Reasearch On Phase Transformation Of A508-3 Steel During Continuous Cooling Process Under Stress

A508-3 steel is a kind of steel with low-carbon low-alloy,Its forging is widely used in nuclear pressure vessels.Due to the great volume,the heat transfer rate is different in different parts,resulting in thermal stress and phase transformation stress.Therefore heat treatment relates to the interaction of temperature,stress and phase transformation,Most previous studies have ignored stress.In response to this real phenomenon,we took A508-3 steel as research subjects,and focus on the effect of phase transformation kinetics and microstructure of different cooling rate under stress.Not only expect of explaining the mechanism of the phase transition in theory,but also provideing a reference for practical engineering,The main test procedures and conclusions are summarized as follows:Continuous heating thermal expansion curve was measured by means of dilatometer.We measured the phase transition point, which guides the continuous cooling process.Combined with observation of microstructure and hardness measurement,we draw a continuous cooling transformation curve through continuous cooling expansion curves.According to CCT curve,proeutectoid ferrite, pearlite, bainite and martensitic transformation has been discussed respectively. The result founds that relationship between the starting point of proeutectoid ferritic phase transition and the cooling rate can satisfy the lange model which is disposed by Scheil; Test data of pearlite and bainite transformation meet exponential model satisfactorily; Martensitic transformation meets the varying temperature martensitic phase transformation kinetics equation; As is shown in CCT curve,critical cooling rate of pearlite and bainite is below 0.1℃/s; Critical cooling rate of martensite is between 2℃/s and 5℃/s.Gleeble-1500 D thermo-mechanical simulator is used to finish study on phase transformation during continuous cooling under the stress. It shows that:stress expand cooling range of proeutectoid.Stress promote transformation course of proeutectoid and pearlite.The greater the stress is, the more significant role in promoting; Although stress can increase the starting temperature of bainite transformation, but it do little effect to the finish temperature of bainite transformation. Overall proeutectoid ferrite generates granular structure durning the cooling process following-up.Stress promote nucleation,but restrain growth.Finally it make the rate of bainite transformation slow.Pearlite microstructure is not obvious changed as stress increasing, the number of M/A islands linearly arranged along a certain direction decreases and the same as its chord length. The number of granular M/A islands in granular bainite increases with stress increasing.Gleeble-1500 D thermo-mechanical simulator is used to finish studying on bainite transformation during continuous cooling under the stress,it shows that: uniaxial tensile stress promote bainite transfomation when cooling rate is 0.5℃/s and does the opposite effect on that when cooling rate is 1~2℃/s.Tensile stress play a same role in morphology of bainite as compressive stress.