Cast iron blacksmiths forge repair

[SOHC]

I have this old cracked blacksmiths forge, its 1000mm x700mm and 45kgs,

I have a packet of high nickel ARC welding rods Apart from drilling the ends of the cracks should i pre heat it or just do very short stiches and let it cool? And after should i normalize the whole thing? I can wrap it in Kawool, i have done a lot of brazing on cast iron but never ARC welding.

 

[johnoder]

I've used "Nickel 55" - as you say, very short beads and immediately peen. Preheat to at least 350 F - use temp crayons to check. And cool slow with suitable blanket. The preheat ideally would be the entire piece - but since such is generally impractical very localized is used instead

It usually remains to be seen if such goings on are actually durable - but they do in fact stick things back together - though very likely with some loss of dimensions / geometry

 

[Rob F.]

I would bolt a piece of steel plate to the bottom, fill in the missing piece if you feel the need to and not waste your ni-rod on it.

 

[memphisjed]

I would bolt a piece of steel plate to the bottom, fill in the missing piece if you feel the need to and not waste your ni-rod on it.

a thick stainless bowl tacked to top. fire pots are consumables- granted slow but still consumables.

 

[fciron]

Jed is not wrong. Those cast iron tables need to have a clay lining in the fire pot or they will crack again at the first firing. (Voice of experience).

I'd find a proper cast iron fire pot and cut an opening to fit or bolt straps across the crack/ gap and line with clay. (I think clay lined forges are the origin of the term "duck's nest" for the fire pot.)

If you're set on welding, John Oder described my technique. Be sure to pre-heat the all the way around the tuyere opening otherwise it will try to crack somewhere else while you're welding.

 

[magneticanomaly]

I have been welding CI for a long time, with excelent success. But i will second the suggestion to just strap or plate your firepot. It cracked because of differential expansion..hot around the tuyere or duck's nest, cooler elsewhere. Lots of thermal stress. Will almost certainly crack again. Does not need to b e watertight. Cast iron that has a lot of "thermal history" also is often impossible to weld with stick electrode, needs oxyacetylene torch, CI rod, and flux. I think this is because there is a lot of iron oxide in the metal, that crept in along the graphite flakes over time.

YES also to luting with mud or clay.

 

[gbent]

Here in the USA, most of those old forges had cast into the top "clay before use". It is not a difficult task, and doesn't require special clay. Clay from a stream bank would be fine, get some that doesn't have organic mater. Mix it pretty dry, and try for 1/4 to 1/2 inch thick. Allow to dry completely, may be a few days to a few weeks. Mix up a little more and fill the cracks that occurred while drying. Patch as necessary to keep the extreme heat off the cast iron.

I agree that welding burnt cast iron is difficult. Mechanical fixes have a better chance of working longer than a few heating cycles.

 

[Servicar rider]

I have had the best luck welding cast iron using cheap baling wire from the farm store as filler using tig! It has worked for me when other stuff failed. I have tried nickel rod and some other cast rods and still had cracking , hard spots and it was too hard to machine or even grind in one case. I preheat and slow cool as much as you can and peen if you can. Gbent is right when he says welding burnt cast iron is difficult.

 

[Gazz]

I once repaired a large gear box cover - about 36" or so - for a mixing machine. While I brazed it and did not weld, I did the preheat by building a fire in the yard and set the piece on top of the coals and brazed it in place on the coals.