Precipitation behavior of M(23)C(6)carbides and its effect on mechanical properties of Ni-based Alloy 690

Abstract

The effect of solution annealing and aging treatment on the intergranular carbide precipitation behavior of alloy 690 tubes for steam generators is investigated. The carbides precipitated on the grain boundaries were identified as chromium-rich M(23)C(6)carbide, which has a cube-cube orientation relationship with the matrix on one side of neighboring grains. The carbide precipitation behavior is closely related to the interaction between the solubility of carbon in Ni-based matrix and diffusion of alloying elements. The ultimate tensile strength and elongation of the solution annealed alloy 690 tubes at 300 degrees C are 638.8 +/- 12.4 MPa and 43.2 +/- 4.2%, respectively. In the early stage of precipitation, the ultimate tensile strength increased to 720.4 +/- 13.5 MPa. The carbides function as reinforcement in alloy 690 and provide strength enhancement with aging time reached 15 hours. However, when the aging time passed 15 hours, this property gradually decreased to 710.6 +/- 12.6 MPa due to the low bonding strength between the carbide and matrix.