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In this paper, the effect of the electro gas arc welding (EGW) process with a heat input of 600 kJ/cm on the microstructural and mechanical characteristics was investigated for two types of EH40 high strength hull structural steels, containing Ti and Nb. The coarse grain heat affected zone (CGHAZ), with a peak temperature of 1450 °C, was simulated for both alloys using a material thermal cycle simulator. Tensile and Charpy impact toughness tests were performed to determine the mechanical properties of the CGHAZ, and microstructural analysis was performed using scanning electron microscopy and transmission electron microscopy. Both alloys exhibited a significant loss of impact toughness because of the welding thermal cycle, causing the formation of a brittle phase and the growth of precipitates. The experimental results indicated that although the addition of Ti and Nb had several benefits on mechanical properties, this might be unsuitable for the EGW process owing to the resulting deterioration in impact toughness.

This study examined the effect of the line-heating process on the microstructure and mechanical properties of 320-MPa-grade high-strength steel designed for low-temperature applications. The samples were heated thrice to the maximum temperature condition of 900 ℃ prior to air cooling or water quenching. Tensile strength and Charpy impact toughness tests were performed to understand the variation in the mechanical characteristics across the thermal histories. The decrease in the tensile strength of the air-cooled specimen was attributed to grain growth and phase transformation caused by slow cooling, while the water quenched specimen exhibited grain refinement. The experimental results indicated that the line-heating process was suitable because all the specimens with various thermal histories satisfied the international classification standard.