brazing axle tubes

I've got an 8.8 I'm swapping in and I was wondering if anyone has tried brazing the axle tube instead of welding. Our welder at work suggested this. I looked at the tensile strength of brazing vs welding and it was quite impressive. This avoids some of the dangers of welding to the cast. I've got a torch, so I could preheat the cast, but from what I've read, unless you can oven heat the entire part, it is kind of a crap shoot. Any thoughts or ideas?

Where are you doing this - from the tubes to the pumpkin, or the tube to the knucks?

Thinking about it, the idea does have some merit (I've use brazed joins in some light-duty structures before,) but bear in mind that the HAZ (Heat Affected Zone) is actually rather larger with brazing than with arc welding (while it's a lower level of heat, it's still going to be higher than the transformation point of the iron or steel alloy, and could still call for post-fusion heat treatment/normalisation.)

I might be willing to braze tubes to the centre section - with a "torch-boy helper" to slow the cooling process - but I'd rather have the knucks or end brackets welded on. If you can get them TIGged, that's the way to go (since it would minimise the HAZ significantly.)

I had my mountain bike frame repaired with brazing one time and it broke much more quickly than the regular welds. Sure I'm a fatass, but I'd want more information before I drove around on that...

Beej said:

I had my mountain bike frame repaired with brazing one time and it broke much more quickly than the regular welds. Sure I'm a fatass, but I'd want more information before I drove around on that...

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Largely why I said what I said. The tube-centre section join would have more area to be bonded by the filler metal, so it would make for a stronger join. Also (relatively speaking,) there is somewhat less stress there.

The outer join (to the knuckle or the end bracket) has rather less material to work with, and therefore would require the stronger join.

Not saying anything against fxdlrider's welder buddy, but he may not have had enough experience with this sort of thing to make an informed decision (is he a structural welder? Steamfitter? What's his primary area of expertise?)

Besides, welding is a "true fusion" process where the two parts are melted together to make a single unit. Brazing is a "filler fusion" process akin to soldering, where the two parts are heated and a filler metal is applied - this filler provides the strength of the join. That, Beej, is why your brazed joins broke on your bicycle - they were "butt brazed" into place, most likely, and weren't fused into a single part like welding would have. While there are some very strong brazing alloys out there, it's still just a metallic answer to glue, and I can't think of any brazing alloy would be be as strong as an actual weld.

fxdlrider said:

I've got an 8.8 I'm swapping in and I was wondering if anyone has tried brazing the axle tube instead of welding. Our welder at work suggested this. I looked at the tensile strength of brazing vs welding and it was quite impressive. This avoids some of the dangers of welding to the cast. I've got a torch, so I could preheat the cast, but from what I've read, unless you can oven heat the entire part, it is kind of a crap shoot. Any thoughts or ideas?

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I don't know enough about welding to advice you, all I can do is share some personal experience. When welding cast a high nickel rod works best.

falcon556 said:

I don't know enough about welding to advice you, all I can do is share some personal experience. When welding cast a high nickel rod works best.

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Yes, it does. Along with pre-heat and post-heat (or thermal stresses in the part will cause fractures.)

5-90's right. You would have to oven heat the whole housing to get a proper braze joint. Welding is cheaper, quicker, and will produce a much stronger joint.

jeeperjohn said:

5-90's right. You would have to oven heat the whole housing to get a proper braze joint. Welding is cheaper, quicker, and will produce a much stronger joint.

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And the smaller you can keep the HAZ, the better all around. That's why I suggested having it TIGged...

Yep...you'll be fine with nickel rod. A few shots from my 14-bolt experience:





Did this around it to burn out impurities and to preheat the joint, so that a wax crayon would melt anywhere on the joint.



Proceed as normal, but the Ni55 rod welds rather more "liquid" than a 60- or 70- series rod.



Then continue to stress-relieve and post-heat the joint for a while. I could touch mine after a few hours. So far it's held up fine to some rather agressive wheeing.

my welding buddy is a welder in an aircraft repair shop. He is used to working with exotic metals and stuff, so I am going to assume he knows what he is talking about. Maybe I'm wrong too, but wouldn't less heat over a wider area spread that stress out more?

We used Nickle rods when welding mine. It worked well, but they don't give those rods away!

I preheated mine and mig'ed it. The welds aren't near as nice as the pic shown above with the high nickel rod. But they work. I also added a truss just to make sure the housing doesn't flex, and the tubes don't spin.

Something else to think about:

How would you clean the joint without pulling the housing apart?

I've never pulled a tube out of a housing, but I've pulled several inner "C"s off of front axles. All of them had rust in the joint thick enough to mess up a braze or solder joint.

I had a friend(builds late models, and modifieds) MIG my 8.8, and it has held up fine....Just my opinion

Justin

Mig or Tig really doesn't matter. Although the Haz will probably be smaller with the Mig and Mig works better with thicker material. Although it's easier to Tig with a nickel rod then Mig with a whole spool of nickel wire. Colling rate is what you need to watch so the cast doesn't become brittle, hence preheat or post heat. The nickel is a softer metal and it will help keep the weld from cracking.

fxdlrider said:

but wouldn't less heat over a wider area spread that stress out more?

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No. It's not about spreading the stress out. It's about thermal expansion/contraction.

Fact is, the cast iron center section has a different rate of thermal expansion than the tubes. Having the filler material expand or contract faster than the base material is what results in cracks and fractures. Basically, you're heating the cast part so there's not as much "thermal shock" to it while welding, and you stress-relieve and post-heat so that the two metals cool at similar rates and reduce the amount of post-cooling stress you might have. The nickel rod is great for this because the filler is extremely ductile - it has a lot more "give" than your regular MIG wire or 60- / 70- series rod. That's why you experience less cracking with the nickel stuff.

fxdlrider said:

my welding buddy is a welder in an aircraft repair shop. He is used to working with exotic metals and stuff, so I am going to assume he knows what he is talking about. Maybe I'm wrong too, but wouldn't less heat over a wider area spread that stress out more?

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Before you draw any conclusions, I'd suggest you look up <Heat Affected Zone> and research the heat-treatment, tempering, hardening, annealing, and normalizing processes. You'll be able to find it explained online rather better than I'm about to do here (because it would take up far too much space for me to do properly...)

However, what you end up with in the HAZ is an area where the heat-treatment and/or basic "hardness" of the iron or steel has been altered - usually increased - by the fusion heat and relatively rapid cooling (there are some "air-hardening" tool steels out there - I've worked with a couple.) Yes, letting iron/steel sections cool in open fresh air counts as "rapid" cooling from a metallurgical point of view - so you wind up with a hard "band" in between two zones of normal metal.

"Hard" - in a metallic sense, means "strong." "Hard" also means "brittle" - like glass. That's why I sometimes advice against using harder screws in some applications - the softer grades don't fail as dramatically (read: "explode under stress") and give you more warning...)

Post-weld heat-treatment is a variation of annealing which is done to reduce the relative hardness in the HAZ (are your eyes glazing over yet?) This is especially important in cast sections, due to the microscopic grain structure of the metal.

In "wrought" stock (like the tubes,) the metal is hot- and/or cold-worked into shape. This results in a definite microscopic grain structure to the metal - which can be maximised in a structural sense, and you take advantage of it when you need strength in a particular direction (in beams, tubes, bars, and the like - it's invariably a "long grain." Sheets can be cut either way.) This is just like the grain in wood - but much smaller.

In "cast" stock (like the centre section or the knuckles,) the metal is melted and poured into a mould. It cools in no particular manner, leading to what is called an "amourphous" grain structure. Think "particle board" here, and you're on the right track.

Cast items can be heated and formed (called "forging") or heated to a critical temperature (the "transformation point" of the alloy) and the microscopic grains can be realigned. "Forging" is similar to what is done to "wrought" stock - it can even be done "hot" or "cold" (above or below transformation temperature points.) "Hot working" is better from a structural point of view, but results in scale formation on the metal (which must be removed before fusion processes can be used.) "Cold working" doesn't give you the scale, but doesn't allow for as much refinement of the grain structure, either.)

The transformation point of the alloy is always below the melting point. The welding ("fusion") temperature is always at the melting point or slightly higher. So, welding can instantly wreck heat treatment and hardening - and you'll have to do it over again. Also, the weld zone itself is more like "cast" material - even if welding two pieces of "wrought" stock together. The only way to get away from that - "forge welding" (what blacksmiths used to do on the anvil.)

I've probably given you a headache by now, so I'll keep shut. However, searches for <iron metallurgy> or just pillaging around aisi.org should prove rewarding, if you want further information.

Pre/Post heating helps control the microstructure of the metal. Thick section cool faster and fast cooling leads to martensite formation which is a very brittle structure. There are two types of cracking you need to worry about when welding, cold cracks and hot cracks. Cold cracking has to due with the cooling rate, hot cracks tend to deal more with what vetteboy stated about the different ductilaity of the filler metal. Cold cracks occur after the welding process, hot cracking often occurs during cooling or as it's welded.

Brazing might be better for heat treated parts as the might be less chance of altering the treatment. But there could be a problem as already mentioned of a rust between the two. Not really up on brazing though. Seems like it would just be a glue like already stated.

Weasel said:

Pre/Post heating helps control the microstructure of the metal. Thick section cool faster and fast cooling leads to martensite formation which is a very brittle structure. There are two types of cracking you need to worry about when welding, cold cracks and hot cracks. Cold cracking has to due with the cooling rate, hot cracks tend to deal more with what vetteboy stated about the different ductilaity of the filler metal. Cold cracks occur after the welding process, hot cracking often occurs during cooling or as it's welded.

Brazing might be better for heat treated parts as the might be less chance of altering the treatment. But there could be a problem as already mentioned of a rust between the two. Not really up on brazing though. Seems like it would just be a glue like already stated.

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Pretty much. Think of "brazing" as "soldering" with higher temperatures and a harder filler metal. You still don't get the "one-pieceness" of a good weld - simply because you don't get the base metal hot enough to melt.

Brazing is good for joining dissimilar metals (that can't be welded together anyhow,) for working with an alloy that will catch fire at welding temperatures (like magnesium?) or for when the join is not "structurally critical." I've used brazing for attaching decorative goodies - and it works well for that. You can also select a brazing alloy that has a melting temperature below the critical transformation temperature of the base metal (usually) if that is a concern.

Besides, with brazing you'll have to use a gas torch. The heat is lower - but it's still "transformation temperature" for cast iron, and the HAZ will be larger (because the heat is not as concentrated.)

The idea behind pre-/post-heating weldments of a cast section isn't to reduce the size of the HAZ, but to reduce the net effect of the HAZ. By spreading out the temperature changes, you don't have as drastic a change - so you can get away with more.

5-90 said:

The idea behind pre-/post-heating weldments of a cast section isn't to reduce the size of the HAZ, but to reduce the net effect of the HAZ. By spreading out the temperature changes, you don't have as drastic a change - so you can get away with more.

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Right you want to slow the cooling rate so the filler metal can fill all the voids as it cools with no cracking and avoid getting brittle.