Rear torsion bar rust, how much is too much?

[Wildthings]

In a recent post a member had a rusty rear torsion bar which he treated with a phosphoric acid product. My question: is a rusty bar suitable for reuse? or does the pitting from the rust render the bar unsuitable for reuse? When I replaced my rear torsion arm bushings a few years back I noticed a small area of paint failure and the accompanying rust on one of the bars and have wondered since if it might lead to a fatigue failure?

[TomWesty]

I wouldn’t think surface rust would cause fatigue failure. If it’s gone deeper then you might have a problem.
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[wcfvw69]

This would be a great question for a metallurgist.
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[busdaddy]

A pit will eventually propagate a crack, how long that will take is a mystery, but when it happens it'll be sudden and possibly catastrophic.

You could grind and sand the pits so they are smooth depressions, remove the sharp corners that are the stress risers and the load is distributed evenly, no cracks (unless one has already started from years of action).
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[Wasted youth]

Without seeing any pictures of the problem area, I would offer this with a grain of salt...

Prepare the pitted area for welding; lay down a bead or two of weld. Build this area back up with steel.

I would say the same wire feed setup one would use for sheet metal work would be appropriate.

[vwwestyman]

I'm no metalurgist, but I would be way more wary of a torsion bar that had welding done to it than I would one that had a little surface rust or some gentle grinding (as described above) to get rid of the rust.

I just know that the heat does some funky stuff (that I don't fully understand how it works) to the atomic structure of the metal, and being a type of spring, those things were forged (?) in a special way.
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[Wildthings]

[Just us buses]

I know this ain't your first rodeo but my split Bentley says to make sure there are no signs off rust; replace if necessary. Bay Bentley just says to paint any scratches/chips to protect against corrosion.
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[busdaddy]

Yeah, welding (or heat of any kind) is a giant no no.

Think of it like an aircraft propeller, they flex constantly and a scratch or gouge will result in blade loss if left alone. The rule for aviation is dress an area 10X the depth of the divot so it tapers gradually to the lowest point.

For example: http://www.avcanada.ca/forums2/viewtopic.php?f=27&t=101883
You can skip the part about restoring aerodynamic shape, but the rest still applies to any forged metal part under stress and flex.
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[Wasted youth]

I’m sorry! I was thinking of the torsion trailing arm. You guys are referring to the actual spring steel torsion rods?

If so, then absolutely no welding on spring steel at this level.

[SGKent]

I would guess that 99% of the torsion bars have some rust on them.
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[MorkC68]

[Starbucket]

[Wasted youth]

I think I’m confused about which parts are being discussed. I thought originally it was the torsion tube then I thought it was the trailing arm; which was why I made my comment. Then I was thinking we were talking about the stack of bars that make up the torsion assembly.

Somebody help me out please and explain exactly which part are we talking about. Is it the part that is internal and is normally saturated in grease, or is this part of an assembly that is external and usually has some sort of body coating and paint?

[Abscate]

It’s a Bay rear torsion bar which is a solid rod with two splined ends, no stack of leaves like the front
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[SGKent]

my take is this:

[Wasted youth]

Thanks. That’s an area I’ve never ventured.

If the carbon or other non-ferrous make up is higher than what I’d call mild or structural steel (very loose idea here) than I’d not weld on anything like that unless it follows an approved process with normalizing.

I thought for a bit we were talking about external parts that see welding as part of their assembly process. Clearly, we are not. Thank you for the clarification.

[Joey]

Mine looked like this...

I removed the rust with a wire wheel then painted it with Tremclad paint. That was over ten years ago and many off-road camping trips.
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[busdaddy]

Well there you go, real life beta testing!

But is there a small crack starting that might let go one day?, who knows?, and how long until it does?
Other than going full NDT on one of yours with all the tools available to see if there's anything starting we'll likely never know. VW said to keep them painted and avoid nicks, how many thousands of hours in a test rig did it take for them to get one to actually snap? Front leaves break regularly, but I've never seen a rear bar broken myself that I can recall.

Either way I'll still follow the recommended protocol, non rusty bars are easy to find if one is too messed up to trust. If I decide to run one that's not up to par and it defys the odds and actually breaks it'll be a major pain in the a** that I could have avoided by just installing a clean one.
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[Amskeptic]