Bainite Transformation During Continuous Cooling: Analysis of Dilatation Data
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INTRODUCTION
EVER since the discovery of bainite in steels, there has been an intense debate on the mechanism of its transformation.[1,2] However, the displacive approach has been shown to be quite appropriate and the theory of transformation under this framework has advanced sufficiently; the quantitative theory is used extensively to design new grades of steels, such as TRIP-aided steels, bainitic steels, and nanobainitic steels, with an encouraging combination of properties.[1–10] Like other phases, the ability of bainite to affect the final combination of properties offered by a steel depends on its volume fraction, composition, and size. There are some experimental results that indicate that the carbon concentration in bainitic ferrite is higher than that dictated by transformation under paraequilibrium condition.[11,12] At first, the measured high carbon concentration in bainitic ferrite was attributed to carbon atoms trapped at dislocations generated as a result of the transformation.[10,13–15] However, recent experimental results obtained using three-dimensional atom probe studies indicate that excess carbon, over and above the solubility limit in body-centered-cubic (bcc) ferrite (in equilibrium with austenite), remains in solid solution in bainitic ferrite.[11,12] Higher solid solubility of carbon in bainitic ferrite has been attributed to a change in crystal structure from bcc to body-centered-tetragonal (bct).[16–19]
RAVI RANJAN and SHIV BRAT SINGH are with the Department of Metallurgical and Materials Engineering, Indian Institute of Technology Kharagpur, Kharagpur, 721302, India. Contact e-mail: [email protected] Manuscript submitted 16 August, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
In general, bainite shares many features, e.g., carbon supersaturation, morphology, and crystallographic features, with martensite.[1,2,4,11,12,20] Therefore, it is often difficult to distinguish bainitic ferrite from martensite and quantify their relative fractions in a multiphase microstructure through conventional microscopy. Dilatometry can be a useful technique in such cases to detect the presence of bainite and martensite as these transformations usually occur in different temperature ranges. A dilatometer records the change in length of a material for a given thermal treatment. Therefore, the recorded data include the contribution from the thermal effect and associated phase transformation, if any. In general, the formation of a phase with bcc or bct crystal structure from face-centered-cubic (fcc) austenitic phase leads to an expansion, which is clearly reflected in the dilatation curve. The expansion observed is proportional to the amount of phase formed from austenite. However, in general, this qualitative relationship alone is not sufficient to extract the quantitative details of the phases involved such as their amount and the composition. Therefore, a method is presented in this work to extract these details from the experimental dilatation curve for the transformation of austenite to bainitic ferrite
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