DIL 805A type fully automatic phase transformer was used to determine the expansion curve of LC3 cryogenic cast steel under different cooling rates between 0.05-150 ℃/s,and the tangent method was used to determine the phase transformation point Ac,of 700 ℃and Ac3 of 849 ℃.Combined with microstructure analysis and microhardness to determine the actual transformation products,and the CCT curve was plotted using Origin software.The results show that the microstructure is ferrite+pearlite when the cooling rate is 0.05-2 ℃/s,ferrite+pearlite+bainite when it is 5-20 ℃/s,bainite+martensite when it is 30-50 ℃/s,martensite+retained austenite when it is 80 ℃/s,and martensite when it is 100-150 ℃/s.Hardness of the LC3 cryogenic cast steel gradually increases from 168 HV10 to 453 HV10 with the increasing of cooling rate.Martensite critical cooling rate is about 80 ℃/s.
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