Diffusion on Hydrogen in Quenched and Tempered 21/4 Cr-1 Mo Steel

An investigation of hydrogen diffusion in 21/4 Cr-1Mo steel subjected to quenching and tempering treatments has been carried out at room temperature in an attempt to establish the effect of microstructures developed by the heat treatment on the diffusivity and solubility of hydrogen by means of the electrochemical permeation technique. The results obtained are as follows: (1) The diffusion coefficient in the mixed structure of martensite and ferrite phases obtained by quenching from a low austenizing temperature is higher than that in the as-quenched martensitic structure obtained from a higher austenizing temperature. On the other hand, the solubility of hydrogen shows a behavior opposite to the diffusion coefficient. When tempered at 550℃, the diffusion coefficient to the precipitation of fine Mo and Cr carbides at the interface of the ferrite phase, thus increasing the interfacial areas. The interface acts predominantly as the hydrogen trapping site, and hence an increase in solubility and a corresponding decrease in diffusion coefficient occur. (2) With regard to the effect of tempering temperature on hydrogen diffusion, the as-quenched martensitic structure has a low value of diffusion coefficient and has the maximum at the tempering of about 350℃. The value of tempered structure decreases to a maximum at near 550℃ and increases again at higher temperatures. The solubility of hydrogen is high for the as-quenched martensitic structure and decreases with increasing tempering temperature to a minimum at 350℃ after which it increases until it reaches a maximum around at 550℃ and decreases again at higher temperatures. (3) The diffusion coefficient and solubility of hydrogen do not change even though the tempering time is increased up to 100hrs at 350℃. However, in the specimen tempered at 550℃, the diffusion coefficient decreases with increasing tempering time and the solubility of hydrogen increases with it. (4) In the temperature range of 7 to 60℃, the diffusion coefficient and solubility of hydrogen in martensite tempered at 350℃ can be described as follows: D=5.86×10^exp[(-5530±310)/RT] (cm^2/S) C=7.27×10^exp[(-16670±980)/RT] (mol/cm^3) The hydrogen absorption occurs by the exothermic reaction.X線材料強度小特