Mechanical Properties and Microstructural Evolution of Simulated Heat-Affected Zones in Wrought Eglin Steel
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EGLIN steel is an ultra-high strength steel alloy that was developed at Eglin Air Force Base in the early 2000s and has since been patented in 2009.[1] The steel has strength levels similar to AerMet100, AF1410, and HP9-4-30, but at a reduced cost due to a reduction or elimination of expensive alloying elements such as nickel and cobalt, which can both range from 10 to 14 wt pct in the previously mentioned alloys. Table I shows the alloy composition for Eglin steel used in this work. Silicon is added to enhance toughness and stabilize austenite. Silicon is well known to make cementite precipitation difficult at tempering temperatures used for Eglin steel.[2–6] This is critical because similar alloys can experience tempered martensite embrittlement due to cementite formation at temperatures as low as 548 K (275 C).[2] In order to ensure that Eglin steel does not form cementite while being tempered at 473 K (200 C), the increased silicon content is BRETT M. LEISTER, Associate, is with Exponent Failure Analysis Associates, Menlo Park, CA, and also with the Department of Materials Science and Engineering, Lehigh University, Bethlehem, PA 18015. Contact e-mail: [email protected] JOHN N. DUPONT and MASASHI WATANABE, Professors, are with the Department of Materials Science and Engineering, Lehigh University. RACHEL ABRAHAMS, Research Scientist, is with Eglin Air Force Base, Eglin AFB, FL 32542. Manuscript submitted October 16, 2014. Article published online September 10, 2015 METALLURGICAL AND MATERIALS TRANSACTIONS A
necessary. According to the patent document, chromium is added to increase strength and hardenability, while molybdenum is also added to increase hardenability. Nickel is used to increase toughness, and tungsten is added to increase strength and wear resistance.[1] Eglin steel typically has a quenched and tempered microstructure consisting of tempered martensite with a variety of carbide sizes and morphologies. Paules et al.[7] have reported M3C, M6C, and MC carbides that form after heat treatment with sizes of 180, 250, and 10 to 20 nm, respectively. Two heat treatment schedules were investigated. In the first heat treatment, the samples were normalized at 1363 K (1090 C) for 1 hour, followed by a sub-critical anneal at 923 K (650 C) for 1.5 hours. The samples were then austenitized at 1173 K, 1223 K, and 1283 K (900 C, 950 C, or 1010 C) for 0.5 hours, oil quenched, and then tempered at 533 K (260 C) for 1 hour. The second heat treatment was the same as the first, but did not contain the normalization and sub-critical anneal. M3C and MC carbides were found in all heat-treated samples, but the M6C carbides were only present following the low-temperature austenitization treatment. Increasing the austenitization temperature from 1173 K to 1283 K (900 C to 1010 C) caused the dissolution of the M6C carbides. Eglin steel will undergo a variety of fabrication processes such as casting, rolling, forging, fusion welding, and heat treating. Welding of Eglin steel will be especially necessary, and a comprehen
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