The effect of quenched and tempered microstructures on the diffusion coefficient and solubility of hydrogen in the commercial alloy tool steels have been investigated at room temperature by means of the electrochemical permeation technique. The results obtained are as follows; (1) The as-quenched structure of martensite plus undissolved carbides gives a low diffusion coefficient, and the diffusion coefficient does not show the remarkable change with tempering temperature up to 300℃, after which in the case of SKS 3 steel, it increases with increasing temperature. However, in the case of SKD 4 and SKD 11 steels, the diffusion coefficient decreases with increasing temperature, reaching a minimum around 600℃ and 500℃ respectively and these decreases in diffusivity correspond to the "secondary hardening" in the tempering curve. (2) The solubility of hydrogen is high for the structure of quenched martensite plus undissolved carbides and there is an opposite relation between the diffusion coefficient and solubility, when the steels are tempered. (3) The variations in diffusivity and solubility can be explained in terms of hydrogen trapping process involving the lattice imperfections such as dislocations, faults, lattice vacancies and subgrain boundaries etc., introduced by martensitic transformation and or precipitation of carbides on tempering, togethet with the interfaces such as quenched martensite/undissolved carbides and the ferrite/precipitated carbides, depending upon tempering temperature.
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