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Abstract
In recent years, the increase in the development of petroleum resources and the increase in transport efficiency in polar regions have led to the demand for higher strength and low temperature toughness of steels for vessel. Weld metals are made by adding alloying elements to improve toughness. Among them, Nickel (Ni) is known to increase the low temperature toughness. However, it is known as an element promoting solidification cracking of weld metal, but the influence of carbon steel is not clearly defined. The purpose of this study is to evaluate the effect of Ni content on solidification cracking in the weld metal of low carbon steel. Four types of flux-cored arc welding (FCAW) wires with different Ni contents (0 to 3.5 wt.%) were prepared and subsequently welded on plate. Although longitudinal solidification cracks were detected for every wire, it was found that the cracking susceptibility depends on the Ni content. That No., the total crack length as well as maximum crack length decreased with an increase in Ni content up to 2.5 wt.%, But the weld with 3.5 wt.% Ni resulted in high susceptibility on solidification cracking. The thermodynamic calculation showed that solidification mode was changed from F mode to FA mode, when the Ni content increased from 0 to 2.5 wt.%. This result indicated that the finer columnar grains and low interfacial energy could be obtained by increasing Ni content in this range, which was helpful to reduce the susceptibility to solidification cracking. On the other hand, the weld metal with 3.5 wt.%
Ni was solidified by A mode, where the primary phase was changed from -ferrite to austenite. Since the austenite had a relatively low solubility for Sulphur (S), the S segregation was readily produced during solidification and the cracking susceptibility increased in this region. Compositional analysis result also confirmed the S segregation near the crack tip region in the weld metal with 3.5 wt.% Ni. These results reveal that the maximum allowable Ni content is 2.5 wt.% to prevent the solidification cracking in the low carbon steel weld metal.