THE RELATIONSHIP OF MICROSTRUCTURE TO STRENGTH AND TOUGHNESS IN HIGH-STRENGTH STEEL.

Abstract : This report deals with the relationship of elements of microstructure to the fracture characteristics of a group of commercially produced steels, heat-treated to ultra-high strength levels. The microstructures were characterized in terms of optical microscopy, electron diffraction, and electron fractography. Fractographic studies were made of fractured surfaces broken in impact at +25 and -196 C as a function of tempering temperature, in slow bending at ambient temperature, in a modified bend test in which slow and fast propagation stages were effected, and on surfaces produced in tensile fractures. The fracture characteristics were studied by means of an instrumented bend test and in terms of the plastic instability generated in the torsion test. The instrumented bend test yielded data on criteria for the onset of slow crack propagation and for the onset of rapid propagation, including the effect of the total stored energy in the system. Plastic instability in torsion was studied as a function of specimen section size in AISI 4340 steel. A 250,000 psi yield strength Maraging steel was studied in a standard size test bar. (Author)