A study on Characterizations on Microstructure of Coarse Grain HAZ and Phase Transformation Mechanism of Acicular Ferrite in High Heat Input Steel Plates
Date Issued
2011
Date
2011
Author(s)
Chen, San-Ho
Abstract
To improve the welding efficiency and to reduce economic cost, the advanced steel plates for the high-heat-input welding have been developed from the conventional ferrite–pearlite steel plates with a yield strength of about 460
MPa for shipbuilding and construction industries. The newly developed steel plates, after high-heat-input welding, have a novel microstructure in HAZ, where great amounts of coarse acicular ferrite are enveloped by the networks of allotrimorphic ferrite forming around the prior austenite boundaries. Such a novel microstructure improves the toughness in HAZ and enables the high heat input welding to be a pratical application for join steel thick plates. In
fact, different from the conventional idea that the refinement of microstructure could improve the toughness in HAZ, the coarse acicular ferrite formed in HAZ during the high-heat-input welding in the present steel plates also provides required mechanical properties.
The present study attempts to explore the mechanism to form coarse acicular ferrite in steels during the high-heat-input welding. The transformation of coarse acicular ferrite was studied via the simulated heat treatments using a dilatometer to control several parameters: prior austenite grain size, transformation temperature, transformation interval, and nucleation sites. The corresponding microstructure of acicular ferrite and the nanoscaled carbonitrides (as the nucleation sites for acicular ferrite) were investigated using transmission electron microscopy (TEM). More attention has been paid to the morphology of acicular ferrite. The effect of the grain size of prior austenite on acicular ferrite transformation has also been studied. Moreover, the macrostructure and the corresponding crystallography were analyzed by electron back scattering pattern (EBSD) using scanning electron microscopy (SEM).
The results have been compared with the microstructure in the real HAZ of weld plates. It is believed that understanding the transformation of coarse acicular ferrite facilitates the development of higher grades of steel plates for the high-heat-input welding.
Subjects
High-heat-input Weld
Acicular Ferrite
TEM
EBSD
Steel Plate
Type
thesis
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