Morphology and Crystallography of Bainite Transformation in a Single Prior-Austenite Grain of Low-Carbon Steel
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INTRODUCTION
ONE of the characteristics of a weld microstructure is the coarse grain size.[1,2] Especially in the coarse-grained heat-affected zone (CGHAZ) of welds, the grain size of prior austenite reaches a few hundred micrometers as a result of the large heat input. Understanding the morphology and crystallography of a coarse-grained microstructure is essential to secure the mechanical properties of a weld. In a low-carbon steel weld, austenite can transform to bainite on account of the fast weld cooling rates. If local hardenability is sufficiently high, austenite can be stabilized, and a martensite-austenite (M-A) constituent can be formed.[3,4] Much research on the microstructure of bainite and the M-A constituent has focused on the morphology and quantity of the M-A constituent under different cooling conditions,[5,6] the position in the HAZ,[7,8] and the chemical composition.[3] However, in coarse-grained HAZs, many blocks coexist in a single prior-austenite grain under the same experimental conditions, such as cooling rate. The ‘‘term’’ block refers to a bainitic ferrite group the elements of which share a similar crystal orientation.[9–11] Few studies have focused on the difference in microstructural development of bainite and the M-A constituent between blocks in a single prior-austenite grain. In the present study, microstructural development in one coarse prior-austenite grain under a thermal cycle in a simulated CGHAZ is tracked in situ by high-temperature laser-scanning confocal microscopy (LSCM).[12–14] HIDENORI TERASAKI, Associate Professor, and YU-ICHI KOMIZO, Professor, are with the Joining and Welding Research Institute, Osaka University, 11-1 Mihogaoka, Ibaraki, Osaka 567-0047, Japan. Contact e-mail: [email protected] Manuscript submitted October 7, 2012. Article published online January 24, 2013 METALLURGICAL AND MATERIALS TRANSACTIONS A
Furthermore, the morphology and crystallography of the coarse grain after bainite transformation are analyzed in detail by an electron back-scattering diffraction (EBSD) method. These methods allow us to focus on the difference in microstructure development of bainite and the M-A constituent between blocks in a single prioraustenite grain.
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EXPERIMENTAL PROCEDURE
Fe-0.1C-1Ni-0.8Mn-0.4Mo (mass pct) was used in this investigation of the transformation microstructure in a simulated HAZ. The specimens were austenitized at 1623 K (1350 C) for 30 seconds and then cooled at 1.67 K/s until they reached room temperature, which simulated a weld HAZ in which the heat input is large. Microstructural development under the thermal cycle was observed in situ using LSCM for high-temperature applications.[12–14] A schematic illustration of the system is shown in Figure 1. The specimens, 5 mm in diameter and 2 mm in thickness, were placed in an alumina crucible, which was inserted into the furnace. The temperature, measured by a thermocouple incorporated into the crucible holder, was displayed on a monitor and simultaneously recorded along with the LSCM image, which
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