Well when we talk about ironwork welding, usually we are referring to cast iron welding
It is thought to be that welding of cast iron is not only difficult but at times ruins the product under consideration.
There are conflict views out there when it comes to ironwork welding. Some prefer to undertake machine welding where as some electric welding. Both techniques have their pro and cons. But many believe using an arc welding will provide too much stress on the product under welding may lead to cranking the object and in turn be not be repairable any more.
Many experts agree on the fact that using an arc or electric welding technique would be the very worst decision you could make to attempt to repair your cracked cast iron part. There reasoning that is used here is - Cast iron cannot stretch and withstand the contraction and hardening caused by cast welding. Plus the temperatures in these ovens are 1200 deg. F.
Some welder may counter argue that – “that would depend upon the brand of welding rod.” But in reality the brand does not make any difference. To explain this there is a simple reason - it's the heat that causes the changes to the cast iron and not a specific brand, and all arc welding have similar heating patterns.
Now if you use a nickel weld, which is machine able, it can be used on a cast iron. That said we still need to understand that the cast iron will become as hard as a drill bit or tap and as a result will foil the appropriate machining that is regularly needed. With this fact in sight, experts comment that almost 50% of the casting repairs conducted via arc welded have disastrous results and cost the owner at least twice as much more to repair properly.
A good tip regarding ironwork welding is contact a good experienced welder. Ironwork welding should not be attempted by a novice and certainly not on a high temperature oven welding. Very few crafted welders can attempt and succeed in such ironwork welding. It is suggested that cast iron requires preheat of at least 900 deg. F. for brazing and 1300 deg. F. for fusion welding
Now, to counter argue that there are welders that have come with two options, such as arc electrodes and gas torch rods, they say that it better to use these technique rather then try and heat the oven at an optimum preheating as even the slightest error can lead to the cast iron to re-crack and especially if done frequently.
But, should one use electrodes, may be the question now. As long as the electrodes are elongated well, say up to 300% higher than nickel-type electrodes it would allow the weld to stretch and absorb weld contraction. In turn it would prevent cracking. Also try using a Ni rod rather than nickel rod, as they are softer and cools and forms a hard spot in the cast iron, which acts as a magnet for cracks.
In case you need to undertake thin castings, exhaust manifolds, and small cast iron cracks, then it may be bets that you use torch welders, which has a silver solder for cast iron repair.