Welding cast iron

[The Metal Butcher]

Very likely. My magical procedure for dealing with cast machine parts (be it engine blocks, exhaust manifolds, or machine tool components) goes something like this:

1) Soak in concentrated Simple Green for as long as I have patience (ideally days)
2) Scrub and rinse with water
3) Immediately dump in container of acetone and soak for another long spell
4) Scrub and evaluate
5) (Optional) Soak in container of fresh acetone for another long spell
6) Heat gently with a torch and verify no oil seeping out and no smoke - if oil seeps to the surface, you forgot to sacrifice a goat under a full moon and must repeat step 1
6) Etch in acid for a few minutes up to a few hours depending on the part and acid used. Muriatic works but be very careful as it can be very aggressive. Picric acid in ethanol is better.
7) Rinse in water until you think you can't rinse anymore and then rinse some more
8) Soak in a solution of water and baking soda to neutralize any remaining acid and prevent flash rusting
9) Preheat to 600F and observe closely for oil weeping to surface. If oil weeps to the surface, you forgot to dance naked under the starlight and must repeat step 1
10) Braze away

The trouble, IMO, with trying to bake out oil is that you sometimes build up a burnt on layer of carbon that is particularly difficult to wet. I suppose the finicky (and time consuming) soaks could be replaced by a couple of high heat sessions until it stops smoking and then etch it if wetting becomes a problem. The acid etch helps remove those stubborn carbon deposits (and rust, paint, etc.) by dissolving the iron. Done correctly, it provides a wonderfully clean surface with a lot of microscopic surface area, which really improves wettability and joint performance. Done incorrectly, it basically ruins the part and rusts the rest of your shop (i.e. when using muriatic acid).

But yeah, the main takeaway is that cast iron is like a sponge for oil. IMO, 99% of the problems people have with brazing cast iron comes from contamination, either directly by oil (e.g. bubbling) or due to a sooty carbon build up as the oil burns (e.g. poor wetting).

Cross posted from my lathe thread.

[eKretz]

All I know about this is that I have some cast iron machine parts that were brazed (not by me). They are as good as the cast iron, there has been no sign of them moving or failing. One of them holds a dovetail on a mill knee. Works fine.

The whole temperature thing is talked up a lot....but maybe not everyone know why. The parts were cast, and even casting runs the risk of cracking in the mold due to temperature differentials.

So, if you "weld" you are melting just "part of" the material, in other words you are introducing a temperature differential that is much worse than the one when the part was molded. Even with pre-heating, you are not going to get the conditions when it was cast, because you will be hundreds of degrees colder with the preheat.

So why screw around with that huge differential when brazing has much less, and preheat can be done more easily if needed? Braze is also almost surely more ductile than the iron, so it will likely absorb the contraction as everything cools.

A lot of people seem to forget that heated metal expands, and it contracts when it cools. So welding of steel even, causes issues of warps and moving on account of that. A small area has to cool down from just below the melting temp to room temp. Folks forget the amount of shrinking that small welded are is going to do.

If the muggyweld will hold up, it melts way below even braze, and the stuff is ductile. It's just surely going to do better as far as cracking and distortion. Strength might be another issue.

Yes, right on. This is exactly what I was talking about earlier.

[dian]

you guys talk as if all cast iron was the same.

[Doozer]

When it comes to some things, women are all the same.
When it comes to other things, women are vastly different.

Same rules apply to cast iron.
But carbon content is the biggest factor to
ability to weld, or more specifically, deal with
rapid localized changes in heat, which effects
thermal expansion stresses and the critical
cooling curve of the material to make martensite
and cementite.

-D

[nickel-city-fab]

you guys talk as if all cast iron was the same.

White cast iron (chilled iron) is going to be turned into gray cast iron simply by trying to weld it. Likewise for malleable cast fittings, and pearlitic or "nodular". It's all about the amount of carbon, and the thermal gradients. Welding any kind of cast iron will turn the heat affected zone into normal Gray iron, if it isn't already.

[J Tiers]

White cast iron (chilled iron) is going to be turned into gray cast iron simply by trying to weld it. Likewise for malleable cast fittings, and pearlitic or "nodular". It's all about the amount of carbon, and the thermal gradients. Welding any kind of cast iron will turn the heat affected zone into normal Gray iron, if it isn't already.

Really?

You get white iron form quick cooling, typically from using a "chill" in the mold. So a preheat that is considerably under the "soak temp" to make malleable from white should have the potential to quick-cool the melted area and make a spot of hard white iron.

Is that not essentially similar in effect to the process of "flame hardening" gray iron? In that, depending on the surrounding mass, the use of a water chill may not even be needed, as I understand it.

[Doozer]

White cast iron (chilled iron) is going to be turned into gray cast iron simply by trying to weld it. Likewise for malleable cast fittings, and pearlitic or "nodular". It's all about the amount of carbon, and the thermal gradients. Welding any kind of cast iron will turn the heat affected zone into normal Gray iron, if it isn't already.

Iron castings of any size act like a heat sink or thermal mass more correctly.
If you weld a cold casting, the weld pretty much instantly chills, so the critical cooling curve has been satisfied, and you get
essentially quenched and therefore localized hardening in the heat effected zone, and maybe cracking, etc...
If you weld a preheated casting, the weld stays hotter longer, and the weld zone stays warmer than the critical cooling curve
needs, so the weld area stays soft and is not quenched.
Soooo it all depends on the cooling of temp vs time, and preheat helps make that cooling take longer, and then no
quenching (cooling faster than the critical cooling curve) takes place, no cracky cracky.

-Doozer

[eKretz]

Again, it's not all about the hardening or not, though that certainly can and does play a role. The thermal expansion and contraction can cause cracks that are nowhere near the weld. I have seen with my own eyes (more than once) iron gears and other large items crack in half because someone preheated them wrong. Both things are a factor. When welding on something with fairly large size/volume you have to be very very careful to get an even preheat and not to do any significant uneven heating that could cause a constraint. This is one of those things that you might get away with 10 times and then bang! You've got a cracked part. Better to be careful from the start, better safe than sorry, etc.

[JRouche]

People at work (when I still worked) would bring small delicate jobs to us mere welders and I would break out the 3/64" (that's not a typo) stick and have it done befog others could have the TIG gear ready to go.

No kidding. Thats some small stick. Conversly my teacher at welding class was the previous Boss at AWS. So he was a stickler for guys running the stick machines that were getting certs to do it correctly.

He gave us a demonstration of his old hands and eyes one day on structural steel. Took a 3/16" stick (core) and dapped that puddle around and made a nice solid 1/16" bead after cleaning. A full weld with really fat rod and just work that pool. He was a good teacher.

I love the way you guys can weld. I could never stick weld. Thats what I was taught on, by my uncle at 11 working on the frame of my moms station wagon (too many cross country trips for that ol car).

JR

[darryl]

Well, the piece came back today. The guy did an excellent job of it- you really can't tell where the joins are. So now I get to install it, and another part of this reno job will be complete.

[dian]

White cast iron (chilled iron) is going to be turned into gray cast iron simply by trying to weld it. Likewise for malleable cast fittings, and pearlitic or "nodular". It's all about the amount of carbon, and the thermal gradients. Welding any kind of cast iron will turn the heat affected zone into normal Gray iron, if it isn't already.

so have you welded white iron and succeded in joining it?

there are so many types of (even of gray) cast iron, not only by composition but by their structure. the proper way of welding them includes a complete heat treat. welding of critical components is prohibited anyway. it makes a difference if you just want stuff to stick together or work as intended.

btw, if you weld ci with a nickel filler its brazing, right?

[JRouche]

btw, if you weld ci with a nickel filler its brazing, right?

Yes.. JR

Well, with AWS Welding, brazing and soldering are temp determined.

Welding would mean at a temp where we have many metals fluid. Not with Brazing. JR

[dian]

well, another view is that welding is fusing "identical" metals as opposed to using a different metal. different brasing techniques then can be differenciated by temp.