Problems with Silver-Brazing Bronze Casting

[KIMBALLB]

I am working on my first loco boiler, 6" diameter copper tube for a 2-1/2" scale 0-4-0 7-1/2" gauge steamer that some of you may recognize as the Don Young designed "marie estelle" as sold by Power Models. I bought the casting kit and plans many years ago. I have recently made four attempts to braze the main steam dome bushing to the copper boiler tube without success. Nearby smaller bushings of alloy 936 bronze (McMaster-Carr) made proper joints at the same time. In fact, later attempts on the staem dome bushing found the braze metal to prefer to run over to those joints instead of staying in the area where it was applied. The result was was grossly over-filled joints in the nearby bushings, and NO JOINT at all in the steam dome bush. I was using 75% silver alloy as these are my first sub-assemblies in the all copper boiler. Later joints wil use 45% (Easy-Flo 45) but I wanted the safety margin. The metal of the bushing looked oddly textured after several heats, as if perhaps lead or some other metal had come out. Has anyone made a successful boiler using casting form Power Model? Would I maybe be successful if I used only Easy-Flo 45? How can I clean-up the now rough surface of the bushing? Any suggestions? This is a real heart-breaker.

[Harold_V]

Cleaning up the surface would likely be no big deal if you can either sand or glass bead blast the item, but it sure must be a big disappointment for you to be going through this. It's entirely possible that you oxidized the casting the first time you heated it, then just made it worse with each further attempt. You also may have driven zinc from the casting by the repeated heating, it's hard to say what's going on for sure. One thing I would do, though, is blast the thing by some means, so you can guarantee that you've cut through any scaling that my be part of your problem. With some luck, one of the guys that have soldered copper boilers will have the exact cause and cure for your problem.

I recently soft soldered some "brass" plugs in copper tubing. The stuff didn't machine well at all, and was even less happy to solder, though with a little screwing around I did get a respectable solder joint. Seems not all of this stuff is happy to solder.

Harold

[Bruce_Mowbray]

Kimball,
It sounds like in your initial attempt you didn't pre heat your boiler and you didn't focus your heat on the big bronze steam dome casting and your got the copper shell to hot too fast. Copper has a way of sucking so much heat from the soldering area that not enough heat will be absorbed into a large casting such as your steam dome. Also, make sure you have enough BTU/Hour coming from your torch. A tiny torch tip won't make silver soldering easy. A big fat flame is better than a tiny concentrated flame. You don't want the heat from the flame to melt the solder. You want the boiler material itself to be hot enough to melt the solder.
My suggestions are two fold.
First, Get your casting and the area where it is to be silver soldered as clean as possible. Bead blast, grind, sandpaper all of the latent grime and oxidation off as possible. The cleaner the better!! Apply a coating of silver solder flux (I like the hi temp "black flux") to both the casting and the area where it is to be soldered.
Then, Place the dome on top of the boiler and heat up your entire boiler with either a large propane torch (weed burner) or blowpipe. Bring it up to almost soldering temperature. Keep your steam dowm on top of the boiler so it heats up too. Then place a small (1/2" long) piece of silver solder at the joint between the dome and the boiler. The copper boiler should stay warm from the preheat. Then at first, concentrate your soldering heat on the dome casting and watch to see when the piece of silver solder melts. You then know that the casting is up to temperature. Don't play the flame on the silver solder itself as this will cause it to oxidize and it will not melt as soon as it should. Instead, work the flame around on the casting and every now and then onto the copper boiler. When the piece of solder melts, you can then finish the joint between the dome and boiler by adding more silver solder and using a scratch rod to coax it around the seam. Keep the casting up to temperature and you souldn't have any problems. Always keep your flame moving. This will prevent over heating and will distribute the heat more evenly around your soldering area.

Did I mention cleaning!!!

[Greg_Lewis]

Great advice from Bruce M. Note his recommendation of the black flux. Also, I have had better success with cadmium alloy solder than from the cadmium-free stuff. You can get the cadmium alloy stuff from McMaster Carr.

[UnkaJesse]

Kimball, I don't recall you saying what kind of heat you were using, Propane, Oxy-Acet? If you are using oxy-acet be sure to keep a "feather" in the center of the flame. A "hard" (hissing sound) flame will cause excess oxidation because of the oversupply of oxygen in the flame. Be sure you have the mating surfaces well coated with flux also. If you use a proper eutectic flux, when it melts and cleans the surface, the silversolder should flow right into the joint. Doing as Bruce says and putting a piece of the solder at the joint will tell you definitely when the proper temperature is reached. [img]/ubb/images/graemlins/cool.gif"%20alt="[/img]

Bruce, what is this "black" flux? Something you Yankees are using and keeping a secret from us Rebs? [img]/ubb/images/graemlins/frown.gif"%20alt="[/img]

Unka (has used a lot of silver solder in his day) Jesse

[KIMBALLB]

Thanks Guys! I am using a big propane weed burner torch. Yes, it is possible that I heated the 6" copper tube too much on my first attempt. I was trying to not overheat the bush itself. I was using the white flux as it had worked for about 10 other sub-assy joints with the 75% silver solder. I do have black flux, however. Has anyone out there sucessfuly silver-soldered a boiler using the castings supplied by Power Models? It is difficult to believe that I am the only guy that had problems, so I am left to wonder if it was something I did or something I bought?

[Bruce_Mowbray]

Jesse,
I have been using The Harris' Black Flux for years. I could always find it next to the white flux at my welding supplier. If it's available south of the Mason-Dixon line? That I don't know. It works real good when the mass of the two objects you are silver brazing together are very different. One piece almost always gets too hot for the white flux and it burns before the second piece gets hot enough to melt the solder. The attachment contains a pic of the bottle of The Black Flux (actually very dark brown) so you know what to look for.

[Rolly]

UnkaJesse
Hear is the web site.
http://www.jwharris.com/jwprod/brazesolderfluxes/
Go down the page a bit.
Rolly

[davestea]

Ok - so far so good - very good information guys thanks - I learn so much on this site.

Now - while I do a very minor amount of silver soldering I do do (do-do?) a fair amount of brazing. Now here in California what I learned as brazing is with a some form of bronze "brazing rod" as apposed to "silver solder". I have a can of blue brazing flux that I dip the hot rod into - I also have some flux coated rods.

My problem is similar to that of the original poster here. The brazing rod puddles and does not flow in the joint. I do know that the joint was clean and I THINK it was hot enough - low cherry red - I was joining rebar to some kind of stainless for a fire place poker. I am well versed in general soldering and know and understand the need for clean pieces.

I try to take care not to hit the rod with the torch (I use oxy-propane most of the time but have acet. also.

Can anyone offer any help with my problem? Can you (UnkaJesse ) give some more detail about the "feather" in the flame - this is new to me but makes sense that if you have too much oxygen in the flame you would tend to oxidize the joint.

[jpfalt]

My experience has been that the rod type has a serious effect on the metal wetting. When brazing carbide to steel, silver solder wouldn't wet it at all, but 70 nickel/30 copper worked really well. Probably too hot for a bronze casting though. Stainless is notoriously bad for oxide filming up even without heat. I have always used Sta-Silv silver soldering flux for both silver soldering and brazing. It's pretty much just straight borax in water and does a consistently good job.

[UnkaJesse]

Dave, what I (and other torch welders) call a feather is when the flame has a very slight excess of acetylene (or propane). This causes the little bright blue jet right at the tip of the torch to sort of branch out a bit and "feather". When using this flame, it is far less likely to form an oxide on the workpieces than when the flame is "hard" (IE: Makes a hissing sound). There is one disadvantage (minor one) in that carbon more easily forms on the tip of the torch and causes what the old welder who taught me, called a "star" . Stars are extremely bright whiteish color which has to be either popped off or wiped away or the flame will become hard to aim properly. No problem as I said, and it usually only occurs when you have been using the torch for a long time at once.

Good idea to sprinkle some flux on the joint as soon as the material is hot enough to cause it to adhere to the workpieces. Then do as you said, and dip the rod into the flux as you start to braze.

For silversoldering, I use a paste flux that is mixed with water until it can be painted on the joint before any heat is applied at all. I then dip the silversolder wire into it as is done with brazing flux.

For brazing, I have used, and got the best results, with a flux that is actually made for cast iron welding with real cast iron rod. [img]/ubb/images/graemlins/smile.gif"%20alt="[/img] (Yes indeedy do, we had cast iron rods about 3/8" square that we used for welding cast iron.) [img]/ubb/images/graemlins/cool.gif"%20alt="[/img] The flux looks like powdered red brick dust, but it works wonders when brazing cast iron also. My mentor at the railroad kept a can of the red stuff hidden under his work bench and got it out whenever he had a particularly tough brazing job. [img]/ubb/images/graemlins/grin.gif"%20alt="[/img]

Unka (brazing fool) Jesse

[davestea]

thanks guys - very well written and very infomative - will try next time I need to do some brazing.