Research On Low Temperature Toughening Behavior Of 9Ni Steel Large Forgings

As the key material of LNG storage tank,9Ni steel is widely studied by domestic and foreign scholars,but most of the research object is board,and there has not been a systematic research on the industrial development of large specification forgings of9Ni steel at home.9Ni steel still has good toughness and high strength at very low temperature.Compared with austenitic stainless steel and aluminum alloy,it has the advantages of low thermal expansion coefficient,low cost,low temperature?up to-196 C?and so on.Since 1960,it has been proved that it is safe to use after the non-welding heat treatment is also used,and the 9Ni steel is made to be used in the system.One of the main materials for making large LNG storage tanks.The main characteristics of the steel are high nickel content,high purity,high strength,high and low temperature impact toughness,good welding performance and so on.It has been fully proved that 9Ni steel is suitable for all kinds of low temperature conditions.However,the current domestic techn ology conditions limit the section thickness of 9Ni steel forgings to less than 130mm,which is thinner than other steels.Obviously,there are very few data about the perfection,mechanical properties and uses of 9Ni steel forgings.Aiming at this practical problem,this paper has carried out a certain research on the industrial trial production of 9Ni large forgings,and successfully developed a large size section 9Ni forging of?400mm x 1500mm in the end..In this paper,The research of this paper is co mbined with the actual production of Shanghai Electric Heavy Forging Industry Group.The main contents of this paper are the composition optimization of small furnace test steel,the exploration of the optimal thermal processing technology,the comparison of mechanical properties after QT and QLT heat treatment and the analysis of its mechanism,using the tensile experiment at room temperature and low temperature,the impact test,the analysis of the CCT curve,the thermal compression experiment,the thermo dynamic calculation and so on,and the use of optical microscope and SEM.EBSD,TEM and other small size forgings were tested,observed and analyzed.Finally,in the industrial production,the 9Ni steel forgings with a size of?400mm x 1500mm were develope d.The conclusions are as follows:1)through the vacuum smelting and electroslag remelting process,two different components of 9Ni diameter 100 small size forged steel are smelted.In order to ensure the low temperature toughness,the optimum inner compo nent interval of some components is obtained.The content of C and Mo should be under 0.05%and below0.1%respectively.2)Through the thermal simulation experiment of the optimized 9Ni steel,the constitutive equation is obtained:???,the accuracy is as high as 99%by verification.The energy dissipation diagram is obtained.Through the verification of microstructure,It is concluded that the optimum hot working interval is at 1100¹150 C and the strain rate is 0.1⁰.3s^-1,which provides a guiding role for the selection of temperature and forging rate in the process of industrial trial.3)comparing the microstructure,phase composition and mechanical properties of the steel after heat treatment of QT and QLT,the main factors affe cting the plasticity and toughness are the reverse austenite content,size,morphology and stability.Because of the reversal process of QLT,austenite is mostly distributed between tempered martensitic lath,and its morphology is thin film and its size is small and dispersed.Therefore,the chemical stability and mechanical stability of austenite are relatively high,which is not easy to happen martensitic transformation under low plastic deformation.The low temperature impact of QLT process is obviously higher than that of QT process.4)industrial trial production of 9Ni steel after the optimization is made.The best hot working temperature and strain rate are used for forging.After forging,the improved QLT heat treatment is carried out,and the large forging parts of the 9Ni steel with a size of?400 x 1500mm are developed at last.Its performance is all up to the national standard,Especially,the impact toughness is 200J,which is much higher than that of the national plate standard.Provide some suggestions for the production and further research of 9Ni steel large forgings.