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THE KINETICS AND MECHANISMS OF THE STRENGTHENING OF MARAGING STEELS.
UNITED STATES STEEL CORP MONROEVILLE PA
The fine-scale microstructures of the 18Ni250 and 18Ni300 grades of maraging steel have been examined using electron microscopy of thin foils and extraction replicas in conjunction with electron-diffraction and electron-probe analyses of particles extracted from these steels. The microstructure of both steels in the annealed condition consists of small, equiaxed martensite grains containing a high dislocation density but few microtwins. The main source of the strengthening that occurs in these steels during aging is the precipitation of extremely fine particles of intermetallic compounds. The major precipitate, Ni3Mo, appears to nucleate on dislocations, and during the early stages of aging it occurs in the form of ribbons or rods. A second precipitate, which nucleates homogeneously, appears as small spheres in the early stage of aging. The identity of this precipitate is not certain, but indirect evidence suggests that it is Ni3Ti. Upon continued aging, both types of precipitates assume the form of platelets or discs. The softening that occurs in these steels upon overaging is believed to result from the rapid growth of reverted austenite plates and the resolution of precipitate particles in the vicinity of the austenite plates. Author
Summary rept. for 1 Apr 63-30 Sep 64,
AF33 657 11149