Study On Technique Of Cold Stretched Austenitic Stainless Steel Pressure Vessle And Its Performance Evaluation In Typical Media Environment

Solid solution austenitic stainless steel has high tensile strength and good plasticity, but exhibits low yield strength, thus causing thicker wall thickness of austenitic stainless pressure vessel designed by conventional method with lower material utilization and poorer economic benefit. Cold stretching technique can improve the yield strength of material and thereby reduce the wall thickness of vessel to achieve lightweight, while the security can be guaranteed, so it is a kind of green manufacturing technology.In this paper, systematic theoretical and experimental studies were carried out for cold stretching technique applied to austenitic stainless steel pressure vessel. The effects of cold stretching on mechanical properties of austenitic stainless steel at room temperature, cryogenic and elevated temperature were investigated respectively. Test vessels were designed and manufactured, and the time, treatment mode, pressure controlling and strain measurement methods of cold stretching were discussed. The distributions of stress and strain on the key parts of the vessel during cold stretching were obtained by finite element simulations. The influence of cold stretching on stress corrosion cracking susceptibility at high-temperature chloride environment for austenitic stainless steel was discussed, and its effect on ratcheting behavior of the material at high-temperature was also summarized. All investigations can be concluded as follows:Strain rate and strain level were two key parameters in the process of cold stretching and their effects on the mechanical behavior of austenitic stainless steel were investigated. The results show that the strain rate should not be too slow, otherwise the serrated yielding effect will occur which have an adverse effect on the properties of the material. When strain level was limited below10%at room temperature, it has little effect on the mechanical properties of S30408austenitic stainless steel while its yield strength can be improved significantly and the stretching effect in the temperature range of100～400℃will still be maintained.J-integral was taken as evaluation index to determine the fracture toughness of austenitic stainless steel weld metal with defect before and after cold stretching, and their apparent values of initiation toughness J1C at room temperature were obtained. Compared with the original weld metal, The yield strength of stretched one (8%level) just increased by19%, whereas its apparent value of J1C dropped nearly33%, which indicate that the properties of weld metal are sensitive to the cold stretching effect, and thus more strictly requirements should be implemented for welding process, welding quality and post welding inspection.The austenitic stainless steel cryogenic test vessels were designed and manufactured according to the current standard of cold stretching. The stretching processes were enforced and three measurements such as resistance strain gauge, dial indicator and circumference measurement were used to measuring and controlling hoop strain of the vessel. The merits and demerits of three methods were compared, and their respective applicable conditions were also discussed. The burst tests for cold stretched vessels demonstrate that the vessel still maintain good plastic reservation and the safety of the cold stretched vessel can be guaranteed.The finite element model of cold stretching process for austenitic stainless steel pressure vessel was established on the basis of material and geometric nonlinearity. The reliability of the FE model was validated before simulation, and then the variation of stress and deformation on the key parts of the vessel during cold stretching were analyzed.In view of the phenomenon that stress corrosion cracking is apt to occur for austenitic stainless steel vessel under corrosive media environment, slow strain rate tests (SSRT) were adopted to investigate the change of stress corrosion cracking susceptibility in high-temperature chloride environment for base metal and welded joint of austenitic stainless steel. The research indicates that the stress corrosion cracking susceptibility of base metal show rising trend with the increase of strain level and the stress corrosion cracking susceptibility index for stretched (8%level) base metal is nearly twice as solid solution one. Due to a certain amount of ferrite in weld metal, the stress corrosion cracking susceptibility of it is relatively low. Moreover, cold stretching is beneficial to improve the welding residual stress.The influence of cold stretching on ratcheting behavior at high-temperature for austenitic stainless steel was discussed by developing fatigue test under stress controlling mode. The results show that the ratcheting behavior of solid solution material is more notable with the increasing of strain amplitude while the ratcheting effect of stretched material is inhibited under the same condition. The degree of inhibition is increasingly obvious with the increase of strain level and also the fatigue life is gradually improved under stress controlling. Based on the Hull-Rimmer theory, a life prediction equation applicable to different pre-strain in high temperature was developed, and the predicted lives coincided well with the test results.