Effect of Process Variables for Reducing Assist Gas Pressure in 50 mm-Thick Stainless Steel Underwater Laser Cutting

Abstract

While dismantling nuclear power plants, the reactor vessel internal is cut underwater using mechanical and thermal cutting. In laser thermal cutting, assist gas must be used to remove melted metal; consequently, a large number of radioactive aerosols can be generated. To reduce the generation of aerosols, the assist gas pressure should be lowered. However, below the pressure limit, the molten metal is not well-removed from the cut surface and fails to cut. In this study, an assist gas visualization experiment was performed to find a condition for the gas to flow well inside the cut surface, even at low pressures. The top kerf width, nozzle type, distance between nozzle and specimen, and assist gas pressure were selected as process parameters, and in the case of large top kerf width condition, assist gas was able to penetrate deeply. In the actual laser-cutting experiment, the laser beam focus position was set to −20 mm and −30 mm. In the case of −30 mm, the top kerf width was widened due to the characteristics of the laser beam profile, and cutting was successful even though the assist gas pressure was lowered by 20%.