Different between Sensitization and reheat Cracking on 316 Stainless Steel 1

[Hlengani]

Good Day

What are the obvious features to differentiate the failure which was as the result of sensitization or reheat cracking on the 316 stainless steel weld. The crack initiated on the weld toe and sensitization on the HAZ was observed. The failure happened after 36hours of operation. The failed pipe transport oil and which is below 80 degrees.

The filler metal used was ERNiCr-3 (NiCr20Mn3Nb).
The materials being joined it was a carbon steel and a 316 s/s. The crack was on the HAZ of the 316 s/s

Regards
Hlengani

 

[EdStainless]

Is there indication of corrosion in the Cr depleted zone along the sensitized grain boundaries?
If so then blame the sensitization.
If not then more likely welding/reheat issues.

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[Hlengani]

@EDStainless, I attached the picture showing sensitized grain boundaries. Meaning this weld failed because of sensitization.

 

[EdStainless]

If there is oxidation in the cracks then they occurred during welding/reheating.
If they cracks are filled with corrosion products (you may find S and Cl) then corrosion due to sensitization.

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P.E. Metallurgy, consulting work welcomed

 

[Hlengani]

Thank you very much, I will use the Scanning Electron Microscope (SEM) to check if there are corrosive products. But looking at the timing, would it be possible for corrosion to cause the failure in less that 36 hours of operation at less than 80 degrees of operating temperature, taking into consideration that the crack initiated from the outside of the pipe. Would this be not another sign that the cracks occurred during welding.

So what would you call this failure mechanism?

Another thing since the sample has been polished to see sensitization using polishing medium, the cracks will be contaminated for SEM analysis?

 

[TugboatEng]

What kind of testing was done on the pipe after welding and before placing in service? Was it hydrostatic tested, dye penetrant, etc...?

 

[Hlengani]

Radiographic test was done after welding and the indications were acceptable

 

[SJones]

80 deg C is not a service temperature that would be considered to induce reheat cracking.

Steve Jones
Corrosion Management Consultant

http://www.linkedin.com/in/drstevejones

All answers are personal opinions only and are in no way connected with any employer.

 

[EdStainless]

A small amount of contamination in the weld could sure cause this.

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P.E. Metallurgy, consulting work welcomed

 

[weldstan]

Was the weld subject to PWHT?

 

[Hlengani]

There was no PWHT done.

 

[EdStainless]

Did you preheat the CS?
Is this more sensitization than you saw in your weld qualifications?

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P.E. Metallurgy, consulting work welcomed

 

[Dhurjati Sen]

1. Was there a WPS?

2. Was the welder qualified?

3. Why was Inconel electrode used?

Guess the answers to 1 is a NO, 2 is may be and 3 is boss said so.

Now the boss needs to satisfy his bosses with some exotic failure mechanism.

History goes on repeating...

DHURJATI SEN
Kolkata, India

 

[weldstan]

I have heard of similar failures when the cap pass was washed to make it look prettier by the welder(s). I' not sure why you didn't use ER309(L) for this weld based on your stated operating conditions and no PWHT.