Study On The Strengthening And Toughening Mechanism And Fatigue Performance Of 2200MPa Ultra-high Strength Stainless Steel

In order to meet the needs of future equipment development of aeronautics and astronautics,especially the urgent demand for weight reduction of structural parts,it is necessary to develop a new type of ultra high strength stainless steel with strength at the 2200MPa level.In this paper,the influence of C and Cr on the mechanical properties of three kinds of ultra high strength stainless steels was studied.The matrix structure,precipitation phase and interfacial properties were characterized by OM,SEM,TEM,APT and XRD,and the relationship between the microstructure and the strength and toughness was also established.The influence of the smelting process on the fatigue performance was studied,and the influence of the smelting process on the inclusions and fatigue performance was initially explored.The main results are as follows:Under optimal heart treatment conditions,the tensile strength of 0.1C-11.5Cr steel can reach2126MPa,the tensile strength of 0.1C-9.5Cr steel can reach 2256MPa,the tensile strength of 0.2C-9.5Cr steel can reach 2206MPa,and the fracture toughness can be 67.2 MPa·m^1/2,33.4 MPa·m^1/2,50.9 MPa·m^1/2 respectively.When the carbon content is the similarity,the decrease of the Cr content will increase the strength of the steel but lower the toughness.The Ms point of the 0.2C-9.5Cr steel is lowest than other sample,this is the main factor for the maximum austenite content after refrigeration treatment and solution treatment and is also the main reason for better fracture toughness than 0.1C-9.5Cr steel.The structure of steel is lath martensite with high density dislocation mainly and a small amount of thin-film austenite.A large number of fine Laves phases about 4-5nm are dispersed and distributed on the martensitic lath.The good common relation between Laves and martensitic matrix is the main reason for the tensile strength to 2200MPa and the film austenite distributed on the martensite lath boundary maintain the good toughness of the test steel.The enrichment of Cr,Mo and C in the martensitic lath boundary and the poor of Fe,Co and Ni,the mainly segregation is Cr,and spinodal decomposition of Cr also exists in the matrix.The calculation of elemental atomic ratio indicate that Fe:Mo is about 2:1 and is proved that the precipitated phase is Fe₂Mo type Laves phase maybe.The non-metallic inclusions in the steel with three different smelting processes are mainly oxide and a small amount of CaS,MnS,silicon and so on.The inclusions in the VIM+ESR process are smaller than those of other processes,and tension and pressure fatigue properties is better than other processes.VIM+ESR+VAR can reduce aspect ratio resultful.