| As a low carbon and low alloy steel, the SA508-3steel was used widely in nuclearreaction pressure container due to its ideal weldability, plastic property and ductility,forgeability, resistance to neutron irradiation and impact flexibility at low temperature. In thispaper, the pulse electric current and magnetic field as a supplementary mean was firstlyemployed to influence the bainite transformation of SA508-3steel. The bainite transformationprocess of SA508-3steel during continous cooling was in-situ observed by high temperatureconfocal scanning laser microscope. The effect of pulse electric current and pulse magneticfield complex pretreatment on the subsequent bainite transformation during continuouscooling in SA508-3steel was studied.For investigating the influence of pulse electric current and pulse magnetic field complextreatment on the ferrite, M/A island and dislocation configuration in granular bainite, the highdensity pulse electric current was emplyed to heat the SA508-3steel, and the pulse magneticfield is applied during continous cooling and isothermal treatment.The results show that thestarting and finishing temperature (BS) and (Bf) of bainite transformation is increased about35℃and60℃when1.5Tpulse magnetic field is appled during air cooling. The range ofbainite transformation temperature is decreased from140℃during air cooling to115℃during air cooling with1.5T pulse magnetic field. The microstructure of SA508-3steel afterholding at920℃for10min and air cooling consists of self-tempering martensite andgranular bainite. But, the microstructure is composed of martensite, granular bainite and lowerbainite when1.5T pulse magnetic field was applied during air cooling, at the same time, thequantity and size of M/A islands are increased and bainite ferrite laths become thinner. Withthe increase of pulse magnetic field intensity, BSis increased gradually. The BSis580℃、620℃、625℃、635℃c orrespondent to0T，0.5T，1T，1.5T, respectively. The formationof isothermal granular bainite will be accelerated when pulse magnetic field is appled duringisothermal bainite transformation.For studying the influence of pulse electric current and pulse magnetic field complexpretreatment on bainite transformation dynamics, the SA508-3steel is pretreated undernormal heating furnace and the pulse electric current adding pulse magnetic field, then in-situobserve the formation and growth of bainite during continous cooling. The results show thatthe precipitation temperature isdecreased, the quantity and size of nitrides are increased inSA508-3steel after pulse electric current and pulse magnetic field complex pretreatment.Compared with the normal pretreatment, the starting and finishing temperature of bainitetransformation are increased during continous cooling and the time of phase transformation isprolonged after pulse electric current and pulse magnetic field complex pretreatment. Thegrowth rate of bainite is increased, the length of bainite ferrite lath is raised and the spacelength between ferrite laths is reduced. In the room-temperature microsturcture after in-situ observation, the pulse electric current and pulse magnetic field complex pretreatment makethe quantity and length of bainite ferrite lath increase, the length of M/A islands in granularbainite is increased. |
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