Comparison of Experimental Methods to Measure the Isothermal Precipitation Kinetics of Nb(C,N) in Austenite of a Nb-Micr
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recipitation kinetics of carbonitrides has been the main subject of numerous studies about microalloyed steels, especially in austenite during hot rolling, because the precipitation influences the recrystallization and decomposition of austenite and mechanical properties of the steels. The precipitates formed in austenite during hot rolling suppress austenite recrystallization, refine austenite grain size, and improve yield and tensile strengths.[1–3] Therefore, the various experimental methods, such as transmission electron microscopy (TEM),[4–6] hot compression,[7–10] and electrolytic dissolution,[11–13] have been attempted to quantitatively measure the precipitation kinetics of carbonitrides in austenite of microalloyed steels. Transmission electron microscopy is a useful method, because tiny precipitates can be directly observed, which is helpful when determining the precipitation start time (Ps). Ashby and Ebeling[14] suggested the equations for calculating the number, size, spacing, and volume fraction of particles from replica images. Nevertheless, it is difficult to determine the precipitation finish time (Pf) and to measure the average number and volume fraction of precipitates due to small observable area.
J.S. PARK and Y.S. HA, Graduate Students, and YOUNGKOOK LEE, Associate Professor, are with Department of Materials Science and Engineering, Yonsei University, Seoul 120-749, Korea. Contact e-mail: [email protected] Manuscript submitted January 17, 2009. Article published online May 7, 2009 METALLURGICAL AND MATERIALS TRANSACTIONS A
The hot compression method is useful in examining the interaction between precipitation and recrystallization based on the softening phenomenon of the materials.[15] However, it provides only Ps and Pf, not entire precipitation kinetics. It is also impossible to measure the precipitation kinetics in strain-free austenite. Nonetheless, this method has been widely used for the determination of the strain-induced isothermal Ps and Pf. For the electrolytic dissolution method, a lump of sample is dissolved in an acid solution. The particles are filtered from the solution using a fine membrane filter. The weights of the dissolved sample and the filtered particles are measured. The Nb element in the particles is detected using an inductively coupled plasma–atomic emission spectrometer, and the solute Nb content in the filtered particles is quantified, leading to the quantitative amount of the Nb(C,N) precipitates in a bulk specimen. However, because incomplete filtering of tiny particles can cause an inaccurate amount of the precipitates, recently, this method has not been preferentially adopted. Recently, the authors measured the dissolution temperature and precipitation kinetics of Nb(C,N) in both strain-free and strained austenite and in bainite using the electrical resistivity (ER) method.[16–19] It is difficult to compare the precipitation kinetics in austenite measured using the aforementioned experimental methods, especially in the steels with different chemistries. In this study, therefor
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