SA-765 Gr.Ⅱ钢过冷奥氏体的连续冷却转变行为

Continuous cooling transformation behavior of supercooled austenite of SA-765 Gr.Ⅱsteel

为了深入了解SA-765 Gr.Ⅱ钢在冷却过程中的相变行为和组织演变规律,采用FTF-340型Formastor-FⅡ型全自动相变测定仪测定了SA-765 Gr.Ⅱ钢在不同冷却速度下的膨胀量-温度曲线,使用切线法确定了临界转变温度点,并结合金相检测与硬度测试,绘制出了试验钢的CCT曲线,并分析了冷却速度对其显微组织、显微硬度和CCT曲线的影响。结果表明:试验钢的临界转变温度A_(c1)为729℃,A_(c3)为830℃。在冷速为0.05~0.1℃/s时,主要发生铁素体和珠光体转变;在冷速为0.2~1℃/s时,主要发生铁素体、贝氏体和珠光体转变;在冷速为2~5℃/s时,主要发生铁素体和贝氏体转变;在冷速为10℃/s时,只发生贝氏体转变;在冷速为30~45℃/s时,主要发生贝氏体和马氏体转变。SA-765 Gr.Ⅱ钢的维氏硬度由冷却速度为0.05℃/s时的164 HV2缓慢增加到2℃/s时的207 HV2,后又快速增加至45℃/s时的365 HV2。

In order to gain a deeper understanding of the phase transformation behavior and microstructural evolution of SA-765 Gr.II steel during cooling,the FTF-340 Formastor-FⅡautomatic phase transformation analyzer was used to measure the expansion-temperature curve of the SA-765 Gr.Ⅱsteel 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 A_(c1)and A_(c3)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 bainite 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.Ⅱsteel 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.