Research On Microstructures And Properties In Super Martensitic Stainless Steel And Formation Mechanism Of Reversed Austenite

On the basis of traditional martensitic stainless steel, super martensitic stainless steel (SMSS) decreases the content of carbon (wt.%≤0.02%) and increases the content of Ni (4%～6%) and Mo (0.5%～2.5%). Because of the feature of composition and application, super martensitic stainless steel has high strength, high toughness and good corrosion resistance and other properties.The excellent of properties of this steel is related with the reversed austenite. In this thesis, three kinds of SMSS are designed. The effect of heat treatment processing on the microstructures and mechanical properties of different content of Cr tested steels have been researched. The formation rules and transformation mechanism of reversed austenite in tested steels have been focused. The influence of alloying elements (Ni and Cr) on formation of reversed austenite have been studied.The matrix microstructure of13Cr and15Cr tested steels is lath martensite after900-1100℃quenching, while there are retained austenite and δ-ferrite in15Cr because of higher Cr content. The sizes of austenite grain increases with quenching temperature increasing. When the quenching temperature is below1050℃, the growth of austenite grain13Cr is faster. However, when the quenching temperature is above1050℃, the growth of austenite grain in15Cr increases rapidly due to dissolution of the carbide and the second phase. The variation of HRC in two tested steels is similar after quenching. HRC of15Cr is bigger and more gently than13Cr. The microstructures are tempered martensite, reversed austenite and ε-Cu after tempering at400-750℃in two tested steels. The volume fraction of reversed austenite increases first, and then decreases with the increasing tempering temperature and reaches the maximum value at650-700℃. Reversed austenite in15Cr is more than that in13Cr and distributes more dense in the matrix. Since there are more amount of alloying element and reversed austenite, so HRC, tensile strength and elongation are higher than those in13Cr steel. Moreover, the variation of the mechanical properties in15Cr is more smooth with the increasing tempering temperature.The formation mechanism of reversed austenite is diffusion in super martensitic stainless steel. Ni diffusion plays an important role in nucleation and growth of reversed austenite. The formation of reversed austenite in13Cr and15Cr steels is similar, but there are still some difference. The sameness:(1) Both the quantity and the size of reversed austenite increases first, and then decreases with the increasing tempering temperature and reacher the maximum value at650-700℃(2) The formation rule of reversed austenite in two tested steels in the same. Reversed austenite nucleates at low tempering temperature; With the increasing tempering temperature, the degree of Ni enrichment in reversed austenite increases constantly, so that the reversed austenite size becomes bigger; When the tempering temperature continues to rise, reversed austenite dissolved back and its size becomes smaller. The dissolution of reversed austenite in the martensitic lath will separate the lath into two or several, which leading to refinement of matrix.(3) The variation trend of mechanical properties is the same. The tensile decreases first, and then increases with the tempering temperature, which is opposite with the variation of the reversed austenite quantity. However, elongation and the amount of reversed austenite are both increase first, and then decrease with the increasing tempering temperatureThe formation of reversed austenite in13Cr and15Cr steels still exist some differents:(1) The reversed austenite in13Cr forms at550℃, while forms at450℃in15Cr.(2) The nucleation site of reversed austenite are at the boundary of martensitic lath of in the lath in both13Cr and15Cr. In addition, reversed austenite in15Cr also can attach to the M23C6and nucleate.(3) Due to the more content of Cr and Ni, the amount and size of reversed austenite in15Cr are bigger than those in13Cr.(4) The degree of Ni enrichment in15Cr reversed austienite is bigger than that in13Cr.(5) When the reversed austenite dissolve, the refinement effect on matrix in15Cr is more obvious than that in13Cr.This paper compares two tested steel with1.5Cu and3Cu. The results show that there is a synergistic effect between Ni and Cu on formation of the reversed austenite. When Cu content is higher, it can increase the degree of Ni enrichment in reversed austenite and the quantity of enrichment areas. Thus it also can promote the increasing of amount of reversed austenite. Moreover, in the area of high Ni concentration, there also is the high degree enrichment of Cu and ε-Cu precipitation. This synergy can make reversed austenite content increased and the mechanical properties of super martensitic stainless steel improved.