Yukiteru Asayama1
1Nagoya Aircraft Works, Mitsubishi Heavy Industries, Ltd., Nagoya
Precipitation processes were investigated with precipitation hardening stainless steels, 15-5 PH, PH13-8Mo and 17-7PH, by means of the measurement of electrical resistivity.
Metastable precipitation, P1, was found at about 690 K in all the steels in the measurement of electrical resistivity change during isochronal aging. P1 was detected more evidently during re-aging at low temperature (670∼750 K) after high temperature aging at 833 K and 873 K. The value of apparent activation energy for P1 was obtained as 200 kJ/mol.
From the precipitation behavior of the steel 15-5PH, the metastable precipitation, P1, was assumed to be due to the formation of Ni-rich zone.
Embrittlement characterized by the decrease in notch tensile strength occurred when the steels were aged at temperatures ranging roughly from 670 K to 750 K. The embrittlement was also caused by the re-aging at low temperatures after the high temperature aging. The decrease in hardness due to over-aging raised again the notch tensile strength.
From these experimental results it was suggested that the embrittlement is closely related to the precipitation of Ni-rich zone, and that the increase in hardness is necessary for the occurrence of the embrittlement.
(Received 1981/01/30)
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