The effects of quenched and tempered microstructures on the diffusion coefficient and solubility of hydrogen in high tensile strength bolt steel have been investigated by means of the electrochemical permeation technique. The results obtained are as follows : (1) The diffusion coefficient was minimum when the steel had the as-quenched martensitic structure or the low temperature tempered martensite, and it increased with increasing tempering temperature. The mixed structure of ferrite and lamellar pearlite obtained by furnace cooling from a temperature above Ac_3 gave a higher diffusion coefficient than the tempered spheroidal pearlite. On the other hand, the solubility of hydrogen showed a behavior opposite to the diffusion coefficient. (2) The low diffusion coefficient and high solubility are attributed predominantly to hydrogen trapping at the lattice imperfections, such as dislocations and faults etc., which were introduced by martensitic transformation during quenching. (3) With regard to the effect of cathodic current density, i. e., the hydrogen concentration directly below the surface on hydrogen diffusion in tempered steel, the diffusion coefficient hardly depended upon the hydrogen concentration but the solubility of hydrogen increased up to a certain value with increasing cathodic current density. On the other hand, the hydrogen permeation current efficiency decreased with increasing cathodic current density. (4) The activation energies for hydrogen diffusion in the tempered martensite, tempered fine and/or spheroidal pearlites in the temperature range of 6 to 61℃, were almost constant (about 8. 1 kcal/mol) but the pre-exponential factor increased with tempering temperature. The heats of solution of hydrogen in the tempered martensite and tempered pearlites were almost the same (about - 10. 3 kcal/mol) , indicating the exothermic reaction. However, the pre-exponential factor decreased with increaing tempering temperature.
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