| Mobile tankers are widely used in the transportation of liquefied gases.The requirements for mobile pressure vessels are both safety and economy.Therefore,the S30408 stainless steel used in mobile pressure vessels needs to be subjected to strain hardening treatment to achieve the purpose of lightweight,and the study on the performance of S30408 stainless steel after strain hardening is of great significance for the security guarantee of equipment use.In this paper,the uniaxial tensile properties and cyclic properties of S30408 stainless steel hardened by various pre-strain conditions at room temperature were studied.The effect of strain hardening on mechanical properties was obtained.The fatigue properties of S30408 stainless steel after strain hardening were studied.The relationship between fatigue life and strain hardening is explored,and a theory for judging the effect of strain hardening on material properties is proposed in combination with martensitic transformation.At the same time,the properties of S30408 stainless steel were also studied at low temperature,and compared with the data at room temperature.The main conclusions of this paper are as follows:(1)Both torsional pre-strain and tensional pre-strain can enhance the strength of S30408 stainless steel,and the yield strength and cyclic stress response increase correspondingly with the increase of strainstrength amplitude,Among them,the hardening way of torsion prestrain is isotropic hardening,and the hardening way of tensile prestrain is kinematic hardening,which will lead to the asymmetry of tensile and compressive properties of materials.(2)The fatigue life of S30408 stainless steel shows the "S" pattern of first decrease,then increase and then decrease with the increase of the amplitude of strain hardening.The strain hardening leads to the reduction of material ductility,thus reducing the service life.The martensitic phase transition at the intercrystalline position will hinder the crack initiation and propagation,thus extending the service life.The interaction of ductility and martensitic phase transition leads to the "S" type change of service life(3)The distribution position of martensite is mainly related to the strain hardening and the strain amplitude of cyclic test.The stress amplitude response can comprehensively reflect the influence of strain hardening and cyclic strain amplitude on the distribution position of martensite,so as to establish the relationship between the stress amplitude response and the fatigue life.(4)The yield strength and tensile strength of S30408 stainless steel at low temperature is greater than the strength at room temperature.This is mainly because the low temperature causes the martensitic transformation speed and the total amount to increase significantly,thus hardening the material.The life of the material at low temperature is longer than that at room temperature.Because the stress strengthening at low temperature leads to the decrease of the plastic strain accumulation rate in the cycling process,which has a positive impact on the fatigue life.It is also verified that strain hardening can also improve the yield strength and cyclic stress level of materials at low temperature. |
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