Technical Any tips on filling in pitting on frame with brazing rod?

Done!

 

Don’t you feel better doing it the right way?

 

Haha! Yeah. Once I saw that the cross member had warped a bit I knew it had to be replaced. Lots of work though. The damn patch panel cost CAN$70.00! Guy at metal shop said steel prices were almost 3x pre-Covid and said they are busier than ever.

 

While I tend to agree with the majority that replacement of metal is sometimes the better option, I'm going to offer up some commentary in support of your original efforts and how you might have improved your chances of a greater success with it.

First off, back before GTAW, GMAW, you had two choices really. Shielded Metal Arc Welding (stick), or Oxy-Fuel Welding/brazing (gas).

When you look at all that has been crafted with those two welding processes over the years you have to ask, what's the problem using them?
Answer is nothing at all. Except as your discovered, it takes more practice to be good at it.
What you attempted was the way most would have fixed the problem if not back in the day, today on the cheap with limited expense.

But like any tool, the skill of the operator and his understanding of the tool in hand does effect the out come when using it for a task.

But to address your problem using what's commonly called the low temperature brazing process...you were excessively heating the base material and the brazing filler metal. This increased heat volume, flattened and spread the filler, while at the same time causing the expansion leading to the warping and distortion.

It could also be the material wasn't clean enough, the torch held back to far, too close and for too long, incorrect flame adjustment, or gas pressures adding to the problem?

In round numbers the torch flame temperature was around 5600 degrees F. The filler metal started to soften around 1800 degrees F and was flowing and spreading by 2400 degrees F. By 2800 degrees F it was boiling and burning brass, zinc and flux.

When it comes to welding, in this effort brazing, you have to understand that with heating of the filler metal you are controlling the metals state of transition from a solid to a liquid. More sustained heat, or higher temps makes it liquid, less keeps it solid.
The more fluid it becomes, the harder it is to control.

During low temperature brazing the base material isn't required to melt, but it has to be sufficiently heated to prevent cooling of the filler when it's being added and heated enough to allow for flow to occur.

Problem arise when excessive heat is used thinning the filler, burning the flux, preventing build up, as well because outside the protection of the by products of combustion and the melted flux shielding, the over heated metal oxidizes. Red hot is to hot. Dull red at best and only under the flames protection.

When joining body panels with a low temperature brazing filler metal, excess heating of a joint results in a brass weld that joins two pieces thinly and the oxidized base metal rusting on either side.

A braze weld can be a suitable repair if done properly and back before welding advancement made it easy for all of us to be "welders", it was an option many chose, myself included.

This was my very first attempt back in the day of panel replacement. Mind you I was young, fresh from welder training as well pretty sharp and steady with my eye hand control. But it was new ground for me and back then all you had was car mags to learn from in how to do stuff. This side was lapped enough to set a screw and brazed. The other side which I failed to take a picture of was done by gas welding with a R-45 rod, hammer and dolly. Both proved successful although the hammer welded side did experience a bit more distortion where it joined the wheel arch. That could have been my inexperience combined with lacking an extra set of hands?
Recently repairing window regulators parts with the brazing process, it gets easier with practice and understanding.

What you tried should have worked. Brazing like soldering requires a few things to remember.

1) Clean and prep. Remove contaminants to clean shiny metal. This also ruffs up the surface to expose surface area. The metal when heated expands ever so slightly, once filled with filler on cooling it contracts completing the bond.
2) Flux. Most brazing rods are flux coated but the mixture on some leads to different cleaning actions, regulated by the application of heat. Fluxes act as cleaners, flow agents, and surface protectants during the brazing process. Again, action of the flux types are heat/ temperature sensitive. To much is as bad as not enough.
3) Control the heat. While a torch that is too "hot", can be pulled away, what that does is spread the heat taking longer to bring the melting/activation temperatures up. Incorrect flame setting can result in excess temps. or introduce contamination. Excess gas pressure can blow/ spread the filler metal excessively. Bigger flame or smaller flames all are adjusted to bring about a flame type. Carburizing, neutral, oxidizing. After that it's volume of heat energy being used and released to control temperatures. If you see it's getting to hot, hard part it seems is to just stop and let it cool.

Interesting to note that brazing, braze welding is still being used but with electric arc processes. The last picture I've attached was from a screen shot of a frame for a very high end car and it has bronze welded pipe chassis parts that end up powder coated. I'm going to assume it was GTAW with a silicon bronze wire but still, it could have been done much the same with Oxy-fuel gas instead.

Google said... A properly made brazed joint (like a welded joint) will in many cases be as strong or stronger than the metals being joined. ... This base metal integrity is characteristic of all brazed joints, including both thin- and thick-section joints. Also, the lower heat minimizes danger of metal distortion or warping.

Admittedly, I wondered why they didn't braze it all but I'd guess it's because someone with a piece of paper thought the other welding process were better suited when you take into account road shock, vibration, stress, strain, ductility, rigidity ( the things they call mechanical properties) with maybe accessibility for welding of those areas as well?

Hope you find this helpful.

 

Thanks. Excellent summary. In the end, I’m sure I used excessive heat as I was trying to add lots of material and to keep it flowing (and was getting impatient). You’re right about the gas “pressure” pushing the liquid braze material “away”. In the end, the parent metal was weak due to corrosion, but my lack of skills didn’t help much. Where I used to work (Mobile Climate Control) we had a bunch of guys hand brazing all our copper tube heat exchangers. Most were Vietnamese, and were masters. We also had a tool & guy die that was an excellent welder and also did some cast iron repairs on our rotary vane AC compressors. Wish I still had access to those guys/shop. Your pic reminds me when my cousin and I “repaired” his aunt’s 72 Duster in the late 80’s. We replaced the rear quarters and brazed them on. What an unholy mess. There was a good amount of warping and we ended up having to take it to a pro. Turned out nice after he massaged our handi-work. Lol!

 

See... your figuring it out. Well if you don't get out there and try it again you'll probably be disappointed you didn't. I'm guessing you'll notice the improvement.

I should have mentioned the tip size... while the flame temperature doesn't change with tip size the volume does. Not saying you can't braze or gas weld with a larger tip size, sometimes requiring the patience to stop and allow for cooling, just what makes it easier is sometimes a smaller tip size.
It's all about controlling the volume of heat to bring about consistent results.

 

I used a #2 tip. I definitely want to learn more about brazing as I would really like to braze the frame togther on my speedster project. We’ll see. I have a good book somewhere in the house on brazing I got from one of suppliers back in the day.