Laser Multi-Pass Narrow-Gap Welding – A Promising Technology for Joining Thick-Walled Components of Future Power Plants

Abstract

Today, the average worldwide efficiency of coal-fired power plants stands at about 33 percent. The consistent use of state of the art technologies would enable an increase of the average efficiency of up to 47 percent and thus a sharp reduction of greenhouse gas emissions. The importance of improvements in this field becomes apparent when reviewing e.g. plans in Europe in 2017 for new power plants to be built across the continent. About 44 percent of the envisaged 153 gigawatts are still to be generated by fossil-fuel power plants [1]. One technical solution is to increase the steam turbine inlet temperature to 700°C. This, however, requires the use of nickel-based superalloys. Only these alloys satisfy all the requirements with regard to high-temperature, corrosion and oxidation resistance and creep behavior [2], [3]. Due to their relatively poor machinability, forgeability and high material costs compared to the steel-based alloys they are to replace, a more effective welding technology is needed to overcome the disadvantages of conventional welding technologies, i.e. large quantities of filler metal required and high energy input per unit length resulting in distortion and the potential reduction of high-temperature properties.

Extension of the process limits in laser beam welding of thick-walled components using the Laser Multi-Pass Narrow-Gap welding (Laser-MPNG) on the example of the nickel-based material Alloy 617 occ

“…Differences result from the fact that higher energies per unit length are used in the upper welding passes. However, heat treatment of the specimens generally leads to a homogenization, which means that a homogeneous precipitation of carbides is set both in the grain inner and at the grain boundaries, and as a result, the locally higher hardness of the weld drops to the level of the base metal [11,12].…”

“…Fig. 17 Laser-MPNG sample P6 with a weld defect in the form of a lack of fusion to the sidewall as a crack initiator [11] Fig. 18…”

“…6 Left: cross section of the pipe; right: longitudinal cut Fig. 7 remoweld®MPNG weld head prototype [11]…”

“…15 Top left: weld seam stabilization annealing (980°C/3h); top right: detailed view of the weld seam structure; bottom left: detailed view of the HAZ, fusion line, and weld seam structure; bottom right: SEM detailed view of the precipitated carbides in the weld metal Fig. 16 Comparison of LCF properties of base material and Laser-MPNG samples [11] morphology of the carbides inside the grain and at the grain boundaries, which were investigated in more detail below for a creep test specimen.…”

Interlaminar Microstructure and Mechanical Properties of Narrow Gap Laser Welding of 40-mm-Thick Ti-6Al-4V Alloy

“…Nevertheless, the application of EBW is significantly limited by its vacuum condition [ 13 , 14 ]. Narrow gap laser welding applies a laser to melt a filler material and fill a narrow U-shaped groove between two plates, which is a highly efficient process especially for welding thick metal plates [ 15 , 16 ]. Many researches have reported that NGLW has superior ascendancy over other processes related to the welding of thick plates.…”