Study On Microstructure Evolution And Mechanical Properties Of9Ni Steel

Recently, more and more attention has been paid to liquefied natural gas which is used as a sort of high quality and efficient clean energy sources at home and abroad. And the demand for liquefied natural gas is increasing. However, the particularities of liquefied natural gas demand for its constructional materials with better low-temperature property, welding performance and high strength and so on. According to these performance requirements, the9Ni steel with high strength and toughness which has become key materials for the construction of vessels and tanks of liquefied natural gas was well established by American in1942. Through the thermal simulation experiment, the recrystallization behaviors of9Ni steel were studied during hot deformation and the phase change rule was also investigated during continuous cooling in this paper. The effects of heat treatment parameters on microstructure and properties of9Ni steel were also studied by heat treatment experiment. At the same time, its mechanism of low-temperature toughness was explored.The results as follows are obtained by the study on microstructure evolution and mechanical properties of9Ni steel in this paper.1. The curves of continuous cooling transformation of9Ni steel are gained by thermal simulation experiment of CCT and the martensite and bainite microstructure with more bainite is observed under set processes. The recrystallization behaviors were investigated by single and double pass thermal simulation experiment with dynamic recrystallization activation energy Qd=377.22kJ/mol and static recrystallization activation energy Qrex=219.34kJ/mol, relations between critical strain, peak strain and peak stress and Zener_ollomon parameter being determined and the functiones of dynamic recrystallization dynamics and static recrystallization dynamics being established.2. The effects of heat treatment process parameters on microstructure and properties of9Ni steel were studied.(1) The microstructure of tempered samples all keep martensite lath pattern after QT (quenching and tempering) and QLT (quenching, intermediate quenching and tempering) treatment.(2) The effects of quenching processing parameters on strength and plasticity are not great, but those on toughness are larger. The quasi-cleavage fracture pattern can be observed on all low-temperature impact sections, if the quenching temperature is lower or the holding time is shorter. The average size of initial austenite grain (15.4μm) is least under the quenching process that the quenching temperature is800℃and the holding time is60minutes, when the QT-procedure is adopted. So reversed austenite precipitating at grain and packet boundary is fine and dispersion after tempering treatment and the value of impact absorbing energy (test temperature:-196℃) reaches221Joule.(3) Yield strength sharply declines, and tensile strength gradually declines and both percentage elongation and impact absorbing energy fistly increase and descend subsequently with tempering temperature increasing, when the QT-procedure is used, but the strength all meet En10028-4and ASTM/ASEM standard. The main reason is that reversed austenite increases with tempering temperature increasing, resulting in yield strength falling off and the value of impact absorbing energy swelling. But when the tempering temperature reaches580℃, the value of impact absorbing energy reaches maximum. After that, toughness will decline because of heat stabilization of reversed austenite declining with tempering temperature increasing.(4) The effects of tempering time on increasing low-temperature toughness of9Ni steel are not great and those on strength and plasticity are also not obvious with tempering time increasing on the basis of that tempering time making alloying element enough diffuse in QT and QLT heat treatment. The better toughness after intermediate quenching treatment can be gained, even if the tempering time is relatively shorter compared with QT-procedure.(5) The effects of intermediate quenching temperature on strength and plasticity are not great. The low-temperature toughness of9Ni steel can be increased obviously after intermediate quenching treatment and the values of impact absorbing energy all reach above230Joule. After the two-phase region α+γ treatment, on the one hand, the distribution of reversed austenite precipitating at martensite lath boundary is homogeneous, on the other hand, that the higher content of nickel in reversed austenite increases heat stability of reversed austenite. These make grain boundary and matrix toughened and the low temperature toughness of9Ni steel is increased greatly. 3. Through the analysis of transmission electron microscope, the block reversed austenite precipitates at grain boundary in QT samples and there is a certain orientation relationship which is [001]α//[011]γ and011}α//{lll}γ between reversed austenite and matrix.The laminar reversed austenite precipitates at martensite lath boundary in QLT samples and the orientation differences of these reversed austenite are not great. And the nickel content of reversed austenite reaches19.3wt%in QLT samples, but the reversed austenite with so high nickel content is not observed in QT samples.