Studies on hydrogen permeability of 2.25% Cr-1% Mo ferritic steel: correlation with microstructure

The influence of microstructure on the hydrogen permeability, diffusivity a nd solubility in 2.25% Cr-1% Mo ferritic steel was investigated using elect rochemical permeation technique. Varieties of microstructures ranging from martensite in water-quenched (WQ) steel to a predominant ferrite structure in annealed steel were characterised using analytical transmission electron microscopy. In the tempered structures, continuous precipitation of a vari ety of carbides of different morphologies and sizes was also characterised. The hydrogen diffusivity showed a continuous increase as the structure cha nged from martensite to ferrite and also with increasing extent of temperin g. Solubility showed a corresponding decrease. The trends have been underst ood in terms of the number of reversible traps available for hydrogen in th ese different structures. Accordingly, martensite structure offered the max imum resistance to hydrogen diffusivity and tempered structure the least re sistance due to the annihilation of defects during tempering and reduction in the solute content of the matrix due to precipitation. (C) 2001 Elsevier Science B.V. All rights reserved.