Straightening bent aluminum bar stock

The cheap roller boat trailer I picked up has a couple bent roller bars. These are the roughly I-beam shaped solid aluminum bars that the rollers sit on, as seen below.

On mine one rear set of rollers looks like it got slammed hard by a heavy boat.  It appears that the mount bars are bent nearly flat at the inboard bend nearest the clamp. 

My first attempt will be to take the new rollers off and slip a heavy steel tube over the bar and try to cold bend it back. If that fails (it's a stout bar), what are the consequences of heating the desired bend point with a MAP torch? is aluminum heat treated like steel? If I heat it to bend it do I risk softening it enough that it will just rebend with my boat on it?  My boat is way lighter than the trailer was built to hold, but most of the weight will be on those rollers near the outboard.

Replacement bars are $10 ea, so I could drop $50 on it and be done, but bending is free, right?

Aluminum hates to bend, and hates to unbend even more.  Can you remove bars and find someone with a press?  

Aluminum can be heat treated, and it will loose that temper if heated too much.  Like if it's welded, it's returned to dead-soft in that area.  Needs to be heat treated again for any useful strength.

Those boat trailer parts look pretty cheap, and they may not be anything particularly special.  You may be running into work-hardening.  When you bend aluminum it actually gets harder and stronger with each bend (until it breaks!).  So first bend is easy, second bend is much harder, third bend is almost impossible.

You could try warming it a little.  Just know with aluminum the difference between "warm" and "molten on the floor" can happen pretty fast!  I'd probably put the part in my vise and get a bigger cheater bar.

This seriously depends on what alloy these were made from in the beginning. All aluminum is not the same. The fact that they were originally a bent part tells me that they are not 6061T6, which is super common in structural applications, but is too hard. Probably 6061T4 or 5052, which bends OK once, but will fracture if bent back. If 6061 you cannot anneal it. Take the worst one and anneal it, then bend it back and examine the outside of the bend very carefully for striations indicating the beginning of cracks. The technique for annealing: Heat the part to 500 degrees F and chill it in water. This gives full soft. With this thick part you need to leave it in the oven for 20 minutes to get full heat penetration. When working thin sheet the MAP torch works, and air cooling gives 1/4 hard. Try one and learn, but be prepared to buy new

TurnerX19 said:

This seriously depends on what alloy these were made from in the beginning. All aluminum is not the same. The fact that they were originally a bent part tells me that they are not 6061T6, which is super common in structural applications, but is too hard. Probably 6061T4 or 5052, which bends OK once, but will fracture if bent back. If 6061 you cannot anneal it. Take the worst one and anneal it, then bend it back and examine the outside of the bend very carefully for striations indicating the beginning of cracks. The technique for annealing: Heat the part to 500 degrees F and chill it in water. This gives full soft. With this thick part you need to leave it in the oven for 20 minutes to get full heat penetration. When working thin sheet the MAP torch works, and air cooling gives 1/4 hard. Try one and learn, but be prepared to buy new

That's more or less my view as well. I know very little about aluminum heat treatment except that it actually takes weeks for it to harden after it's been heated. 

Like Scott said, it goes from "everything's fine" to molten with almost no warning. (AL will not glow like steel) I do a little bit of AL forging and have melted a lot of bar stock on accident. 

A little more technical info on aluminum for the nerdy types:

When you buy stock the numbers tell you a lot:

6061 is the alloy designation, and refers to the metals added into the base metal.  Super common, super nice to work with.  Weld it, machine it, yes - you can bend it.  Thin sections will definitely crack though.

You'll see 2024 in the aircraft world.  Strong and great for sheet metal parts.  Rivet it, because it can't be welded.

7075 is sort of the super-metal in aluminum land, but it has certain downsides too.  Machine, but don't plan on welding.

You'll see some 3-series and 5-series.  Non-heat-treatable.  Good formability.  Not super strong.

1000-series: lawn chairs.

The next number indicates the temper:

Zero temper -0 is dead soft or annealed.  This stuff is ready to bend or fold.  If you weld aluminum, it's at zero-temper at the weld zone.  No strength, no fatigue resistance.  It will need secondary heat treating if you want it strong.

You can indeed anneal hard aluminum by heating and letting it cool.  Traditionally, you'd set an oxy-acetylene torch to rich acetylene, and then cover the part with soot.  Change the torch to neutral flame and heat the part until the soot burns off.  It's now annealed.  No quenching needed.  Just let it cool off.

T4 - Heat treated and naturally aged.  This is what Crackers was saying above about taking weeks to fully strengthen.  The material is heated to the temp where the alloys will go back in solution - I can't find an exact number at the moment, but say 600 F.  After the part cools a little of the alloying elements will drop out of suspension over time and increase the strength.  This is natural aging.  How long this can take depends on the alloy.

T6 - Same as T4, but the with an "artificial aging" process.  After heat treat, the part is held for hours to days at a higher temp - 240 F to 375 F.  This makes the aging process happen quickly.

This is only a quick overview using Carroll Smith's "Engineer to Win" as crib-notes.  Not all inclusive, not the last word.  For your boat parts, I'll bet they'll just bend for you!

Sounds like my best bet is to buy new parts if I can't cold bend it without cracking it. 

TBH, I don't have much money in the trailer yet, so there's a pretty low threshold of PITA before I'm willing to drop another $50 on it.  I do have a 3 ton arbor press, but I'm not sure it would be any better than a 6' cheater bar with it still attached to the trailer...

Take torch, heat up aluminum, bend back into shape.  Might have to reheat after bending a bit to prevent cracking.  If it bent without cracking you can likely bend it back.  

Maybe the easy button? 

Tonight I stuck a six foot piece of 2x2 steel tube over the end of the bars and bent them back. Worked fine but sure as hell wasnt easy!

I replaced the rest of the rollers and moved the winch post back as far as it would go. Get the tires and bearings done and it's ready to put a boat on it.