Influence of microstructure on the hydrogen permeability of 9%Cr-1%Mo ferritic steel

The influence of microstructure of 9%Cr-1%Mo steel on the hydrogen diffusiv ity, solubility and hence the permeability was investigated using electroch emical permeation technique. This steel was austenitised and cooled at vari ous cooling rates to produce different microstructures. Tempering behaviour was also studied by heat treating for different durations at 1023 K. Chara cterisation of microstructures was carried out using scanning electron micr oscopy and analytical transmission electron microscopy. A fully martensitic product was obtained during fast cooling and a mixture of proeutectoid fer rite and martensite during slow cooling. Tempering the normalised steel res ulted in the formation of fine intragranular M2X precipitates and M23C6 On the boundaries. The hydrogen diffusivity and solubility showed a regular tr end with the amount of strain in the lattice. Lattice defects and precipita tes act as trap sites for hydrogen. Increase in lattice strain either due t o increase in defect density, substructure or coherent precipitates resulte d in decrease in diffusivity due to increase in trap sites. Martensite stru cture offered the maximum resistance to hydrogen diffusivity and tempered m artensite the least resistance due to the annihilation of defects during te mpering. (C) 1999 Published by Elsevier Science B.V. All rights reserved.