Abstract
Superconducting magnets in fusion reactor system produce enormous electromagnetic force which generates very high stress working on the containers of these magnets at cryogenic temperature. The candidate alloys for the structural material of containers are non magnetic stainless steels with improved yield strength and toughness at cryogenic temperature. Containers are produced as welded structures, so that these properties on the structural material are also indispensable for the weld metal.
The chemical compositions of stainless steel weld metal are usually controlled to include a small quantity of δ-ferrite for the prevention of hot cracking in welds. The δ-ferrite is, however, ferromagnetic and causes the reduction in cryogenic toughness of weld metal. On the other hand, the weld metal of 19Cr-13Ni-5Mn type is known as fully austenitic one with very low susceptability on hot cracking in welds. The improvement of weld metal of this type resulted in the development of the fully austenitic weld metal of l7.5Cr-l6.5Ni-6.5Mn type with improvde cryogenic strengths. The effects of carbon and molybdenum contents on the tensile properties and Charpy impact properties were examined on this type of weld metal.
Conclusions are as follows. It is possible to assure the yield strength of the weld metal of 2.3% molybdenum content higher than 1000 MPa when carbon content is higher than 0.07% and carbon plus nitrogen content is higher than 0.13%. While the weld metal decreases in the elongation and reduction in area as the addition of carbon, the weld metal which contains 0.13% of carbon and 2.3% of molybdenum indicates about 40% of elongation and about 300% of reduction in area at 4.2 K. The Charpy absorved energy and lateral expansion of the weld metal at 4.2 K are equal to those at 77 K. The weld metal decreases in the absorved energy and lateral expansion as the addition of carbon content, but the weld metal which contains 0.13% of carbon and 2.3% of molybdenum indicates about 60J of absorved energy and 0.8 mm of lateral expansion at 4.2 K. The relation between yield strength and absorbed energy at 4.2 K on the weld metal coincides with the lower limit line of absorbed energy of the stainless steels SUS304LN and SUS316LN of high yield strength.
