Atom-Probe Tomographic Investigations of a Precipitation-Strengthened HSLA-115 Steel and a Ballistic-Resistant 10 wt. % Ni Steel for Naval Applications

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High performance structural materials are needed for Naval applications which require an excellent combination of yield strength, low-temperature impact toughness, ductility, ballistic-resistance, and weldability. This research investigates precipitation-strengthened HSLA-115 steels and ballistic-resistant 10 wt. % Ni steels, which have emerged as promising alternatives to the widely used HSLA-100 steels for Naval applications. HSLA-115 is a Cu-bearing high-strength low-carbon martensitic steel and has been used in the flight deck of the recently built U.S. Navy CVN-78 aircraft carrier. It is typically used in conditions with overaged Cu precipitates, to obtain acceptable impact toughness and ductility at 115 ksi (793 MPa) yield strength. However, overaging of Cu precipitates limits its strength and applications. This research demonstrates that aging at 550 oC facilitates the co-precipitation of sub-nanometer sized M2C carbides and Cu precipitates in high number density (~1023 m-3) in HSLA-115. 3-D atom-probe tomography (APT) investigation reveals that Cu precipitates form first, followed by the nucleation of M2C carbides, which are co-located with Cu precipitates and are distributed heterogeneously at lath-boundaries and dislocations, indicating heterogeneous nucleation of M2C. Carbon redistribution during quenching (following the austenitization) and subsequent aging at 550 oC is followed using APT. Segregation of C (3-6 at. % C) is observed at martensitic lath-boundaries in the as-quenched and 0.12 h aged conditions. On further aging, C redistributes, forming cementite and M2C carbides, whose composition and morphology evolves with aging time. Precipitation kinetics of M2C carbides is intertwined with Cu precipitates; temporal evolution of Cu precipitates and M2C carbides is characterized in terms of their mean radii, number densities, and volume fractions and correlated with the bulk mechanical properties. </DISS_para> <DISS_para>Precipitation of M2C carbides offsets the softening due to overaging of Cu precipitates and tempering of the martensitic matrix. This results in an extended yield strength plateau, compared to alloys relying solely on Cu precipitation strengthening (for example, NUCu-140 steels) and is highly beneficial as impact toughness improves significantly in overaged conditions with respect to Cu precipitates. Optimum mechanical properties (yield strength 141 ksi or 972.1 MPa, elongation to failure 24.8 %, and impact toughness 188.0 J at −18 oC) are attained after 3 h aging at 550 oC. Incorporating finely dispersed M2C carbides with Cu precipitates, thus provides a promising pathway for use of Cu-bearing Naval HSLA-115 steels in higher strength applications, while still meting toughness and ductility requirements. Low-carbon 10 wt. % Ni steels are optimally processed via a multi-step intercritical Quench Lamellarizing Tempering (QLT)-treatment to form a fine dispersion of thermally stable Ni-enriched austenite in a tempered martensitic matrix. Deformation-induced martensitic transformation of this austenite is key to its superior overall mechanical properties, specifically ballistic resistance over HSLA-100 steels. This research elucidates the basic physical principles controlling the thermal stability and kinetics of Ni-stabilized austenite, formed during the QLT-treatment. The role of Ni-enriched austenite and fresh martensitic regions, inherited from the first isothermal intercritical step (L) at 650 oC, in forming thermally stable austenite during the second isothermal intercritical step (T) at 590 oC is highlighted using dilatometry, synchrotron X-ray diffraction, 3-D atom-probe tomography (APT), and thermodynamic and kinetic modeling using ThermoCalc and Dictra. Results indicate the growth of nm-thick austenite layers during T-step tempering (predominantly in the Ni-enriched fresh martensitic regions), with austenite retained from L-step acting as a nucleation template. Thermal stability of austenite is estimated by predicting its martensite-start (Ms) temperature, using the approach formulated by Ghosh and Olson. This approach is particularly useful as empirical relations cannot be extrapolated for the highly Ni-enriched austenite investigated in the present study. Co-located and mixed MC/M2C-type carbides (M is Mo, Cr, V), comprising of a M2C carbide shell and a MC carbide core are observed after isothermal tempering at both 590 and 650 oC. Since MC carbides are inherited from the as-quenched condition, the nucleation of M2C-type carbides is likely assisted by the MC carbides during tempering at these temperatures. Local redistribution of C, Ni, and Mn in the single-pass heat-affected-zone (HAZ) microstructures of QLT-treated 10 wt. % Ni steels is investigated using site-specific 3-D APT to assess the thermal stability of austenite (Ni-rich regions) in HAZs. Ms temperature calculations predict that austenite in the HAZs is susceptible to martensitic transformation upon cooling to room temperature, unlike the austenite in the QLT-treated base metal. While C in the QLT-treated base metal is consumed primarily in MC/M2C-type carbides, its higher concentration in the Ni-rich regions observed in the HAZs indicates the dissolution of carbides, particularly the M2C carbides. The role of M2C carbides and austenite stability is discussed in relation to the increase in microhardness in single-pass HAZs relative to the QLT-treated base metal. A better understanding of austenite stability and C redistribution after single-pass weld cycles will assist in designing multiple-weld cycles and Gleeble-weld simulations for these novel 10 wt. % Ni steels.

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  • 10/09/2018
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