9Cr-1Mo강에 대한 Laser 및 SMAW 용접부의 열 및 역학적 거동에 관한 기초적 연구

Abstract

The pressure vessel and pipe laying used for atomic power plant and petro-chemical plant is subjected to harsh environments such as high temperature, high pressure, hydrogenates environment etc. Low alloy Cr-Mo steel known to be considerably superior for heat, pressure and corrosion resistance is used for the manufacturing above structures. However, welding process generates deterioration of material and welding residual stresses caused by thermal cycles of heating and cooling, which reduce the stability of the weldment. These effects are quite different according to the welding methods (i.e. according to the various heat sources) [1]. Latest technical advance has made welding for thick plates capable by high output of laser welding equipment. Laser welding gives a minimum thermal shock to adjacent quality of the material, accurate welding, minimum welding contraction and transformation and has an advantage obtaining high quality of welding with minimum heating of the material quality when compared with arc welding process. In spite of the merits of laser welding when compared to arc welding, laser welding is mainly used in joining of thin steel parts of electronics industry. Laser welding is getting widely used in joining thick plate and special kinds of steel due to its high power. While the arc welding is still applied for 9Cr-1Mo steel which is the essential materials of atomic power generation equipment, the laser welding is not yet applied despite of its high quality. So it has been applied in a special case demanding high welding quality, such as atomic power plant. Therefore, in this present study, in order to confirm the possibility of applying the laser welding to heat and pressure resistance steel, low alloy 9Cr-1Mo steel was fabricated by SMAW and CO2 laser welding, and the characteristic of welding heat distribution and residual stress in weldments of each welding process have been investigated and analyzed using the two dimensional heat conduction and elasto-plastic simulation. The strength of welded joints has been evaluated through the mechanical tests(hardness, low thermal toughness, bending and tensile). The Charpy test was carried out to understand the effect on the fracture toughness of weldments. The results of examination and test of the mechanical properties showed the validity of this research. Accordingly, in this research, the mechanical properties of weldments by arc and laser welding were investigated using FEM to confirm the applicability of laser welding to 9Cr-1Mo steel.