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I was worried for that same reason to weld on a cast steel item, but upon close inspection I could see that the casting had minor voids and imperfections, and it looks like those had been filled by welding and ground flush, and that's why I decided to weld on it at all. Also, the size of the manifold is so massive that with the amount of heat I used, except for the immediate weld area, the temperature of the casting did not even get close to the temperature it reaches during operation of the car, so I thought it would be ok as long I took breaks and didn't let it get too hot overall. I know aluminum is completely different, but I have tig welded cast aluminum with success, and no subsequent cracking. I know aluminum disperses heat better, but since I didn't let this heat up too much, and also because while in operation, the header will constantly heat up and cool down, I thought it'd be ok. From experience I know that welds don't have to be stress relieved unless the welded metal thickness is over 1". In this case, neither the weld itself nor the welded area is anywhere close to 1", so I didn't think cracking would be an issue. The only thing that slightly worries me is the thickness of the local weld area compared to the surrounding header total thickness, and while hot and under pressure (24 psi at times), it could pose a strength issue, but judging from the EGT bung thickness, I think it should be ok in that regard as well. I will heat it up and do the ball peen hammer crack dispersion thing just to make sure that won't be an issue either. Thanks for your help and suggestion/input again.
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