Is this welding method ok?

[mutantZ]

Just came across this vid of some guys putting together a chassis. I thought it was prefered to have the tubing flush fit against each other rather than leaving gaps. Though to be fair, it is just for a charity auction.

 

Edited May 15, 2013 by mutantZ

[Tony D]

Listen to the first few seconds "we cut the tube too short leaving a big gap..."

 

Not ideal, not preferred, but it's a bracket for holding internal parts...not anything critical like part of the crash structure.

[johnc]

Looks like the bracket or brace for the brake pedal/MCs. 

[LLave]

Not something I would want to show off, but also not something I wouldn't do myself. It looks like he got a plenty strong enough connection for that part in question.

[johnc]

The issue with filling gap like that is that the weld itself work hardens and gets very brittle. There is almost no parent material in the weld, just filler.

[Snailed]

The weld does become harder but I don't believe there is any work hardening in the weld.  Work hardening comes from employing the slip systems in the crystals, compressive forces like hammering work harden the metal, strain from tension also work harden metal.

 

A steel weld becomes harder/more brittle because the rapid heating and cooling of the crystals turns it into harder forms like martensite, a much less ductile material than the base metal (think of bending a drill bit vs. bending a paperclip). This is essentially to how heat treating works and is not desirable in a welded structure like a tube chassis/ cage.

[johnc]

Because of the large gap and large amount of filler, the weld work hardens as it cools from tension. That was the explanation given to me by the instructors at the Lincoln Electric welding school.

[Snailed]

The end result is similar but I'm pretty sure the increased weld hardness is because it's a different form of carbon steel once it has solidified. There is a difference between work hardening and the structure of the crystals changing during the recrystallization (heating and cooling) process. One you can do with a hammer, the other could be done with a torch.  A re-solidified piece of filler wire (imagine you just melted some on it's own, no parent metal) is a harder metal than some cold rolled 1008 steel, right?

 

Both processes result in a harder, less ductile steel.  If the Lincoln school didn't get into the atomic structure; face centered cubic, body centered cubic, close packed hex etc.. then the explanation given to you might be an easy way of understanding the resulting change without the welders needing an understanding of the crystalline structures of steel.  I'm not a metallurgist so don't take my word for it, but I'm pretty sure this is whats going on in the weld.

[Chris Duncan]

Another problem with a large gap like this is that the back side of the weld is not shielded.

 

With a very small gap or no gap there is little atmospheric exposure to the back side of the weld, with a large gap there is large exposure. Subsequently you get weld oxidation, essentially the weld puddle is burning which causes impurities and inclusions.

[nautamaran]

The end result is similar but I'm pretty sure the increased weld hardness is because it's a different form of carbon steel once it has solidified. There is a difference between work hardening and the structure of the crystals changing during the recrystallization (heating and cooling) process. One you can do with a hammer, the other could be done with a torch.  A re-solidified piece of filler wire (imagine you just melted some on it's own, no parent metal) is a harder metal than some cold rolled 1008 steel, right?

 

Both processes result in a harder, less ductile steel.  If the Lincoln school didn't get into the atomic structure; face centered cubic, body centered cubic, close packed hex etc.. then the explanation given to you might be an easy way of understanding the resulting change without the welders needing an understanding of the crystalline structures of steel.  I'm not a metallurgist so don't take my word for it, but I'm pretty sure this is whats going on in the weld.

 

 

Because of the large gap and large amount of filler, the weld work hardens as it cools from tension. That was the explanation given to me by the instructors at the Lincoln Electric welding school.

 

You're both right. 

The initial temperature and rate of cooling will determine the micro-crystal structure of the metal, and the change in volume as the metal cools can result in internal stresses in the material, or in the part "pulling" as it cools.  This movement can work-harden the metal in the weld area.

 

Some metal alloys (like tool steels) can go through many phase changes as they cool from the liquid state, and a fast quench can lock in harder structures (e.g. martensite) as the metal crystals are denied the time required to reorganize into softer structures.  Some of these harder structures can also be brittle, and there may be an advantage in heat treating the material and allowing other crystal structures to form.

 

Work hardening occurs when the ordered crystals within the metal are shattered during deformation (e.g. bending soft copper tubing, or rolling out a metal plate).  These shattered crystals then resist movement better than larger crystals because the slip-planes within them are no longer continuous, and the atomic bonds are under internal stresses.  Reheating the metal to a high temperature and allowing the metal crystals to reorganize will ease these internal stresses and soften the metal again and is called annealing.

 

Some alloys used for rolled stock get their hardness from being cold formed during manufacture, but they become locally annealed by the heat of welding with resulting weakness in the heat affected zone.  With some base metal/filler metal alloy combinations, part of the heat affected zone is immediately work-hardened as the filler metal cools, deforming the surrounding base metal and strengthening the finished weld.  In the extreme, the movement of the cooling filler can overstress the joint and crack the weld.