Microstructural Evolution in an Fe-10Ni-0.1C Steel During Heat Treatment and High Strain-Rate Deformation
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THE presence of thermally stable but mechanically unstable austenite in martensitic steels increases the work-hardening response and damage tolerance through transformation-induced plasticity (TRIP). Considerable work has been done on Mn- and C-containing steels as both Mn and C are austenite stabilizers[1–17]; these alloys can be heat-treated to produce thermally stable austenite either by austenite formation (and Mn enrichment) through intercritical annealing or C enrichment of residual austenite through quench and partitioning treatments. Development of these TRIP steels has required a multifaceted approach including fine tuning the alloy composition and designing novel thermo-mechanical processing techniques that together influence the morphology,[11,12,16] size,[11,16] volume fraction,[4,9]
I. HARDING and K. S. KUMAR are with the School of Engineering, Brown University, Providence, RI, 02912. Contact email: [email protected] I. MOUTON is with the Max Planck Institut f¨ur Eisenforschung, 40237, D¨usseldorf, Germany. B. GAULT is with the Max Planck Institut f¨ur Eisenforschung and also with the Department of Materials, Royal School of Mines, Imperial College London, Exhibition Road, London, SW7 2AZ Manuscript submitted April 24, 2020.
METALLURGICAL AND MATERIALS TRANSACTIONS A
composition,[4,6,7,9,15,17] and hence the thermal[4] and mechanical[7,11,16] stability of the austenite phase. Likewise, Ni-containing, low-C steels can also be tempered to produce thermally stable, mechanically unstable austenite, which makes them promising candidates when high strength and toughness at low temperature are necessary.[18–34] Such steels were extensively studied for their low temperature toughness, and factors that controlled austenite stability as a function of isothermal tempering temperature and times were examined.[18,32,33,35] Subsequently, multistage tempering was also considered and its influence on austenite composition, size and morphology was characterized.[28–31] The mechanisms by which this precipitated austenite enhanced the Charpy toughness of these steels were analyzed and attributed variously to austenite size, composition, austenite connectivity, its high interstitial solubility (primarily, carbon), and to mechanically transformed martensite variants that differed from the matrix variants, thereby influencing crack paths.[18,29,30,32] One such steel that has drawn recent attention is an Fe-10Ni-1.0Mo-0.6Mn-0.6Cr-0.08V-0.1C (wt pct) steel (hereafter referred to as the ‘‘10Ni steel’’ for brevity) that has been variously heat-treated including a twostage temper called the quench-lamellerize-temper (QLT) treatment.[36–44] In this QLT condition, this steel has been shown to have high quasi-static strength and
toughness as well as superior ballistic resistance.[37] The QLT treatment on the 10Ni steel produces a dispersion of fine, Ni-rich, thermally stable austenite precipitates in a ferritic matrix[37,40]; it is thought that the high quasi-static toughness and the resistance to shear localization during dyn
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