Dissimilar materials fillet weld PWHT requirements 4

[gtimind]

Dear Engineers,

Currently we had a problem with one of our pressure vessel build under asme div2 code.
Fillet weld to attach circular plate on the internal side of vessel which act as supports for coalescer filters had found with cracks extending about 1/3 of vessel circumferential. Root cause not yet finalized, but based on PAMI, the filler material was detected as 316 instead of 309L per the fabrication WPS. (the vessel shell is 516Gr70N 57mm thickness and the attachment is SS316 70mm thickness.
During the fabrication, the vessel had undergone PWHT by putting the whole vessel inside furnace (holding period 2hrs 15 minutes @ 620 degree celcius).

Now we are going to repair the cracks. My questions are
1. How reliable is PAMI result?
2. shall we PWHT the fillet repair area again? although the joint thk of fillet weld may not exceed 38mm?
3. this vessel is meant for sour service gas, does NACE requirements spell out any welding shall be PWHT less the weld thk?

Thank you for your cooperation.

 

[metengr]

gtimind;
If this is a new vessel build, you need to understand that this is a nonconformance and the AI and the Engineer that will certify the design and fabrication of this vessel must be I agreement. I know what I would do, however, any guidance received does not matter. Here is some advice below to help you in this problem;

1. Normally, a PMI test using a portable alloy analyzer is fine for distinguishing between 316 and 309 filler metal. So, I see no problem there. The cracking is caused by dilution effects between the carbon steel and 316 filler metal.

2. As I understand your situation, I would verify complete removal of the original 316 stainless weld deposit and perform a liquid penetrant exam prior to re-depositing the fillet weld using 309 stainless steel.

3. Your nonconformance report will need to address the root cause (incorrect filler metal selection) and followed up be corrective measures as I stated in 2) above.

4. I would use the position that PWHT is not required if the throat dimension of the fillet weld does not exceed the exemption requirements. However, you need to confirm this with the requirements in ASME Section VIII, Div 2. Also, you need to run the corrective actions past the AI, Design Engineer and the client for agreement.

 

[gtimind]

thank you metengr

FYI, we are on the client side. the vessel had been in use fr the past 6 months in which still covered under fabricator warranty of 18 months.
Thank you for your help.

 

[strider6]

dissimilar metal weld in sour service can give you some problems of SSC..
RELIABILITY OF DISSIMILAR METAL WELDS
SUBJECTED TO SULFIDE STRESS CRACKING

https://eprints.kfupm.edu.sa/1702/1/P297.pdf

http://www.ludwigassociatesltd.com/...9664DEF68CC68=0&bcsi_scan_filename=P1toP8.pdf

S

Corrosion Prevention & Corrosion Control

 

[gtimind]

thank you strider 6. you had been very helpful.

 

[0707]

First you have to finalize you root cause analysis.

The weld of 516GR70N57mm thick shell with a ring SS316 70mm thick when submitted to PWHT because of different thermal coephicient has a great possibility of cracking.

You also have to consider that when welding C.S materials with SS, the use of 309 electrodes is less prone to cracking than the welding of 316.



Luis marques

 

[stanweld]

Dillution of 316 weld filler metal with carbon steel produces a martensitic structure highly prone to cracking especially when using high heat input welding processes like SAW. The addition of PWHT could have readily led to crack growth. PWHT of these dissimilar welds will not really reduce welding residual stress due to the thermal coefficient mismatch. The PWHT will temper any martensite and lower hardness in the carbon steel HAZ.

If service conditions permit, a suitable nickel base alloy weld filler metal is advised.

Advise recommendations by metengr.

 

[gtimind]

Gents,
thank you for the feedback.

 

[robsalv]

All the guiding material I have on hand specifies a 309 filler metal between CS and SS... at what level was the unauthorised filler metal change made?? (Just curious)



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[MikeMet]

Go to the following website for an excellent explanation of why you got cracking:

http://www.gowelding.com/met/diss.html

Grinding out the weld and using E309 will solve the problem whether you PWHT or not.