Production and property evaluation of single crystal austenitic stainless steels

Austenitic stainless steels are currently known to have irradiation-assisted stress corrosion cracking (IASCC) susceptibility when exposed to critical neutron irradiation in pure water environment at high temperatures. Since the radiation induced segregation (RIS) at grain boundaries (GBs) has been considered as a predominant factor for IASCC, the use of single crystals may be an effective way to prevent RIS. In this paper, relations between the single crystal growth and chemical compositions were studied. The single crystal austenitic stainless steels were manufactured by a mold withdrawal unidirectional solidification method which can provide large size single crystals. The single crystal growth was observed to depend on equivalent ratio (Cr-eq/Ni-eq). The single crystals with equivalent ratio (Cr-eq/Ni-eq) below 1.83 were manufactured. Steels with equivalent ratio (Creq/Nieq) over 1.87 were not suitable for manufacturing single crystals. The interdendritic delta phases were observed in the as-cast state of the single crystals. Furthermore, several properties of single crystals were evaluated. The resistance against SCC were examined by the creviced bent beam test (CBB test) in pure water at high temperature. No crack was observed in the CBB test specimens of the single crystal, even at the gamma/delta boundaries, whereas polycrystals were slightly cracked. However, 0.2% proof stresses of these single crystals were lower than those of polycrystalline austenitic stainless steels because grain boundary strength can not be expected in a single crystal.