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| Intermediate-temperature transformation behavior of high-nitrogen austenite generated by nitriding thin pure-iron sheets |
| 华南理工大学机械工程学院 |
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Cite this article:
. Intermediate-temperature transformation behavior of high-nitrogen austenite generated by nitriding thin pure-iron sheets. Acta Metall Sin, 2007, 43(4): 337-343 .
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Abstract Abstract A microstructural study was carried out to clarify the nature and morphology of the microstructure produced in high-nitrogen austenite generated by nitriding thin pure-iron sheets and subjected to an intermediate-temperature isothermal transformation at 225C, using optical microscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM). It was found that an upper bainitic transformation, γ-Fe +γ′-Fe4N, occurred at either the grain boundaries or the dislocation lines in the interior of austenite grains, producing a microstructure which, although morphologically looking markedly different, consisted of bainitic colonies composed of parallel bainitic ferrite (-Fe ) laths with γ′-Fe4N laths entrapped between them, and was interspersed with discrete patches of retained austenite(γr). Transformations occurring at the grain boundaries initiated earlier, proceeded faster, and produced a coarser microstructure, than those occurring in the interior of austenite grains did, due to the difference between the two types of nucleation sites in thermodynamics (nucleation energy) and kinetics (diffusivity). The bainitic transformation was diffusional in nature, with γ′-Fe4N serving as the leading phase. The striking similarity of the γ′-Fe4N to the high-nitrogen austenite in crystal structure, and the good coherence of the γ/γ′-Fe4N interface greatly facilitated the precipitation of γ′-Fe4N from austenite.
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Received: 23 August 2006
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