Cast Iron - Recommendations?

[Jon W]

Hello. I'm an architect by day (and a grateful reader who learns a lot from the accumulated wisdom here at Chaski).

We recently wrapped up a project that involved modifications to some cast iron stairwell guard railing posts. Back in the '40s they built them a lot shorter than building codes require these days. Our task was to cut off the upper portion of the existing posts, add new steel extensions and rails/balusters to bring the railing heights up to code. Our drawings called for new HSS 3x3 steel post extensions to be sleeved and welded to the existing cast iron. Had warnings all over the drawings cautioning the Contractors those joints would be CI to steel welds.

Have done this in numerous projects where welders used preheating and rods appropriate to the old CI makeup with excellent results. Our structural PE agreed with and approved this approach for this project. The General Contractor had a sample of the existing CI tested to determine the best rod to use, and the steel sub's response to the test results was simply "you can't weld cast iron".

I'm but a humble architect, but I don't see anything in the test results that would suggest this is un-weldable... a headache for sure, but do-able to my layman's eyes. In the end the sub came up with a very clever alternate to welding that worked out well in the end (Porok in the hollow CI, sleeves and internal shims), but still holds to this CI having been un-weldable.

I'm hoping someone here can weigh in on if you feel this CI could have been be welded, and if so what rod/method would you have used? Not to beat up the steel sub with, just hoping to learn from those in the know.

Here's the tested composition:
C 3.33% by weight
Mn 0.47%
P 0.53%
S 0.12%
Si 2.34%
Mg <0.001%
Fe balance

Thank you.

[Russ Hanscom]

There are multiple procedures for welding cast-iron and it has been done successfully for many years. One of the welding handbooks, or a metallurgical lab, can provide samples of acceptable procedures. A test, using the applicable bits, is a good place to start.

There are several possible excuses for the sub, either he has had a bad experience, or he does not trust his employees, or he is unwilling to overcome a prejudice.

P.E.

[Steggy]

I have successfully welded grey cast iron numerous times over the years. I see no reason why anyone who knows iron and steel fabrication would say cast iron can’t be welded.

[Harold_V]

The chief problem with welding gray iron is the resulting cracking from uneven heating and cooling. If those issues are addressed, it certainly can be welded.

It would be my opinion that an alternative method that offers success would be desirable in this situation, as it would likely avoid the need to heat the pieces. Done in situ, that could prove to be troublesome, if not hazardous.

If, by chance, the posts can be removed from the assembly without issue, welding might be acceptable as they could be heated (and even cooled) in an oven, eliminating the problems of heating in place. Just a thought.

H

[NP317]

And if the cast iron posts can be removed, it might be better to replace them entirely with new steel posts...
RussN

[318J]

This is a common trope I hear.

"You really don't want to have to weld cast iron" has simply become the short "you can't weld cast iron". It's an answer that can save the trouble of having to do it, BUT you absolutely can in the right situation, with a good quality cast iron to weld to, with the right welding procedure to include pre and post heating. It sounds like you have done so in the past, and have had success.

I've told people people before that "you don't want to try welding this [cast iron item]", because of the risk of further damaging (namely cracking) it. Often I feel like they walk away thinkimg it can't be done. Even with pre and post heating there's no 100% guarantee- its the nature of cast iron. I tell them this, not that it can't be done, but I don't want to be the guy to try, and be responsible for breaking it. The larger the item and repair is, the trickier it gets. Smaller things are a lot less of a hassle and have more consistent results, imo.

(I can, and have welded cast iron, but trust me, I steer away from it if at all possible, I much prefer brazing as the go-to option. If welding is insisted, I make it clear that I can't guarantee success)

I prefer the use of a Nickel-Iron rods with careful, generous pre-heating up to 500°F, wrapped in ceramic blankets to prevent from cooling too quickly and cracking, with only the area to be welded exposed. The trick is to weld in very small incidents- never running more than an inch lond bead at a time. This will help to prevent the thermal gradient beyween the weld and base material from becoming too extreme (also resulting in cracking). Don't forget to maintain the preheat, particularly on larger items.

Once the welding is done, completely insulate, and do whatever you can to let the item cool down as slowly as possible.

[Jon W]

Thank you - I knew Chaski (and by extension you all) was the right place to turn. Truly appreciate you taking the time to respond.
Jon