Strengthening of SS316LN for Development of Superconductor Jacket

Abstract

In recent years SS316LN stainless steel is preferred for use as a jacket material for Nb3Sn strands/wires. This superconductor is used in ITER (International Thermonuclear Experimental Reactor), a current driven plasma experiment that sets a milestone towards the demonstration of fusion reaction as a source of energy for future. In Indian scenarios, IPR (Institute for Plasma Research) is currently attempting to fabricate fusion grade Nb3Sn strands and cables for such purposes. So far, the Jacket material development and characterisation for Nb3Sn CICC (Cable-In-Conduit-Conductor) has been attempted at IMMT, Bhubaneswar. In my project work micro-alloyed SS316LN is prepared in a vacuum induction melting furnace; Niobium and molybdenum in their ferroalloy stage are considered as alloying element. These micro-alloyed steels are then subjected to cold rolling and heat treatment to minimize stress and defects in cast ingots. These micro-alloyed steels are expected to improved mechanical properties, compared to pure SS316LN, at liquid Helium temperature (i.e. 4K).
The various properties of the processed steel are examined using XRD, SEM and optical Microscopy. Chemical test such as OES is carried out to know the chemical composition of the micro-alloyed samples and for comparison with pure SS316LN. Similarly mechanical tests such as tensile and hardness tests are done to evaluate UTS, YS and Percentage elongation etc. for both as cast and heat treated samples. From this study it is found that, the Nb/Mo micro-alloyed steels are more suitable for used as jacket material for Nb3Sn superconductor strands to be used at cryogenic temperature.
Keywords: strengthening; micro-alloying; jacket material; CICC superconductor.