화력발전설비 리턴휠(Return Wheel)의 이종재 SS400/SCM440에 대한 레이저-아크 하이브리드 용접기술 개발

Abstract

Coal-fired thermal power plants are made of materials that are superior in corrosion resistance and heat resistance due to high capacity of power plants, high temperature and high pressure of steam conditions, and high efficiency due to the combined cycle system, and the application of dissimilar materials made by mixing these materials is expanding. The return wheel, which is a component of coal-fired power generation plant, is produced by applying flux cored arc welding, which is advantageous for cost reduction and welding automation, to dissimilar materials of mechanical structural alloy steel and general structural alloy steel, which are excellent in abrasion resistance and toughness. However, it is required to secure the reliability of the welded part due to the occurrence of welding defects such as pores and cracks in the welded metal part and the heat affected part of the arc welding when producing the heterogeneous return wheel. In this study, laser-arc hybrid welding was applied to heterogeneous materials (SCM440 / SS400). Laser power, feed rate, laser-arc distance, arc current and voltage were selected as main process variables and mechanical and metallurgical characteristics were evaluated according to process variables. Tensile tests and hardness tests were carried out, weld quality was evaluated according to international standards, and metallurgical properties were confirmed by optical microscope.

From a result of the tensile test of the welded joints, it was confirmed that the tensile strength increases and the elongation rate increases as the welding speed is fast and the laser power is high. Especially, the maximum tensile strength of 424MPa and the SS400 base material under the condition of laser output 12kW welding speed 2m / min (# 9) showed ductile fracture and elongation of 24.5%. # 5, # 6 and # 8 showed a low elongation of 2.7 ~ 8.5%, which is considered to have affected the elongation due to the low elongation and high hardness.

From a result of the hardness test of the welded joints, the hardness value was slightly lower at the bottom of the weld, and the maximum hardness value occurred at the heat affected zone of SCM. It is considered that the structure of acicular shape was formed at the SCM heat affected part and showed high hardness value.

From a result of observation of the microstructure of the welded joints, the arc zone was affected by the laser and the arc heat source at the same time, resulting in a relatively coarse columnar crystal due to high heat input per unit area. On the other hand, as the heat source of the arc has almost no effect on the laser zone, the microstructure of the equiaxed dendrite is observed due to the rapid cooling rate compared with the arc zone due to the influence of the heat source of the laser.