UNS S20100 stainless steel weld cracking

[grayseal]

SA-240, Type 201 14 gauge stainless steel blanks are rolled and welded (autogenous) on a Jetline seam welder. This becomes the shell to which heads (SA-240, Type 304) are autogenously welded to make autoclaves.
One shell is 9" in diameter with a 12" long weld seam. Another shell is 11" in diameter with a 14.5" weld seam. The entire perimeter of both shells is blanked on a die and washed prior to welding.
We are experiencing periodic weld cracking on the larger shell's long seam. The crack is just under 3/16" long, located at the centerline of the weld bead, approximately half way along the length. When a crack appears, it is always in the same location.
Both shell blanks come from the same heat of material. Both blanks are processed identically. The burr from the die is controlled and is the same for both blanks. Yet we only experience cracking on the larger blank. The only difference that can be seen is the sheared edge has a rougher appearance on the larger blank.
This appears to be some kind of contamination issue. But why does it only occur on the larger shell?
Any thoughts are appreciated.
grayseal

 

[unclesyd]

Usually when a weld cracks as in your description it is an indication that there is insufficient strength at that point.

This can be caused by the mechanics of the welding process are putting constraints on the actual weld area.

 

[stanweld]

The cracking locations is at a high point of stress and subjectto hot cracking. One way to mitigate the problem is to tack weld the seam at approximate 1" to 1.5" intervals.

 

[EdStainless]

Are the cracks longitudinal or transverse?
Sounds like a hot tear.
You might be able to tack weld at ends and center, then weld from center out.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[grayseal]

Thanks for the suggestions.
The crack is longitudinal at the centerline of the weld and I agree it is a hot tear.
The weld seam is held very tightly together in the Jetline seam welder. The copper hold down fingers are designed to push the material down on the water cooled back up bar as well as squeeze the seam together.
I'm still at a loss as why we only see this on the longer weld seam. We have welded more than 130,000 of these parts and only recently encountered the cracking problem.
Thanks again for your suggestions.
grayseal

 

[EdStainless]

Something has changed, you hold downs, cooling, weld speed or heat input are the most likely issues.
Clamping something tightly to weld is usually a disaster. Thermal expansion is a lot stronger.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[stanweld]

If you are not already using pulsed GTAW or pulsed plasma arc processes, try switching to pulsed. This will provide additional help to mitigate cracking.