Elimination of δ-ferrite in N50 steel and its effect on cryogenic mechanical properties

Abstract

δ ferrite serves as a detrimental phase in TF (Toroidal Field) superconducting magnet jacket pipe materials. The distribution features of ferrite in N50 casting and forging were investigated, and the cryogenic mechanical properties with different ferrite content were studied accordingly. The results indicated that the non-equilibrium crystallization leads to the solidification mode gradually changing from AF (Austenitic-ferritic) at the surface to the FA (Ferritic-austenitic) at the center of the casting. The skeleton δ ferrites induced by interdendritic segregation could disappear almostly after annealing while the tangled lath δ ferrite at the center of casting caused by the more serious segregation seems to be stable. Based on thermodynamics calculations and through changing the Cr and Ni content according to the Espy diagram, the stable ferrites can be eliminated when the Creq/Nieq is less than 1.25. From fractography, the cryogenic impact toughness of N50 steel would be deteriorated because more secondary cracks were taken place by the mismatched deformation between the harder residual δ ferrites and matrix. Though the elimination of δ ferrites caused a slight decrease in cryogenic tensile strength, it greatly improved the cryogenic impact toughness.