Abstract
The welding performance of shipbuilding steel under large heat input could be improved greatly by the addition of Mg to the steel, but the impact toughness of the heat affect zone (HAZ) is not stable. According to the three different thickness steel plates obtained in the industrial experiment, the large heat input welding was carried out by different heat input, and the impact toughness analysis, impact fracture analysis, metallographic microstructure analysis and inclusions analysis were carried out. The results showed that, the HAZ of three kinds of thickness plates induced much intragranular acicular ferrite (IAF); with Mg addition, the inclusion dimension had been reduced effectively, and the IAF-induced ability of the inclusions had also been improved. The difference of HAZ impact toughness with different welding heat input and different impact temperature is significant; considering the influence of welding heat input and metallographic microstructure on the impact toughness of HAZ, the welding heat load had a far greater effect than the metallographic microstructure on ductile–brittle transition temperature. At the same time, if the original metallographic microstructure of steel was coarse, the pinning effect of the inclusions would be reduced significantly, and the microstructure of HAZ would be coarsened and the impact toughness of HAZ would be decreased, so there is a certain matching relationship between the metallographic microstructure and the inclusion dimension.
Highlights
In recent years, the shipbuilding industry has developed rapidly, especially in eastern Asia.In order to further shorten the shipbuilding time and improve competitive power, high-efficiency large heat input welding technology has been brought into the industry
As shown in the experimental results, it can be seen that both the macroscopic appearance and the microscopic appearance of the fracture were matched with the corresponding impact absorbed energy values, and there was no sharp drop in the impact absorbed energy value caused by the steel plate defect, which indicated that the impact energy value could completely represent the impact toughness of the heat affect zone (HAZ) of the steel plate under the corresponding welding and impact conditions
The welding line energy characterizes the heat input per welding area, but for different thickness steel plates, the heat load in the local area does not necessarily increase with the increase of weld heat input, it depends on the steel plate thickness
Summary
The shipbuilding industry has developed rapidly, especially in eastern Asia. Under the high input energy of the welding line, the traditional HAZ (heat affect zone) of the shipbuilding steel would undergo severe grain coarsening, localized softening and embrittlement, and result in a significant reduction in impact toughness and a serious threat to the safety of the ship structure. Kasamatsu Y [8] found that when the temperature exceeds 1350 ◦ C, TiN would partially solid-solubilize, the amount of TiN will drop sharply after cooling and precipitation, and lead to a significant decrease in its inducing ability This method limits the input line energy during welding. The morphology of the impact fractures, microstructures, and the presence of inclusions were systematically studied, to obtain of the impact toughness influence mechanism of HAZ for Mg-based shipbuilding steel, an effective way to improve the energy weldability of shipbuilding steel would be explored
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