Continuous cooling transformation curve of a weathering steel

Continuous cooling transformation(CCT) curve of Al-Si weathering steel was obtained by measuring the dilation of specimen during cooling on Gleeble-1500 thermal mechanical simulator combining with microstructure observation.The results show that the transformation zones of pearlite and bainite are separated in the CCT diagram.And there is no metastable austenite zone between the transformation zones of pearlite and bainite.The microstructure of ferrite and pearlite is obtained at the cooling rate below 1℃/s.Small amount of granular bainite is observed at the cooling rate of 1℃/s.Volume fraction of bainite increases while the volume fractions of ferrite and pearlite decrease as the cooling rate increases.The microstructure mainly consists of ferrite and bainite when the cooling rate ranges between 5℃/s and 30℃/s.For the rate beyond 30℃/s,martensite begin to appear.Bainite disappears and the microstructure mainly consists of martensite when the cooling rate reachs 80℃/s.And only martensite can be obtained after water quenching.Ferritic transformation zone moves up to left and bainitic transformation zone moves down to left because of deformation of austenite at high temperature.