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

Abstract

The formation of delta-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 gamma -> delta 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 gamma-austenite completely transforms to delta-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 delta-ferrite revealed a significant reduction in low-angle and coincidence site lattice (CSL) grain boundaries of the martensite matrix. There was no reverse transformation (delta-ferrite ->gamma-austenite) observed during heating in DSC. Further, the Kolmogorov-Johnson-Mehl-Avrami (KJMA) model was employed to study the kinetics of the gamma-austenite -> delta-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 delta-ferrite, once formed in the material, does not dissolve by heat treatment. [GRAPHICS]