Evolution Of Structure And Property Of High Si F/M Steels And Heat Deformation Behavior Of Constructed Billets

Ferritic martensitic（F/M）steel has become one of the main candidates for the in-rear components of Generation IV advanced nuclear power systems due to its good resistance to neutron irradiation swelling and excellent fatigue resistance.Currently,F/M steels are inadequate in terms of resistance to liquid metal corrosion in service environments,and there is an urgent need to design nuclear materials with better performance.In addition,the large size of the components in nuclear reactors is usually prepared by welding technology,but the performance of the welded joints is not guaranteed in the service environment,and there is an urgent need to find a new connection technology for the integrated preparation of large size components.Therefore,this paper designs and prepares four different Si content F/M steels,discusses the influence of Si on the tissue and properties of F/M steels,investigates the evolution of the tissue and properties of F/M steels with different Si content after long aging at 550°C,and also uses the heat deformation joining technique to explore the best heat deformation joining process in order to realize the preparation of large size F/M steels.According to the design principles of F/M steel,the ratios of elements were reasonably regulated,the compositions of F/M steels with different Si contents were designed based on Thremo-Calc data,and forgings with different Si contents were prepared by vacuum melting and three-way forging,after which the best heat treatment process for F/M steels was determined.F/M steels meet the design requirements of composition and tissue.The main conclusions are that Si can promote the precipitation of M₂₃C₆ carbide and Laves phase,and the high-resolution electron diffraction technique shows that the M₂₃C₆ carbide and the matrix are in a co-lattice relationship;with the increase of Si content,the F/M steel’s The room temperature strength of F/M steel decreases and then increases with increasing Si content,which is caused by the competition between the solid solution strengthening effect of the elements and the precipitation strengthening effect of the precipitated phases;with the increase of Si content,the room temperature impact work continues to decay,and the large amount of precipitation of Laves phase is the main reason for the decrease of the impact performance of the material.The F/M steels with different Si contents were subjected to long aging treatment at 550°C to study the evolution of the organization and properties of F/M steels after high temperature long aging.The results show that the F/M steel has good stability of tissue and mechanical properties,which is mainly the result of the dual action of the precipitation phase and the matrix tissue.During high-temperature aging,the martensitic slats revert to form small-angle grain boundaries,which grow to form high-angle grain boundaries and finally transform to subcrystalline grains,which grow to form recrystallized grains.At the same time,the precipitated phase also undergoes dissolution,nucleation,coarsening and other evolution,and these microstructural evolution processes cause multiple gradients of mechanical behavior changes.Trace-free joining of F/M steel construction blanks can be achieved by using heat deformation joining techniques.The main conclusions include increasing the deformation temperature,deformation volume and high-temperature insulation measures can effectively enhance the bonding effect at the thermal deformation connection interface;the optimal process parameters for thermal deformation connection of F/M steel are determined as follows:the deformation temperature is about 1150°C,the deformation volume is greater than 30%,and the insulation time is not less than 12 h at 1100°C,when the mechanical properties of the connection joint are comparable to those of the substrate;the initial connection interface The presence of a variety of oxides,in the thermal coupling effect of the interface oxide broken and transformation,the final interface only residual nanometer Si O₂ solution in the matrix,the thermal deformation of the joint interface will not have an impact.