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Metallurgical and Materials Transactions A

, Volume 38, Issue 2, pp 244–260 | Cite as

A Study of Diffusion- and Interface-Controlled Migration of the Austenite/Ferrite Front during Austenitization of a Case-Hardenable Alloy Steel

  • Eric D. SchmidtEmail author
  • E. Buddy Damm
  • Seetharaman Sridhar
Article

Abstract

Migrating austenite/ferrite interfaces in the ferrite regions of an alloy steel, containing 0.20 wt pct C, 0.87 wt pct Mn, and 0.57 wt pct Cr, with a ferrite/pearlite microstructure have been observed during austenitization by a high-temperature confocal scanning laser microscope in order to determine the mechanisms of transformation. The samples were subjected to isothermal (790 °C to 850 °C) and nonisothermal (0.5 °C to 20 °C/s) temperature profiles. The kinetic rates extracted from the observations were compared to models for long-range diffusion-controlled and interface reaction-controlled migration. The transition between the two mechanisms was found to occur at T 0, which defines the temperature and composition at which a partitionless phase transformation is possible. Occurrence of the partitionless, interface-controlled transformation was confirmed by an analysis of carbon distribution and microstructure before and after a sample was subjected to a particular thermal profile. The mobility of such interfaces was found to be in the range 1.6·10−13 to 2·10−12 m4·J−1·s−1, which is consistent with previous studies on interface-controlled migration of the reverse reaction, α to γ, during cooling of dilute substitutional iron alloys. The diffusion-controlled migration, at temperatures below T 0, was found to occur in two stages: an initial stage, at which the growth rate can be predicted by a semi-infinite diffusion model; and a second stage, at which the growth slows more rapidly, possibly due to the overlap of diffusion fields.

Keywords

Ferrite Austenite Martensite Bainite Pearlite 
These keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Notes

Acknowledgments

This project was performed through financial support from the National Science Foundation under CAREER Grant No. DMR 0348818. The authors thank the Timken Steel Corporation for materials used in the investigation.

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Copyright information

© THE MINERALS, METALS & MATERIALS SOCIETY and ASM INTERNATIONAL 2007

Authors and Affiliations

  • Eric D. Schmidt
    • 1
    Email author
  • E. Buddy Damm
    • 2
  • Seetharaman Sridhar
    • 1
  1. 1.Department of Materials Science and EngineeringCarnegie Mellon UniversityPittsburghUSA
  2. 2.The Timken CorporationCantonUSA

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