Metallurgical and Materials Transactions A

, Volume 38, Issue 2, pp 244–260

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

  • Eric D. Schmidt
  • E. Buddy Damm
  • Seetharaman Sridhar
Article

DOI: 10.1007/s11661-006-9029-x

Cite this article as:
Schmidt, E.D., Damm, E.B. & Sridhar, S. Metall and Mat Trans A (2007) 38: 244. doi:10.1007/s11661-006-9029-x

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 T0, 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 T0, 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.

Copyright information

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

Authors and Affiliations

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