Thanks for the info.

I'm new at this so I'm probably using the wrong term. Here's an example of what I'm talking about - it's a piece of iron pipe from Lowes called "black iron coupling":

http://www.lowes.com/lowes/lkn?action=productDetail&productId=75607-44860-70605

The sell all their iron pipe components in either "black" or "galvanized" varieties.

Yes, an old arc welder is all I've got at present. But the epoxy idea sounds possible. Any suggestions on something that would make a reasonably strong bond?

I have welded black malleable iron pipe and fittings to mild steel using arc and mig, so unless it is different in US to UK it should be OK using rods for dissimilar metals or st/st rods with the arc welder.LWS will advise which rod.

heat it to a dull red and hit it with 7018 it'll hold itself.
matt

Since this is not a structural weld, you can get away with a few different procedures. However, the key issue I would be looking at from an aesthetic point of view would be how the weld will look with time. The problem you run into when welding high carbon (particularly cast iron) to stainless steel, is carbon migration. The chromium in the stainless and carbon in the steel have a very high affinity for each other at elevated temperatures. What will happen is the carbon and chromium will combine to form chromium carbide. This is a very hard, brittle material that will also rust. Essentially you’ve taken the very component in the stainless steel that prevent corrosion and entrapped it in this new material. What you will get is a weld zone that not only can look bad overtime if not painted or protected from the atmosphere, but also has a high possibility to crack and fail due to its brittleness.

With that said, the best way to join these two materials is with filler material that prevents this migration. If you simply weld a layer of nickel filler material (I would suggest a Ni99 alloy since strength isn’t a huge concern in this case) to the iron. This is procedure is called “buttering”. Then you can weld the buttered surface to the stainless without the adverse effects mentioned above.
Hope this helps.

John Swartz
Product Manager
Miller Electric Mfg. Co.

Your best bet is to use a "merchant coupling" instead of the maleable iron. Merchant couplings are usually mild steel and are found on one end of full sticks of black iron pipe or can usually be purchased individually at a plumbing specialty retailer. The mild steel is much easier to weld and will look better. They can be identified by a smooth exterior, no casting marks, bands or seams.
That said, I have also made use of many maleable fittings in boilers, tanks and filter housings. The trick seems to be preheating to a certain degree but also a clean grind surface prep on the maleable item. I use MIG predominantly and have had no trouble getting leak free welds to mild steel tanks (10ga - 5/16") and plate.

Boilers - Piping - Pumps - Controls
Functional Sculpture in copper-brass-stainless and cast.
-=Smartfly=-

1.what is the preheat temp.
2.what is the welding current to be use.
3.post weld treatment is required or not.

If you will scroll up ou will see that this has all been covered, except that there is no absolute number for welding current as this varies with the size of the material, electrode size, and how you hold your mouth.

Rich

One of the big problems with any welding of cast iron is cracking in the heat affected zone next to the bead.Arc welding cast properly requires proper pre-heat,a well planned welding procedure to include short beads and peening those short weld beads to counteract weld shrinkage then post heating.Anything less is setting yourself up for failure and even when everything is done close to perfect things may still crack,especially if the part is stressed in use.
Brazing is a far easier and more forgiving procedure and much less likely to crack.If you can`t braze small cast parts then IMO you are not likely to be able to effectively arc weld cast parts.
The best approach here is to bring the cast part back to " the Borg"(Lowes) and then go to a real plumbing supply house and get a steel or stainless threaded coupler and thread the stainless pipe into the steel coupling.
If it`s an emergency and it absolutely HAS to be done using the wrong parts(and the parts described are the wrong ones for this) then the best chance of success is brazing NOT arc welding.
For those interested,I base this on 30+ years of real world experience in factories,plants and steel ship building.

In theory there is no difference between theory and practice.
In practice there is.

I was told that if you drill a small hole at the end of the crack it will not get bigger when you weld it, is this true

It helps to prevent propagation of a crack, but it isn't a guarantee. The real trick is to not build up too much heat in one small area. Thus the call for pre-heat of the whole piece and careful skip welding, followed by a post-heat.