Welding AISI 430

Hello,
We have a pot of AISI430 which contains nitric melted salt at temperature of about 260 °C. the shape of the section is a "reverse omega", something like this: __ __ |__|
Dimension are: 200mm of "lips" 150mm height, 150mm width, 17.000mm length, thickness 4 mm.
It's realized with 6 sections 3m approx. long, welded together by TIG with AISI430.
The pot is linked on position on a rigid structure and free to extend (on the length) but can't twist.
It is contained in a "cover" and insulated but the lips are cooler than the bottom of the pot, 260°C the bottom, approx. 100°C the external sides of the lips, so they have a strong traction stress and a welding joint today has broken.
Looking the fracture I can see quite big "crystals" in the steel, approx 2 mm, so my question is: could it be due to a project mistake only and it would happened anyway, or the quality of the welding is not correct?
Thanks,
Stefano

Hi Stefano, I am not sure I fully understood your description. You do not specify if the failure is in the weld or in the heat affected zone. Grain coarsening in ferritic stainless is a common consequence of welding. However it seems to me that the as welded structure was too brittle if it was not stress relieve annealed after welding at 790 - 840 C. Good luck.
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