silver soldering steel to steel

[JohnHudak]

Hi All, I've silver soldered a lot of brass, bronze and copper, and I've gotten pretty good at it.. However, my experiences with 1018 CRS have been so-so at best. It seems like it throws off so much carbon that the flux gets contaminated with it and spoils the joint. I've only used the white flux, but I now have the black flux on hand. Any tips?
I have to fabricate some parts for the PRR G5 and I don't want to use brass.. 954 Aluminum bronze would be my next choice, but steel would be my first..
Thanks; John

[Fender]

My experience is that ss on steel is very easy, but requires a lot of heat. It’s easy to overheat with oxy/acetylene so I use air/acetylene or propane. The surfaces must be very clean, either filed or sanded to remove surface oxidation, and scrubbed with detergent to remove oil.
I believe it is possible to ss al bronze, but it is much more difficult.

[JohnHudak]

Thanks Dan, White or Black flux?

[FLtenwheeler]

Use the black flux on steel.

Tim

[RET]

Hi John,

Everything that's been said so far is good but there is a little more to it.

First, with big parts especially, you can overheat and burn the white flux. When you do, it turns black and doesn't work any more, but I have never been able to do the same thing with the black stuff.

Second, Use lots of heat over the whole part. Having multiple tips for your torch helps a lot. I use propane/air for most of my silver soldering, but for the Big Boy copper boiler, we had to use a combination of propane/air for general heating together with just the wash from an oxy/acetylene torch for the actual soldering. The large amount of copper pulls the heat away at a fantastic rate. The wash protects the work, but generally, you need to use lots of heat and don't rush it. Wait until the entire area is at a fairly bright red heat.

You will find that you don't need very much solder and you will also see that the solder will follow the heat. Once it has melted, the solder will run toward the hotter part of the joint. Once you master the technique, you will find its easy. I've also found that the more I do it, the better I get and the easier it seems. Conversely, if you haven't done it for a while, you have to relearn the details.

Its pretty easy to do, but one of the factors seems to be the thermal conductivity of the base metal. Stainless isn't hard, neither is steel but with large amounts of copper, you have to heat the entire part. In all cases, don't be in a hurry.

John, as you can see we're still working on Big Boy; we've done a lot, but there always seems to be more to do.

Have fun and best of luck.

Richard Trounce.

[gwrdriver]

John,
I've had the same difficulty with steel . . . perfect conditions, spotty results.

[rkcarguy]

I've had pretty good luck silver soldering steel to steel with propane, I use the white flux interestingly enough. The trick is to grind both parts so there is no mill scale and the to-be-soldered areas are shiny. Then preheat them well, you might see some oils and moisture appear, keep going until that burns off and wire brush clean if needed. Get the parts up to soldering temp and THEN quickly apply the flux and solder the joint.
I apply the flux with a brush, and then grab the torch and solder and lay it in there as fast as I can. I was making these thermostat housings from steel and the only one I had to redo (flux turned black), was because I didn't preheat it hot enough before I added the flux.

[Fender]

Thanks Dan, White or Black flux?

John, I’ve mostly used the white flux. I usually put it on before heating. It bubbles up and then becomes transparent, protecting the surface from oxidation. I seldom have trouble with overheating the flux, but prefer using a big propane torch, which doesn’t have as high a temperature as other gas fuels.
You may want to try stainless steel, as it solders well and doesn’t conduct heat away as fast as copper alloys.

[jcbrock]

Hi John,

Richard has summed up what I do. Black flux, I use propane heat and heat the larger parts slowly. I get my parts clean and shiny before heating. It's taken me a bit of practice and experience, but I can do it pretty successfully at this point with 1018. Hardest part is getting the mill scale off the HRS parts.

[Dave_Johnson]

I use the white flux (never tried the black flux because the white seems to work). Propane or acetylene, with a small tip, on smaller parts. Acetylene (large tip) on larger steel parts. (I'm not talking about huge parts here, but typically parts that would just be better off using a little more heat). Before heating, I apply the flux and pre-cut bits of the silver solder at the same time on the joint. It all heats up real nice and rarely do I have any issues with the process. Just start the heating process slow so that the silver solder is not boiled off by the bubbling flux. Once the flux stop bubbling the silver solder will stay put. I also use the same process for copper, and brass. On jobs when the Acetylene is not hot enough, I have fired up the propane torch to add more heat.

[Andypullen]

There is special flux for aluminum bronze. The regular stuff won't work.

Andy

[Greg_Lewis]

One more vote for the black flux. I also prefer the cadmium alloy silver solder; it seems to flow better. You can still get this from McMaster. Something that hasn't been mentioned is the size of the parts. For something not bigger than the end of your thumb, I set it on a piece of angle iron held in the bench vise. Then I heat the underside of the angle and let the heat soak through to the parts. I set bits of solder on the joint areas — I find feeding solder in as you would with electrical work results in too much solder all over the joint. By heating under the angle, the metal heats up and melts the solder, not the direct application of the flame, which could just melt the solder without heating the base metal enough. For this I use an o/a torch with a long feather on the flame.

For big stuff, say the size of a scoop of ice cream (I once fabbed some journal boxes for arch bar trucks), I set the parts inside a cave of fire brick and use a propane weed burner for heat.

Regardless, the secret is clean, clean parts and being patient enough to let them get up to temperature so they melt the solder, not the torch flame.