Silver soldering cast iron

Bill D · Sep 28, 2016

I have a small broken cast iron part. I plan to silver solder it back together on the clean break. I have read here I should lightly sandblast the area so the solder sticks to the iron and not beads up on the carbon. How about wire brushing,by hand, instead?
Solder is really air conditioning silver solder so about 10% silver. That should be strong enough. I will probably fixture in some simple wood jaws and a c-clamp or two. In my experience wood is easy to shape and lasts long enough for a one time deal. Also wood will not act as a heat sink.
Bill D.

Cbcues · Sep 28, 2016

I just used some Easy weld tig wire to repair a Wilton vise. It works quite well. Just another option if you have a tig welder.

SIP6A · Sep 28, 2016

I have silver soldered cast iron. It has to be super clean, a wire brush won't do it. I used acetone and a stiff brush then dried in an oven to preheat the work and drive all the moisture out.

partsproduction · Sep 28, 2016

Bill, please come back with results, with photos? I've been thinking of this too as I've read people saying it could be done. They have paste silver solder mixed with black flux, it might work to fixture (with metal) so that it would have pressure squeezing each broken edge into the other, then heat to a dull red, or until silver solder squeezes out.

JST · Sep 28, 2016

Have soldered a number of chunks of CI.

Do not bother with mechanical cleaning. You are going to get it red hot to solder it, so it will at that point cook out oils and goo, whatever has soaked-in over the years. With broken surfaces, you get mostly carbon, but may get oil around the edges.

Since you will get it hot to solder, I suggest getting it that hot a couple times FIRST, in order to cook out any oil, and to decarb the surfaces. Hold it hot long enough to get the carbon burnt off.

If you do anything mechanical, the pieces will not fit together well. The broken surface is a great way to get the pieces aligned, even if you just leave a couple spots, and cut the surface back elsewhere for 4 or 5 thou of solder space.

WizardOfBoz · Sep 28, 2016

In another thread here, people advocate multiple cycles of heating. Some recommend doing this in a bed of cornmeal. I would suggest a bed of kitty litter. After this, sandblast.

Another method is heating in a mixture of Dawn liquid dish soap (or TSP) and water, Rinse, repeat.

Another method is to use repeated acetone soaks.

Another method is do use ultrasonic cleaning with trichloromethane.

Ok, here's a never-tried, but I think probably good way to get the oil and crude out of your casting. Butanol has been used for degreasing. First, do an oven cycle or two with the part in kitty litter (the litter absorbs oil and grease). Then put your part(s) in a stainless bowl of butanol and put the bowl in a vacuum oven (you'll need a trap for the butanol vapors) and do multiple cycles of vacuum at a warm (120° F) temperature with the casting in the bowl.

I'm thinking that the heat and vacuum will draw some oil and any air out of the casting. Repressurizing pulls the butanol in, dissolving any oil, resulting in a lower viscosity liquid that will drain more easily.

Finish with a wash of acetone and leave the thing in the vacuum oven overnight.

Bill D · Sep 28, 2016

This is part of a wood sander machine so grease and oil are not a problem. Sounds like I have to sandblast to get enough carbon off. I will try soldering on some broken scraps and see how it looks. My main concern with blasting is removing too much metal so it no longer stays tight together in the clamps.
Bill D.

Jeremy · Sep 28, 2016

Have a look at the pictures I posted here: Image Gallery

I broke the cast iron part with a rubber mallet by accident. After doing a lot of reading, I bought Superior #601 silver brazing paste flux and Harris Safety-Silv 45 45% silver brazing alloy, 1/16" dia.

One of the pictures shows the part horizontally and it looks like there is a gap in the joint. That was the first attempt when I tried someone's suggestion of flattening a piece of silver solder with a hammer and sandwiching it in the joint before heating. Didn't work. Heated it back up and took it apart. Brushed off the silver solder while it was hot.

Attempt #2: Wire brushed the outside of the parts, figured out how to jig it together, slathered all surfaces and down the sides with the paste flux, and cut off a length of the silver solder. Fired up the rosebud tip and heated the area around the break to a red heat. The flux turned a clear color and a short while later, around read heat, the silver solder slumped on the joint and then was drawn into the joint. Really fascinating to watch. Made what appears to be a perfectly sound silver-soldered joint. I didn't get fancy with the cleaning, just made sure it was clean, and didn't sandblast anything so the parts would self-align.

I dipped the part in boiling water to clean off the flux...worked great.

I bought the flux and solder off Amazon.

TGTool · Sep 28, 2016

That was my experience too when I broke a gear. Cleaned it up, no wire brush, no sand blast, wired the OD to hold the parts in place and silver soldered. Same Harris solder and it wicks in nicely. You can just make out the line of the break and the gear works just like it ever did.

true temper · Sep 28, 2016

Tom Waltz said use oven cleaner "lye" to clean parts before silver brazing.
Bill I don't know for sure but I am not sure your silver braze rod is the best stuff for this application.
Keep the comments coming this is good stuff.

JST · Sep 28, 2016

Not sure sandblasting is a good idea, nor that it will remove carbon. It WILL mess up the surface so it doesn;t fit well. But some seem to be saying not to bother removing carbon anyway. My pieces are typically oily, so the heating may do more to carbonize the oil, and then burn off the carbon from that. Might not be needed to remove inherent carbon from the gray iron..

Demon69 · Sep 29, 2016

I recently had to braze a cast iron lathe apron. Testing with a torch on some old weights that had I couldnt get it to flow at all, just balled up. I tried on a fresh cracked surface, grinding, TC buur, wire brushing, acetone. Was just about to give up when I read about how graphite shards can mess with wetting, that shot blasting / heating with an oxidising flame help. I tried a needle gun to pound the surface, instant success!

I tried tig with Sbronze, couldnt not melt the parent metal some which lead to a weak join. Silver solder comes in all sorts of grades btw.
Have fun.

stephen thomas · Sep 29, 2016

I've silver soldered a lot of CI. I generally use a version of Jeremy's approach, and sometimes (if small parts that fit slop sink) will scrub with a lye base cleaner like ZEP and rinse, rinse, rinse, while scrubbing with a stainless steel wire brush.

Then its lots of flux, and as mentioned, red heat. So it will be necessary to do a very slow cool down with the torch to prevent brittle metal. If the surfaces are open, I scrub the flux in with a 1/16" stainless TIG rod. Then as the metal is being heated, I will wait until the silver runs, but if it is beading up anywhere, scrub it in with the stainless rod. Just keep picking and scratching.

Some of these old breaks were completely apart and could be scrubbed out, others had to depend on copius flux and heat.

Aligment is a critical factor if the machine is to run well afterwards, of course.

One thing I often do, if the area needs built up, I will silver solder the crack to get "penetration" and then switch to regular brazing rod for filler and fillets.

I keep both white flux and brown flux. The white seems to clean better but the brown will stand much more heat. I use Harris 45 for "larger" stuff, and 505 (more silver) for delicate castings and thin tight cracks. It is essential to actually get the flux to run in the cracks, or the solder won't either.

smt

richard newman · Sep 29, 2016

This is really interesting, had no idea one could silver solder cast iron, thought it had to be welded or brazed. Have to go round up all my busted machine parts and play around.

awander · Sep 29, 2016

richard newman said:
This is really interesting, had no idea one could silver solder cast iron, thought it had to be welded or brazed. Have to go round up all my busted machine parts and play around.

Ummm-silver soldering and brazing are really the same thing...

Bill D · Sep 29, 2016

awander said:
Ummm-silver soldering and brazing are really the same thing...

My understanding is the only difference is the liquid temperature 842 F for solder higher is called braze.. Why that temp I do not know. Kind of like the biology definition of fats and oils. Liquid or solid at room temperature.
Bill D.

Zinc melts at 787 F so maybe that is the difference. Lead is 621 so I do not t think that is a reason. it is probably the temperature of some common silver bearing solder at 842

sfriedberg · Sep 29, 2016

Bill D said:
My understanding is the only difference is the liquid temperature 842 F for solder higher is called braze.. Why that temp I do not know. Kind of like the biology definition of fats and oils. Liquid or solid at room temperature.
Bil lD.

Silver solder melts at brazing temperatures. To take one common example, Harris 50N has a solidus of 1220F and a liquidus of 1305F, very comfortably above 842F. Which is why awander said they were really the same thing.

(Obligatory disclaimer: There are over 50 common alloys of "silver solder" and probably thousands on the market althogether. No doubt a few have a solidus below 842F. Even less doubt that some ignorant vendor calls any shiny melt-it-stick-it material "silver solder". Does not invalidate awander's point!)

partsproduction · Sep 29, 2016

I had troubles silver soldering mild steel and was told by the welding supply guy that I should be using the black flux, I had been using the white stuff. The black (Dark brown) is when you want it to flow in deeper he said. Also,I should try the soldering bearing paste, if I can use gravity to bring it "home" after it gets dull red and tap very lightly with a bolt or something to align the break halves together after the solder is wetted.

I like the idea of silver solder over anything else for looks but have questions about strength, if the part broke because of some weird snag or something unusual it might not need lots of strength, but if it broke during normal usage then it was under engineered and needs a mechanical repair that incorporates extra steel in the weak area.

awander · Sep 29, 2016

High-temperature soldering is more properly called brazing. Look up silver-brazing, for example.

Demon69 · Sep 29, 2016

The main thing i got, If the parent metal melts, its welding. If if dont, its brazing/soldering.

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