X. J. Zhang, Carderock Division, Naval Surface Warfare Center, W. Bethesda, MD

Summary:

Four 500 lb vacuum induction melt heats of low carbon-10Ni-Mo-V steels with different combinations of C, Mo, and V contents were produced recently. Systematic mechanical testing and metallurgical characterization were carried out to investigate processing-structure-property relations of these steels using various analytical techniques. It was found that slight differences in chemical composition significantly alter their mechanical properties and the heat containing 0.1% C, 10%Ni, 1.20% Mo, and 0.08% V exhibited an optimum combination of strength and impact toughness under proper heat treatments: 160 ksi yield strength at room temperature and 80 ft-lb Charpy impact energy at -94 ºF. Strengthening mechanisms and toughening mechanisms were also studied. The evolution of martensite lath morphology with an increasing tempering time, revealed by improved metallographic and SEM techniques, indicated that the first substantial drop in strength is mainly associated with the dulling of the laths, as illustrated in Fig. 1. On the other hand, primary precipitates played an important role in controlling the impact toughness. Comparisons in microstructure and mechanical properties between this low carbon-10Ni-Mo-V steel class and other ultra-high strength high toughness steels are also discussed.