Where to repair a cast iron casting (transmission)

Not that it's something I know about from experience, but it seems that you need really even heating to not create more cracks, so how about an induction furnace for the heat in furnace brazing? I had some thin-to-thick steel parts brazed that way years ago, spring perches for a Lotus Elan race car suspension. Offset the springs inboard a bit to clear wider tires...it worked.

I bought a used industrial machine (a huge pipe threader).

It is a super nice machine in all respects and was rebuilt in 2007.

However, the reason why it was sold for nothing is that the cast iron transmission case in it is cracked due to a past accident.

The picture, with the crack highlighted, is here:

You can visualize this as a rectangular casting with one corner broken off. The three highlighted areas are in fact one crack separating only two pieces.

The size of this is about 1x1x2ft.

What I need to do is put it back together. Put in funny terms, I could take these pieces, clean them and "glue" them together, the question is how.

Real glue, obviously, would not work.

Nickel welding would likely not work well either.

It would seem that the pieces could be taken out, really cleaned up, pushed together, heated red hot, and brazed with brass.

************ I have no real experience with cast iron, but that is what I have read more than once by those who do. Get it hot all over. Braze it. Cool it very slow.

I bought a used industrial machine (a huge pipe threader).

It is a super nice machine in all respects and was rebuilt in 2007.

However, the reason why it was sold for nothing is that the cast iron transmission case in it is cracked due to a past accident.

The picture, with the crack highlighted, is here:

You can visualize this as a rectangular casting with one corner broken off. The three highlighted areas are in fact one crack separating only two pieces.

The size of this is about 1x1x2ft.

What I need to do is put it back together. Put in funny terms, I could take these pieces, clean them and "glue" them together, the question is how.

Real glue, obviously, would not work.

Nickel welding would likely not work well either.

It would seem that the pieces could be taken out, really cleaned up, pushed together, heated red hot, and brazed with brass.

Does that make any sense?

What would be the best approach?

Thanks

i ======================================================

If you can get it out and apart, I would try drilling and tapping some holes and bolt it back together so you don't have to worry about thermal stresses, more cracks, heat treat, keeping it square and aligned, etc. In the picture the bottom crack would need bolts perpendicular to the shaft with the gear but each hole would only total about 2" or 2.5" deep. For the upper crack the holes will be parallel to the gear shaft and the non-threaded holes are going to be 3" or more, then the shorter threaded holes, so not trivial but maybe you can find someone who owns a big mill :-) :-). Maybe someone can recommend something better but I'd use bolts that are 1/3 to 1/2 of the plate thickness when you go into an edge, so 3/8 or maybe 1/2" bolts if your casting is 1" thick. Counterbore and use socket head cap screws if you don't have clearance for hex heads on the surface. Grade 5 bolts just to avoid the horrible variable quality of anything less, thread engagement of double the bolt diameter since the bolt material is stronger than the cast iron, and torque with a torque wrench in a couple of steps.

However you try to do it, it will be a challenge. You will need to totally dissassemble it and totally degrease it, then uniformly preheat it whether you arc weld it ot braze it. With the location of the break bolting plates on it to hold it together really isn't a viable option either. Possibly vapor degreasing, applying a metal filled epoxy like JB, then drilling through and putting a big bolt through, with spreader plates on the outside MIGHT work.

If you decide to go the welding route, I've had good luck TIG welding with stainless steel wire on grey cast - but you have a HUGE mass to keep it hot - and it will radiate a LOT of heat back at you.

That mostly agrees with my experience. I think the vapor degreasing is probably unnecessary; regular solvent cleaning should do. To successfully weld or braze a complex cast iron gear housing with minimal distortion requires enough preheat to burn off all traces of oil in the pores of the casting; this happens at a lower temperature than is required for distortion control and additional crack prevention.

I did over a foot of cracks in a bulldozer transmission casing a couple of decades ago by building a preheat oven on the floor consisting of a layer of insulating brick from a kiln supply and a top box of 1" ceramic fiber insulating board cut to size with a hand saw and tied together with bailing wire. Propped casing on insul bricks, thermocouple bolted to one corner, propane burner under, preheat to ~800 F IIRC, turn off burner, remove box lid and weld till temp drops to 600 F or so then reheat. Double gloves and heavy shirts advised. I used 55% Ni stick, which is somewhat difficult, practice on scrap if you want to use it.

For your part I think that brazing might be better. I would try preforming brazing rods or better sheet if you can get it to fit the joint, possibly hammer rod flat, preheat with broken piece in place, remove cover of oven, lift out broken pieces and brush flux on both surfaces, insert flux covered preforms, heat with torch until preforms fully melt and broken part can be pushed down to fully seated and cool slowly. This requires more heat input than arc welding into ground out groves on both sides of the crack, so it may require more preheat to prevent distortion.

A search on "cast iron welding preheat temperature" or similar should turn up a lot of information on how to select the best preheat temperature for the job, likewise I would do a search to find the best brazing alloy for filling a thin crack in CI; you want something that will flow well.

That would depend on how you did it. If you look for posts by me, here, from years ago, I have described "cold welding with nickel rod" as I was taught (specifically for repairing cracked large castings without heating them) at least twice (OK, the second time was probably just a cut and paste, and I'm not bothering to do that for a third time, since you seem hostile to the idea from the get-go.)

It works. You have to peen the weld while hot until it cools, work in short stitches, and you absolutely do NOT want the part to get hot.

But feel free to spend a lot of time and money heating everything red hot if that's what you like.

Nickel welding works just fine and can be done cold. You just need to peen the weld and use short stitches on alternating areas to keep the part cold. The reason it works is simple. The weld heat the localized area, the weld is peened while hot, it does two things, relieves stresses from the heat AND more important it slightly expands the nickel in the weld, counteracting it's shrinkage rate and keeping it from pulling the cast apart. I use nickel wire in the MIG for this and it works really well.

Or you can opt for the same repair method and instead get the part HOT, which will burn the oil out of the casting and give you a cleaner weld.

Brazing also works and can also be done hot or cold.

Have done a lot of cast welding over the years using all of those as well as using mechanical methods like stitch-n-lock and keying.

Combined them a few times as well. You could do that with this as well. Cook it clean, then get the broken area back in tight, cast likes to lock in at a break pretty well. Now drill as close as possible down the center of the web following the angles of the crack as best as you can. Thread the hole and screw some common all thread in. Cut it so it ends up just below the surface. Then run some vs down the crack and over the key you just installed. Braze over it all. The rod and braze will work together to lock it all in place.