On Sun, 13 Dec 2015 20:03:38 -0600, "Terry Coombs"
wrote:

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"
vertical - kind of like an angle iron but not at 90° . I attempted a repair
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 ./
Are you anywhere near a pottery supply company? If so, get some
ceramic wool and wrap the piece after heating. As to how hot in my
experience if I heat cast iron to about 700 degrees it won't crack. I
have had the best luck heating to a dull red heat when observed in low
light, wrapping in ceramic wool, and welding. It's a pain because
everything is so hot but on the other hand no peening is required and
no cracking is happening. I have used this method with brazing rod,
aluminum bronze rod, stainless steel rod, and nickel alloy rods.
Eric