View Full Version : Silver solder & Cast Iron

06-09-2004, 02:21 PM
I was told by a guy at a welding shop that a silver soldered cast iron repair is NOT a good idea. That comment surprised me. And I kinda think he just really didn't want to take it on. The job in question needs to be done very precisely, and without slopping any excess filler material into the inside bore.
So I thought I'd poll the collective wisdom here for opinions.

This is to repair a drill press spindle sleeve (about 7/8" OD, and maybe 9/16" ID). It's cleanly broken around the circumference.

06-09-2004, 02:43 PM
You need the right stuff like this:


06-09-2004, 03:53 PM
Have you (or anyone here) used any of that?

Forrest Addy
06-09-2004, 03:58 PM
BS. Cast iron silver brazes superbly once the surface carbon is bead blasted away. Flux and braze immediately afterwards.

Don't etch, don't grind, don't file, don sandblast except as a preliminary joint prep operation. Fine media bead blast last. The tiny grits remove the distribited graphite from the surface leaving behind the iron which wets and takes silver.

06-09-2004, 04:15 PM
If I flux both surfaces and then clamp into position (just tight enough to hold the position dependably), will the silver wick into the joint? Or should I tin the surfaces first?

Mike W
06-09-2004, 05:23 PM
I silver solder bandsaw blades. It wicks into the joint quite well.

J Tiers
06-09-2004, 05:30 PM
It will wick fine.

It follows heat, so let the heat sneak up on the solder through the part you want to solder. Then it will suck in perfectly.

Paul Alciatore
06-10-2004, 06:23 PM
The ad Evan referenced says,

"1" of our high content silver solder will flow 18"--the only silver solder manufactured with this capability".

Exactly what does this mean?

Paul A.

06-10-2004, 07:04 PM
I expect it means the molten alloy has very low surface tension and flows extremely easily.

Ragarsed Raglan
06-11-2004, 10:59 AM

I would normally always use a cast iron repair rod with a high nickel content. Silver brazing is just that....brazing, a correct C.I filler rod and flux will give you a joint as good as the original parent metal - if not better. My bet is that a brazed joint (any brazed joint come to that!) will always fail prior to the parent metal failing; the only exception to this may be a nickel bronze brazed joint when applied to high tensile steel tubing. Then the failure will usually be in the parent metal in the austenitic/martenitic phases out from the joint boundaries.


06-11-2004, 11:46 AM
Hey RR, we've missed your inputs. Good to have you back.

But I'm a little confused. You say '...will give a joint as good as original parent metal - if not better...' But then you continue and say '...will always fail prior to the parent metal failing..'
Is your first comment only applying to that "...cast iron repair rod with a high nickel content"?


(added later) Oh, I think I misinterpreted RR's comment. I was reading that high nickel rod suggestion as a brazing rod, not as an arc welding rod.

[This message has been edited by lynnl (edited 06-11-2004).]

06-11-2004, 12:15 PM
The problem with arc welding cast iron is the high temps involved. Silver brazing uses temps below 1000° and is much less likely to crack the cast iron or decarburize it locally. Some of the silver brazing alloys are stronger than the cast iron. I built a Can/Am race car for a client back in the 70's including the motor mount frame and front suspension frame. They were made from 4130 tube and the client specified that they be brazed instead of welded. Apparently it produces a structure that will withstand repeated shock and vibration better than welding that alloy. I'm not sure as I totally agree with that as I used to torch weld aircraft control column assemblies and they were certified no problem.

06-11-2004, 02:10 PM
A big problem I'm gonna have doing this is having only propane torches to heat it. Doubtful I can bring it to brazing temp with just one torch. The plan I'm working on is to preheat the whole thing in the oven, then rush it out to the garage (maybe 10 steps away) and put it in an inverted bucket with cutouts for two torches. Maybe the bucket will help retain enough heat to make a difference. Of course I'll make some trial runs beforehand. Anybody else ever do something like that? (successfully?)

I've wanted an oxy-acet rig for a long time, but just hate to give up the space for storing it. Been tempted to get one of those little outfits sold for the refrigeration/HVAC crowd, but afraid I'd find the small capacity too frustrating.

I do have a forge tho and some coal/coke. Have though about using that, but it seems like that would burn out all the flux. Or possibly burn up my part if not careful. Any experiences in brazing/soldering that way?

06-11-2004, 02:56 PM

It won't work with propane torches. Not hot enough for the type of brazing alloy you need to use.

06-11-2004, 03:16 PM
Well I've successfully silver soldered other items ok. Tho none with quite as much mass as this.

06-11-2004, 03:41 PM
Yeah but, what alloys of silver solder? The kind you need will have a pretty high melting temp.

06-11-2004, 04:56 PM
I don't know the exact analysis, but it is hard, hi-temp stuff bought from a welding supply place. Not that stuff called silver solder from Wal Mart. AIRC it cost about about $10-12 for about a 3 or 4ft piece. It does require getting the target metal up to at least a dull red heat before it'll melt the solder. It solidifies into a yellowish, silver color.

I know just one propane torch won't be enough for this size item. But I'm hopeful that by using two, and by capturing some of the heat in an enclosure I can make it work.

Several years ago I had a little torch set that used solid oxygen pellets. Now that little sucker would generate enough heat/temp for brazing..easily. But it was troublesome to use. Hard to get the pellet ignited, and then you had to stuff it into the handle and close it up and light the torch and get to work before the pellet was all burned up.

Hmmm, now that I think about it, I believe I may have another little outfit with a dual hose and fittings for using one of the small oxygen bottles. I did at one time, but haven't seen it lately.

I know MAPP is supposed to burn hotter than propane. But it's never seemed that way to me. In fact the propane has always seemed noticeably more effective.

06-21-2004, 11:11 AM
You can MIG cast iron with high Nickel content wire.
The cast iron component must be heated well before welding to prevent cracking, I usually get the part dull red before welding.
I've only used this technique for exhaust manifolds but it appears to work very well indeed,

06-21-2004, 12:55 PM
I was reluctant to try welding on this, because the part fit was too close to allow any distortion whatsoever.

I found my little oxy/Mapp rig and was able to silver solder it. Tho even with that it took a long time to heat it up enough to melt the solder. It's now stuck together, and hopefully has enough solder wicked into the joint to hold it.

It took a little filing and sanding and lapping the inside bore, even tho visually it looked perfect, but I was able to finally get a nice sliding fit.