Study On The Transformation Mechanism Of Pearlitic During Isothermal Condition After Deep Undercooling (DUIT)

The isothermal transformation kinetic curve of pearlite performed at the deep undercooling(DUIT) was studied in this paper by rapid phase change apparatus DIL805 high purity helium instead of lead bath quenching process. And the OM, SEM and TEM were applied to study the controlling of the pearlite microstructure on the fine lamellar and small group size. The effect on DUIT curve and microstructure of pearlite at different temperature or time of deep undercooling based on the fact that the process of DUIT have a longer time in undercooling than IT. The experiment results showing as follows:1. IT curve was a smooth curve C-shaped, the isothermal transformation curve of pearlite and bainite were not separated, the nose temperature at about 560?. The incubation period of hypoeutectoid steel increased with the increasing of Mn content in the process of undercooling austenite IT, With the increase of IT temperature, the microstructure is martensite, bainite, a mix microstructure containing bainite and fine pearlite and completely pearlite, respectively. The uniform sorbite obtained at 530~560? isothermal, and the larger pearlite lamellar spacing and significantly proeutectoid ferrite were observed at 620? isothermal process.2. The incubation period increased and C curve moved to right with the decrease of isothermal temperature and the extension of isothermal time of the DUIT kinetics curve, Transformation start line tends high isothermal transformation temperature and straight, but the incubation of the end of the line from 560? to 590?,Meanwhile, the time consumed in pearlite transformation process was reduced, specifically in the kinetics. and the size of sorbate become uniformed and fining with the decrease of temperature and increase of time. Compared the microstructure of steel after DUIT process with the IT process, it is found that the pearlite lamellar spacing decrease significantly with the decrease of isothermal temperature at the IT process, and the pearlite lamellar spacing changes slightly in DUIT process with the decrease of isothermal temperature and the extension of isothermal time.3. The model of the eutectic growth controlled by diffusion and the eutectoid growth controlled by interfacial diffusion revealed that the lamellar spacing size are inversely to the undercooling. Undercooling and the diffusion coefficient combined effect of the theory nucleation rate equation with the information of great driving force beneficial to improve the nucleation rate and growth rate. Combining the kinetics curves and microstructure results: the undercooling increased the driving force, and it improve the nucleation rate and growth rate, which refinement the group size and spacing size respectively. 4.The isothermal transformation after deep undercooling were the supplement and correction for the isothermal transformation. The lamellar spacing size are inversely to the undercooled, and also refinement by improve the driving force in some form to increasing growth rate.