Investigation on Influence Factors of Mechanical Properties of Austenitic Stainless Steels for Cold Stretched Pressure Vessels

Cold stretched pressure vessels from austenitic stainless steels (ASS) have been widely used all over the world for storage and transportation of cryogenic liquefied gases. Cold stretching (CS) is performed by pressurizing the finished vessels to a specific pressure to produce the required stress which in turn gives an amount of plastic deformation to withstand the pressure load. Nickel equivalent (Ni_eq) and preloading, which is introduced in welding procedure qualification for cold stretched pressure vessels, are considered to be important factors to mechanical behavior of ASS. During the qualification, welded joint will be preloaded considering the effect of CS on pressure vessels. After unloading, the preloaded welded joint will go through tensile test according to standard requirements. There are two kinds of preloading method. One is to apply required tensile stress $σk$ on specimen and maintain it for a long time (stress-controlled preloading). The other is to stretch specimen to a specific strain of 9% (strain-controlled preloading). Different preloading and preloading rates may lead to differences in mechanical behavior of preloaded welded joint. In order to understand the effects of nickel equivalent, preloading and preloading rate on the mechanical behavior of ASS for cold stretched pressure vessels, a series of tests were conducted on base metal, welded joint, and preloaded welded joint of ASS EN1.4301 (equivalent to S30408 and AISI 304). As regards to the preloaded welded joint, the ultimate tensile strength (UTS) decreased as the nickel equivalent increased, while the elongation to fracture increased. It was more difficult to meet the available mechanical requirements with strain-controlled preloading case than with stress-controlled preloading case. Rates of preloading had some effect on the mechanical properties of welded joint but nearly no effect on the mechanical properties of preloaded welded joint. These results are helpful for choosing appropriate material and determining a proper preloading method for welding procedure qualification.