Natsuo Yukawa1, Masayoshi Mizutani2 and Hiroyasu Saka1
1Department of Metallurgy, Faculty of Engineering, Nagoya University, Nagoya
In the previous papers, it has been reported that the age-hardening of 17-7 PH steel is attributable to the precipitation of α ' of the CsCl type. In the present paper, the morphology of α ' precipitation in the specimens used in the previous papers was studied by means of transmission electron microscopy, for purpose of confirming the age-hardening mechanism mentioned above. The technique of extraction replica was also applied to the problem.
The results obtained were as follows: (1) On aging at 400°C to 600°C after martensite formation, spherical α ' precipitates from αM as well as from δ with complete coherency with the parent matrix. (2) At the αM-δ interfaces, there also takes place a discontinuous precipitation of feather-like α ', which appears to grow into δ grains from interfaces. (3) The spherical α ' in the grains grows with aging time, and the peak hardness is obtained within an aging stage in which the diameter of α ' particles is less than 100 Å, regardless of the aging temperature ordinarily used. (4) On aging at about 600°C and above, a reverse transformation of austenite from martensite occurs, which results in rapid softening of materials concurrently with coagulation of α '.
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