Computational Design and Analysis of High Strength Austenitic TRIP Steels for Blast Protection Applications

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Recent assessment of material property requirements for blast resistant applications, especially for the naval ship hulls, has defined the need to design steels with high stretch ductility and fragment penetration resistance, along with high strength and adequate toughness. Using a system based computational materials design approach, two series of austenitic (γ) steels have been designed - BA120 to exhibit high uniform ductility in tension (>20%) and SA120 to exhibit high tensile (>20%) and shear strains (>50%), with both alloys maintaining high levels of yield strength (120 ksi/827 MPa) at room temperature under Tensile and Shear stress states. BA120 is low chromium (4 wt %) high nickel (23.5 wt %) alloy while the SA120 is a high chromium design (10 wt %), both designed for non-magnetic behavior. The Thermo-Calc computational thermodynamics software in conjunction with a Ni-DATA 7 thermodynamic database has been used to model precipitation strengthening of the alloy, by quantifying the dependence of yield stress of austenitic steels on the mole fraction of the precipitated γ' (Gamma Prime) Ni3(Ti, Al) phase. The required high strength has been achieved by the precipitation of spheroidal intermetallic γ' - phase of optimum diameter (15 nm) in equilibrium with the matrix at the standard aging temperature. Adequate Al and Ti with respect to the Ni in the matrix ensure enough γ' phase fraction and number density of precipitates to provide the necessary strength. The predicted γ' precipitation strengthening to 120-130 ksi for both BA120 and SA120 has been validated through both microhardness as well as static and dynamic tensile and shear tests conducted at room temperature. 3-D LEAP analysis of the aged specimens has shown the expected size and distribution of γ' - precipitates with good compositional accuracy of predicted values from the thermodynamic models, for both matrix austenite and γ'. Metastable austenitic steels have been known to exhibit high uniform elongation, tensile strength under static and dynamic loads, and high fracture toughness due to mechanically induced martensitic transformation. The phenomenon of Transformation-Induced Plasticity (TRIP) arising from the FCC  BCC martensitic transformation has been used to create theoretical parametric models of matrix stability, flow stabilization and fragment resistance under tension and shear loads which were then applied to obtain significant improvements in uniform ductility for both stress states. These stability models have then been calibrated through experimental data from static and dynamic/adiabatic tensile tests and characteristic MSσ temperature measurements from an earlier TRIP prototype to support the new alloy designs. BA120 and SA120 alloys are designed to undergo stress-assisted martensite transformation at a pre-determined critical temperature (MSσ) thereby optimizing transformation plasticity to achieve the desired performance improvements. The new prototype alloy BA120 has demonstrated improved mechanical properties with a high strength of 124 ksi (845 MPa) and ~ 150 ksi (1040 MPa) under static and dynamic tensile loading at room temperature. The measured uniform ductility for BA120 under quasi-static tensile loading is 21% at room temperature with high strain hardening leading to UTS of 246 ksi (1696 MPa). The UTS under dynamic loading is ~ 195 ksi (1344 MPa). The uniform ductility is consistent (21% - 24%) over a wide range of temperature (25oC - 65oC). Mechanical testing demonstrates the required MSσ temperature, and 3-D LEAP microanalysis confirms the predicted matrix composition as well as the particle size and distribution of strengthening precipitates. FSI simulation experiments conducted on BA120 to analyze the material behavior under actual blast loading have shown promising results in terms of strains exceeding 40%. A prescribed simple heat treatment process comprising of Solutionizing treatment at 950oC for 1 hour followed by a single-step temper at 750oC for 10 hr has achieved the desired performance goals of strength and uniform ductility.

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  • 09/19/2018
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