The Effect of Ultrafast Heating on Cold-Rolled Low Carbon Steel: Formation and Decomposition of Austenite

Abstract

Abstract:The effect of heating rate on the formation and decomposition of austenite was investigated on cold-rolled low carbon steel. Experiments were performed at two heating rates, 150 • C/s and 1500 • C/s, respectively. The microstructures were characterized by means of scanning electron microscopy (SEM) and electron backscattered diffraction (EBSD). Experimental evidence of nucleation of austenite in α/θ, as well as in α/α boundaries is analyzed from the thermodynamic point of view. The increase in the heating rates from 150 • C/s to 1500 • C/s has an impact on the morphology of austenite in the intercritical range. The effect of heating rate on the austenite formation mechanism is analyzed combining thermodynamic calculations and experimental data. The results provide indirect evidence of a transition in the mechanism of austenite formation, from carbon diffusion control to interface control mode. The resulting microstructure after the application of ultrafast heating rates is complex and consists of a mixture of ferrite with different morphologies, undissolved cementite, martensite, and retained austenite.

‘Flash’ Annealing in a Cold‐Rolled Low Carbon Steel Alloyed With Cr, Mn, Mo, and Nb: Part I ‐ Continuous Phase Transformations

“…The dilatometric curve of 10 °C s −1 (dashed black line in Figure ) indicates that the transition from diffusion‐controlled to massive formation of austenite is smooth. This was already predicted in previous studies of austenite formation under ultrafast heating and discussed in the framework of the mixed‐mode model . The fraction of martensite above A m showed in Figure should not be directly associated to the fraction of austenite formed on heating.…”

“…The constitution of the microstructure is revealed in more detail in Figure . It should be noticed that the microstructure is consistent with the observations in previous works . The ferrite is not completely recrystallized in samples heated below 791 °C.…”

“…This is consistent with observations of austenite nucleation in steels with ferrite‐pearlite initial microstructure . Although it was shown that the austenite nucleation at the α / θ interface is the most occurring, under UFH rates it is common to find austenite nucleation at α / α boundaries . It has been shown that the nucleation of ferrite on γ / γ boundaries occurs with double K‐S orientation relationship (OR).…”

Effect of Ultra-Fast Heat Treatment on the Subsequent Formation of Mixed Martensitic/Bainitic Microstructure with Carbides in a CrMo Medium Carbon Steel

“…Specifically, most of the PAGs have a diameter between 8 and 14 µm (Figure 4c). The reason for these smaller austenite grains is most likely that austenite nucleates at the interfaces of undissolved carbides (cementite and M 7 C 3 ) with ferrite and at ferrite/ferrite interfaces [15,34,35]. Moreover, undissolved carbides have a pinning effect, thus, impeding further the growth of austenite grains.…”

Ferrite-to-Austenite and Austenite-to-Martensite Phase Transformations in the Vicinity of a Cementite Particle: A Molecular Dynamics Approach

“…The microstructure development of medium-carbon low-alloy steels containing Cr and Mo under ultrafast heat treatment conditions (rapid heating, peak austenitization followed by quenching) considers the role of undissolved carbides, such as cementite, which influence the austenite formation. Thus, the role of the cementite interface on the microstructure formation in quench-partitioning (QP) and also in ultrafast heating (UFH) steel are under intense discussion [16][17][18][19][20][21][22][23][24][25].…”