Study On The Evolution Of Isothermal Transformation Of Medium Carbon Alloy Steel

This work mainly studied the isothermal transformation for50Si2Mn3steel at higher temperature under19.8T magnetic field, isothermal bainitic transformation for50Si2Mn3and30Si2Mn2steel without magnetic field. The evolution process of medium carbon alloy steel from high to low temperatures were investigated, including the nucleation positions, growth, microstructure, thermodynamic of magnetic field-induced pearlite, etc. And it was discussed that the influence of magnetic effect on solid-state phase transformation.The result that pearlite isothermal transformation for50Si2Mn3steel at higher temperatures above the eutectoid point with19.8T high magnetic field applied suggested:Compared with that by applying12T magnetic field, the magnetic induced pearlite transformation can occur at higher temperatures under19.8T magnetic field. The pearlite, ferrite and cementite fractions transformed which are associated with isothermal temperature and holding time follow the phase transformation theory. The transformed volume fraction increases with holding time under the conditions of the same isothermal temperature, and decreases with the isothermal temperature when holding time is the same. And it is larger on the longitudinal section that parallels to the direction of magnetic field. Pearlite ferrite as first leading phase under strong magnetic field can grow at a faster speed compared with cementite. The fraction ratio of two constituents has been in a dynamic change during the evolution of pearlite. At995and1008K, lamella spacing of pearlite increased with holding time, and stabilized at983K. Spacing of cementite also showed the same changes, while spacing of ferrite has been stable at995K due to ferrite grows preferentially. For the same holding time, microhardness of matix and pearlite increased at first, lowered later, and peaked in1008and995K respectively as the isothermal temperature increased. At the same isothermal temperature, microhardness of the matrix is slightly lower with the extension of holding time, almost kept the same at1008and1018K. Microhardness of pearlite decreases with the holding time. Furthermore, spheroidal pearlite was promoted under high magnetic field. The result of isothermal bainitic transformation of30Si2Mn2and50Si2Mn3steel indicates:Pearlite ferrite is the leading phase in pearlite phase transformation. In according with this, bainite ferrite nucleated as the leading phase in bainite transformation for these two medium carbon alloy steels. And the ferromagnetic phase, ferrite account for a significant proportion. The isothermal bainite transformation was studied from the point of magnetic energy. When plused magnetic field being applied, magnetic energy offered by magnetic field is the main items of magnetic driving force. In the absence of additional magnetic field, ferromagnetic material has spontaneous magnetization while the temperature dropped below the Curie temperature. Spontaneous magnetization should also be taken into consideration as a item of magnetic driving force source. It is found γ→α in all phase transformation during austenite decomposition from high to low temperature in steels, namely paramagnetic to ferromagnetic phase transition. Although the bainite isothermal transformation under the strong magnetic field cannot measure smoothly, we can also dig through the pearlite, bainite and martensite phase transformation in magnetism sense to gradually explore the correlation of magnetic effect in the role of solid-state phase transformation.