The Precipitation Behavior Of Alloy Carbides In Heat Resistant Steels Under High Magnetic Field

The ring, which is made of reduced activation steel, is to magnetically confine plasma innuclear fusion reactor. It is operated in the condition of high temperature and high magnetic field.Carbide precipitation and evolution play an extremely important role in high temperature creepresistance in the nuclear power and fossil power heat-resistant steels. Insights into carbideprecipitation will contribute to a better understanding of the precipitation behavior of alloycarbides under high magnetic fields.This paper systematically studied the characteristics and laws of the carbide precipitation inFe-C-Mo and2.25Cr-Mo alloy steels. The two kinds of steels were treated under the conditionsof a12-T strong magnetic field in the heat treatment furnace of the vacuum magnetic field.Carbide precipitation type, size and distribution were observed and analyzed by means of opticalmicroscope （OM） and transmission electron microscope （TEM）. Theoretically the magnetizationof a variety of carbides was calculated according to the first principle. The Weiss molecular fieldtheory was used to investigate the magnetic properties of different temperatures of carbides. Theresults are as follows:（1） The alloy carbide precipitation was analyzed in the austenite-ferrite isothermaltransformation of Fe-C-Mo alloy below the “bay temperature”（Tb）. Results show that the strongmagnetic field promoted the precipitation of M₆C carbide. Meanwhile, the Fe atom concentrationwas increased in M₆C carbide by means of measurements on gold replicas specimens. Themagnetic moment was calculated on the basis of the Wien2k software. Theoretically thevariations of the magnetic free energy change for the carbide M₆C and Fe atom concentrationwas very consistent with the experimental results.（2） Strong magnetic field had almost no influence on the morphology of M₆C carbide.Mininum energy principle was used to calculate the magnetic field morphology of carbide. Theresults show that the aspect ratio was approximately equal to1:1, which indicates that themorphology the M₆C carbide is spherical. Actually, the Fe3Mo3C was appeared to be equiaxed inshape.（3） The12-T magnetic field promoted the precipitation of χ-Fe₅C₂, which was not observedin the traditional heat treatment. χ-Fe₅C₂has the higher moment, which enhanced the stability under the high magnetic field through magnetization. Therefore, the influence of the magneticfield on the Gibbs free energy of χ-Fe₅C₂is the most remarkable. The high magnetic field had nonoticeable effect on the hardness of the steels which were tempered at low temperature. It isbelieved that any increase in hardness due to a denser distribution of metastable carbides, causedby the high magnetic field, was counterbalanced by the reduction in carbon content of the matrix.（4） The remarkable reduction of the magnetic Gibbs free energy of M₆C carbides resulted inits higher stability under high magnetic field in Fe-C-Mo alloy tempered at530°C. Therefore, theprecipitation of M₆C carbide was promoted by high magnetic field.（5） The concentration of Fe increased, whereas the concentration of Mo decreased in M₆Calloy carbides according to the measurements on gold replica specimens when specimens weretempered at530°C under12-T magnetic field. The substitutional solute atom concentration hadno obvious change for M₂C and M₃C carbides. The substitutional solute atom concentrationchange in high magnetic field was mainly attributed to the change of magnetic free energyresulted from the magnetization differences of Fe and Mo atoms in each alloy carbide.（6） The effect of a12-T high magnetic field on alloy carbide precipitation in the2.25Cr-Mosteel during tempering at low and high temperature was investigated, which leads to thefollowing conclusions. The concentration of substitutional solute atoms Fe in the carbide ofM₂₃C₆and M₇C₃was greatly influenced by strong magnetic field. The higher Fe atom contents inthe M₂₃C6and M₇C₃carbides make the magnetization increase remarkably, which thus reducethe magnetic Gibbs free energy of alloy carbides. Therefore, the magnetic heat treatment canmake stable alloy carbide precipitate in advance, which can provide a new approach to improvethe creep properties for the power plant steels.（7） At the high tempering temperature of700°C, high magnetic field had almost noinfluence on the morphology, nucleation site, the precipitation amounts, the precipitationsuequence and the substitutional solute concentration of different carbides in Fe-C-Mo and2.25Cr-Mo alloy steels, because the carbides were changed into paramagnetism at the hightemperature.（8） Regarding the theoretical calculation of the magnetic Gibbs free energy, thedemagnetization field of the carbide was considered when a high external magnetic field wasapplied. The internal field induction of carbides was accordingly calculated. Insights into the effect of the magnetization and the internal field induction on the magnetic Gibbs free energycontributed to the deep understanding of the carbide precipitation mechanisms.