Hybrid laser arc welding of thick high-strength pipeline steels of grade X120 with adapted heat input

The influence of heat input and welding speed on the microstructure and mechanical properties of single-pass hybrid laser arc welded 20 mm thick plates of high-strength pipeline steel X120 were presented. The heat input was varied in the range of 1.4 kJ mm(-1) to 2.9 kJ mm(-1), while the welding speed was changed between 0.5 m min(-1) and 1.5 m min(-1). A novel technique of bath support based on external oscillating electromagnetic field was used to compensate the hydrostatic pressure at low welding velocities. A major advantage of this technology is, that the welding speed and thus the cooling time t(8/5) can be variated in a wide parameter window without issues regarding the weld root quality. The recommended welding thermal cycles for the pipeline steel X120 can be met by that way. All tested Charpy-V specimens meet the requirements of API 5 L regarding the impact energy. For higher heat inputs the average impact energy was 144 +/- 37 J at a testing temperature of - 40 degrees C. High heat input above 1.6 kJ mm(-1) leads to softening in the weld metal and heat-affected-zone resulting in loss of strength. The minimum tensile strength of 915 MPa could be achieved at heat inputs between 1.4 kJ mm(-1) and 1.6 kJ mm(-1).