High-temperature phase stability, γ → δ transformation of ferritic/martensitic steel studied by differential scanning calorimetry and electron backscatter diffraction

Raj Narayan Hajra, Manmath Kumar Dash, Woong Chu, A. N. Singh, Kyung Wan Nam, Jeoung Han Kim

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Abstract

The formation of δ-ferrite in advanced structural materials is well known to impair mechanical and corrosion properties. In the present work, high-temperature phase stability and phase transformation characteristics of the γ → δ phase transformation of ferritic/martensitic steel were studied by differential scanning calorimetry (DSC) and electron backscatter diffraction (EBSD). The characteristics of martensitic transformation were studied by varying the temperature and time of austenitization. The results show that γ-austenite completely transforms to δ-ferrite during austenitization beyond 1523 K. The absence of martensite transformation was observed in the DSC thermogram for the sample annealed at 1523 K for 5 h. A detailed EBSD study of the time-dependent evolution of δ-ferrite revealed a significant reduction in low-angle and coincidence site lattice (CSL) grain boundaries of the martensite matrix. There was no reverse transformation (δ-ferrite → γ-austenite) observed during heating in DSC. Further, the Kolmogorov–Johnson–Mehl–Avrami (KJMA) model was employed to study the kinetics of the γ-austenite → δ-ferrite transformation. The activation energy and growth exponent obtained for this transformation were 335 kJ mol−1 and 2.1, respectively. Tis result has significant technological implications as it revealed an important fact that δ-ferrite, once formed in the material, does not dissolve by heat treatment. Graphical Abstract: The characteristics of martensitic transformation were studied by varying the temperature and time of austenitization. It is observed that the γ-austenite completely transforms to δ-ferrite during austenitization beyond 5 h at 1523 K. A detailed EBSD study of the time-dependent evolution of δ-ferrite revealed a significant reduction in low-angle and coincidence site lattice (CSL) grain boundaries of the martensite matrix. For δ-ferrite, there was no reverse transformation (δ-ferrite → γ-austenite) observed during heating in DSC which signifies the difficulty associated with its dissolution by heat treatment. Further, Kolmogorov–Johnson–Mehl–Avrami (KJMA) model was employed to study the kinetics of the γ-austenite → δ-ferrite transformation. The activation energy and growth exponent obtained for this transformation were 335 kJ mol−1 and 2.1, respectively. [Figure not available: see fulltext.].

Original languageEnglish
Pages (from-to)3357-3371
Number of pages15
JournalJournal of Thermal Analysis and Calorimetry
Volume148
Issue number9
DOIs
StatePublished - May 2023

Keywords

  • Differential scanning calorimetry
  • Ferritic/martensitic steel
  • KJMA
  • Phase transformation
  • Thermo-Calc
  • δ-ferrite

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