Effects of Cooling Rate on Microstructure Evolution and Hardness of EH40 Steel
The effects of cooling rates on the microstructure evolution,precipitation and hardness of EH36 were studied by using OM,TEM and hardness test instrument.The results show that the microstructure of the steel changes with the change of cooling rate.When the cooling rate is ≤ 1.0 ℃/s,the microstructure of the steel is polygonal ferrite and pearlite.When the cooling rate is 1.0-3.0 ℃/s,granular bainite appears.When the cooling rate is 3.0-10.0 ℃/s,polygonal ferrite changes to quasi-polygonal ferrite and bainite appears.When the cooling rate is 10.0-50.0 ℃/s,only bainite exists in the microstructure.The precipitation of the steel include small size precipitation with the diameter less than 10 nm and large size precipitation with the diameter between 80 nm and 150 nm.The number of small precipitation decreases while the number of large precipitation does not change significantly.Furthermore,as the cooling rate increases,the hardness of the steel first increases rapidly and then increases slowly,and at last trends to be stable.
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