STK 400 합금의 마찰접합 특성에 관한 연구

Abstract

We evaluate the properties of solid-state-welded STK400 steel tube in terms of the relationship between microstructures and mechanical properties. As a solid-state-welding process, the friction welding which is suitable for tube and rod type metals was introduced in this study. Prior to this, STK400 steel tube with a diameter of 60.5 mm, which has a thickness of 3.3 and 2.2 mm, was prepared by length of 170 and 70 mm, and friction welding was conducted at a rotation speed of 2,000 rpm and upset time of 3 - 7 sec., respectively. To analyse the grain boundary characteristic distributions (GBCDs) such as grain size, shape, misorientation and phase distributions for the welded materials, electron backscattering diffraction (EBSD) method was introduced. As a result, the application of friction welding on STK 400 alloy led to the sound weld without any weld defects such as crack, distortion, fracture and so on. In addition, the welded zone consists of fully equiaxed grains due to dynamic recrystallization occurred by friction heat and metallic flow during welding. These refined grains achieved the significantly enhanced mechanical properties such as microhardness and yield and tensile strengths. Consequently, Vickers microhardness and yield and tensile strengths are increased to 10, 12 and 30% relative to the base material, respectively. In particular, all the tensile tested specimens showed the fracture aspect at the base material zone not at welded zone, which means the soundly welded state at all conditions.