Strength and ductility of bainitic steels

Donald H. Desy, J. O. Brittain, M. Gensamer
naca-tn-3989
Aug 1957


Some of the factors believed to affect the strength and ductile-to-brittle transition temperature of bainitic steels, including mean ferrite path and degree of internal strain, have been studied. Strength was measured by diamond pyramid hardness, and transition temperature was obtained from a tensile impact test on small notched specimens. Seven laboratory heats of carbon-molybdenum steel and two heats of carbon steel were transformed isothermally to bainite at various temperatures and tested. Mean-ferrite-path measurements were made on electron micrographs of three of these steels. The mean ferrite path was found to have only a slight effect on the strength of bainite and no effect on the transition temperature. Preliminary measurements of X-ray line broadening indicate that the degree of internal strain may be the controlling factor in determining the strength of bainite. The transition temperatures of the bainites fell within a band between -80 and -160 degrees C and did not vary regularly with hardness, carbon or alloy content, or mean ferrite path. In the SAE 1062 steel at high strength levels, bainite has a transition temperature lower than that of tempered martensite at the same strength level.

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