Influence of Heat Input on the Microstructure and Impact Toughness in Weld Metal by High-Efficiency Submerged Arc Welding

Abstract

The development of high-efficiency multi-wire submerged arc welding technology in bridge engineering has been limited due to the high mechanical performance standards required. In this paper, weld metal was obtained by welding at three different high heat inputs with the laboratory-developed high-efficiency submerged arc welding wire for bridges. The effect of changing different high heat inputs on the microstructure and impact toughness of high efficiency submerged arc weld metal was systematically investigated by cutting and Charpy V-notch impact tests at −40 °C, using optical microscopy, scanning electron microscopy, energy-dispersive electron spectroscopy, electron backscatter diffraction, and transmission electron microscopy to characterize and analyze. With the increase in heat input from 50 kJ/cm to 100 kJ/cm, the impact absorption energy decreased significantly from 130 J to 38 J. The number of inclusions in the weld metal significantly decreased and the size increased, which led to a significant decrease in the number of inclusions that effectively promote acicular ferrite nucleation, further leading to a decrease in the proportion of acicular ferrite in the weld metal. At the same time, the microstructure of the weld metal was significantly coarsened, the percentage of high-angle grain boundaries was decreased, and the size of martensite/austenite constituents was significantly increased monotonically. The crack initiation energy was reduced by the coarsened martensite/austenite constituents and inclusions, which produced larger local stress concentrations, and the crack propagation was easier due to the coarsened microstructure and lower critical stress for crack instability propagation. The martensite/austenite constituents and inclusions in large sizes worked together to cause premature cleavage fracture of the impact specimen, which significantly deteriorated the impact toughness. The heat input should not exceed 75 kJ/cm for high-efficiency submerged arc welding wires for bridges.

Insight into the Role of Mo Content on the Microstructure and Impact Toughness of X80 Thick-Walled Low-Temperature Pipeline Steel

“…Generally, grain refinement can improve the impact toughness [14,25,29,30]. In this study, MED was found to decrease with increasing Mo content, but rather than enhancement, the impact energy deteriorated.…”

Effect of Mg Addition on Inclusions in the Welding Heat-Affected Zone of Pressure Vessel Steels

“…As production demands increase, traditional welding methods can no longer meet the requirements. In contrast, high-energy welding can significantly improve welding efficiency [3], reduce the number of passes, and shorten the welding cycle due to its advantage of higher heat input [4,5]. However, the higher wire energy and slower cooling rate lead to a weakening of the lowtemperature impact properties of pressure vessel steels in the welding heat-affected zone.…”