Tempering of Low-Temperature Bainite
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INTRODUCTION
PROCEDURES for the design of bainitic nanostructured steels in bulk form have been established.[1–4] Modeling the effects of composition on transformation kinetics enables tailored alloying to allow isothermal transformation at suppressed temperatures, while silicon additions prevent the precipitation of coarse carbides during the bainite transformation.[1,2] Isothermal transformation circa 473 K (200 C) then results in a structure of fine bainitic ferrite plates (width 20 to 40 nm) separated by thin-films of austenite.[1,2,5] The fine scale of the structure is a consequence of the displacive transformation mechanism[6] and contributes significantly to the strength and hardness,[7] while retained austenite contributes to the toughness and elongation.[8] Carbide precipitation in a nanobainitic steel has been observed using atom probe tomography to be associated with the formation of carbon-depleted austenite after isothermal transformation at 473 K (200 C) in composition Fe-0.79C-1.5Si-1.98Mn-0.24Mo-1.06Al1.59Co wt pct.[9] Therefore, the tempering behavior of these alloys is of interest both to understand the thermal stability and the scope for tailoring the mechanical properties. Softening may also be necessary to allow thermo-mechanical processing.[10,11] Previous work has shown that these steels are relatively resilient to tempering.[1,8,12] The fine plate size provides the dominant strength contribution, so high hardness is maintained until coarsening of the bainitic ferrite occurs.[12,13]
MATHEW J. PEET and H.K.D.H. BHADESHIA are with the Department of Materials Science and Metallurgy, University of Cambridge, 27 Charles Babbage Road, Cambridge CB3 0FS, UK. Contact e-mail: [email protected] SUDARSANAM SURESH BABU is with the University of Tennessee, Knoxville, TN 37916, and also with the Oak Ridge National Laboratory, Oak Ridge, TN 37831. MIKE K. MILLER is with the Oak Ridge National Laboratory. Manuscript submitted September 24, 2016. METALLURGICAL AND MATERIALS TRANSACTIONS A
Development of alloys with increased temperature stability is in progress; this will rely on further suppressing carbide precipitation to increase the stability of the nanostructure.[14] So there are a several strong motivations to characterize the tempering behavior of these nanostructured steels. Previous work has found that nanostructured bainitic steels exhibit excellent tempering resistance, retaining much of the hardness to temperatures above 723 K to 773 K (450 C to 500 C) for 1 hour. In the current study, the structures after 30 minutes tempering treatments at 673 K (400 C) and 773 K (500 C) on the alloy with composition Fe-0.75C-1.63Si-1.95Mn-1.48Cr-0.28Mo-0.1V wt pct (Table I) designated A2 transformed at 473 K (200 C) were characterized using atom probe tomography. Both heat treatments are in the range of tempering temperatures where retained austenite decomposes as determined using ex situ X-ray diffraction. Tempering heat treatments are commonly applied to quenched martensitic steels, at temperatures belo
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