이종재 SCM440/SS275의 CO2 레이저- GMA 하이브리드 용접시 와이어 종류에 따른 용접 특성

Abstract

Recently, materials with superior corrosion resistance and heat resistance are used due to high capacity of power plants and high temperature and pressure increase of steam conditions. In the case of power generation equipment, a high Cr – Mo steel material is applied in order to simultaneously improve mechanical efficiency, safety and durability to improve thermal efficiency.

In the case of dissimilar materials, cracks due to different physical properties occur, and strict quality control is required. During welding of dissimilar materials, damage to the dissimilar material welds often occurs, mainly due to welding defects in the welds and degradation of strength due to corrosion between dissimilar materials.

In this study, laser – arc hybrid welding test of dissimilar materials (SCM440 / SS275) was carried out to investigate the welding characteristics of dissimilar materials, and to select optimal welding conditions and to study mechanical properties. In order to overcome the problems of FCAW, the welding characteristics of butt welds by laser – arc hybrid welding were compared and evaluated. We used heat resistant steel wire and FC carbon steel wire and observed the mechanical properties and non – destructive testing of the weld to investigate the welding characteristics and structural stability of welded part. The optimum conditions of welding were investigated by evaluating the bead shape and welding phenomenon according to the laser output and moving speed of the main variables.

For optimum welding conditions, the fused part test was performed for the surface bead and back bead. In the hybrid welding of heat resistant steel wire rather than the use of FC carbon steel wire, many low – temperature cracks occurred at a slow moving speed. As the heat input increased with different chemical composition of different species, cold cracking occurred along grain boundaries formed by recrystallization of the structure. Nondestructive tests confirmed the presence of defects in the welds, and no other welding defects were found in the welds, such as circular marks and linear markings.

The tensile, fatigue, and hardness were measured to evaluate the mechanical properties of the welds according to the type of wire. As the welding speed increases, the tensile strength increases. As the welding speed increased, the cooling rate became faster and the tensile and fatigue were increased due to the refining action by grain refinement due to the relatively rapid solidification. The tensile test was carried out at a cross-head speed of 3 mm / min according to ASTM E8M standard, and the fatigue life was considered to be indefinite life span over 2 X 10 ⁶ cycles until fracture and separation of welds.