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Author Topic: How to "Braze" Cast Iron --  (Read 17328 times)

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December 03, 2009, 02:19:50 PM

goodfellow

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This is one of those threads that "old timers" will ask "so what?" because many of us were taught to weld and braze with an O/A torch. Younger guys however may never have been exposed to a proper procedure for brazing cast iron -- so here goes.

Cast iron is one of the hardest materials to securely weld and braze because of its unique properties. It has a crystaline structure that when heated and welded does not cool evenly and as a result cracks and inclusions will form to render the repair useless. Most repairs only last a few weeks before they will again crack at the welded seams.

There are three common ways to fix a broken cast iron casting -- Arc welding (either with nickel rod, or nickel free cast iron rod), fusion welding with O/A, and brazing with O/A.

I've tried them all with a modicum of success (used to fix many engine blocks with nickel rod and lots of peening), but for highly stressed parts the best and most effective way is to braze the castings. Most ductile cast iron has a tensile strength ranging from 25,000 psi to 50,000 psi, while a brazing rod has a tensile strength over 60,000 psi. So the logical step is to use that 60,000 psi to our advantage.

This is my 40 year old Buffalo 6" vise. Last week I was bending a big bracket and the casting let go at the body junction





The crack was almost completely around the circumference of the casting, so i just seperated the two with a hammer









Preparation AND speed are key to brazing cast iron -- here are a few critical steps to take

1) When cleaning/prepping the cast pieces for brazing, don't use an aluminum oxide wheel (or any other abrasive compound). Alum. oxide will deposit itself in the pores of the cast iron and create huge inclusions in the brazing material (sort of like "fish-eyes" in paint). Instead I first degrease the casting with lacquer thinner, and then use an aggressive flast bastard file to expose clean fresh metal on the castings to be joined. After the filing, I reclean the area with thinner

2) I set up a nice waist high work area with fire brick so that I can position and turn the castings as needed. I also makes sure that I have an extinguisher nearby, along with a can of brazing flux, a digital thermometer, flux coated brazing rod, some thick welding gloves for repositioning the casting, and a safe torch rest to hold the lit torch while I'm moving the casting around.

Why the cannned flux and the flux coated rod? Because the castings need to be heated to a high temperature for brazing, and at that heat level you need massive amounts of flux to make the brazing rod flow -- much of the flux will be dissipated by the heat, so that little bit of flux on the rod is NOT enough to effect any significant material flow from the rod. Here is my can of Marquette brazing flux -- I use a tea spoon to sprinkle the flux on the heated area.



To start with, cast iron needs to be pre-heated. If you don't preheat, the castings will develop cracks and the brazing material won't flow properly. In my case I use a medium rosebud torch set at 8-10 psi acetylene and 20-25 psi oxygen. I also use a very slight carburized flame (not a neutral flame as would be the case for welding). Carburized flames have a bit of an acetylene feather on them and are a great way to control the heat by "licking" the feather over a part in order to maintain a precise heat.



I used my digitital thermometer to check and recheck that both castings are brought up to around 850 degrees -- this isn't gospel, as long as the temperature of all the pieces are uniform and within 40-50 dgerees.



Once that temperature range is achieved, I take the torch (same rosebud tip) and concentrate the heat on a 1-2 inch section of cast iron. When both pieces of cast iron start to glow "cherry red" I sprinkle a spoonful of brazing flux on the joint. You'll know the flux is working because it will bubble like a soda drink. Once the bubbles subside, I reheat to cherry red color and start adding the brazing rod. At this juncture the temperature should be between 1100-1200 degrees at the "cherry red" zone.

NOTE: It's also at this point that brazing becomes hard for most people. The key is to use the feathered (carburizing) portion of the flame to melt the rod and to help if flow out. Too much heat, the rod will melt and the now liquid brazing material will simply flow away. By using the tip of the feathered flame, the rod can be softened just enough to flow across both joints and saturate the area-- heat control is key here, so use the feathered flame. Repeat this step by turning the casting (use the gloves and the torch rest) and brazing the next 1-2 inches, until the entire circumference is completely brazed.

Also, when fixing a casting you DO NOT want a "dime stacked" fusion welding-type bead. The purpose of brazing a casting is to have the filler metal spread out and wash over a large area as possible so that you can get the most surface area covered with filler and promote strong adhesion. At this temperature (1100-1200 degrees) the brazing metal will actually penetrate the pores of the cast iron -- so the more surface area that you can cover with the filler metal, the stronger the joint will become.

Speed is required, because you don't want the castings to cool down to any great degree. If the overall temperature drops significantly, then there's a great chance that the castings will crack around the brazed joint.

I don't have any pics of the interim steps, because of the speed requirement -- (since I was alone), but here are the pics of the finished brazed joint. The black/white "splotchy" areas are from the flux "burn off". Once the casting has cooled, it will be properly cleaned with a wire brush and the brass will be filed to a nice shine.







I used three 1/8" flux coated rods to complete this joint. This thing is not going anywhere soon  :toothy9:



Please NOTE: Once the brazing is complete, DON'T shock cool the brazed casting. Let the piece rest and come back to room temeprature on it's own. The longer and easier the casting can come back to ambient temperature, the less of a chance there is that cracks will develop later on. So once the brazing is done, just walk away from the casting and let it cool by itself
If the temperature is freezing, then you must insulate the brazed casting, or it will cool too fast. One way to insulate is to take a metal bucket and bury the casting in clean sand -- totally covered up. The sand will insulate the casting and it could take days to come down to a normal ambient temperature. That's a good thing!!


In my case, it's very warm and sunny outside today and the casting will take four/five hours to come back to working temparature. That's good enough --

Good luck -- let me know if this writeup has helped your own cast iron brazing project be a success.

EDIT:

The casting cooled down and here is the result after a little grinding and wire wheel work to clean up the brazing slag and "burn off". The ram was lubed and it works quite well again. I'll file it next week to make it more cosmetically pleasing -- but so far so good!!



No cracks!!! in the castings, and the brazed section is stronger than the surrounding cast iron.
« Last Edit: December 04, 2009, 07:52:19 AM by goodfellow »
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December 03, 2009, 06:43:59 PM
Reply #1

rusty

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That brings back nightmares of brazing tractor parts at night so we could run them the next day.

We had one old John Deere that was more braze then cast. The brazing held up. The cast found new places to fail.
Tool do job=Good tool

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payable to 'The United States of America,' for an amount of 'up to, and
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December 04, 2009, 08:01:42 AM
Reply #2

goodfellow

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That brings back nightmares of brazing tractor parts at night so we could run them the next day.

We had one old John Deere that was more braze then cast. The brazing held up. The cast found new places to fail.

I hear ya Rusty. Farmers do some of the most innovative repairs with brazing. I've actually seen a guy braze a broken main bearing journal on his tractor (Russian made tractor -- so parts were no longer available) and the joint held for the life of that POS.

It's hard to convince younger mechanics and machinists that brass/bronze brazing and hard silver brazing will often yield a much stronger repair than a fusion weld under certain conditions.  Just goes to show that there is still a lot of value to owning an O/A torch outfit -- LOL
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December 04, 2009, 02:31:11 PM
Reply #3

goodfellow

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I came home a little early from work and dressed the joint with a file and then lubed the entire ram -- It actually turns much more smoothly now than it did before  -- yeah!!!!

I stress tested the casting by bending the same bracket that caused this thing to give way in the first place -- the bracket bent, and the vise stayed intact  :salide:

It's not a super premium 6" class vise, but after almost 40 years of excellent service, it didn't deserve to be tossed for scrap.







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December 04, 2009, 03:47:55 PM
Reply #4

B

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Woo hooo someone who understands brazing as I understand it.  I know more delicate brazing as in bicycle frame where we lather up the joints with lots of flux and use a fine tip to silver or brass braze the joints together. One friend had an old pizza oven to heat treat AL frame and to slowly cool brazed joints of frames. 
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December 04, 2009, 04:06:22 PM
Reply #5

goodfellow

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Yes, that fine brazing is an art. The pizza oven idea however is an inspiration. For years I preheated cylinder heads and other smaller parts in my outdoor gas grill  :clapbouncie: 

That worked pretty well and would get parts to a nice even temp.

Most folks don't realize that many race car frames and exotic sports car sub-frames were actually brazed at the factory (My Jag subframe comes to mind)
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December 04, 2009, 05:05:49 PM
Reply #6

rusty

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Brazing is used in many high strength applications. Usually shocks the crap out of every one the first time they realize it.
Tool do job=Good tool

A 'Veteran'is someone who, at one point in his life, wrote a blank check made
payable to 'The United States of America,' for an amount of 'up to, and
including his life.'
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December 05, 2009, 01:03:24 AM
Reply #7

B

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pizza ovens have heavy stone liners turn it off and by morning depending on leaving temp it will be warm still
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