chromoly and stainless steel

[Wladi labraña]

Hello everyone.
I have this, maybe, stupid question. I'm doing some research about if chromoly and stainless steel can be silver brazing together? I was looking at some welding forums but couldn't find any answer.
Any help?

cheers.

Wladimir.

[Mark Bulgier]

Yes, it is done all the time. Brazing stainless is tricky though.

I hope it doesn't sound mean or disrespectful, but if you have to ask that question here then you probably aren't ready. Get some training before trying to build something that someone's life depends on.

Mark Bulgier
Sent from my phone.

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From: Wladi labraña <wladi...@gmail.com>
Sent: Nov 16, 2017 6:12 PM
To: Framebuilders
Subject: [Frame] chromoly and stainless steel

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[Jon Norstog]

Wladimir,

I use stainless braze-ons, like water bottle stars, dropout Shineys  and "divers helmet" cable stops, also stainless fork crowns and lugs, with chrome-moly tubing.  56% silver will stick the stuff together - flux well and keep the temperature down - but not TOO far down.  A little practice first is a good thing.

jn

"Thursday"

On Thu, Nov 16, 2017 at 6:17 PM, Mark Bulgier <Ma...@bulgier.net> wrote:

Yes, it is done all the time. Brazing stainless is tricky though.

I hope it doesn't sound mean or disrespectful, but if you have to ask that question here then you probably aren't ready. Get some training before trying to build something that someone's life depends on.

Mark Bulgier
Sent from my phone.

---

From: Wladi labraña <wladi...@gmail.com>
Sent: Nov 16, 2017 6:12 PM
To: Framebuilders
Subject: [Frame] chromoly and stainless steel

Hello everyone.
I have this, maybe, stupid question. I'm doing some research about if chromoly and stainless steel can be silver brazing together? I was looking at some welding forums but couldn't find any answer.
Any help?

cheers.

Wladimir.

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[Andrew R Stewart]

Yes, one aspect of brazing that I haven’t completely wrapped my head around is the different heat up rate of SS vs steel and the different coloring as it does warm up. Watch your flux comes to mind for the thousandth time. Andy

 

Andrew R Stewart
Rochester, New York
USA

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[Ethan Labowitz]

Hey Wladimir, I learned to braze largely thanks to this list maybe 12 years ago. Since then I've worked in the welding industry and now I teach a brazing class at a makerspace here in the Boston area.

Stainless oxidizes differently than other steels. When you melt stainless with TIG, you have to blanket it with argon all around it, even on the backside, to prevent the formation of chromium oxides (sugaring is the redneck word). I suspect for similar reasons, the oxide layer that forms even at (much lower) silver brazing temps may be a little more tenacious, in the sense that stainless oxides  (likely chromium oxides) seem to inhibit braze flow/wetting more than iron oxides.

Also the thermal conductivity of the common 300-series stainlesses is around one third that of mild steel, so it tends to spot heat even more than steel, which is already a fairly poor thermal conductor as metals go.

My advice on stainless brazing if you already know how to silver braze non-stainless steels like 4130 is:

- Be even more obsessive about pre-braze cleaning. Really shiny and bright all over. In welding, it's best practice to use dedicated cleaning supplies for stainless, e.g. abrasives or wire brushes (which should be made with stainless bristles).

- Be even more careful about it spot-heating and therefore burning the flux. Keep the torch moving even more. You might try a slightly reducing/carbeurizing flame, the kind with a little bit of feather to it, also called a secondary cone.

- You may need to let the heat "soak" a bit longer before you can successfully braze. One of the harder things to learn with brazing is to bring a part to brazing temp and then let it sit there at brazing temp for a few seconds while the heat conducts to the inside of the part. You only have access to the outside of the part with your torch, but you need heat throughout the whole part, even the inside, and that can take a little time. Stainless conducts heat more slowly, so it needs a even more time. During this soak, you need to have pretty good temperature control, because it's possible to overheat it (and burn the flux, and then the braze won't flow) or let the temp drop too low (i.e. below brazing temp). Until it gets to brazing temp, your goal is usually to get it hot pretty quickly so you can get the job done fast, but during this little soak, you can relax a little bit and add slightly less heat per second, because you just need to replace the heat being lost to the air around the part, not increase its temperature much. 

- Use 50N filler if possible. I believe it's made especially for stainless. I've had 56% silver fail to wet well when brazing stainless, but 50N has been reliable for me. Pricing these days seems fairly cheap--around $30/troy oz., which is about what I remember it being 10 years ago.

- Also get some Cycle Design Stainless Light flux. I'm a fanboy for Wade's (/Freddy's?) stuff in general, but the Stainless Light might be my favorite product of his. I buy a steady supply of Fillet Pro, Stainless Light, LFB flux, and LFB rod for my class and for hobby framebuilding projects. It's not much more expensive than the mainstream brazing industry stuff, and it's noticeably better, especially the Stainless Light. I'm just a happy customer, no other connection to Cycle Design.

Ethan

http://ethanlabowitz.com

[Mark Bulgier]

Ethan,

Thanks for taking the time to explain it so clearly. That one’s a keeper!

Wladimir , I'd like to emphasize the importance of sanding the surface of the stainless bits to remove the oxide layer right before brazing, because it comes back. Flux can lift some oxide but it has limited capacity, so don't wear it out. As Ethan said, cleanliness is more important with SS, so don't handle the work with dirty gloves or oily hands. Clean your filler rod if it's been lying around, or don't let it get dirty to begin with.

Use any tricks you know to keep the flux covering the work, and not cooked. Waving the flame, rotating the work so gravity is helping, using enough flux to begin with. With SS lugs it can be extra bad to get burnt flux at the shoreline -- the silver will not flow there afterward. You may have to soak the flux off, riffle file the edge down to clean metal, and re-flow that area.

The advice to use a slightly reducing flame is excellent, as is the recommendation for Cycle Design Stainless Light flux.

-Mark