Study On The Factors Affecting The Microstructure And The Austenite Stability Of Ni Low Temperature Steel

The content of Ni was high in traditional Ni series cryogenic steel, and its alloying cost was high as well. Thus reducing the content of Ni became an important development direction of cryogenic steel. The existence of the stable austenite in steel can obviously improve the toughness of the steel, and this kind of austenite was mainly generated in the process of tempering. Therefore, the tempering process had a greater influence on the microstructure and mechanical properties of cryogenic steel. On the other hand, the mechanical properties of cryogenic steel was closely related to the stability of austenite. The aim of this dissertation is to study the tempering process on the microstructure and mechanical properties of 5.5Ni and 3.5Mn cryogenic steel as well as the factors that affecting the stability of metastable austenitic of Mn-Ni alloy steel.XRD(X-Ray Diffraction), SEM(Scanning Electron Microscope) and TEM(Transmission Electron Microscope) were conducted to investigate the volume infraction, morphology and size of reversed austenite of 5.5Ni steel tempered at different temperatures and the influence of tempering temperature on mechanical properties of 5.5Ni steel was discussed. The results show that, when the experimental steel was tempered in the range from 580℃to 600 ℃, there was no significant change in either the tensile strength or yield strength. When the tempering temperature increased to 620 ℃, there was a slight increment in tensile strength but a great decrement in yield strength, in addition, a maximum elongation was obtained. As the tempering temperature increased from 580 ℃ to 620 ℃, the volume fraction of reversed austenite of 5.5Ni steel increased gradually but impact energy decreased. Whether the low temperature toughness was pros and cons of 5.5Ni steel, it depends not only on the volume infraction of reversed austenite, but depends more on the morphology and stability of reversed austenite. Two types of reversed austenite were detected in this steel including film and block shape. The former had different length with a width of about 20 nm which could improve the low temperature toughness of steel. The latter had a size of about 200 nm and tended to gather in the form of clusters which were detrimental to the low temperature toughness of steel. Stable, homogeneous, dispersive and fine "film-type" reversed austenite was the main reason of the high impact energy of 148 J when the experimental steel tempered at 580 ℃.The microstructure, mechanical property and recrystallization behavior of the lath martensite of 3.5Mn steel tempered at 650 ℃ for different time were discussed by Optical microscope, X-Ray Diffraction and Scanning Electron Microscope. Some lath martensite in the rolling steel became recrystallization at tempered process. With the increase of tempering time, more grains of recrystallization were observed. The sample that after quenching and tempering heat treatment, mostly remains lath martensite structure. Even if the time of heat preservation is 100 h, the recrystallization of lath martensite was rarely occurred. The recrystallization behavior of the lath martensite was related to the state of the sample before tempering. When the state of the sample was rolling before tempering, the recrystallization behavior of the lath martensite was easy occurred. The hardness of the specimen was high before tempering, however, it decreased dramatically after tempering, and the hardness of the rolling sample after tempering heat treatment decreased faster.The effects of tempering temperatures and morphology, distribution, size of reversed austenite and the content of the austenite stabilizing elements on the stability of reversed austenite were nvestigated by X-Ray Diffraction, Scanning Electron Microscope and Transmission Electron Microscope. With the increase of tempering temperature, the volume fraction of the reversed austenite increased first then decreased, and the reversed austenite became unstable when tempered at 650 . The reversed austenite formed on the small-angle grain boundaries or between laths of martensite exhibited the highest stability, followed by which formed on the large-angle grain boundaries, and the most unstable reversed austenite was formed at the triple junctions and the stability was enhanced with the decrease of grain size of the reversed austenite. The stability of reversed austenite was also associated with the stabilizing element like C, Mn and Ni in the reversed austenite, the higher the content of the stabilizing element, the higher the stability of the austenite.