Synchrotron Based Observations of Sigma Phase Formation and Dissolution in Duplex Stainless Steel
- Lawrence Livermore National Laboratory (LLNL)
- ORNL
The formation and growth of sigma ({sigma}) phase in 2205 duplex stainless steel was observed and measured in real time using synchrotron radiation during isothermal heat treating at temperatures between 700 C and 850 C. Synchrotron experiments were performed on this material at the Advanced Photon Source (APS) while isothermally holding the samples for times of up to 10 hr. During the isothermal hold, sigma formed in quantities up to 22% as the ferrite transformed to a mixture of sigma and austenite phases. In addition, sigma formed at 850 C was heated to 1000 C to observe its dissolution. The amounts of sigma that formed, and the dissolution temperature of sigma were compared to the results predicted by Thermocalc, showing differences between the calculated and measured values. The synchrotron data was further modeled using a modified Johnson-Mehl-Avrami analysis to determine kinetic parameters for sigma formation. The initial JMA exponent, n, at low fractions of sigma was found to be approximately 7.0, however, towards the end of the transformation, n decreased to values of approximately 0.75. Because of the variable value of n, it was not possible to determine reliable values for the activation energy and pre-exponential terms for the JMA equation. During cooling to room temperature, the high temperature austenite partially transformed to ferrite, substantially increasing the ferrite content while the sigma phase kept its high temperature value.
- Research Organization:
- Oak Ridge National Lab. (ORNL), Oak Ridge, TN (United States)
- Sponsoring Organization:
- USDOE Office of Science (SC)
- DOE Contract Number:
- DE-AC05-00OR22725
- OSTI ID:
- 1028144
- Resource Relation:
- Conference: Numerical Analyses of Weldability, Graz, Austria, 20060925, 20060927
- Country of Publication:
- United States
- Language:
- English
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