Cu Content Of The Steel Microstructure And Mechanical Performance Of The 9ni Low Temperature

9Ni steel is being used for Chinese LNG Terminals in recent years. With the booming demand for the overseas LNG, the size of LNG tanks becomes larger and larger. In consideration of the stringent requirements for ensuringsafety, more attention shall be paied to higher technical requirements for strength and toughness of the9Ni steel, with increasing thickness of the steel plates. In this study, the effect of Cu alloying on the microstructure characteristics and mechanical properties of the9Ni steels heat-treated by quenching+lamellarizing+tempering(QLT) process and quenching+tempering(QT) process has been studied. Furthermore, the mechanism of strengthening and toughening of Cu-bearing9Ni steel was discussed.The results showed that both the strength and the yield ratio of the test steels were increased with Cu increased. And the impact energy at-196℃was increased as Cu content increases within1%, but decreased slightly when Cu content is increased as high as1.5%. The highest impact energy reached157J for1.0%Cu steel while the steels with other Cu contents also showed high level impact energy. The effect of Cu content on strength and cryogenic toughness of9Ni steels heat-treated by QT process was similar to the steels by QLT process, however, the yield ratio of9Ni steels heat-heated by QT were higher than that by QLT process, and cryogenic toughness significantly reduced.The microstructure characteristics, the fine structures and precipitates of Cu-bearing steels have been observed by means of Optical Microscopy(OM), Scanning Electron Microscopy(SEM), Transmission Electron Microscopy(TEM), Electron Backscatter Diffraction(EBSD) and X-Ray Diffraction(XRD). The result revealed that tempered martensite and the mixed microstructure of quenched martensite and reversed austenite were gained in QT process, and the mixed microstructure of tempered martensite+ferrite, together with granular and strip reverssed austenite along original austenite grain boundaries and martensitic lath boundaries were obtained in QLT process. Fine observation shows that, with increasing Cu, mixed microstructure of tempered martensite and reversed austenite tended to increase and ferrite to decrease. Cu addition increased the volume fraction of reversed austenite of9Ni steels heat-treated both by QT and QLT, but as a whole, the content of reversed austenite by QLT process was much more than that by QT process. Cu content also had a remarkable effect on Cu particles precipitation. There were no Cu precipitates observed for0.5%Cu steel while there were a certain number of Cu precipitates for1.0%Cu steel. When Cu content increased to as high as1.5%, both the number and the size of Cu precipitates were significantly increased.The mechanism of strengthening and toughening of Cu-bearing9Ni steel heat-treated by QLT was discussed. Cu addition would drive Cu particles precipitated, reinforcing the strength. In addition, increasing Cu content has apparently increased the amount of secondary tempered martensite which has higher hardness, and refined prior austenite grain size. Cu-bearing9Ni steels were strengthened by precipitation strengthening together with fine grain strengthening and microstructure strengthening mentioned above. On the other hand, Cu addition refined effective particle size, increased high angle grain boundary which hinder crack propagation and increased the volume fraction of reversed austenite and improved the stability of formed reversed austenite resulting in cryogenic toughness improvement.