Fig. 1. Sketch of pre-treatment process of the test sample.

Fig. 2. Initial microstructures before austenitization: (a) Image quality maps with phase maps marking cementite in yellow; (b) orientation map; (c) SEM image showing the microstructure of one cementite particle in ferrite matrix; (d) Mn profile along the scanning line in Fig. 2(c); (e) Statistics of Mn content in cementite and its surrounding ferrite at a 20 × 20 um2 square in a SEM image.

Fig. 3. Relative length changes of the specimens as a function of temperature: (a) continuous heating at 0.1 °C/s and 100 °C/s to fully austenitizing temperature; (b)continuous heating at 0.1 °C/s and quenched from 775 °C and 797 °C; (c) (b)continuous heating at 100 °C/s and quenched from 827 °C, 837 °C and 857 °C.

Fig. 4. (a) Micrograph of the slow-heated sample quenched from 775 °C; (b) SEM image showing the diffusion field across one cementite particle; (c) Image quality map with phase maps marking retained austenite (RA) and cementite; (d) Crystal orientation imaging maps; (e)C and Mn profiles along the line in Fig. 4(b); (f) SEM image at higher voltage mode showing the distinguished contrast in the surrounding ferrite.

Fig. 5. (a) Micrograph of the fast-heated sample quenched from 827 °C; (b) SEM image showing the diffusion field across one cementite particle; (c) Image quality maps with phase maps marking RA and cementite; (d) Crystal orientation imaging maps; (e)C and Mn profiles along the line in Fig. 5(b).

Fig. 6. (a) Image quality map of multi-diffusion field with phase maps marking RA and cementite microstructure of fast-heated sample quenched at 827 °C; (b) Crystal orientation imaging map; (c) Mn profiles across the RA regions along the scanning line in Fig. 6(b).

Fig. 7. (a-d) Image quality map with phase maps marking RA in green; (e-h) orientation maps of the corresponding region in which RA are highlighted in dark circle; (i-k) SEM pictures of single RA covered with the Mn profile.

Fig. 8. Schematic isothermal sections of Fe-C-Mn phase diagrams to indicate the austenite growth under local equilibrium condition: (a) Mn diffusion-controlled austenite growth, e.g. Partitioning Local Equilibrium (PLE) (b) C diffusion-controlled austenite growth, e.g. Negligible Partitioning Local Equilibrium (NPLE) (PNTT: Partition to Non-partition Transition Temperature).

Fig. 9. (a) Comparison between the kinetics of austenite formation upon heating simulated by DICTRA and measured by dilatation; (b) The γ/α and γ/θ interface position as a function of temperature upon heating with 0.1 °C/s and 100 °C/s.

Fig. 10. (a, b) Evolution of C and Mn profiles during continuous heating at 0.1 °C /s; (c, d) Evolution of C and Mn profiles during continuous heating at 100 °C /s.

Fig. 11. The predicted γ/α and γ/θ interface position as a function of temperature during heating and cooling and the corresponding elements distribution: (a-c) the slow-heated case, (d-f) the fast-heated case.

Fig. 12. A sketch of the evolution of microstructure and Mn distribution during the thermal cycles.