Brittle fracture of T91 steel in liquid lead–bismuth eutectic alloy

• Tempering temperature is important for LBE embrittlement occurrence.
• Brittle behaviour in LBE evidenced by small punch test and fatigue test.
• Brittle behaviour in low oxygen LBE observed for low loading rate.

The mechanical behaviour of the T91 martensitic steel has been studied in liquid lead–bismuth eutectic (LBE) and in inert atmosphere. Several conditions were considered to point out the most sensitive embrittling factors. Smooth and notched specimens were employed for respectively monotonic and cyclic loadings.The present investigation showed that T91 appeared in general as a ductile material, and became brittle in the considered conditions only if at least tests were performed in LBE. It turns out that the loading rate appeared as a critical parameter for the occurrence of liquid metal embrittlement of T91 in LBE. For the standard heat treatment condition, loading monotonically the T91 very slowly instead of rapidly in LBE resulted in brittle fracture. Also, under cyclic loading, the crack propagated in a brittle manner in LBE.