The effect of the microstructure of 9Cr-1%Mo steel upon H diffusivity, solubility and permeability was investigated by using the electrochemical permeation technique. The steel was austenitized, and cooled at various cooling rates to produce differing microstructures. A fully martensitic product was obtained by fast cooling, and a mixture of proeutectoid ferrite and martensite was produced by slow cooling. Tempering the normalized steel resulted in the formation of fine intragranular precipitates at the boundaries. The H diffusivity and solubility exhibited regular trends as a function of the amount of strain in the lattice. The lattice defects and precipitates acted as trapping sites for H. An increase in lattice strain, due to an increase in defect density, sub-structure or coherent precipitation, resulted in a decrease in diffusivity due to an increase in trapping sites. The martensite structure offered the greatest resistance to H diffusivity, while tempered martensite offered the least resistance; due to the annihilation of defects during tempering.

Influence of Microstructure on the Hydrogen Permeability of 9%Cr-1%Mo Ferritic Steel. N.Parvathavarthini, S.Saroja, R.K.Dayal: Journal of Nuclear Materials, 1999, 264[1-2], 35-47