Hydrogen environment embrittlement of an ODS RAF steel – Role of irreversible hydrogen trap sites

The microstructure and the effects of 10 MPa hydrogen atmosphere on the tensile properties of a oxide dispersion strengthened (ODS) reduced activation ferritic (RAF) steel were investigated. The microstructure consists of a fine grained ferritic matrix with Me3O4 (Me = Cr, Fe or Mn), VN and Cr23C6 grain boundary precipitates as well as dispersed yttrium oxide nano precipitates in the ferritic matrix. The yield and ultimate tensile strength were unaffected by the H2 atmosphere whereas elongation at fracture and reduction in area were markedly reduced. In H2 atmosphere, the fracture morphology was found to be a mixture of intergranular H-assisted fracture and a smaller amount of transgranular hydrogen enhanced localized plasticity (HELP) fracture. The sensitivity of the ODS RAF steel to hydrogen embrittlement is attributed to the large number grain boundary precipitates which enhance the tendency for intergranular fracture.