Crack Sensitivity Inconel 601 2

[lcarter157]

A client has a filter vessel of Inconel 601 in HDPE manufacture operating at 5 psig & from 700 F to 1100 F. The filters are tubes screwed into a tubesheet and are separated into 4 groups by 5/16" thick internal baffles, the baffle is not stressed. The baffles are also Inconel 601 and are welded to the 1/2" tubesheet. These welds have cracked after service of several years and we are building a replacement vessel. I want to avoid the same failure, I have indications that Inconel 601 is sensitive to cracking especially if the cooling from 1400 F to 1000 F is slow. The welds on the vessel body have not cracked.
Any recommendations would be appreciated, I am not a metallurgist.
Larger welds?
Different material?
Just fatigue failure?

Thank you,

 

[metengr]

What is the material of the tubesheet? Is is Inconel 601, as well or a different material?

 

[lcarter157]

Same material, all is Inconel 601.

 

[metengr]

lcarter157;
I have included the web site below so that you can review technical information regarding Inconel 601. This material is very weldable but can sensitize during slow cooling. The sensitization has more to do with corrosion resistance and stress rupture properties in elevated temperature service. What type of cracks are you seeing in the baffle fillet welds? These could be fatigue cracks related to service or could be poor quality welds from undercut, etc.

http://www.specialmetals.com/index.php

 

[lcarter157]

My inspector says they appear to be fatigue cracks but some extend into the HAZ. That's why I was concerned. There are four 90 Deg. areas, each baffle is welded to the tubesheet, the baffles stiffen each other so I couldn't see much chance of flexure.

 

[metengr]

Could be flow induced vibration that results in wmechanical fatigue propagation. Listen to your Inspector. You may need to evaluate the baffle arrangement.

 

[weldtek]

Icarter157,
For your new fabrication welds, keep in mind that all fully austenitic materials are prone to centerline cracking. With that in mind, you'll want your welders to make convex, rather than concave beads, fill craters etc.

 

[MikeMet]

I think there is little doubt that the cracking is due to fatigue. Could be a combination of mechanical from even small amounts of vibration and/or thermal fatigue. If the cracks start in the weld, more likely thermal fatigue especially if the welds are true fillet welds with no penetration.

I would make the replacement with full encirclement, full penetration welds of the baffles to the back side of the tubes sheet. You could then TIG wash the the toe of the fillet weld, which is the best method to eliminate the stress raiser at the toe of the fillet weld. You could also grind the toe to a smooth transition with the base metal. If you want high strength in the weld itself use, Inconel 617 instead of Inconel 82 which is the common filler metal used on Inconel 601.