Investigation On Composition Optimization And Microstructure And Properties Of 9Ni Steel Heavy Forgings For Low Temperature Compressor

In the course of service,repeated thousands of times,the complex force situation,therefore,the performance of the main axis needs the high requirements.9Ni Steel is used as the basic material in the design.9Ni steel plate has been widely used in cryogenic field including liquid natural gas tanks and pipelines because of its excellent toughness at low temperature.Recently,it tries to use in the heavy forging,which has a strict demanding on the toughness and strength.However,this technology is blocked by a few developed countries,which seriously restricts the development of special aircraft and even national defense security This paper focuses on the study of 9Ni steel heavy forgings for low temperature compressor by the way of simulation combined with experiment.Main study contents and results are listed as follows:First,the microstructure prediction and performance evaluation of the industrial grade 9Ni steel used in heavy forgings are completed.Through the finite element simulation method,the temperature changing of 9Ni steel forgings 800 C water quenching process was studied,meanwhile the diameter of 1200 mm surface,1/2 radius and center temperature curve was obtained,followed by the heat treatment furnace simulation experiment.The results show that the surface microstructure of 9Ni heavy forging(1200mm)is granular bainite,and the bainite and ferrite in 1/2R and at the center,accompanied by the precipitation of coarse carbides carbide.Eventually,the cryogenic toughness can not meet the design targets.Then,the composition optimization of 9Ni steel heavy forgings was completed.The effects of different C and Mo contents on the microstructure and mechanical properties of 9Ni steel heavy forgings in different positions were analyzed.The results show that the comprehensive mechanical properties of forgings are the best in the quenching process when the C content is between 0.13%~0.15%,With the increase of Mo content,the martensite content at the edge of the heavy forging gradually increases.When the content of C is 0.13%~0.15% and the content of Mo is 0.20%~0.25%,the forging has an ideal strength and toughness matching.Finally,in this paper,the microstructure,grain size and inclusions of 9Ni steel with different rare earth contents were analyzed by OM?TEM?SEM and EDS.Then the effects of different rare earth contents on the low temperature toughness of 9Ni steel were studied.Proper addition of rare earth can improve the mechanical properties of 9Ni steel by modifying inclusions and refining microstructure.Finally through the mechanical properties test,its low temperature toughness,strength,fracture toughness,fatigue strength and other indicators obtained a reasonable chemical composition range of the compressor 9Ni steel spindle forgings,providing a theoretical guidance for the preparation of 9Ni steel for the compressor.