Some phenomena concerning the hydrogen absorbed in a high speed steel are shown as follows: (1) Hydrogen % in the high speed steel ingot is chiefly dependent on the humidity in the air. So the mean rolling yield in a certain month is nearly controlled by the mean humidity in the same month. (2) Hydrogen % in the 150kg high speed steel ingot ranges from about 0.0002 to 0.0001. As compared to the hydrogen in a plain C steel ingot (0.0006-0.0007%), hydrogen in the high speed steel ingot is much less. (3) The special aging phenomenon due to the hydrogen diffusion in a steel at room temperature is observed in a high speed steel. Specimens held at room temperature for a month show the higher impact values than the fresh ones.
To verify quantitatively these phenomena due to the hydrogen diffusion, the diffusion experiments are tried at various temperatures, using the Fick's diffusion equation. Diffusion constant can be estimated from the following formula:
_??_
where
a,
b and
c stand respectively for length, width and height of a block specimen and
q1,
q2 for respectively the evolved hydrogen per unit time at the time of
t1 and
t2. From the results at 800°, it is cleared that the diffusion constants of hydrogen lower gradually in the following order: high C tool steel (No. 4 in Table 3), JES H. S. steel 2nd class (No. 1), JES H. S. steel 3rd class (No. 2) and Mo H. S. steel (No. 3). At room temperature, however, the diffusion constants lower gradually in the following order: plain C steel (No. 5 in Table 3), Mo high speed steel (No. 3), JES H. S. steel 2nd class (No. 1) and JES H. S. steel 3rd class (No. 2). Here it is noted that the total amount of absorbed hydrogen in Mo high speed steel is evidently largest.
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