Papers by Author: Xiang Liang Wan

Abstract: The effect of heat input on toughness of coarse-grained heat-affected zone of an ultra low carbon acicular ferrite steel were investigated when the welding was conducted with high heat input. Microstructural observations, energy dispersive X-ray spectroscopy analyses were conducted using optical microscopy, scanning electron microscopy and transmission electron microscopy, respectively. The microstructures of coarse-grained heat-affected zone consist of predominantly bainitic microstructure and a small proportion of acicular ferrite grains. The bainitic microstructures become coarsened with increasing heat input. The impact toughness of coarse-grained heat-affected zone remained at a higher level when the heat input ranged from 42 to 55 kJ/cm. It became not stable and dropped to a lower level when the heat input increased to 110150 kJ/cm. The enhancement in impact toughness was attributable to the MnS precipitation on the pre-formed Ti oxides as well as the formation of intragranular ferrite. When specimens were welded with higher heat input, the deterioration of impact toughness was caused by the coarsening of austenite grains.

Abstract: The effect of Nb microalloying on microstructure transformation of coarse-grained heat-affected zone of high strength low alloy steels were investigated utilizing different heat input welding simulation. For the low-Nb steel, the microstructures of coarse-grained heat-affected zone mainly consisted of acicular ferrite, bainite and grain boundary ferrite for small heat input welding; the amount of acicular ferrite decreased whereas grain boundary ferrite, polygonal ferrite and pearlite increased with increasing heat input. In constrast, for the high-Nb steel, granular bainite was the dominant microstructure. The formation of granular bainitic microstructure was associated with the solid solution of Nb, which suppressed ferrite transformation and promoted the formation of granular bainite. The hardness of coarse-grained heat-affected zone increased with increasing Nb content, and decreased with decreasing heat input, which was attributed to the microstructural change.