Analysis of a New High-Toughness Ultra-high-Strength Martensitic Steel by Transmission Electron Microscopy and Atom Prob
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HIGH-STRENGTH steel alloys remain in high demand in the modern economy due their combination of excellent mechanical properties. Components such as landing gear, structural members, and fuel injectors require both high strength and high toughness due to the critical load bearing nature of these applications. These needs were met initially through the development of the
MATTHEW I. HARTSHORNE, Research Assistant, and MITRA L. TAHERI, Hoeganaes Associate Professor, are with the Department of Materials Science and Engineering, Drexel University, 3141 Chestnut St, Lebow 344, Philadelphia, PA 19104. CAROLINE MCCORMICK, formerly Research Assistant with the Department of Materials Science and Engineering, Drexel University, is now Metallurgical Engineer with Arcelor Mittal Coatesville, 139 Modena Rd., Coatesville, PA 19320. Contact e-mail: [email protected] MICHAEL SCHMIDT, Manager, Alloy Design, and PAUL NOVOTNY, Manager, Technical and Metallurgy Training, are with Carpenter Technology Corporation, 101 West Bern Street, Reading, PA 19601. DIETER ISHEIM, Research Associate Professor and Manager, is with the Department of Materials Science and Engineering, Northwestern University, 2220 Campus Drive, Evanston, IL 60208-3108, and also with the Northwestern University Center for Atom-Probe Tomography. DAVID N. SEIDMAN, Walter P. Murphy Professor of Materials Science and Engineering, is with the Department of Materials Science and Engineering, Northwestern University, and also Founding Director, Northwestern University Center for Atom-Probe Tomography. Manuscript submitted July 2, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
low-alloy AISI 4340[1,2] and subsequently 300M[1,3] steels, and the new alloys such as the AerMet series and PremoMet analyzed in this work have been developed to meet requirements for even higher fracture toughness.[3–5] Most ultra-high-strength steels utilize a martensitic structure, possess a yield strength in excess of 1380 MPa (200 ksi) and can be separated into three broad categories: maraging steels, secondary hardening steels, and low-alloy steels.[5,6] Maraging steels and secondary hardening steels both contain high alloying additions and low carbon contents, with maraging steels forming intermetallic compounds such as Ni3Mo and Ni3Ti,[5,6] while the secondary hardening steels instead form fine alloy carbide precipitates such as Mo2C and Cr2C.[5,7–10] Both maraging and secondary hardening steels possess KIC fracture toughness above 100 MPam, but typically require alloy additions of approximately 25 wt pct, including large amounts of both nickel (Ni) and cobalt (Co) and are sensitive to sulfur (S) inclusion size and shape, all of which increase cost.[4,5,7,11] In comparison, the relatively low-cost low-alloy steels have an alloy content of approximately 5 wt pct, a medium carbon content, and form iron carbide precipitates during tempering, but possess a fracture toughness of only approximately 60 MPam.[6,7,12,13] The increased cost of maraging and secondary hardening steels is
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