only done a little bit of this in class 10 years ago. But the guy who taught me did it, and spent the summers teaching NYS DOT and town maintenance welders to do it, as part of the Ag Engineering school's mission to save the state taxpayers money by educating local maintenance workers. As he explained it, the difference between preheating and not preheating was the difference between (for instance) stripping an engine block, finding or cobbling up a furnace big enough to preheat it, welding on it, cooling it, then putting it back togther afterwards, .vs. cold-welding the crack in place, or at least without needing to fully strip the block.
Having both done it and seen it done, I do think it actually works. Welding a stich and goofing off for 15 seconds to find your hammer probably would not work.
Real handy item, both while doin the work and for cooling, is one of the black insulating blankets (welding shops will have them-they are made for this).
Been doing buildup on some large cast parts lately, and the blankets have been a godsend. Preheat slowly, set on blanket, leaving enougn to fold over top during cooling, braze, then fold blanket over for slow cooling. The blankets are about 1/8" thick, and the parts are still too hot to touch after 8 hours (40 lb castings in a 50 degree shop, preheat is about 400 to 500)
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