Austenite Nucleation and Growth Observed on the Level of Individual Grains by Three-Dimensional X-Ray Diffraction Microscopy |
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Authors: | V I Savran S E Offerman J Sietsma |
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Affiliation: | (1) Materials Innovation Institute M2i, 2628 CD Delft, The Netherlands;(2) Department of Materials Science and Engineering, Delft University of Technology, 2628 CD Delft, The Netherlands |
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Abstract: | Austenite nucleation and growth is studied during continuous heating using three-dimensional X-ray diffraction (3-D XRD) microscopy
at the European Synchrotron Radiation Facility (ESRF) (Grenoble, France). Unique in-situ observations of austenite nucleation and growth kinetics were made for two commercial medium-carbon low-alloy steels (0.21
and 0.35 wt pct carbon with an initial microstructure of ferrite and pearlite). The measured austenite volume fraction as
a function of temperature shows a two-step behavior for both steel grades: it starts with a rather fast pearlite-to-austenite
transformation, which is followed by a more gradual ferrite-to-austenite transformation. The austenite nucleus density exhibits
similar behavior, with a sharp increase during the first stage of the transformation and a more gradual increase in the nucleus
density in the second stage for the 0.21 wt pct carbon alloy. For the 0.35 wt pct carbon alloy, no new nuclei form during
the second stage. Three different types of growth of austenite grains in the ferrite/pearlite matrix were observed. The combination
of detailed separate observations of both nucleation and growth provides unique quantitative information on the phase transformation
kinetics during heating, i.e., austenite formation from ferrite and pearlite. |
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