Microstructure and deformation characteristics of inconel 690 alloy for steam generator tubes; (I) Precipitation behavior of carbides

摘要

The effects of thermal treatment on themicrostructure evolution of Inconel 690 alloy were investigated.The carbides were precipitated mainly at grain boundaries bythermal treatment at 720℃. The intergranular carbides wereidentified as M26C6 type and have cube-to-cube orientationrelationship with the matrix. The size and shape of intergranularcarbides were sensitively dependent on the thermal treatment timeat 720℃. The average length and thickness of intergranularcarbides increased with increasing thermal treatment time. Theintergranular carbides showed inhomogeneous distribution afterthermal treatment for 1hr, and were changed into semicontinuousafter thermal treatment for 10hrs. The average distance betweenintergranular carbides showed the minimum value after thermaltreatment for 10 hrs due to the change of carbides fromsemicontinuous into discontinuous morphologies. Themorphology of intergranular carbides did not affect the mechanicalproperties, such as yield strength, ultimate tensile strength andelongation. The solid solution temperature of intergranularcarbides was measured as 1120-1140℃. The intergranularcarbides hindered the grain growth during thermal treatment belowthe solution treatment temperature due to the Zener dragging ofgrain boundaries.