In-situ Observation Of Bainite Phase Transformation Process In Low Alloy Steel

SA508-3steel, a low-carbon and low-alloy steel, is widely used in nuclear reactorpressure facilities due to its sufficient strength, high fracture toughness, forgeability,anti-neutron irradiation, low temperature impact toughness, and excellent welding properties.The main structure in SA508-3steel is bainite, but the bainite phase transformation oflow-alloy steel during continuous cooling is not very distinct. In this paper, in-situobservation of bainite transformation during continuous cooling of SA508-3steel wasconducted using high temperature confocal laser-scanning microscopy. At the same time, theinfluences of nitride, cooling rate, cooling manner and electropulsing pretreatment on thebainite transformation process were studied.Nitrides on the sample surface can be obtained by atmosphere protection in the hightemperature confocal laser-scanning microscopy. The results showed that the amount and sizeof precipitated nitrides increased, the precipitation temperature decreased with increasingcooling rate. Cooling manner can change the precipitation of nitrides. The density ofprecipitated nitrides increased during two stages cooling. Electropulsing pretreatmentpromoted the growth of nitrides during continuous cooling. The bainite nucleated at nitrides,austenite grain boundaries and formed bainitic ferrite interface when the sample was cooled atthe rate of60℃/min and120℃/min. However, nitrides did not play an important role onthe induction nucleation of bainitic ferrite. With the decrease of temperature, the growth rateof bainite group and sheaves increased at the cooling rate of60℃/min,120℃/min and120℃/min. A bainite group grew at a constant lengthening speed. The growth speed of abainite group gradually increased with the increase of cooling rate at the same temperaturerange. Compared with the sample with normal pretreatment, the start temperature of bainitetransformation in the sample with electropulsing pretreatment increased about28.3℃. Theaustenite grain boundaries played an important role on the nucleation of bainitic ferrite.Compared with the layer growth of single bainite sheaf in the conventional pretreatmentsample, a bainite group pushed forward into the grain in the sample that underwent theelectropulsing pretreatment at the same temperature range. Electropulsing pretreatmentincreased the lengthening speed of bainite sheaf. The lengthening speed of bainite sheafnucleated at the austenite grain boundaries in the sample with conventional pretreatment washigher than the lengthening speed of bainite sheaf nucleated at nitrides at the sametemperature range.