why'd it crack? (tig + cast iron)

cast iron exhaust manifold with a ~4" long hairline crack. if it weren't for the smoke, the crack would've been difficult to spot.
wanted to weld the cast iron with the tig welder. thought i could fuse the crack without filler. first pass, closed the crack. just to be certain, i ran a second pass with SS filler (308L)
now i know SS isn't the right filler for cast, but.. i recall hearing somewhere that SS filler could join cast iron to mild steel. maybe i misheard.
after laying down a beautiful bead, we reinstalled the manifold and fired the engine up. 10 minutes at idle and the bead split right down the center. its like the original crack floated to the surface. i couldn't draw a line down the bead that accurately!
did it crack because of incompatible (stainless) filler? if yes, why didnt it crack along side the bead? i'd think the weak spot'd be the interface between the stainless and the cast iron.
i pre- and post-heated the manifold. drilled out both ends of the crack. let it cool down for over an hour in a draftless shop (lunch time).
-tony
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Sounds like you just welded over the crack. You need to grind it out and fill the whole crack. Failure to do so can result in the kind of crack you experienced.
Austenitic stainless filler can sometimes be used. More often a nickel filler is used.
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If you are welding up a crack in cast, the rule that is best for that is brill a 1/8" - 1/4" hole at both ends of the crack. Then when you weld it up on the crack seem and then fill in the drilled hole it will not open up or lengthen as that is what the drilled hole prevents. Just remember, 1) clean the crack. 2) drill the holes. 3) do the welding on the crack in short steps so as not to over heat the cast. 4) Once done the crack weld, fill in the drilled hole properly. 5) Clean up what you have done and you should not have a re-crack in the cast. THOMAS' Wrought Iron Works.

you

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Hi Thomas
Thomas wrote:

Rod: I agree with your statement. Save, welding. It is always better to braze the cast iron then weld it. Reason's: 1. More penetration. 2. Less strain produced.
Rod Ryker... It is reasoning and faith that bind truth.

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...

Hi. Maybe not always. I think that Tom's advice is pretty good. I've seen similar advice on the Internet.
I found a cast iron vise with a cracked base, and brazed it with an acetylene torch and bernzomatic brass rods from Home Depot. The job was marginal because the heat spread too far and the joint did not get hot enough for full penetration. In the old days, when these things were brazed, a forge was used, and a big blob of brass was stuck on. You sometimes see these on Ebay, and the repair has been holding for generations.
Anyway, the modern recommendation seems to be to weld the joint. My marginal brazed joint finally gave way after less than a year of use. I smacked the vise with a 3 lb. drilling hammer and it cracked at the repair =:-0. So, I paid more attention to the experts, and welded the break. I ground out the crack and used 3/32 6013 rod. I sunk the vise in gravel and preheated it with a large propane torch. Then, I laid the beads in 1/4" runs, peening really hard (not hard enough to crack the cast iron, but pretty hard) with a 5/8 x 12" piece of hard rebar. This is when I could have used an autodark helmet. After the job was done, I buried the vise all the way. The repair was much stronger. I read that for large critical repairs where it is inconvenient to preheat, one would use studs and a more suitable rod. Anyway, this repair seemed to hold.
Maybe brazing would have worked if I had a bigger heat source. Also, I picked up a nice heavy forged post vise in case I need to do more, er, wailing with that hammer. Have you had better luck with brazing? How do you do it? It must work, since I see all those old time repairs which have stood the test of time.
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For smaller parts that I where I can heat up the entire part, I always braze it. You need to get the whole part up to around 600 degrees F and then just have at it. Brazing also has things in common with soldering, ie you have a flux that must work it's magic. For cast iron parts, the flux must go all the way down into the crack plus the walls of the crack have to get to the melting point of the filler, usually somewhere around 1000 F.
Eric Chang wrote:

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Hello Eric
Eric Chang wrote:

Rod: Fuck up any joint you wish. As for my suggestion, oblige me. ;) For thinner sections can be _SAFELY_ welded, if properly done. However, thicker sections should be brazed period.
Rod Ryker... It is faith and reasoning that binds faith.
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