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

The influence of microstructure on the hydrogen permeability, diffusivity and solubility in 2.25% Cr–1% Mo ferritic steel was investigated using electrochemical 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 variety of carbides of different morphologies and sizes was also characterised. The hydrogen diffusivity showed a continuous increase as the structure changed from martensite to ferrite and also with increasing extent of tempering. Solubility showed a corresponding decrease. The trends have been understood in terms of the number of reversible traps available for hydrogen in these different structures. Accordingly, martensite structure offered the maximum resistance to hydrogen diffusivity and tempered structure the least resistance due to the annihilation of defects during tempering and reduction in the solute content of the matrix due to precipitation.