Cracks Appear in Structural Tube When Welded 1

[ManicalEngineer]

Good Afternoon!

I work for a company that uses a lot of structural tube in our product. Recently we had some 4" X 6" X 1/2" A500 Grade B tubing show cracks after welding, a number of these cracks are away from the weld site. I've spent sometime thinking about this, my weld lead (25+ years experience) thinks they started as hairline cracks that opened up due to the heat/cool cycle during welding. Has anyone ever received material cracked from the mill before?

I've spent some time working with cast turbine shells and cracks in those are typically repaired by grinding smooth and filling with weld. Is that an option on this type of tube?

The mill's initial response is it is just mill scale, I disagree, but if it were just mill scale cracking I feel like that is still an issue, am I wrong?

Thanks in advance

 

[Once20036]

where are the cracks occurring? Any pics?
Is the steel galvanized?

 

[le99]

NDT may help in clarifying mill scale vs microcrack.

 

[ManicalEngineer]

Sorry should have included the crack location in my original post, they are occurring on the outside corners of the tube, always on the weld seam side (weld seam of the tube not our weld). I've added two pictures, they are close up to highlight the cracks.

Thanks,

 

[EdStainless]

I would reject the material.
I presume that you bought it to a spec that requires flattening tests?
Cut a sample adjacent to the crack and put it through the spec testing.

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P.E. Metallurgy, consulting work welcomed

 

[david339933]

Sure doesn't look like mill scale. Did you send the mill these pics? Grind it out and see how deep it goes.

 

[le99]

How could the supplier claim mill scale for such a wild continuous crack!

 

[ManicalEngineer]

@EdStainless - We didn't find the cracks till we welded it in to a larger assembly, if the cracks had been noticed before hand we would have surely rejected it. As far as our requirements, we call out A500 Grade B for the part, we don't by the material just the finished part that goes in to our assembly/weldment. We have fairly limited testing equipment, just bought an MPI kit that arrived today.

@David - Similar pictures were sent to the mill by our supplier (who buys the material and cuts it), it looks like one of the guys on the floor ground part of the crack out already, it created a pretty significant flat, I haven't had a chance to measure it.

@le99 - Yeah I pushed back on our part supplier when he gave me that feedback from the mill, I was pretty blunt about it. We will be talk to the mill on Monday.

As a safety minded engineer my preferred method to dealing with these cracked assemblies/weldments is to scrap them, the business part of me wants to investigate doing weld repairs, anyone have any experience in weld repairs on structural tube? Any strong opinions either way?

Thanks again everyone!

 

[Once20036]

With the close up nature of those photos, I can't tell if they`re galvanized or not, but this type of cracking is well documents with Galvi.
I can post some links if it's applicable here.

 

[ManicalEngineer]

@Once - Its not galvanized, no coatings, but those links might be good to have for future information. We do sometimes produce product that is galvanized.

Thanks,

 

[TugboatEng]

If you can determine that the cracks are caused by stress and not defect, the welded structure could be stress relieved and then the cracks can be repaired by welding.

 

[Once20036]

Haha, I suppose I should have just posted it initially.
The website doesn't exist anymore, but I had a PDF copy filed away.

 

[Once20036]

Here was my most recent experience on the subject

 

[Once20036]

One more...

 

[ManicalEngineer]

@Once - That's a great resource and good to know! Thanks for sharing!

@TugboatEng - Yeah we are working on getting feedback from the tube mill about their impression of what the cause is.

 

[dauwerda]

The steel tube institute information is still available online, just in a different spot:

https://steeltubeinstitute.org/resources/galvanizing-hss/

They also have some other good info there, such as this:

https://steeltubeinstitute.org/resources/mechanical-properties-hss-corners-versus-flats/

 

[MotorCity]

How does a crack like that go unnoticed?? In any case, even though it is not galvanized I would bet that the cracking is due to the same phenomenon described in the paper posted above. If such a crack is possible with galvanizing, surely it is possible from welding which has much higher temperatures than a galvanizing bath.

 

[Once20036]

Thanks for the links Dauwerda. I looked but couldn't find those papers.
I think it's too bad they got rid of the Elmer Fudd / exploded shotgun photo. That one was pretty dramatic.