ultra low temp annealing?

[PagoMitch]

Have a friend building a motorcycle swingarm out of mild steel. Several welds, hammering into final shape, final fitment, then powdercoated and baked for 4 minutes at about 225C. The part comes out looking great... but deformed. See pic.
How can this happen at such a low temp? I was taught (I'm not a metal guy, by the way...) that annealing starts at about 500C.
So, if(?) this is whats happening, could he 1)weld and hammer, 2) "ultra low temp anneal", 3) trial fitment, and then 4) powdercoat and bake? If this is not the deformation process, how does he resolve it?
I'm stumped...

 

[NickE]

The residual stresses introduced due to the welding and cold forming are releasing and causing twist when heated. Even at room temp, with enough time, this part would likely twist.

I would introduce 2-3 stress relieving cycles during the production.

First after welding to release the stresses present due to the differential heating.

Then do a coarse forming with the hammer to get the basic shape. And then stress relieve.

Do a finish forming to finalize the dimensions. test fit it and then stress relieve.

Test fit again and dust up the dimensions.

is the part sandblasted to clean it before powder coat? That can put all kinds of stresses into the part, causing more warping.

nick

 

[PagoMitch]

Thanks Nick for the reply. Just noticed that the baking time for powder coating should be 45 minutes, not 4...

Yep, the part is indeed bead blasted after welding and before powder coat in order to improve powder coat adhesion.

So, would the time/temps he has been using for powder coat baking be sufficient to relieve the stresses? I'm still amazed that this is occurring at such low temps.

If this is happening here, how the heck do the commercial powder coat guys avoid warping customers parts with their process?

Mitch

 

[swall]

Another option would to be assemble in place after fabrication and use a torch and Tempil Stick to stress relief in-situ. I would aim for 800F. Obviously, anything combustible, such as oil, grease and tire would need to be removed from the vicinity. Then bead blast and powdercoat.

 

[NickE]

So, would the time/temps he has been using for powder coat baking be sufficient to relieve the stresses?

Yes

how the heck do the commercial powder coat guys avoid warping customers parts with their process?

by using multiple stress relief cycles during the manufacture of the part.

I would use as a final stress relief, around 300C for 2hrs. that would assure that any stresses that remain from the bead blasting are low enough to be inconsequential when the part is baked at 225C.

(Did you not read my prior post?)

 

[PagoMitch]

Thanks for the replys.
NickE: Yep, I read your post-very informative answers. But I was trying to confirm the temps, as (metal) failure/fatigue of this device while in service would likely result in a fatality... More than likely mine! Thanks again.
Mitch

 

[NickE]

The big question to answer with regards to failure is the carbon content of the steel. You stated mild steel, which I would expect to have <0.20%. Steel with carbon this low is generally used in either a cold worked condition, or a condition with no added strengthening, normalized/annealed or some such.

Basically you are using the steel in its weakest, but toughest condition.

Hopefully you've picked a strong enough geometry to support the loading you will be placing on the component. The heating you are doing to the steel is not going to ultimately affect the fatigue resistance/or final strength of the design.

A proper mechanical design is far more important than material when considering flex/fatigue/failure. Once a design is made then material sufficient to provide the performace needed to realize the goals of the design is selected. Usually the manufacturing process is then determined.

Sometimes all of the above steps are performed simultaneously by either someone with the requisite background, or a multi-disciplinary team.

nick