Effect of heat treatment on liquid sodium embrittlement of T91 martensitic steel

The sensitivity to liquid sodium embrittlement, in the temperature range of 300–550 °C, of T91 martensitic steel has been studied according to tempering temperature. In air, the T91 steel is ductile. A ductile to brittle transition induced by liquid sodium has been observed. Liquid sodium acts as well on the crack initiation period as on the crack propagation one. In absence of external defect, it is found that sodium affects grain boundary strength and initiates an intergranular crack which then propagates by cleavage in the bulk.