Study On Microstructure And Properties Of High Nitrogen Martensitic Heat Resistant Steel

In the context of energy shortage and environmental pollution,green power generation technology is believed to be the most feasible way to break through the dilemma.As the support of green power generation technology,steel materials(used for units)should possess outstanding mechanical properties for long-term service in high temperature.Based on the recent studies of 9Cr-based heat-resistant steel,a novel high N heat-resistant steel was designed and prepared:Experimental Steel A(0.25 wt%N)and Experimental Steel B(0.3wt%N).In this dissertation,the influence of heat treatment on microstructure,phase and mechanical behavior of high nitrogen heat-resistant steels were systematically investigated.The results of structure characterization indicate:the as-cast structure of experimental steel A is a mixed structure of martensite and a small amount of high-temperature ferrite structure,and the as-cast structure of experimental steel B is a complete martensite structure.while the parameters of homogenization treatments for Experimental Steel A and Experimental Steel B were 1050?-10h and 1100?-10h,respectively,the as-segregated microstructure of the two experimental steels decomposes and the high-temperature ferrite content is low.When the normalizing processes of Experimental Steel A and Experimental Steel B are 1000?-2h and1100?-2h,respectively,high-temperature ferrite content decreases in two types of experimental cast steel,their primary austenite grains are refined,the strip nitride content is reduced and square nitrides has hardly changed.It was found that the type and size of the precipitated phase in the steel can be adjusted by adjusting the tempering temperature.When the tempering temperature is between 700?and 760?,a small amount of Fe₃C phases with a length of about 20nm are distributed in the matrix of the tempering structure of Experimental Steel B,and(V,Nb)N,Cr N and Cr₂N phases with a size of 20?150 nm Cr₂N phase distributed in martensite lath and its size decreases with increasing temperature.In addition,Laves(Fe₂W)phase with a size of 150?300nm is distributed on the martensite lath boundary.When the tempering temperature exceeds 760?,the precipitated phase Fe₃C phase will completely decompose.The results of the mechanical property test show that the experimental steel B always shows lower impact toughness,plasticity and higher tensile strength at room temperature,and shows higher tensile strength and good high-temperature durability at high temperatures above 600?.It is because the precipitation phases of VN,Cr N,Cr₂N,and Laves in experiment steel B interact with dislocations to effectively pin the dislocations,which increases the strength of the matrix,and at the same time,its structure contains a large number of large-sized nitrides,thereby reducing the toughness.