High-Strength High-Toughness 10%Ni Naval Steels

Research problem and objectives: We are proposing to use a fundamental physical metallurgy scientific approach to elucidate the complex transformations and microstructures occurring in 10 wt.% Ni steels, which happen during the quenching, lamellarization and tempering (QLT) heat treatments and welding processes. Specifically, austenite reversion, and martensite formation and the tempering of martensite, and carbide precipitates will be studied with a focus on how these processes affect the mechanical properties: tensile and ultimate tensile strengths, plasticity and fracture toughness values as a function of temperature. Homo- and heterophase interfaces between each of the phases or precipitates present, including martensite lath boundaries and prior austenite grain boundaries, and interfacial segregation at these boundaries, will be a focus because they are key factors governing phase transformations and the resulting mechanical properties. We will perform studies of how the microstructures are affected by welding, specifically in the fusion zone, which also will enlighten the potential of this family of steels for additive manufacturing (AM). We will correlate the microstructural and atomistic results obtained from experiments, calculations and simulations with the mechanical properties (yield strength, ultimate tensile strength, plasticity, toughness, and high-strain-rate deformation) of these steels, and also focus on the interrelationships between microstructure and performance in high-strain rate deformation. Technical approaches: We will utilize correlated atom-probe tomography (APT) and transmission electron microscopy (TEM), optical microscopy, scanning electron microscopy (SEM), electron back-scatter diffraction (EBSD), focused ion-beam (FIB) microscopy, synchrotron x-ray diffraction to capture the details of the microstructure and phase dispersions on all relevant length scales in QLT treated specimens as well as welds and weld simulated 10 wt.% Ni steels, and correlate these results with mechanical testing (microhardness, tensile and Charpy V-notch testing, high-strain rate deformation). Computational thermodynamics (ThermoCalc) and first-principles calculations will be performed to model the atomistic energetics underlying phase transformations and interfacial segregation. The investigations will be performed in close collaboration with Dr. Paul Lambert and Mr. Matthew Sinfield, Naval Surface Warfare Center Carderock Division (NSWCCD), Dr. Fred Fletcher, ArcelorMittal Global R&D, Prof. John DuPont (Lehigh University), Prof. K. Sharvan Kumar (Brown University, high strain-rate deformation) and Prof. Todd Hufnagel (Johns Hopkins University). Anticipated outcome of the research, if successful: The results of the detailed micro-structural characterization of samples after the relevant heat treatments will be compared and correlated with their mechanical properties, primarily microhardness, tensile testing and Charpy V-notch toughness for selected samples, to identify the physical metallurgical principles underlying the optimized microstructure and properties. The results will provide a guide as to how the beneficial properties can be achieved and preserved during fabrication and welding processes. Impact on DoD capabilities: The detailed fundamental physical metallurgical understanding of microstructure and mechanical properties in the 10 wt.% Ni steels and how they are affected by heat treatment and welding will describe the optimized use and fabrication processes of these steels in high-performance Naval applicatio