Continuous cooling transformation behavior of supercooled austenite of SA-765 Gr.Ⅱ steel
In order to gain a deeper understanding of the phase transformation behavior and microstructural evolution of SA-765 Gr.11 steel during cooling,the FTF-340 Formastor-F Ⅱ automatic phase transformation analyzer was used to measure the expansion-temperature curve of the SA-765 Gr.Ⅱ steel at different cooling rates.The critical transformation temperature points were determined using the tangent method,and the CCT curve of the experimental steel was drawn by combining metallographic observation and hardness testing.The effect of cooling rate on its microstructure,microhardness and CCT curve was analyzed.The results show that the critical transformation temperature Ac1 and Ac3 of the experimental steel is 729 ℃ and 830 ℃,respectively.At the cooling rate of 0.05-0.1 ℃/s,ferrite and pearlite transformations mainly occur.At the cooling rate of 0.2-1 ℃/s,ferrite,bainite and pearlite transformations mainly occur.At the cooling rate of 2-5 ℃/s,ferrite and hainite transformations mainly occur.At the cooling rate of 10 ℃/s,only bainite transformation occurs.At the cooling rate of 30-45 ℃/s,bainite and martensite transformations mainly occur.The Vickers hardness of the SA-765 Gr.Ⅱ steel slowly increases from 164 HV2 at the cooling rate of 0.05 ℃/s to 207 HV2 at 2 ℃/s,and then rapidly increases to 365 HV2 at 45 ℃/s.
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