Characterization of eutectic borides formed during solidification of borated stainless steel 304B4

Abstract

AISI 304B4 stainless steel (SS) containing 1.3 wt.% boron was cracking extensively during fabrication of neutron shields for Prototype Fast Breeder Reactor (PFBR). However, earlier study on 304B4 steel showed good resistance to weld solidification cracking due to eutectic backfilling in the cracks during welding. In order to understand the backfilling mechanism in this steel, microstructural characterization was performed on 304B4 SS in solution-annealed and re-melted and solidified conditions. In solution-annealed condition, microstructure of 304B4 SS consists of austenite matrix with random distribution of blocky Cr2B-type borides. On contrary, as-solidified microstructure reveals the primary austenite dendrites and interdendritic eutectic of (Fe,Cr)2B-type borides of varying morphologies. In addition to (Fe,Cr)2B, spheroidal non-equilibrium Fe23(CB)6-type borides were also found inside the austenite dendrites. The eutectic liquid rich in boron and Fe, which has low solidus temperature, contributes significantly to backfilling of hot cracks formed during welding. This was confirmed through characterization of eutectic borides in the as-solidified structure which shows Fe-rich (Fe,Cr)2B and Fe23(CB)6 types of borides. Finally, reasons behind severe liquation cracking of 304B4 SS observed during fabrication of neutron shield has been explained based on Scheil’s solidification simulations.