Welding Issue? 1

[JHWC]

Hi All,

Recently, two rectangle hollow bar which was welded together was detached from each other after years of operation. Kindly see attached.

I would like to seek professional advise here on what could be the possible reasons for such failure?

It seem to me this was probably due to poor welding (the welded area seem to be lifted off from the base material) and hence, it could not take the operation load.

On the side note, the shiny portion seem to be brittle failure and the left side suffered quite a bit of deformation.

Kindly advise.

Thanks,
JHWC

 

[metengr]

Looks like there may have been lack of fusion or underbead cracking but send this failed piece into a metallurgical lab for proper failure analysis.

 

[JHWC]

Thanks metengr..

Just curious, usually under sem or high resolution microscopy, what are the things to look up for if there is underbead cracking?

Thanks.

 

[metengr]

Normally, under-bead cracks will appear intergranular and follow the coarse grained heat affected zone of the base material.

 

[weldstan]

Appears to be some large areas of lack of fusion amongst some other weld discontinuities. Quite poor workmanship.

 

[JHWC]

Thanks Metengr and weldstan! Greatly appreciated. =)

 

[Tmoose]

"Recently, ............after years of operation."

So, despite the poor welding, a pretty long service life was attained?

Could you PLEASE provide information on the rest of the structure, environment, and expected loading?

Even Higher quality welding is not going to compensate for the basic limitations of that style joint if loading is significant.

A welded square tube light pole fails like this in a windy environment -

https://www.hobartwelders.com/weldtalk/attachment....

The crack initiation points are the corners. Higher nominal stress, plus the concentrated stress at the toe of the welds make it so.
The welds in the middle of the flat faces are on vacation until the very end, when cracks have slowly propagated well in from the corners, and they are all that is left.

Sometimes triangular gussets are proudly added to the middle of the tube faces to "beef up" the joint.
The late, great Omer Blodgett explains why that is not a real good idea.

http://weldingdesign.com/processes/using-gussets-and-other-stiffeners-correctly

2 down, 18 to go.

 

[dik]

It looks like the failure initiated at the weld on the 'inside angle of the connection. This is often a difficult location to weld. From the surfaces, I'm surprised it lasted so long.

Dik

 

[JHWC]

Hi Tmoose,

Let me provide you with more information:

(a) It acts as a support bracket for a bar which weighs around 100 kg.
(b) There will be 2 support brackets to support the bar.
(c) The maximum distance apart between 2 bracket is 5m (max)
(d) The whole system is out in the open environment
(e) The bracket is made of mild steel, with tensile strength around 280 MPa.

I suspect why it can last so long is probably due to the compressive forces acting against the crack propagation.

Could you please further elaborate what do you mean by
- The crack initation are the corners - which corner specifically? the left or right?
- What do you mean by the toe of the weld?

 

[JHWC]

Hi dik,

Could you explain little further on "inside angle of the connection?"

Thanks.

 

[JHWC]

Would anybody be able to differentiate the difference in colour? Any corrosion issue here?

Need to add on the bracket.

1. The supporting bracket is made of mild steel, welded at 49 degree
2. The supporting bracket is coated with galvanising zinc (hot-dipped)
3. The supporting bracket is able to hold at least 100 kg in weight.

Thanks.

 

[metengr]

I don't see corrosion playing a part in the failure. You have light surface rust that will form on any freshly exposed steel fracture surface when the relative humidity exceeds 40%.

 

[JHWC]

Thanks Metengr,

How about those darker in colour?

I attached one file here for better viewing.

 

[metengr]

Those regions you have highlighted are mostly lack of fusion and the dark color on those surfaces is from oxidation caused by the exposure from heat during welding.

 

[JHWC]

Thanks Metengr! It is very helpful.

Last question, do you reckon any discolouration from the surface since it is dipcoat with galvanising zinc.

Thanks.

 

[metengr]

No, but you can check for Zn by using SEM/EDX on the fracture surface.

 

[JHWC]

Thank you .. =)