Welding vs Bending 2

[Wolfman7665]

A question arose today... If 2 plates welded together at a right angle with fillet welds, are stronger than a single plate of the same thickness and material bent on a brake to form a right angle. I have always been informed that a welded part is stronger at the welds than the material itself if welded correctly. I have also been told that when metal is bent it alters the structure of it in a way that it is weakened due to the plastic deformation. Does anyone have any documented proof of this or perhaps some simple calculations to prove it.

 

[metengr]

If 2 plates welded together at a right angle with fillet welds, are stronger than a single plate of the same thickness and material bent on a brake to form a right angle. I have always been informed that a welded part is stronger at the welds than the material itself if welded correctly

This is not totally correct. Weld metal strength should be equal to or stronger than either of the base metals being joined. In some cases the weld metal strength may exceed the base metal strength, but you still have the affects from heat of welding that alters the mechanical properties of the plate material surrounding the weld region.

So, in essence you still have a local region in the base metal adjacent to the weld region that could be weaker in comparison to the weld or base metal. Also, fillet welds by design have limited load carrying capacity in comparison to a full penetration weld (which is what I believe you meant in your OP).

 

[metengr]

Regarding bending, and the affects to the plate base material; by locally working the material over a break you have permanently deformed it (introduced a local plastic strain). To remove the affects of plastic strain introduced into the base metal as a result of bending, one can locally stress relieve this region, and the material will suffer no ill affects.

 

[MiketheEngineer]

Its also cheaper to bend than weld

 

[MintJulep]

It depends.

How thick are the plates. What is the material. What is the bend radius or weld detail.

Some materials work-harden when bent, and may actually get stronger. Some materials weaken in the heat affected zone.

 

[Wolfman7665]

The plates in question are 3/4" thick and 5" wide. They are a low carbon steel 1008/1010. I assume the fillet welds, on either side of the plate, are comprised of a very similar grade of steel. The radius of the bend is roughly 1" formed in a large brake press.

 

[Dinosaur]

I agree that bending a plate in a break almost always reduces the pieces strength. If two plates are joined by fillet welding with properly proportioned welds by a capeable welder then the part should not have reduced load carrying capacity. Bending a plate of the thickness you describe at the radius you mention will substantially yield the material. I would not be surprised if you could see and feel small tears in the outer tension face of the bend. These tears will be a point of initiation if the part is ever loaded near its capacity. They will also substantially reduce the fatigue stength.

However, the welded joint will have to be checked and performed by a capeable welder to have capacity. I also agree it should cost more to have the welded joint.

 

[metengr]

If two plates are joined by fillet welding with properly proportioned welds by a capeable welder then the part should not have reduced load carrying capacity.

Not true. Fillet welds have inherently poor fatigue properties in comparison to full penetration welds. Also, most code and standards limit the use of fillet welds and require a hefty penalty for their use versus a full penetration weld.

 

[0707]

In my opinion if your plates were thin, bending would be the preference. As your plates are about 19mm thick I am not so sure which would be the good choice, welding or bending. It will be very difficult to answer you which will be the most secure option, in this case you have to qualify both situation and then after NDT testing chose the less prone for your purpose.

Good luck

luis marques

 

[HgTX]

A 1" radius on 1" plate is pretty tight. That's the radius on the inside of the bend or at the centerline of the plate? Either way, you're talking 20-30% strain, far in excess of what is likely to be the ultimate strain for the material.

But on the other hand, depending on your loading condition, those fillet welds could have poor performance as well. What's the application?

Hg

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[Dinosaur]

Well Metengr, we'll just have to agree to disagree. However, I consider a "properly proportioned weld" to mean a weld that was designed for the application. If a weld that was designed for the application does not perform, how else could we build structures with any confidence?

 

[HgTX]

"Porperly proportioned" doesn't mean designed for the application. There's a whole lot more to welds than just their size.

Hg

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[Wolfman7665]

The radius I was referring to was on the inside of a bent plate 3/4" thick. I can see from everyone’s responses that my question can not derive a concrete rule of thumb. It seems that the application and material used determine the proper method of fabrication.

 

[TpaRAF]

Hi, Wolfman :

Why bend this or weld this? It seems like angle iron 5 x 5 x 3/4in would meet your needs. Is the material grade exotic?

Regards, RAF

 

[Dinosaur]

How do you properly proportion a structural component without designing it?

 

[vesselfab]

I think one of the questions that must be asked to give a good answer would be...""is this a pressure or non-pressure application?""

I routinely bend plates to form angles for misc. attachments of carbon steel, but very often for stainless steels or other alloys where shapes are not available.

But for pressure applications, like maybe a rectangular shaped chill box or something of that nature, I would select welding the corners as my preferred method.

If you bend plates for shapes, like angles, so many factors are involved such as grain stucture, grain direction, radius of bend, radius of bend die all vs. thickness of plate being bent. Often times thick pieces start requiring such a large radius bend that either asthetically or physically it become impractical to bend them.

all things considered, if it a short piece, I would just go ahead ad place 2 plates corner to corner and weld both sides. takes longer to move through shop to pressbrakes and to set it up, than to weld it up.

Long pieces like stiffener lips on baffles and such, will be taken to press brakes.

In these "non-pressure" application it is more economics than anything for us and do what suits the particular situation.

 

[ietech]

If the material is bent on a break (or otherwise) there are processes for heat treatment to return the material to, or near to, its original condition. I am sorry, it's been a few years and I cannot remember the process specs for HT on various materials.

Good Luck whichever way you do it.