Hydrogen transport in 34CrMo4 martensitic steel: influence of microstructural defects on H diffusion

The electrochemical permeation technique was used to evaluate the effect of the microstructure on hydrogen diffusivity and hydrogen trapping at room temperature in martensitic steels. A detailed study of the electrochemical permeation technique was first performed in order to identify the boundary conditions of a permeation test in the selected experimental set-up. The validity of the apparent diffusion coefficient derived from this test is also discussed. A 34CrMo4 quenched steel has been selected and designed at three tempering temperatures (200 degrees C, 540 degrees C and 680 degrees C) in order to obtain three different microstructures. According to permeation measurements, H diffusion strongly depends on the microstructure. The material tempered at 540 degrees C exhibits the smallest diffusion coefficient and the largest fraction of reversible traps at room temperature. (authors)