Study On The Material And Forming Process Of Strain Strengthening Austenitic Stainless Steel Cryogenic Vessel

For its good plasticity and toughness, austenitic stainless steel with low yield ratio is commonly used in the manufacture of cryogenic pressure vessels. Using the strain-strengthening technology to design the inter container of cryogenic vessel can reduce the vessel's wall thickness and weight, save the vessel's transportation energy consumption and achieve green manufacture. Plastic deformation occurs in the forming and strengthening process for strain-strengthening austenitic stainless steel cryogenic vessels. Excessive plastic deformation will deteriorate the performance of the vessel; therefore it is difficult to meet the associated manufacturing and testing standards.This thesis is sponsored by the National High-tech Research Development Plan (No: 2009AA044801) and the National Science and Technology Support Program(No: 2006BAK04A02). Based on the consulting of a large number of domestic and foreign literature, by theoretical analysis, numerical simulation and experimental study, the influence law of the chemical composition, plastic deformation and strain-rate on the mechanical properties of strain strengthening austenitic stainless steel is revealed; the dynamic calculating model of the head and cylinder's forming process is established; the stress-strain state and its variation is analyzed for head and cylinder's manufacturing process; nonlinear finite element analysis model is established for strain-strengthening cryogenic vessel with complex structures, and the stress-strain state of the structure is analyzed. The main efforts and results of this study are as follows:(1) Less stable austenitic stainless steel is more prone to martensite transformation during deformation, so the martensite phase is more with the same deformation; the tensile strength is higher but the elongation after fracture is decreased.(2) The martensite transformation content is proportional to the deformation amount. The deformation amount is an important parameter to control for the strain-Strengthen process.(3) Austenitic stainless steel is of higher yield strength, lower tensile strength for rapid stretching process, and the martensite transformation content is lower during the forming process. As the strain rate decrease, the strain rate sensitivity of austenitic stainless steel gradually weakened.(4) Numerical model and theoretical model are established for the forming process of the vessel's head and cylinder respectively, which proves a theoretical basis for determining the need for heat treatment after forming.(5) Using the non-linear finite element analysis model, the numerical analysis result is in good agreement with the experimental data. The model can be used for the strain strengthening stress-strain analysis of complex structure cryogenic vessels.