PWHT Inconel 625 (.625") to C/S (.625) with S/S clad 1

[nejenrod]

Story line:
Vessel built 1994
Vessel diameter 7'0 ID
Top head, shell and bottom c/s 516 gr 70 (1.750"thk) with
s/s clad 304 (.125")=total=(1.875")
Steam jacket total vessel encompassed.

1st failure 2000
bottom head corroded,,,decision made to replace bottom
head with solid Inconel 625 and go to half pipe coils
just on the bottom head. Filler metal ERNICRMO-10
Process GTAW

2nd failure 2003
New Inconel head cir. weld developed through wall
cracking 45" at the toe of the Inconel weld to solid
Inconel head.

3rd failure 2010
Cir welds on bottom head two 30" through wall cracks.

Question: Previous WPS repair never called for PWHT.
Because we have three dissimilar metals with
a range of about 17% coefficient expansion
difference,,,wouldn't a PWHT in the soaking
range for Inconel help?

 

[MikeMet]

A PWHT at the 1100 - 1150F and times typically used for carbon steel will not help or hurt the ERNICRMO-10 filler metal or I625. The temperature is too low for any real metallurgical change. There will be some minor stress relief of the weld and I625 at 1100 - 1200 for 1 hr, but not as much as would occur in a carbon steel weld.

I recommend you butter weld the carbon steel side with ERNiCr3 and then weld that to the I625 solid head with Inconel 112 for Incone A filler, as recommended by Special Metals. Much better from a fatigue and metallurgical aspect. If you want to PWHT to the carbon steel side before making the weld to the I625 head, it can be done.

I also recommend you do an FEA analysis of the weld joint to determine the effects of the heating coils and jacket on the shell with respect to stresses on the weld joint. May show you some issues that are contributing to the failure. I also recommend you get a boat sample of the crack for metallurgical analysis, again more data to show what's happening.

 

[stanweld]

The problem with stress relieving dissimilar metals (dissimilar coefficients of expansion) welds is that stress is re-introduced during cool down. Each thermal cycle represents a potential short term thermal fatigue cycle. What temperature range does this vessel operate? How many times does it cycle per year?

 

[nejenrod]

Stanweld,
Thanks for you input and question.

The reactor is a batch epoxy reactor that swings from 100F to 400F 4-times a day.

 

[stanweld]

I heartily concur with MikeMet's paragraph 3 recommendations. I have a feeling that the heating coils are heating the thinner Alloy 625 heads too rapidly and maybe at a temperature > 400 F relative to the much heavier wall carbon steel shell. Bending/shear type loads may be concentrating at the failure locations in the Alloy 625 head.