Bending AISI 4340 To A Tight Radius 2

[tc7]

We will need to bend 1” thick 4340 normalized bar stock through a 5/8” inside radius. Later in the process, we will be welding on this component and perform a post weld Q&T to increase final strength to ~165-170 uts.

I am worried about the initial 5/8” radius bend and by the way the bar is bent through a full 180 degrees. Is it recommended this be heated as part of the bending operation? If so what might be a recommended temperature?

Is there any reason to think this needs to be bent in two steps with a stress relief (or normalization?) between each step; for instance bend through 90 degrees then SR (or normalize?), then bend through to the full 180 degrees and SR again (or normalize?)

>>>> Regarding the post weld Q&T operations, can anyone advise what the martensite finish temperature is for 4340? I will need to know this as p/o my upcoming weld procedure qualification.


Advice and experience welcomed.
Thankyou

 

[MikeHalloran]

I'm pretty sure you'll have to bend it hot.

Or just have a U shape cut from plate.



Mike Halloran
Pembroke Pines, FL, USA

 

[MikeHalloran]

Oh. You gain nothing by doing the bend in two steps.

Find a blacksmith and watch.



Mike Halloran
Pembroke Pines, FL, USA

 

[metengr]

Is it recommended this be heated as part of the bending operation? If so what might be a recommended temperature?

1500-1600 deg F is a safe hot working temperature

There is no need to do this in steps with any thermal treatment in between.

 

[tc7]

well I don't think we were expecting to work the bend at high temps like that.

How about cold bending then stress relief prior to weld? Is that possible?

 

[MikeHalloran]

At that radius, off the top of my head, I don't think so.
But work out the elongation necessary to make the bend, and compare it with the mechanical properties listed for the alloy and condition you get.



Mike Halloran
Pembroke Pines, FL, USA

 

[cybo11]

I made alot of U-bolts in my last job. The rule of thumb is any radius tighter than 4D gets bent hot. Don't even think of doing it cold with an alloy steel. Worse case: Alloy steel, tight radius, cold bending, PICKLING, then galvanizing. ~TING~!

Steve
CWI UT3 Blacksmith

 

[tc7]

Hello MikeHalloran-
well I did the elongation calculations as you suggested and discovered that to make the bend, we will experience ~44% elongation whereas annealed 4340 offers, at best, ~21% elongation. So we will be working the operations very hot.

My question now boils down to this:
After the hot bending operation, is it suggested that I nomalize or anneal the pieces prior to welding and then do the post weld Q&T operation? If so, what value does that normalizing step provide?

 

[MikeHalloran]

I claim no particular expertise here, but I'd be inclined to leave the part in annealed condition after bending and then normalize, etc., after welding.



Mike Halloran
Pembroke Pines, FL, USA

 

[metengr]

My question now boils down to this:
After the hot bending operation, is it suggested that I nomalize or anneal the pieces prior to welding and then do the post weld Q&T operation? If so, what value does that normalizing step provide?

I would hot form, slow cool, NDT for cracks, weld, reheat treat for Q&T, final NDT.

 

[tc7]

Thanks All-
Your ideas have been very helpful to me.