Prototype evaluation of transformation toughened blast resistant naval hull steels: Part II

Application of a systems approach to computational materials design led to the theoretical design of a transformation toughened ultratough high-strength plate steel for blast-resistant naval hull applications. A first prototype alloy has achieved property goals motivated by projected naval hull applications requiring extreme fracture toughness (Cv > 85 ft-lbs or 115 J corresponding to KId ≥ 200 ksi.in1/2 or 220MPa.m1/2) at strength levels of 150–180 ksi (1,030–1,240MPa) yield strength in weldable, formable plate steels. A continuous casting process was simulated by slab casting the prototype alloy as a 1.75′′ (4.45 cm) plate. Consistent with predictions, compositional banding in the plate was limited to an amplitude of 6–7.5 wt% Ni and 3.5–5 wt% Cu. Examination of the oxide scale showed no evidence of hot shortness in the alloy during hot working. Isothermal transformation kinetics measurements demonstrated achievement of 50% bainite in 4min at 360 ◦C. Hardness and tensile tests confirmed predicted precipitation strengthening behavior in quench and tempered material. Multi-step tempering conditions were employed to achieve the optimal austenite stability resulting in significant increase of impact toughness to 130 ft-lb (176 J) at a strength level of 160 ksi (1,100MPa). Comparison with the baseline toughness–strength combination determined by isochronal tempering studies indicates a transformation toughening increment of 65% in Charpy energy. Predicted Cu particle number densities and the heterogeneous nucleation of optimal stability high Ni 5 nm austenite on nanometer-scale copper precipitates in the multi-step tempered samples was confirmed using three-dimensional atom probe microanalysis. A. Saha (B) · J. Jung · G. B. Olson Department of Materials Science and Engineering, Robert R. McCormick School of Engineering and Applied Science, Northwestern University, 2220 Campus Drive, Evanston IL 60208, USA e-mail: [email protected] A. Saha Intel Corporation, 2501 NW 229th Avenue, RA3-355 Hillsboro, OR 97124, USA J. Jung QuesTek Innovations Inc., 1820 Ridge Avenue, Evanston, IL 60201, USA