Effects of Heat Treatment on Microstructure and Mechanical Properties of Alloy 690

Alloy 690 is especially used as materials for steam generator heat pipe of nuclear plant due to its excellent corrosion resistance. Among steam generator components, heat pipe is the part where the high temperature and pressure steam (60atm, 279 .deg. C) is generated by heat exchange caused when the first coolant, high temperature and high pressure (175atm, 310 .deg. C), and the second coolant with low temperature (227 .deg. C) meet. Since the heat pipe of steam generator is where the primary system and the secondary system directly meets, damage to the heat pipe can enormously affect the stability of nuclear plant. Therefore, the heat pipe material should be greatly corrosion resistant and its mechanical characteristics at room temperature and high temperature should be excellent. However, as a result of long-term operation of nuclear plant which creates Grain Boundary Cr carbide discontinuously, defects, such as stress corrosion crack (SCC) and intergranular stress corrosion crack (IGSCC), have been reported. This paper reviewed the effect of heat-treatment condition on a property of matter and micro-structure by analyzing micro-structure through optical and scanning electron microscope. We water quenched Alloy 690 after proceeding solution heat treatment for 20 minutes to 5 specimens for every 100 .deg. C from 800 .deg. C to 1150 .deg. C. As temperature rose, solubility also rose, and solubility did not affect tensile strength much. After 20 minutes of solution heat treatment from 1150 .deg. C, we controlled the cooling rate at 1 .deg. C/min, 5 .deg. C/min, 10 .deg. C/min to 800 .deg. C which is the medium temperature, and we proceeded 100 minutes of aging treatment. As a result, we found out that as cooling rate was high, it took a shape of continuous carbide and fine grain. Meanwhile, low cooling rate resulted in discontinuous carbide shape and significant grain and Cr depletion. Also, cooling rate did not affect tensile strength much