Research On Some Questions Of Cold Stretching Technology In Austenitic Stainless Steel Pressure Vessels

With the rapid development of industry, cryogenic liquids such as liquid oxygen, liquid nitrogen, liquid helium, liquid hydrogen and liquefied natural gas etc., have been widely used in various fields. As the common equipment for storage and transport of cryogenic liquid gases, austenitic stainless steel pressure vessels have also witnessed fast growth. Due to increasing demand for cryogenic equipment as well as energy conservation requirements, the cold stretching technology (CST) came into existence, which contributes to the lightweight of cryogenic container significantly. Nowadays the technology has been applied extensively, achieving a good social and economic performance.With continual researches on the cold stretching technology in recent years, the development of the CST continues to mature in China and the preparation of the national standard is being carried out. However, several problems remain unsolved and further efforts are essential. Specifically, in terms of the material pre-stretching methods, there are various pre-stretching methods that produce different plastic deformation exists in welding procedure currently. With respect to the strength margin of container, the design temperature of the liquid carbon dioxide (LCO2) storage cans is relevant higher than many other liquefied gas storage equipment. Although the CST of the LCO2 storage meets foreign standards, it has not been applied in China; for the head manufacturing process, the subsequent impact of the stamping process on the material mechanical properties is also not yet clear, and therefore to investigate the problems proposed above is of great significance.On the support of Key Sciences and Technology Team of Zhejiang Province (Number:2010R5001), Special Funds of Central University Basic Scientific Research Business Expenses (Number:2014FZA4024) and National Postdoctoral Science Foundation (Number:2014M551731), a case study of buckling of cold-stretched pressure vessels under external pressure is carried out. The main research of this thesis can be described as following three parts:(1) Starting from the application of cold stretching technology in China, the pre-stretching purpose and the acceptance criteria of welding procedure qualification were clarified. Then considering the plastic deformation of actual vessel during the strain strengthening process, three existing methods of pre-stretching were compared in plastic deformation, operability, standards and regulations, in order to seek for a proper pre-stretching method and evaluation criteria of welding procedure qualification.(2) According to the design temperature and working temperature of austenitic stainless steel pressure vessels for liquid carbon dioxide, the material tensile test at room temperature,-20℃,-40℃ were carried out, the strength margin of vessels was calculated by using finite element method based on material test results. The cold stretching technology for the adaptability of liquid carbon dioxide storage tank was discussed considering of the equivalent plastic deformation, burst pressure and strength margin.(3) According to the relationship between plastic deformation of heads that formed under different stamping process and the amount of the strain induced martensitic transformation, the reasonable method to control the performance of head after forming has been discussed as the specification of standards. Accordingly, the influence of forming stamping process on the material properties has been studied, with the key parts of stamping formed head cut and tensile and impact tests carried out at the room temperature and a cryogenic temperature of-196℃.