Embrittlement of the martensitic steel 91 tested in liquid lead

Two potential problems are encountered in the case of intimate contact between liquid metals and metallic substrates: grain boundary wetting and liquid metal embrittlement (LME) which both induce a degradation of the mechanical properties. Tensile tests were carried out on a 9% Cr 1% Mo martensitic (Grade 91) steel in a liquid lead environment at temperatures ranging between 623 and 773 K. The Grade 91 steel was submitted to heat treatments in order to modify its hardness and also to produce either ferritic or martensitic grains. Smooth and notched specimens were used. We found out that by combining adapted heat treatments and the notch effect, it is possible to create conditions severe enough that lead to LME. Our experimental observations (transgranular failure) are compatible with the expectations of traditional mechanisms based on a reduction of the surface energy and/or adsorption induced chemical bond softening at the steel surface in contact with liquid lead.