Study On The Influence Of Heat Treatment Process On Cryogenic Toughness Of 3.5%Ni And 9%Ni Steel

With the high-speed development of China's petroleum and chemical industry,the demand of liquefied natural gas(LNG)and liquefied ethylene gas(legs)has increased dramatically.Therefore,the demand for nickel-based cryogenic steel is also significantly increasing.Due to the development of the large size of the large steel tank and the LEG vessel,it is not only the strength of the nickel-based cryogenic steel,but also the excellent cryogenic toughness.Therefore,the research and development of low cost and high performance nickel-based cryogenic steel is widely concerned by many experts and enterprises at home and abroad.So this article selected nickel-based cryogenic steel common 3.5%Ni steel and 9%Ni steel as the research object.The influence of the final rolling temperature,QT and QLT heat treatment on the microstructure and mechanical properties of the experimental steel was studied.At the same time scanning electron microscopy(SEM),transmission electron microscopy(TEM)and X-ray diffraction(XRD)method and other test equipment are used to study the effects of fine microstructure of 3.5%Ni and 9%Ni steel structure on cryogenic toughness.The results of this paper are as follows:(1)The relationship between the microstructure and mechanical properties of 3.5%Ni steel is studied by using 8 heat treatments.The microstructure of 3.5%Ni steel is ferrite and pearlite by NT process,and the state of cementite is spheroidized.The grain size is smaller than that of hot rolled state and normalizing state clearly.This particular microstructure organization of 3.5% Ni steel has good low temperature toughness.In the process of the tempering process of QT,QLT and QQT the carbide is produced,and the alloy elements are rearranged in the process to form the element enrichment zone.The small carburizing body is also distributed in the substrate of Ferrite matrix as the second phase particle.That is the main reason for the strength and low temperature toughness,which is also attributed to the vanishing of twin Martensite and grain refining.(2)For 9%Ni steel,different finish rolling temperature is adopted.After hot rolling,the steel plates are carried out QT and QLT heat treatment.The microstructure of 9%Ni steel is Tempered Sorbate by QLT process,which is Tempered Troostite by QT process.Finishing rolling temperature at 860? and 1000? belongs to recrystallization zone rolling,the grain size is of uniformity;815? and 780? belong to partial recrystallization zone;780? is given priority to with the non-recrystallization rolling,and most of the grain sizes are flat.The microstructure of QLT process is smaller than that of QT process,which is reason to improve the strength and toughness of 9%Ni steel.By the QT process,with the increase of the finish rolling temperature,the yield strength and cryogenic impact energy of 9%Ni steel are increasing,and the tensile strength is gradually decreasing,and the QLT process was not monotonous.For the same final rolling temperature,yield strength and cryogenic impact energy of 9%Ni steel by QLT are higher than those by QT.It is one of the key factors to improve the cryogenic toughness of 9%Ni steel by the reversed austenite of the alloy elements of Ni and Mn during the isothermal process of the two phase regions.The number of reversed austenite has influence on cryogenic toughness.