The day I picked up the stove grates I also picked up a small "fire pot" that was cracked/broken . The break is across a 2" horizontal then about 1"
on it yesterday , beginning with a tack at the corner and one at the horizontal edge . That part went well ... but when I tried to fill in between the two tacks it keeps cracking . I have a hammer handy and begin peening before my helmet clears ... I have not preheated this piece because of the size and awkwardness, and I think that's what's killing me . The TIG heat is so localized that the nearby mass of iron is acting as a chill and sucking the heat out so fast I can't peen fast enough to stop the cracking . I can't heat the whole part , but I can heat the repair area . The part is like an oval sleeve , with nothing to restrict expansion in any direction so I think I can safely heat just that area . The question is how hot and will this help keep it from cracking ? I tried some CI strips first , then the Invar 42 , same results with both fillers ./
Two approaches - one is the preheat to something like 800F or more.
The other is cold welding with nickel (55 or 95 ni rod) followed by peening until cold. I was taught that in college and wrote it up on here a while ago. Lincoln briefly mentions it here:
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In either case you also need to grind the crack out to a vee, and drill the ends of the crack or it just keeps cracking.
Reposting myself from 8/27/06:
Cold welding with nickle rod: Drill the ends and V out the crack - use a narrow (60 degree) V to minimize shrinkage, and leave the back of the crack connected if possible. For this job, fixturing to hold the parts tight would probably help with either welding or brazing.
Use DC-, you should not have a crater (pulling iron into the nickle is
about every 2 inches, and then start each new short section on a tack, moving to the cast to lengthen the tacks until the whole crack is covered. Cutting and pasting my own comments from a January 2004 discussion of cast iron repair, with some slight edits:
Use 55 or 99 Ni rod and your stick welder. Weld 3/8 inch, peen the crap out of it RIGHT AWAY, weld 3/8 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 cools, rather than let the cooling weld bead crack the iron. Keep peening until each bead has fully cooled. I personally have only done a little bit of this in class 11 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. Trying to hurry the process definitely won't work.
I was doing pretty much like your description of cold welding, but using TIG and Invar 42 . Peening was started before the helmet lens cleared ... and that wasn't working , so today I preheated it to somewhere
, I still have about half the thickness not fused , but I plan to take care of that in the morning by welding from the other side - after a suitable preheat .
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