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The quench sensitivity of a 6351 alloy was determined by time-temperature-transformation (TTT) curves and time-temperature-property (TTP) curves by an interrupted quench technique with the measurement of the hardness as aged and electro-conductivity as quenched.The microstructure transformation during isothermal treatment was studied by the transmission electron microscope (TEM) and Avrami equation.The results showed that the electro-conductivity increased and the hardness decreased with the holding time prolonged at a certain isothermal temperature.TEM microstructure observation indicated that the supersaturated solid solution decomposed and needles β" precipitated during the initial stage of isothermal holding.With the prolongation of keeping time at the nose temperature, rods β and plate β phases formed.The isothermal transformation rate at 360℃ is the fastest, while that at 280 ℃ slower, and that at 440℃ the slowest.The nose temperature of TTT and TTP curves is about 360℃ and the high quench sensitive temperature range is about 230-430℃.The quench factor analysis indicated that the cooling rate should be more than 15℃/s in the quench sensitive areas during quenching to get optimal mechanical properties.