Brazing Cast Iron?

I am starting a new thread because the following question got lost at the end of the http://www.practicalmachinist.com/vb/antique-machinery-history/mig-weld-cast-iron-292522/ thread.



Okay, I am having second thoughts.

For those who recommended brazing: the reason I did not break out my torch is because I have seen brazed cast iron repairs before and there were always globs or beads of braze sticking out of the joint site. I never liked the look of the repair and I did not want that for my handle.

One recommendation mentioned silver brazing. I forgot that I bought some silver braze last winter for another project. That should flow nicely between the jagged broken edges of the handle.

Here are some questions:

• If I use bronze braze, do I have to leave beads around the joint for strength or will whatever gets between the parts be sufficient? Could it be that the brazed repairs I have seen were just not cleaned up because it was considered "good enough" to get the machine back in production?
• The jagged ends were rusted. Is it sufficient to wire wheel the rust off or should I pickle it in acid or use electrolysis? Or should I clamp the parts together and use a carbide disc to cut through the joint and get down to clean metal on both parts?

Updates:
Here is what I have:

• A Port-A-Torch oxy-acetylene set up.
• A tube of Harris Dynaflow Phos-Copper silver brazing alloy
• A package of generic brazing rod. I noticed no label on the bag. It looks like bronze.
• A container of Harris Stay-Silv white brazing flux.

The brazing rod and flux were sold to me by Baker gas. I bought them last winter as I was investigating the repairing of broken back gear teeth. I did not get a chance to use them before the weather warmed up and I left the shop for fishing season.

The last time I brazed was 34 years ago in shop class. I am willing to learn so that I can do these kinds of repairs as they come up.

Here is something helpful I found about preparing the cast iron joint:


Any helpful suggestions?

How should I test the brazed joint? How much pressure should it be expected to handle?

Frank,

The reason that some cast iron brazing jobs have bronze gobbed up on them is because the parent metal was not pre-heated enough. I have done dome brazing of cast iron and had very good luck as long as the cast iron was pre-heated enough.

I've never tried silver brazing CI but will be keeping an eye on this thread for the education.

Big B

There is bronze sticking out ? Yeah, so what !
Ever heard of the legendary device called a file, god forbid, a die grinder ?

It globs up on the joint if you add too much metal because it will not wet the unprepped area beyond the crack. Nothing to do with heat. The entire thing can be hot enough to boil the bronze and you still get a glob if you add too much.
And if it drains through the joint the glob will then be on the underside.
No magic there....

But it really doesnt matter. If there is a bit of glob you can file and grind it flush and the result is a seamless golden meander in the parent metal. No problem.

Our mechanic did curse me for having to grind down tons of tough bronze on one repair job, but it was not lack of fusion, it was just me getting carried away and adding way too much filler....

Zonko said:

There is bronze sticking out ? Yeah, so what !
Ever heard of the legendary device called a file, god forbid, a die grinder ?

Click to expand...

Drop the attitude please. My impression was that the globs were left on for some mechanical purpose; they contributed to the the strength of the joint. So you are saying that is not true? I am okay with that. I prefer a nice neat brazed joint.

Frank R said:

Drop the attitude please. My impression was that the globs were left on for some mechanical purpose; they contributed to the the strength of the joint. So you are saying that is not true? I am okay with that. I prefer a nice neat brazed joint.

Click to expand...

And depending on how much work you want to put to it, you can have your nice neat brazed joint. (No - you can't keep your doctor - he retired.)

IMHO, brazing has much to recommend it. Given an oxy-acetylene torch just about anyone can develop the technique of "spelter welding." It's also fairly cheap compared to so called non-machinable nickle cast iron weld rod. (I got a quote of $125 a pound for "Eutectic" cast iron weld rod - which is machinable.)

Braze is machinable. Its probably just as strong if not stronger than the cast iron it's done upon. 36K is a typical high quality cast iron. Most cast iron from the 19th century is less than that. And 36K is a good typical strength number for braze.

A lot of people run down brazing somehow because perhaps its the original "farmer fix?" Done fast and just enough to get the machine back in service?

But most everything I have put my mind to I have been able to braze somehow. Sometimes it takes heating the whole part. Sometimes just the area to be brazed.

Well, except for brazing a hole in the bottom of that cast iron kettle - something about the thin section and developed cooling stresses. Braze I might but no matter how careful or how extensively preheated, on cooling there would be a PING and ANOTHER larger crack.

Some things just don't last forever I guess.

Joe in NH

Frank R said:

Drop the attitude please. My impression was that the globs were left on for some mechanical purpose; they contributed to the the strength of the joint. So you are saying that is not true? I am okay with that. I prefer a nice neat brazed joint.

Click to expand...

Sorry!

Basically.....has varying material cross section ever been a good thing ?
Of course not.
Brazing alloy has at least the tensile strength of cast iron and certainly much more ductility. So it is not going to fail in the filler. If it fails again, it would have to fail at the intermetallic bond or in the cast iron.
Brass sticking to itself on top of the joint does nothing to make that bond bigger.
That would be my justification for grinding it down.

Silver brazing and lead soldering entirely rely on the filler in the gap and anything added on top is completely non-functional. Brass and bronze brazing are a bit more like welding in the sense that you can actually lay down kind of a fillet with it. But in the end the idea of Brazing is to fill a gap all the way. The stuff on top.....insignificant.

Thanks guys. What about the joint preparation questions?

Gobs or beads is what I read in the first post. If there are beads the weld is not hot enough or the bronze would flow in with the weld (braze) joint.

If it is just built up too much then yes, too much filler rod. But when I see beads, I see signs of not enough heat.

The most common problem that I have seen with brazing cast iron is not enough heat in the parent metal.

Big B

Zonko said:

There is bronze sticking out ? Yeah, so what !
Ever heard of the legendary device called a file, god forbid, a die grinder ?

It globs up on the joint if you add too much metal because it will not wet the unprepped area beyond the crack. Nothing to do with heat. The entire thing can be hot enough to boil the bronze and you still get a glob if you add too much.
And if it drains through the joint the glob will then be on the underside.
No magic there....

But it really doesnt matter. If there is a bit of glob you can file and grind it flush and the result is a seamless golden meander in the parent metal. No problem.

Our mechanic did curse me for having to grind down tons of tough bronze on one repair job, but it was not lack of fusion, it was just me getting carried away and adding way too much filler....

Click to expand...

Frank R said:

Thanks guys. What about the joint preparation questions?

Click to expand...

I answered that one days ago in the other thread. The best method is abrasive blasting. Partly because it gets the joint as clean as bare metal can get and partly because it works on irregular broken surfaces, which then make locating and fixturing easy. I use a mix of aluminum oxide and glass beads. Grinding is fine for flat surfaces, but it does not assure that oil is removed, so a wash in lacquer thinner before grinding is a good plan.

Once fluxed and carefully heated, silver braze will wick into the joint and give a perfect bond as strong as the iron. The joint gap should be there, say .001, but not over .003 or so in order for the capillary attraction to suck the silver into the joint.

I have not used bronze brazing rod for ages, but my recollection is that it did not wick into a joint like silver. If it does not wick and bond inside the break, then you can only try to build up a big lump on the outside of the part and hope it bonds to the surface. I have seen too many of those ugly repairs on old machines. Grind off the braze and you are left with the original break, which has no braze metal in it. The good news is that you can clean it and repair it right.

Larry

Thanks Larry. What if abrasive blasting is not readily available?

Also, Baker Gas sold me this Harris Dynaflow when I asked for silver braze. I am looking up the specs and it does not look like it has silver-heavy content. Does anybody have much experience with it? Should I prepare the joint like silver braze, meaning a tight fit, or like bronze braze, meaning a less critical fit?

Not sure of what you were sold but you will know it has enough silver when you get the Credit card out! I didn't need much for doing bandsaw blades and what little I had was unbelievably expensive.

I learned to braze large castings over forty years ago... The key to good brazed cast iron repair is grinding a good bevel, the right filler rod, the right flux, enough heat but not too much, good pre heat.. We used Smiths torches, oxy acetylene, and some of the castings were so large that it was necessary to use three torches, two to keep the part hot and one to lay the bronze...

If you bevel the part properly, it may be ground almost flat when finished.. If you think bronze is not strong, just take a brazing rod and try to break it...

You can't just put the two pieces together and braze them.. The part has to be properly prepared first! Ramsay 1

Frank R said:

Thanks Larry. What if abrasive blasting is not readily available?...

Click to expand...

De-grease and then use a toothbrush and toothpaste.

HF sells cheap abrasive blast cabinets. Mine is a Trinco 36" floor model and was not cheap, but the little bench type cabinets are a lot better than nothing. The blast cabinet is also good for cleaning the part after brazing. It leaves a very good surface for painting. If not painting, a wire wheel after blasting often leaves a nice-looking surface, which can be black oxided or just oiled.

I use Harris Safety-Silv 45 or the equivalent from other makers for most of my work, including all the iron and steel. I buy it in coils of 1/16" wire. I have some other grades with less silver that I might use for brazing brass and copper. The various makers probably have good application info on their websites, but you can do your own research.

Larry

ramsay1 said:

I learned to braze large castings over forty years ago... The key to good brazed cast iron repair is grinding a good bevel, the right filler rod, the right flux, enough heat but not too much, good pre heat.. We used Smiths torches, oxy acetylene, and some of the castings were so large that it was necessary to use three torches, two to keep the part hot and one to lay the bronze...

If you bevel the part properly, it may be ground almost flat when finished.. If you think bronze is not strong, just take a brazing rod and try to break it...

You can't just put the two pieces together and braze them.. The part has to be properly prepared first! Ramsay 1

Click to expand...

Absolutely correct! That's the way I was taught to do it when I did my welding tickets. You need a double V prep leaving enough root to key the 2 pieces back into original alignment, lots of pre-heat, decent flux and build up the joint, swapping sides between runs. Slightly convex top making sure you wet out the edges, then grind/file back to original profile. Paint to finish and it should be impossible to tell that there was a break. One of these days I'll have to do this for my Monarch CY tailstock quill lock handle that the freight people broke in transit.

Capillary silver braze should also work, not sure about the strength though.

I thought the V prep was such an obvious step that it never occurred to me that people might just glob filler on top of a crack..... I even use a single V prep and small root gap when butt welding 4mm steel even though I know I'll get better than 80% penetration with a single pass using the MIG.

PDW

As noted earlier, a brazed joint needs proper preparation.

I'll disagree somewhat, however, with the notion that having a bit of braze proud of the original doesn't add strength. Regardless of how the part is prepped, there will be some area where the thickness of braze is essentially mils. At that point a crack can initiate. A slight bit of extra braze can essentially increase the wetted and filled area of the braze.

FWIW, I've been taking a Tig class. Brazing was on the menu; including proof testing a joint in 1/2" square hot rolled rod end to end. Failure was tested by twisting the rods after brazing. Some failed at attempting a single twist (360) of the rod -- and these where joints with poor wetting, prep, and fill. Some (including mine) went 3+ complete turns of the parent metal without failure -- and these were all brazes slightly proud of the square rod. Failures in-between (e.g. 1-2 turns) usually initiated at a thin section of the braze. That would also likely be a site for fatigue crack initiation.

Braze is tough -- as strong as mild steel and stronger in tension than many cast irons. I see no reason not to file or machine it down on an otherwise properly fitted joint if desired for cosmetic reasons. But to the extent that a bit of extra braze allows a bit more of the joint to be grooved, wetted and filled it certainly appeared in our tests that it added to the strength.

Let us know how how the project turns out, I would certainly be interested. My understanding after reading all over the web about cast iron repairs is that unlike steel it does not take well to spot heat. One of the major criteria is the location of the repair in the part itself. As the reply above mentioned trying to fix the middle of a thin cast iron item will result in the heated area shrinking away from the surrounding area after it cools, and cracking. I like the idea of using silver brazing over brass in that you get both the flow and should be able to use lower heat. I would pre heat the entire item as much as you can if possible. There are some very good reads on the web about professional cast iron repairs including stitching, all because of the way cast iron responds to things like arc welding and a torch. If you think you are going to heat a part of a casting red hot to repair it you may be disappointed.

Frank R,
I am by no means a brazing expert, but I have brazed a few cast iron gear teeth and some steel-some successful the first run, some needed more/better/different prep. I will start by answering your questions:

"Here are some questions:

If I use bronze braze, do I have to leave beads around the joint for strength or will whatever gets between the parts be sufficient? Could it be that the brazed repairs I have seen were just not cleaned up because it was considered "good enough" to get the machine back in production?"

There are two possibilities -not enough heat or bad preparation or both. A break in cast iron breaks along the weakest part-the carbon. Carbon does not braze, so when people braze the joint, they bubble the braze on a cold joint and have the mess you describe.



" The jagged ends were rusted. Is it sufficient to wire wheel the rust off or should I pickle it in acid or use electrolysis? Or should I clamp the parts together and use a carbide disc to cut through the joint and get down to clean metal on both parts?"

I would grind anything you want to stick to bare metal using a carbide burr and a die grinder. You want a tight fitting joint, the braze needs only a couple thousandths to flow well.



"Updates:
Here is what I have:

A Port-A-Torch oxy-acetylene set up."

This may work, but you will need get get the parts to a low red heat to get a good braze-may have to borrow a larger torch

"A tube of Harris Dynaflow Phos-Copper silver brazing alloy"
I believe this is for brazing copper-lower temperature and not compatible for ferrous metals

"A package of generic brazing rod. I noticed no label on the bag. It looks like bronze."

the bronze colored stuff is the "normal" brazing rod-this is what you need

"A container of Harris Stay-Silv white brazing flux."
This may work-there are some specifically for brazing cast iron that are a rust color. A regular brazing flux that is white is what I use from the local farm store.

For brazing this part, I would first use a carbide burr and clean up the joint to be brazed.
Get the part hot with the torch (I have a caryalong oxy acetelene, too small for most cast iron work) enough to melt the flux onto both parts.
put the heat on. Get the parts a dull red heat and apply the braze to the joint. The joint should be loosely held together. The braze should flow into the joint very similar to soldering copper pipe, only with a lot more heat. Flow braze into the joint until the brazing rod stops melting, then heat the joint and allow the braze to "level out" and apply more rod until you feel the joint has braze all the way around. Keep the torch moving, the part needs to be this dull red the whole time you are working. Let the part cool slowly (ashes or insulation are better) and clean up any stray braze.
I brazed up a couple of gears for my lathe. Teeth were missing, so I used furnace morter to build a "dam" on both side of the gear and filled the area with braze. The first time I tried, I did not grind the areas where the teeth were broken, and did not dam it, and the new teeth broke. The second time I heated the part more with the dam in place and the area had a nice minuscus of brazing rod, and that gear is working very well in my lathe with 4 teeth missing and replaced with braze. I hope this help you with your repairs.
I have seen a lot of videos of people using a very different technique for brazing-basically like torch welding. I have tied this, and while it looks good and may work for light duty, I have not experienced the same strength as the original part with this technique.
Joe

I use Peterson Flux, but borox 20 mule team is what my buddy uses.
Clean that joint out. Hanging graphite particles will do you no good.
Brazing for me is fairly common...and my braze jobs are always smooth and pretty.
The prep is very important..
But if the material is contaminated, say an old exhaust manifold... sometimes it just won't take.

Years ago there was an excellent article in a hobby magazine on silver brazing cast iron.

Preheat was somewhere around 1200 degrees and a torch was used to make up the temp difference of the part to the silver.
The whole part was put sand and allowed to slowly cool.

Years ago at the Engine shop, we had a welding technologies cast iron welding oven. We could get two engine blocks in it and precise temperature control wasn't a problem. Preheat, grab it out of the oven and weld or braze.

jdavi581 said:

I would grind anything you want to stick to bare metal using a carbide burr and a die grinder. You want a tight fitting joint, the braze needs only a couple thousandths to flow well.

Click to expand...

Joe, you mention it down at the bottom of your post, but I want to emphasize there are two different types of brazing. (Not talking about brazing vs soldering.) There is capillary brazing, where the braze sucks itself into a tight fitting crack. And there is braze welding, which requires the same sort of V- or J- joint preparation you'd use on a fusion weld, and where a significant volume of braze metal is applies. In both cases, the recommendation to take things down to bare metal is a good one.