Welding consumable selection for cryogenic (4 degrees K) application

Brookhaven National Laboratory (BNL) has begun construction of a large (3.8 kilometer circumference) heavy ion collider for the Department of Energy. The collider uses superconducting magnets, operating at 4 degrees K in supercritical helium, which meets the definition of a pressure vessel. The ASME Boiler ampersand Pressure Vessel Code grants an exemption from impact testing to certain metals, but only for operating temperatures down to 20 degrees K. Research and the latest change to ASTM Standard E23 have invalidated Charpy testing at 4 degrees K, thus compliance with the Code is not possible. This effort was undertaken to identify the weld process and weld material necessary to comply with the intent of the Code (impact test) requirements, that is, to design a weld joint that will assure adequate fracture toughness. We will report the results of this development and testing, and conclude that nitrogen and maganese enhanced 385L provides a superior weld metal for 4 degrees K cryogenic applications without the exaggerated purity concerns normally associated with superaustenitic weld materials. This development has been so successful that BNL has procured 15,000 pounds of this material for magnet production. Oxygen content, manifested as inclusion density, has the single most significant effect upon fracture toughness and impact strength. Finally, we report that GMAW is a viable welding process, using off-the-shelf equipment, for 4 degrees K cryogenic applications