Enhancing Mechanical Properties of Hot Wrought Steel by Microalloying and Optimizing Heat Treatments
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JMEPEG https://doi.org/10.1007/s11665-020-05005-3
Enhancing Mechanical Properties of Hot Wrought Steel by Microalloying and Optimizing Heat Treatments Cody Dyar, Shane Brauer, William Williams, Haley Doude, Wilburn Whittington, Andrew Oppedal, Haitham El Kadiri, Mark Tschopp, and Hongjoo Rhee (Submitted May 6, 2020; in revised form July 3, 2020) Novel methods to increase performance of steels without increasing production costs are always sought after. In this study, a chemistry–process–structure–property paradigm was employed to modify rolled homogeneous armor (RHA) steels that contain no nickel (Ni) or chromium (Cr) and instead, microalloying with niobium (Nb). Characterization of 0.02 and 0.05 wt.% Nb additions was targeted in tandem with optimized heat treatments to improve performance without detrimentally increasing carbon equivalence. Designed alloys were cast in a vacuum induction melting furnace and thermo-mechanically processed. Optimal heat treatment conditions were determined with computational simulation software, JMatPro, for materials properties, which resulted in an exploration of three tempering temperatures, 400, 500, and 600 C. Microstructures were investigated by optical and electron microscopy, where Nb additions were determined to reduce average grain area of ferrite, bainite, and martensite by as much as 75, 59, and 58%, respectively. Hardenability was characterized by Jominy end-quench testing and showed a slight increase in average hardness for Nb-bearing alloys due to the precipitation of NbC. Compression and tension tests revealed minimal strain rate sensitivity and a nearly 30% increase in strength in comparison with reference RHA materials. Impact tests showed a moderate increase in energy absorption of up 61%. Fractography of the failed specimens highlighted MnS precipitates as well as NbC and Mo2C carbides. Overall, results showed small ( £ 0.05 wt.%) additions of Nb accompanied by an optimized heat treatment resulted in a modified, less-expensive RHA with similar performance to typical Ni/Cr-RHA. Keywords
heat treatment, microalloying, rolled homogeneous armor, through-thickness hardenability
1. Introduction Steel alloy design has been evolving for many decades and is still very much relevant today with numerous advances that have improved our understanding of the relationship between chemistry, processing, structure, properties, and performance (Ref 1-3), which has led to accelerated development, engineering, certification, and deployment of new grades of steel. For instance, many groups are investigating the influence of alloy and process on high-strength low-alloy (HSLA) steels (Ref 4, 5). Advances in the fidelity of thermodynamic calculation software have led to our ability to design new steel alloys and processing routes that tailor certain properties. Improvement in instrumentation, such as atom probe tomography and high-
Cody Dyar, William Williams, Haley Doude, and Andrew Oppedal, Center for Advanced Vehicular Systems, 200 Research Blvd, Starkville, MS 39759; Shane Brauer, W
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