Abstract:The quench sensitivity of 6351 alloy was determined by the time-temperature-transformation (TTT) curves and time- temperature-property (TTP) curves by an interrupted quench technique with measurement of as-aged hardness and as-quenched electro-conductivity. The microstructure transformation during isothermal treatment was studied by the transmission electron microscopy (TEM) and Avrami equation. The results showed that the electro-conductivity of the 6351 alloy increased and the hardness decreased with prolonging the holding time at a certain isothermal temperature. The TEM observation indicated that the supersaturated solid solution decomposed and needles β″ precipitated at the initial stage of isothermal holding. With the prolongation of holding time at the nose temperature, rod β′ and plate β phases formed. The isothermal transformation rate at 360 °C was the fastest, and became slow at 280 °C and reached the slowest at 440 °C. The nose temperatures of the TTT and TTP curves were about 360 °C and the high quench sensitive temperature range was 230-430 °C. The quench factor analysis indicated that the cooling rate should be more than 15 °C/s in the quench sensitive areas in order to get optimal mechanical properties.

Key words: 6351 alloy; quench sensitivity; microstructure; cooling rate; quench factor analysis; TTT curve; TTP curve