Austenite Nucleation and Growth Observed on the Level of Individual Grains by Three-Dimensional X-Ray Diffraction Microscopy

Abstract

Austenite nucleation and growth is studied during continuous heating using three-dimensional X-ray diffraction (3-D XRD) microscopy at the European Synchrotron Radiation Facility (ESRF) (Grenoble, France). Unique in-situ observations of austenite nucleation and growth kinetics were made for two commercial medium-carbon low-alloy steels (0.21 and 0.35 wt pct carbon with an initial microstructure of ferrite and pearlite). The measured austenite volume fraction as a function of temperature shows a two-step behavior for both steel grades: it starts with a rather fast pearlite-to-austenite transformation, which is followed by a more gradual ferrite-to-austenite transformation. The austenite nucleus density exhibits similar behavior, with a sharp increase during the first stage of the transformation and a more gradual increase in the nucleus density in the second stage for the 0.21 wt pct carbon alloy. For the 0.35 wt pct carbon alloy, no new nuclei form during the second stage. Three different types of growth of austenite grains in the ferrite/pearlite matrix were observed. The combination of detailed separate observations of both nucleation and growth provides unique quantitative information on the phase transformation kinetics during heating, i.e., austenite formation from ferrite and pearlite.

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

“…Therefore, the transition from the zone of fast austenite formation (S1) to the zone of slow austenite formation (S2) is rather smooth. A somewhat sharp transition has been reported in dilatometric experiments of austenite formation during the continuous heating of annealed ferrite-pearlite microstructure [16,17] under heating rates from 1 °C/s to 200 °C/s. One could thus conclude that the cold-rolling of the ferrite-pearlite initial microstructure is enhancing the spheroidization of cementite, which has an impact on the formation of austenite in samples heated at 150 °C/s.…”

“…The change in cementite morphology is thought to be related to the change in austenite distribution in the matrix, as described in Section 3.1. When the cementite morphology in pearlite still remains mainly lamellar, as in samples heated at 1500 °C/s, the formation of austenite takes place quickly in the pearlite regions [16,17]. A banded mixture of ferrite and austenite (martensite) is thus observed in the final microstructure (cf.…”

“…Dilatometric data [16] are in agreement with the different kinetics of austenite growth in pearlite and proeutectoid ferrite. The massive formation of austenite during heating was experimentally measured using in situ techniques [15,17]. The change in the austenite formation mechanism, a consequence of the ultrafast heating rates, will thus have an impact on the microstructure on quenching.…”

Effect of normalizing temperature on microstructure and mechanical properties of a Nb-V microalloyed large forging steel

“…Austenite formation in low carbon steels has been studied extensively in the literature starting from different microstructures [4][5][6]. Previous work has shown that in ferritic-pearlitic microstructures the formation of austenite was described as taking place in three main successive steps: (1) nucleation of austenite in pearlite colonies, ferrite-pearlite grain boundaries or ferrite-ferrite grain boundaries, (2) rapid growth of austenite consuming pearlite, (3) slower growth of austenite consuming ferrite [7,8]. Based on the above theory, as for ferrite-pearlite banded microstructure in large forging steel, austenization in ferrite bands and pearlite bands are asynchronous.…”

Study on Austenitization Kinetics of SA508 Gr.3 Steel Based on Isoconversional Method

“…In addition, it is worth mentioning that some new methods have been used for austenite formation study recent years. For example, Savran et al [10] studied the nucleation and growth of austenite from ferrite/pearlite structures using a three-dimensional high-energy X-ray diffraction (3-D XRD) microscope. Additionally, Esin et al [11] investigated the austenitization during both slow and fast heating for different microstructures of a selected low-alloy steel by simultaneously using in situ high-energy synchrotron X-ray diffraction and dilatometry.…”