Abstract
Due to the constant increase of claims for all materials used in superconducting magnets in "magnetic fusion reactors", the article deals with the possibilities of increasing the mechanical properties of austenitic stainless steel tested at cryogenic conditions that ensure the transport of Helium to magnets. The aim of the experimental plan was to increase the mechanical properties of the steel grade 316LN tested at 4.2K from the original value Steel A: YS = 1045 MPa, UTS = 1528 MPa, A = 33% to the value of YS = 1204 MPa,UTS = 1642 MPa, A = 34% and Steel B: YS = 1173 MPa, UTS = 1541 MPa, A = 28% to the value of YS = 1351 MPa, UTS = 1645 MPa, A = 17%. The increase in mechanical properties of the steel grade under examination has been made by means of heat processing in the conditions of annealing: Th1 = 625 ° C / th1 = 696 h. The mechanical properties of steel were evaluated using static tension tests at 4,2 K. The samples were placed in a cryostat filled with liquid helium. Except for the mechanical properties, there were also evaluated structural changes depending on the conditions of heat processing by light optical microscopy and EBSD (Electron Backscatter Diffraction). The increase of steel properties used in low temperatures was achieved by heat processing.
Highlights
For large-scale superconducting magnets the so-called cable in conduit conductor (CICC) allows a structural reinforcement of the bare superconducting NbTi or Nb3Sn strands by austenitic stainless steel pipes [1]
Due to the constant increase of claims for all materials used in superconducting magnets in „magnetic fusion reactors", the article deals with the possibilities of increasing the mechanical properties of austenitic stainless steel tested at cryogenic conditions that ensure the transport of Helium to magnets
The measurements indicate that the value of yield and ultimate tensile strength measured at 4.2 K approximately doubled as compared to similar materials tested at RT and vice versa, total elongation demonstrated the opposite trend
Summary
For large-scale superconducting magnets the so-called cable in conduit conductor (CICC) allows a structural reinforcement of the bare superconducting NbTi or Nb3Sn strands by austenitic stainless steel pipes [1]. Nitrogen addition resulted in the improvement of mechanical properties, related to short range order SRO, retardation of dynamic strain aging DSA, microstructure change from cellular dislocation to planar dislocation structure, decrease of grain size and carbide precipitation at grain boundaries. Importance of these factors is dependent on stress as well as temperature which was applied during the testing period [2]. This study deals with the comparison of results from tensile tests as well as with the evaluation of mechanical properties and material properties in the cryogenic temperatures [5]
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