TIG Brazing Keeps Cracking

steamandsteel · Mar 1, 2020

Hello,

I'm having a hell of a time trying to braze up this casting.

It's a hollow piece, about fifteen pounds, with a wall thickness around 3/8".

I am proficient in brazing with a torch, and welding with TIG, so I figured I would give TIG Brazing a try. Did my research, kind of figured out how I should run the beads.

Here's the setup:

- A chunk broke out of the casting, triangular in shape. I ground down all the edges on the piece, and on the base casting, and beveled them with a small land and slight gap.

- Preheated to around 300 degrees with a propane torch.

- Machine set to 75 amps, but only using a quarter pedal or so. I'd guess 30-50 amps actually being used.

- Using 1/8" TIG Brazing Rod, ERCuSi, with an 1/8" Thoriated (Red) Tungsten

So, I start brazing up the cracks, and the beads flow alright. Making sure not to melt the cast iron. Keeping the arc over the bronze puddle, and dabbing every so often.

However, once I finish any bead and move to the next, the casting itself forms another hairline or sometimes more than hairline crack. Trying to fix these other cracks has led to the piece looking like a spider's web. No good at all. I've tried additional pre-heat, and even a post heat after brazing.

What the heck am I doing wrong? Should I just break out the O/A torch tomorrow?

Limy Sami · Mar 1, 2020

Sounds like not enough pre heat for the ''instant'' heat of the arc, ……….don't forget the ''old rules'' about keeping the job warm and out of cold drafts etc etc.

cyanidekid · Mar 1, 2020

not sure 30 amps is enough for this job, yea maybe for welding 16 ga or thinner , but sounds like not enough heat for this.

as to the casting and cracking, do you know if this is ductile? if its white cast, its going to crack no matter what.

try GTAW brazing something like mild steel, just to have a baseline for comparison. try some known ductile cast. try a bit more preheat

idacal · Mar 1, 2020

the few times I have attempted cast I was way hotter on the preheat between 650 and 700, degrees with temp sticks. heating with a propane weed burner. but I was using a stick rod designed for cast. one project worked, that one I could keep fully heated, the weed burner on one side while welding on the other. when done wrapped in high temp insulation overnight.that was brutally hot job. the other repair didn't last long blew apart when the centrifugal forces hit it. it was a pulley. dont know what your working on.

Gordon Heaton · Mar 1, 2020

From you description of the casting, it sounds like you have a lot of surface area plus thin sections. Don't know the shape, but certain geometries are exceptionally hard to work without cracking due to stress propagation post-op. I agree with the others that you probably need to go hotter, maybe 400-450, and shield from drafts during/after brazing.

edit: I see that idacal was typing while I was thinking! I agree that for welding the pre-heat should be much higher but I'm not sure its required for brazing. Other fillers might help too.

gbent · Mar 1, 2020

A hollow piece? We need pictures. Its very easy to ignore the overall stresses that occur in a piece due to differential heating. Sometimes you have to break a piece further so the differential heating can be controlled.

Think of a three spoked cast iron pulley, with one spoke broken. That is an impossible weld unless you can bring the entire pulley to near welding temperature.

The brittleness of cast iron presents challenges that don't require any thought in carbon steel.

Ries · Mar 2, 2020

I would second upping the amperage- I would set the machine at least at 150 or so, maybe more. But if that still doesnt work, bite the bullet and buy some nickel tig rod.
Some cast iron alloys are really really tough to weld or braze- cast iron comes in a lot of flavors, and there is usually no way to tell what you have.

Also- how clean is it?
bead blasting works best.
any residual oil in the iron will screw up your brazing or welding- although the cracks arent usually the result of oily cast iron. But still, clean it as well as you can, with whatever you have- alcohol, acetone, ss wire brush, sand or bead blast- get that sucker clean.

steamandsteel · Mar 2, 2020

I've got some pictures as requested.

To clarify, I am trying to TIG Braze and *not* weld. I am not melting the base material.
The casting had a crack in it initially, which I then broke out as a chunk so I could braze it back flush. Perhaps the initial crack (before repairs) was caused by stresses in the casting itself, which may be causing some of the issues I am having now?

@Ries, I also tried running hotter at around 130, as it took some amps to blend the braze beads together. Still got cracking. The material was ground clean, paint removed, oil removed, brakleen'd and also hit with acetone all before preheat. I figured it just might not stick, and wouldn't just crack like it has here.

I know some of this looks bad. While the color of the beads is not golden-red, I assure you that is just brazing pictured. When you hit it with a wirewheel or flap wheel, it polishes right up. I keep getting these little stress fractures right around the edges of a bead.

My current course of thought is to grind down all the brazing, grind into the base material a bit, and use the O/A torch to braze the whole surface, and try and cool it as slowly as possible...

Your thoughts?

Ries · Mar 2, 2020

I wonder about the composition. I think your problem may be the cast iron itself.
I have tig brazed a lot of cast iron, from wood stove parts to auto and tractor parts. Sometimes it works great, and other times it just wont work.
But the location of your cracks, at the iron edge of the braze, does indicate that the iron is cooling much faster than it should. Get it nice and warm, and maybe even post-heat.

but remember- this was never designed to be a repairable part, and it might not be.

Gordon Heaton · Mar 2, 2020

If you pre-heat more, could you use a lower amp setting to braze? Or at least move the torch more broadly/quickly even if you need to go well outside the ground areas? I wonder if 'point concentration' of the heat could be minimized. Just thinking out loud. Like Ries and others have said, some castings just won't braze or weld.

If you're having trouble with bonding at the lower temps, have you tried AC? I sometimes use AC with silicon-bronze when its difficult to bond and the cleaning action seems to make a significant difference.

magneticanomaly · Mar 2, 2020

I see three mistakes. First, 'way too little pre-heat. On a large, fragile, constrained part like yours pre-heat needs to be close to your repair temperature. I would use at least 1000 to 1200 deg F. Preheat serves several purposes. One is to slow cooling do as to prevent formation of brittle martensite, one is to drive off oils and other contaminants, most important is to reduce thermal stress. The primary way to do that is to reduce temperature differences across the part-----so preheat has to be close to the welding/brazing temperature.

Second problem I see is that because you are using low amps, the heat spreads out widely before you can make your braze. So since you have a lot of hot metal in a cool piece, the linear expansion is greater than it would be if your near-fusion zone was smaller. This will cease to be a problem if you have enough overall pre-heat, but if you use low preheat, you also need small HAZ. I favor ENiFe55, and beads no more than 1/2" long, peened while cooling, lay your next bead somewhere else and jump around until done.

Post-heat. A job like this should be done IN a furnace, need be no fancier than a stack of bricks or sheet-metal box with suitable heat sources, propane or charcoal or even a wood fire. As soon as the welding/brazing is finished, add some fuel and seal up the box. Cover it with welding blankets or bury it in ashes and come back tomorrow.

No good shortcuts for large or complex iron castings.

idacal · Mar 2, 2020

magneticanomaly is right, and pre heat means the whole object on a part like that. you have paint less than 2" away that isn't cooked, getting that whole part hot is your best chance of success still may not work. I have a stack of high temp bricks that I use and stack a box around when I need to get something preheated, but thats just steels. Im not patient enough for cast repairs

Limy Sami · Mar 2, 2020

Like I said & followed by others - pre heat, …………..that whole part will have to be preheated - think gas barbecue etc etc to ''only just'' below the braze material melting point, and keep it hot while welding and leave to cool very very slowly.

On tricky jobs like that I often built a nice big wood fire out in the yard, use that at first (cos you fon't want to bring the temp UP too quickly either and then the propane (weedburner size) to pre heat, then keeping the propane going do the weld - as close to the fire as possible (if not in it ??) then back in the fire and cover with burning wood and ashes, ......….and leave to go out AND COOL RIGHT DOWN ......….the longer the better, if it sizzles when ya spit on it, it's still too hot to take out of the ashes.

Oh yes & FWIW I would use OA, simply because I have never TIG brazed nor do I have the equipment

BT Fabrication · Mar 2, 2020

steamandsteel said:
I've got some pictures as requested.

To clarify, I am trying to TIG Braze and *not* weld. I am not melting the base material.
The casting had a crack in it initially, which I then broke out as a chunk so I could braze it back flush. Perhaps the initial crack (before repairs) was caused by stresses in the casting itself, which may be causing some of the issues I am having now?

@Ries, I also tried running hotter at around 130, as it took some amps to blend the braze beads together. Still got cracking. The material was ground clean, paint removed, oil removed, brakleen'd and also hit with acetone all before preheat. I figured it just might not stick, and wouldn't just crack like it has here.

View attachment 280608
View attachment 280609
View attachment 280610

I know some of this looks bad. While the color of the beads is not golden-red, I assure you that is just brazing pictured. When you hit it with a wirewheel or flap wheel, it polishes right up. I keep getting these little stress fractures right around the edges of a bead.

My current course of thought is to grind down all the brazing, grind into the base material a bit, and use the O/A torch to braze the whole surface, and try and cool it as slowly as possible...

Your thoughts?

did that housing hold oil by chance? looks like you are getting hydrogen embrittlement from the hydrocarbons burning off.
and only 300? should be min 550F

gbent · Mar 2, 2020

That is a very highly constrained part. What is probably happening is your broken piece is at high temperature, while the balance of the part is at a much lower temperature. As the small piece starts to cool, it shrinks. But there is no place where the metal can move to relieve the stresses. So it cracks.

You don't have to preheat the entire part. But you must preheat completely around the part where you are going to be brazing, to brazing temperature. When finished brazing, the entire area, all the way around the casting must cool at the same rate.

Something is wrong with your tig brazing procedure. There shouldn't be the great amount of oxidization showing. It may be as simple as loosing your argon shield while the part is still hot.

I would switch to OA. This part is going to be hot, bad hot. With an OA torch you can get further away from the casting, and you don't have to stop and regrind the electrode if you get the filler in the flame. This will also mean going with flux or flux coated rod. Typically, this is a much better process. The argon in tig is just a shielding gas, any fluxing agents have to be in the filler. Therefor, there is little activity to counter any impurities. Flux with the rod is an active flux, it chemically wants to remove impurities.

This looks like a drill press head, and that looks like a non-stressed area. Grinder, bondo, and paint makes a welder what he ain't.

LKeithR · Mar 2, 2020

Ries said:
...but remember- this was never designed to be a repairable part, and it might not be...

Exactly. We've done a lot of oxy-acetylene and tig brazing over the years and, generally, have had pretty good
success. We have, however, encountered some stuff that just couldn't be welded.

I had several neat things to say about welding that part but I see a bunch of others have pretty much hit the nail
on the head. Get it hot--really hot--keep it hot and cool really really slowly. The best thing we've found for cooling
small parts like that is a bucket of ag lime. Even a small part like that will have some heat in it after 20-24 hrs. And
remember, if it's too hot to touch it's too hot to remove...

CarbideBob · Mar 2, 2020

steamandsteel said:
...Should I just break out the O/A torch tomorrow?

Yes.......
Bob

M.B. Naegle · Mar 2, 2020

LKeithR said:
Exactly. We've done a lot of oxy-acetylene and tig brazing over the years and, generally, have had pretty good
success. We have, however, encountered some stuff that just couldn't be welded.

I had several neat things to say about welding that part but I see a bunch of others have pretty much hit the nail
on the head. Get it hot--really hot--keep it hot and cool really really slowly. The best thing we've found for cooling
small parts like that is a bucket of ag lime. Even a small part like that will have some heat in it after 20-24 hrs. And
remember, if it's too hot to touch it's too hot to remove...

We use lime too in a plywood box under the welding table. Doesn't really go bad either as we've had the same batch in that box for the last 20 years.

I tend to braze cast iron more often with a rose-bud tip than anything else as you can keep the whole area hot and not just the bead. Some of the better brazing jobs I've seen had the whole part red hot so that the heat of the part melted the brass in more than the heat of the torch, which was there more to keep the part hot than anything else.

PlasmaOnTheBrain · Mar 2, 2020

As others have noted, more preheat and slow cool. Ceramic fiber insulation worked well for me, but we had it on hand from other jobs. A part that size could stand up to some peening no matter which processes you run. Even a cheap hazard fraud needle scaler would do the trick. Put some dents on the weld/braze its self and maybe like 1/8" beyond the weld zone but dont go overboard or too deep. If you have access to an apprentice/bosses offspring/random co-worker with nothing else to do, have them run the scaler...
As for what and how to weld it, my favorite has become oxy/fuel with cast iron filler and flux. It's a weld but it acts more like a braze or solder, but it seems to take to all sorts of fussy castings without much issue. It looks and acts just like cast iron after welding, but you will likely have to grind it down a bit. Pretty is not one of its jobs...

CarbideBob · Mar 2, 2020

A question to the many who know this stuff far better than me.
What is the advantage of TIG brazing over a torch on cast iron?
TIG localizes the heat much more than a flame but cast needs preheat and a wide area up in temp so I am confused.

1/8 tip in the TIG head and 50 amps seem awfully small to me without big swirl arcs and a lot of time.
300 degrees even if through seems like not much of a preheat.
I see lime "quench" recommend, a bucket of sand dip or a drop and pour a bucket of sand on top fast also works to slow the cooling so that the big M does not puck with you.
Bob

TIG Brazing Keeps Cracking

Aluminum

Diamond

Titanium

Hot Rolled

Titanium

Diamond

Diamond

Aluminum

Diamond

Titanium

Titanium

Hot Rolled

Diamond

Titanium

Diamond

Stainless

Diamond

Diamond

Cast Iron

Diamond

Similar threads