Research On Cold Forming And Bearing Capacity Of Austenitic Cold-stretching Pressure Vessels

Austenitic steel is of great mechanical properties and can perform well under cryogeniccircumstance,so it is always used to produce the inner part of cryogenic vessels. Due to itslow yield stress, the thickness of pressure vessel made of it is relatively high. Normal designmethod can not make full use of plasticity of austenitic steel.Cold-Stretching method applies a pressure in excess of normal test pressure to the vessel,causing parts of it which sustain primary membrane stress to yield to raise the yield stress ofmaterial and decrease the thickness of vessel, Recently, more and more Chinesemanufacturers start using this method. However, this method has not been included innational standard because of its short history in China. More technical details need to beexplored.This thesis is sponsored by “LNG Foundation of Sino-Australia Cooperation”(NO:B1120150) and finished with the help of a pressure vessel manufacturer of Guangdong.After consulting of a large number of literature, through numerical analysis, experiments andtheoretical calculations, Limit-Load analysis method (or plastic analysis method) is checkedand corrected, calculation method of the strengthening pressure is confirmed. finite elementmodel is established to simulate cold stamping process of head and the feasibility of thismethod is testified. Strengthening effect of cold stamping is revealed. Ultimate bearingcapacity and stability of different heads is also studied. Finally, cold rolling process ofcylinders and plastic deformation of rolling is analyzed. The main results of this thesis is asfollows:1.When using plastic analysis method to determine the limit load of head, the FE modelshould contain a part of cylinder, the length of which is longer than the attenuation length ofboundary stress. At the same time, twice elastic slope (TES) method should be used to specifylimit load.2.Strengthening pressurep kshould be determined based on the actual thickness of cylinder, instead of calculation thickness. the range of strengthening pressure is:2s k4skIn this formula,D pk is the actual thickness of cylinder, Di3.Dsiis the diameter,ipkis the strengthening pressure.3.Strengthening Pressure cannot sufficiently strengthen sphere part of head.To solve thisproblem,the strengthening effect of cold stamping is checked. elliptical heads and2:1dishedheads have a better strengthening effect, spherical part of which can be sufficientlystrengthened. However,10%dished head cannot be strengthened enough. So it is advisable tointroduce cold-formed elliptical heads and2:1dished head into cold-stretching vessels.4.In the respect of safety and stability, it is proven that elliptical heads and cold-formedheads are suitable for cold-stretching vessels. The ultimate bearing capacity of elliptical headis similar to that of10%and2:1dished head, and the bearing capacity of cold-formed head issimilar to that of solution heat treated one. In terms of stability, Neither cold-formed orsolution heat treated heads will exhibit local buckling under strengthening pressure.5.Based on the material model featuring power function, the law of stress distributionduring cold-rolling process is revealed. Experiments are conducted to testify the feasibility offormula of plastic deformation under different relative thickness.6. The austenitic vessel sustains obvious plastic deformation during cold-stretching. Tosolve this problem, suggestion is made: If strengthening stress kis chosen slightly lower thanthe maximum value and solution heat treated cylinder is replaced by cold formed one, plasticdeformation during strengthening can be effectively reduced. what is more, Strengtheningprocedure cannot be replaced by cold-rolling process because plastic deformation of coldrolling cannot be manipulated or controlled.