Brazing cast iron

Can someone give me some tips on brazing cast iron? I've brazed a lot of steel and even some cast that worked out good but sometimes cast iron gives me fits. The braze just doesn't want to stick to some cast iron. I use flux coated rod mostly and preheat. Am I doing something wrong or is there cast iron that just doesn't like braze? Thanx, Stevewold

Steve

It sounds like you are not getting the cast hot enough.

Jim

Hi Steve, I remember seeing something to this effect using copper rubbed in a "V"eed out crack in cast. Turned out to be welding cast, not brazing. If interested, it's on page 9, December 1995- GEM

Heed well! I learned a little trick from an elderly gentleman and it works...after grinding or veeing out your crack, ALWAYS wire brush it thoroughly before brazing. Seems the iron and the grinding always leave a carbon dust behind that is pretty much invisible to the eye, but it's there giving the brass a poor bond. Don't just wipe it with solvents...brush it! I've followed this instruction the brass seems to just flow beautifully where it's supposed to. Good luck!

The brush should be stainless steel.

There can be several reasons for your problem, starting with oil embedded in the cast over time. Oil has to be baked out using cat litter to wick the oil from the cast. A second problem may be your selection of brass rod. The melt temperatures of brass cover a few hundred degrees. You might also want to abandon the coated rods for bare rod and employ a paste flux rubbed onto the cast itself. Grinding cast is a tricky process too. the porous nature of cast iron lends itself to impartation of aluminum oxide and the bonding material that held the wheel together. There is also the shape of the groove to be considered, a U shape is superior to a V shape because it minimizes stress when cooling. It's preferable to make the final cut with a carbide burr rather than a fibrous wheel. If at all possible, leave the edges of the U rough so the brass has more purchase. Since cast will carry heat off by conductivity, having a secondary heat source such as a propane/air torch can save you a lot of effort when youheat the casting, allowing the entire work area to be more or less at equal temperature. When applying the filler rod, the cast needs to be hot enough to melt the filler or no adhesion will occurr. Another way to close cracks in cast is to use phosphor bronz (everdur) and TIG or carbon arc.

Kinda like George says....but during the grinding process the carbon present in cast iron collects on the surface of where you grind. Use a flat, round, or rotary file to remove this carbon build up. Then do your welding or brazing! A old trick taught to me by my Dad....a real old time blacksmith.....try it....it works! Warren

I have always used a carbide burr to groove out castiron to a " U " shape and never had a problem welding or brazing it .

I always repair cast with a stick welder using 308 stainless rod

I just had to give you the details of how I do it. First thing is J groove it rather than a V groove which has been said so far.

You have to preheat the cast iron pretty hot. Usually I try to get the area for the braze to begin to turn red. A weed burner works well if you are preheating the whole part. I use the powdered flux in the 1 pound can. Once the metal is preheated I sprinkle the flux over the area to be brazed. Then I get the torch and brazing rod out and start getting it hotter. When it get a brighter shade of red I then begin to braze it up. You know you have it when the brazing rod starts to flow like lead solder. The thing you have to watch out for is if you get it to hot you burn the surface you are trying to braze to. When you do that the rod won't stick to it. When that happens I get the die grinder and grind off the burned area while it's still hot. Then flux it again quick and keep on going.

When you burn the metal by getting the cast iron to hot, I am told you have burned out the iron out and have a carbon surface left. You have to get rid of the carbon surface before the brazing rod will stick. The flux rods work well on small parts, but the larger parts need the flux applied right away. That's been my experience.

One shop I worked at had just had there 24" American lathe rebuilt. They didn't have a crane big enough to handle it so they used 2 forklifts. Any way One guy stoped and the other didn't. It slide off the forks and the forks hit and broke the gear cover door for the spindle to the quick change gear box. It was 1/2 to 5/8" thick cast iron. I got the job of brazing it up. Looked real good and once it was painted you couldn't tell it had been brazed up.

Richard W.

I recommend preheating, but carefully . I use a weed burner but have learned, the hard way, to heat slowly and gradually to avoid cracking something.

There is a certain sound that castings make when they crack. You'd like to think that you didn't hear it and that when you look for the crack it isn't there but.........

You guys are forgrtting one thing,anybody can weld cast iron up, but the cooling off is the trick. I use a 5 gal. bucket or a box if part is to big. Put some dry sand in bottom then put hot part in cover with dry sand, and let it cool for 4 or 5 hr. Worked 30 years for me in the welding business. Paul

Paul you are right we did over look that. And you are correct about the slow cooling required for cast iron. We had a 55 gallon drum full of lime. Once the part was welded we buried it in the stuff. It could take 2 days for it to cool off. Then it would still be warm to the touch.

Richard W.

[Gramps 416]

I noticed no one said anything about using nickle rod. When I work at the IHC plant in Indy they used nickel rods. You have to have the cast iron at the same tempture. I have repaired a lot of CI by using nickel rod with no problems. You just have to ping the area while you weld and let cool slow.
Jeff
Just my 2c

[OzEngines]

Lime works really well, better than sand as it is a finer powder and insulates better. it will hold the heat for days on large items. I used lime at work and at home.
It has been mentioned, there is nothing more scary than the ping sound of casting cracking when you heat wrong and then trying to find where it cracked.

[ramsay]

I learned over thirty years ago to braze cast iron.. You have to grind it so that it is really clean then make sure you use flux and be careful and not overheat your bronze.. We learned to dip our filler rod in a can of flux but nowdays I prefer the prefluxed filler.. Strong deposits are possible if you are careful .. Mike

[Ray Freeman]

If you have difficulty getting the cast to tin after making sure everything is ultra clean you can buy a cast tinning flux. Its black in colour and much more aggressive than the standard stuff. The idea is to just use enough to get a thin coating of bronze on then use ordinary flux after that.
Another trick is not to go ahead and grind the prep but always finish it off with a file. The grinder will burr over the grain structure and the bronze will not penetrate properly. Use a sharp file to finish off the prep. I used to have to bronze up samples of cast for 3 years and test the welds to destruction. Done properly and in the right application there is no stronger joint. These days there is quicker ways to do it. I do a lot of cast iron with nickel/copper mig wire. Its good but at around $2000 for a roll of wire its not for the handyman

[Don Smith]

Check out this site for welding products and videos on Cast Iron Don

http://www.muggyweld.com/castiron.html

[Paul Schultz]

I've had good success brazing cast by grinding, cleaning with a stainless brush, preheating, cleaning again and using not a neutral flame but using a bit more oxygen and making it a slightly oxidizing flame. Old cast quite often has oil/grease soaked into it, hence the second cleaning. Sometimes you'll have to reheat a few more times 'til the part has no more oil.

Instead of lime or sand, I have a bucket of wood ashes from my wood stove that I bury parts in after brazing. A little dusty, yes, but it works well. The main thing is a slow controlled cooling period.

As far as welding the stuff, NiRod 66 usually works well but if there are problems or it needs to be machined after welding, NiRod 99 is what I use. Short welds and immediate peening 'til the part is cool enough to touch between welds is the way to go with larger parts.

[Don Smith]

This site might give you some insight in Cast Iron Welding. Don
http://www.muggyweld.com/castiron.html