Mechanical and microstructural properties of alloy 800

Abstract

The evolution of microstructural and mechanical properties of alloy 800 with respect to operating conditions of the steam generator tubings of fast breeder reactors have been analyzed and presented. On the microstructural side two phenomena have important influence on the mechanical properties, namely γ′ and carbide precipitation. Gamma prime precipitation occurs in alloy compositions containing ≳0.50 pct Ti + Al, inducing mechanical property changes and, in particular, improving the long term creep resistance. Its growth rate follows the exponential law which, when extrapolated, yields an overaging time beyond 4 × 10⁴ h at ≲550°C. M₂₃C₆ carbide precipitation starts in early stages of exposure at 500 to 600°C, being of heterogeneous nature and forming mainly on the grain boundaries. The M₂₃C₆ carbides advance perpendicular to the surface of one of the austenite grains, commonly having a <110>_γ//<110>M₂₃C₈relationship, and occasionally develop into well defined cellular precipitates. On the mechanical side, a sharp creep ductility decline is observed when either, or in particular both, strong γ′ strengthening and discontinuous precipitation develop in the matrix. It is argued that this decline is principally due to the latter phenomenon and is accentuaged by matrix strengthening.