Abstract
9%Ni (as per ASME SA553 type I) is the reference material for the fabrication of large LNG flat bottom above ground storage tanks servicing at −163°C/−260F and industry has then a long experience with that alloy. Considering the case of pressurized applications, the situation is largely different as only few examples of using 9%Ni can be found. AISI 304L austenitic stainless steel is in that case the material of choice, despite its low strength.
On one hand, usual applications in LNG business as well as new low temperature applications linked to the Global Energy Transition could benefit of high performances coming with that alloy. Its high strength and allowable stresses — in comparison to ones on 304L — permit to significantly reduce the wall thickness of pressure vessels while keeping an excellent level of toughness and hence of safety.
But on the other hand, experience with thick sections is very scarce and pressurized equipments fabrication require significantly thicker products than ones encountered in low pressure above ground flat bottom storage tanks. Consequently, demonstration of the industrial feasibility of such equipments needed to be done.
The present paper aims at summarizing recent developments made to produce ultra-thick plates (up to 100mm/4inches thick) and properties achieved on such thick products. Base materials as well as welded joints have been extensively studied, in various thermal states able to replicate all industrial configurations required by customers involved in the fabrication of large cryogenic pressurized vessels. Materials properties after PWHT were measured, as some pressure vessel construction codes may require such treatments after welding or forming operations, depending on design conditions and on considered product thickness. Finally, heterogenous welds between 9%Ni and 304L austenitic stainless steel have been tested with the aim of enabling the use of conventional pressure vessel sub-components (such as nozzles, piping, flanges) with 9%Ni pressure vessels.
Performances down to −196°C/−320F have been addressed by the various tests made during the study. All results achieved are satisfactory and it is demonstrated to be safe to use 9%Ni for the fabrication of large pressure vessels.
