Phase And Transformation Of High Nitrogen Martensitic/Ferritic Heat Resistant Steels

With the shortage of energy and environmental pollution,the ultra-supercritical green power generation technology with high efficiency and energy saving has become the first choice for coal-fired generator sets.Accordingly,the advantages and disadvantages of heat-resistant materials for units have become an important support for realizing this technology.9Cr martensitic/ferritic steel with high strength,high thermal conductivity and low expansion coefficient is the ideal steel grade for cylinders and rotors of ultra-supercritical steam turbines.High-nitrogen steel has excellent strength and corrosion resistance due to the interaction of nitrogen and alloying elements.Combining the advantages of martensite/ferritic heat-resistant steel and high-nitrogen steel,a new type of high-nitrogen martensite/ferritic heat-resistant steel is designed to further improve 9Cr martensite/ferrite using nitride strengthening.The strength and corrosion resistance index of the cast steel material has important theoretical significance and practical value.Based on the Thermo-Clac simulation calculation,this paper designed a 9Cr martensitic/ferritic steel containing 0.2⁰.3N%.Two new types of high nitrogen martensitic/ferritic heat resistant steels were prepared by two different preparation processes:experimental steel A was prepared by pressurized vacuum induction,and experimental steel B was prepared by pressurized vacuum induction,plus by electroslag remelting.The composition and microstructure analysis results showed that the nitrogen content of the experimental steel A was 0.2%,and the nitrogen content of the experimental steel B was0.15%.The as-cast microstructures of the these two steels were composed of two phases of martensite（M）and ferrite（δ-ferrite）.Compared with the experimental steel A,the content of the ferrite phase in the experimental steel B was more,and the distribution of its microstructure was extremely uneven,which was related to the no pressure protection of the electroslag remelting process.The ferrite formation was studied in detail by controlling the homogenization temperature and the cooling rate.The results show that the ferrite content in the experimental steel A decreases first and then increases with the increase of the homogenization temperature.After1150℃-10h-air cooling homogenization treatment,the ferrite content is the least;the ferrite content in the experimental steel B increases with the increase of the homogenization temperature,at 1070℃-10h-air cooling After homogenization.The content of ferrite phase decreases with the increase of cooling rate,but when the cooling rate exceeds 500℃/min,the cooling rate has little effect on the content of ferrite phase.The normalizing temperature（1070¹200℃）and tempering temperature（730⁷80℃）of the experimental steel A were determined by the experimental results of thermal expansion measurement.TEM observations show that after tempering,a large number of dislocation lines and dislocation nets are found in the material,and the microstructure is tempered martensite and ferrite.Among them,there are more（V,Nb）N phases along the martensite.In the slab boundary,a large amount of M₆C（Fe₃W₃C）,Laves phase and Cr₂N are precipitated on the ferrite matrix.Undoubtedly,the existence of these phases can pin dislocation,hinder the movement of dislocations,and provide organizational guarantee for the high temperature performance of materials.The discussion of phase precipitation of tempering at different temperatures will provide theoretical and practical basis for revealing the phase kinetics and phase transition mechanism of high nitrogen martensite/ferritic heat resistant steel.