Nanoscale Analyses of High-Nickel Concentration Martensitic High-Strength Steels
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INTRODUCTION
IT has long been known that austenite reversion in martensitic and ferritic steels by intercritical tempering or annealing can markedly improve low-temperature fracture toughness by lowering the ductile-to-brittle transition temperature (DBTT), mitigating the poor fracture toughness due to the inherent brittleness of martensite.[1–3] Development of a 9 wt pct Ni steel for applications with cryogenic temperatures had been begun as early as 1944 by the International Nickel company.[4] It was soon recognized that the stability of that precipitated (reverted) austenite against transformation to ‘‘fresh’’ martensite and the austenite’s ability to getter embrittling interstitial elements are important factors for the improved mechanical properties.[5] At temperatures greater than Ms, the temperature where DIETER ISHEIM, Research Assistant Professor, and DAVID N. SEIDMAN, Walter P. Murphy Professor, are with the Department of Materials Science and Engineering, Northwestern University, Evanston, IL. Contact e-mail: [email protected] ALLEN H. HUNTER, Postdoctoral Researcher, formerly with the Department of Materials Science and Engineering, Northwestern University, is now with the Department of Materials Science and Engineering, University of Michigan, Ann Arbor. MI. XIAN J. ZHANG, Materials Engineer, is with the Carderock Division, Naval Surface Warfare Center, West Bethesda, MD. Manuscript submitted September 12, 2011. Article published online March 13, 2013 3046—VOLUME 44A, JULY 2013
the martensitic transformation from austenite-to-martensite commences spontaneously upon cooling, the martensitic transformation can be induced by an applied mechanical stress.[6,7] The combination of austenite reversion toughening with secondary hardening by metal-carbide precipitates in a 10 wt pct Ni steel has been demonstrated.[8] For more economical use, a steel with a lower Ni content, 5.5 wt pct, was developed by Nippon Steel.[9] For this steel, the so-called QLT heat treatment was introduced with two heat-treatment steps in the intercritical region after quenching (‘‘Q’’) : a lamellarization (‘‘L’’) anneal at higher temperatures in the intercritical area, followed by intercritical tempering (‘‘T’’) at lower temperatures.[9,10] The correlation between precipitating austenite and increased fracture toughness has been demonstrated to temperatures as low as 77 K (196 C) for 5.5 and 9 wt pct Ni steels.[11–14] Steels relying on the mechanically induced austeniteto-martensite transformation of reverted or precipitated austenite for enhanced ductility are denoted transformation-induced-plasticity (TRIP) steels.[15] It was recognized that this transformation can be used to produce steels with an excellent combination of strength, ductility, and toughness.[15–18] A variety of steels based on the TRIP principle have been developed[1–3,19] and austenite formation during intercritical tempering explored.[20,21] Austenite formation by intercritical tempering in the ferrite plus austenite two-phase field has been studied METALLU
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