An In-Situ LSCM Study on Bainite Formation in a Fe-0.2C-1.5Mn-2.0Cr Alloy

Abstract

Direct microscopic observation of the isothermal bainite evolution in terms of nucleation events, the location of the nuclei, as well as their growth is very valuable for the refinement of models predicting the kinetics of bainite transformation. To this aim, the microstructural evolution in a Fe-0.2C-1.5Mn-2.0Cr alloy during isothermal bainite formation at temperatures between 723 K and 923 K is monitored in situ using high temperature laser scanning confocal microscopy (LSCM). Both the nucleation and the growth kinetics of the bainitic plates are analyzed quantitatively. Bainitic plates are observed to nucleate on three different types of locations in the grain: at austenitic grain boundaries, on newly-formed bainite plates and at unspecific sites within the austenite grains. Grain boundary nucleation is observed to be the dominant nucleation mode at all transformation temperatures. The rate of nucleation is found to vary markedly between different austenite grains. The temperature dependence of the average bainite nucleation rate is in qualitative agreement with the classical nucleation theory. Analysis of plate growth reveals that also the lengthening rates of bainite plates differ strongly between different grains. However, the lengthening rates do not seem to be related to the type of nucleation site. Analysis of the temperature dependence of the growth rate shows that the lengthening rates at high temperatures are in line with a diffusional model when a growth barrier of 400 J mol −1 is considered.

Exploring the Difference in Bainite Transformation with Varying the Prior Austenite Grain Size in Low Carbon Steel

“…Nevertheless, other grains still do not occur in phase transformation, which means the bainite formation is inhomogeneous at the grain scale. This agrees well with the results by Sainis et al [25], who found that the rate of nucleation varies markedly between different austenite grains. For the FGHAZ thermal cycle, the austenite grains are smaller and their size is more uniform (Figure 5a) compared with that of CGHAZ.…”

Study on Bainitic Transformation by Dilatometer and In Situ LSCM

“…Yang et al predicted ferrite transformation with plate-like morphology, complete transformation in the length direction, and incomplete growth in the widening of plates. They proposed that global stasis in ferrite formation with a plate-like geometry is due to the local stasis in the widening of previously nucleated plates, not in lengthening; for isotropic ferrite morphology, stasis occurs in all growth directions [14,26].…”

“…Timokhina et al [11] investigated Si-containing bainitic steel through neutron diffraction and APT, and observed carbon redistribution in high-density dislocations at the bainitic ferrite/austenite phase boundary. Many studies have explored bainitic transformation through in situ observations [12,13,14,15]. Bainitic transformation in low-carbon Si-containing steel was studied using in situ synchrotron X-rays.…”

“…Secondary bainitic ferrite sympathetically nucleated on the preformed bainite laths [13]. Sainis et al [14] studied the nucleation and growth kinetics of bainitic ferrite plates and found that grain boundary nucleation was the dominant nucleation mode at all transformation temperatures. Additionally, the rate of nucleation also varied markedly among austenite grains [14].…”

“…Sainis et al [14] studied the nucleation and growth kinetics of bainitic ferrite plates and found that grain boundary nucleation was the dominant nucleation mode at all transformation temperatures. Additionally, the rate of nucleation also varied markedly among austenite grains [14]. Zhou et al [15] studied the effects of martensite formation on nanostructured low-temperature bainite transformation and found that bainitic laths formed adjacent to a prior martensite plate, grew along the length direction, and hardly grew along the lateral direction.…”