| In recent years,with the increase of energy demand and the development of low temperature technology,the demand for steel used in low temperature environment is increasing rapidly.Among all low-temperature steels,metastable austenitic stainless steel has a series advantages including good corrosion resistance,easy to welding,easy to heat and cold forming performance,and no ductile brittle transition temperature,thus it has been widely used.in medical,petroleum,chemical industry and aerospace fields.The cryogenic fatigue property is one of the most important mechanical characteristics of metastable austenitic stainless steel and the important basis to evaluate the life and security of the equipment.However,because of fatigue test in cryogenic environment need to expend great cost and difficult to implement,moreover,the equipment reliability,low temperature resistance and accuracy of data detection are also been raised,therefore,the cryogenic fatigue performance of the metastable austenitic stainless steel has been rarely research.Both Chinese and foreign standards have not considered the influence of deep cryogenic on material properties in the fatigue design of the metastable austenitic stainless steel cryogenic vessels.On the other hand,due to the instability of the austenitic stainless steel structure,the deformation induced martensitic phase transition is easily affected by the strain and strain temperature during the deformation process,which will affect the fatigue property of the material and the safety of the service of the energy equipment.Therefore,based on the research of the "Life prediction and key technology research project for extended service pressure equipment"(project number:2016YFC0801905),the following work had been completed for the deep cryogenic fatigue performance and the influencing factors of the metastable austenitic stainless steel(S30408):(1)The low cycle fatigue test of the metastable austenitic stainless steel(S30408)at 77K was carried out,and the fatigue stress response,cyclic constitutive relation and the variation of strain life relationship were obtained.By comparing the low cycle fatigue properties of the materials at room temperature and 77K,it was found that when the amplitude is above 0.7%,the fatigue life of the material is reduced by the deep cooling,when the amplitude is below 0.7%,the fatigue life of the material is increased by the deep cooling,and with the decrease of the amplitude,the effect of the deep cooling on the fatigue life is gradually enhanced.On the one hand,the life span of the material is greatly improved as the material is greatly enhanced by the deep cold.On the other hand,the life of the material under high strain amplitude is reduced because of the reduction of the plasticity of the material because of the deep cooling.In addition,the influence mechanism of cryogenic effect on low cycle fatigue properties of materials was discussed from the perspective of the macroscopic fatigue mechanics characteristics,fatigue fracture characteristics,martensitic transformation and dislocation motion.(2)The low-cycle fatigue and tensile tests of 35%pre-strain at room temperature and 35%pre,strain at 90℃ specimens were performed at 77K and the uniaxial stress-strain relationship,fatigue stress response and cyclic constitutive relation.By comparing the cryogenic low-cycle fatigue test results of the base metal and the pre-strained material,it was found that the two pre strain materials all showed the characteristics of cyclic hardening,and the response of stress cycle showed the trend of"hardening-short saturation-second hardening-saturation-failure".Moreover,the stress intensity of the room temperature prestrain material at the initial stage of cycling is higher than that of material prestrained at 363K,both of which are higher than the initial cyclic stress intensity of the base metal.In addition,the effect of pre-strain on fatigue life of materials is preliminarily explored.The results show that the fatigue life of the pre-strain material at room temperature is higher than that of the parent material and the material prestrained at 363K.The fatigue life of the pre strain material at 363 K under the lower strain amplitude is higher than that of the parent material.Since the test data of prestrain materials are limited and the fatigue data are dispersive,the concrete conclusions need to be further verified.Additionally,the effects of two pre-strain methods on the cryogenic low-cycle fatigue performance of the material were discussed from the perspective of the macroscopic fatigue mechanics characteristics,fatigue fracture characteristics,martensitic transformation and dislocation motion.(3)Based on the cryogenic low-cycle fatigue test data of the metastable austenitic stainless steel(S30408),its S-N curve at 77K was obtained.Comparing the fatigue test results with the standard curve,most of the results of cryogenic fatigue tests fall above the standard curve.According to the current standard fatigue design curve,the obtained results are relatively safe In addition,comparing cryogenic fatigue design curves with standard fatigue design curves,it was found that designing with standard curve in low cycle area may lead to more radical results,so it was recommended that the cryogenic fatigue design curve should be adopted in low-cycle areas considering the weakening effect of cryogenic on the plasticity of materials.On the contrary,in the high-cycle areas,the results are too conservative,which is not conducive to fully exerting the fatigue performance of the material under cryogenic conditions.Therefore,it was also recommended that the cryogenic fatigue design curve should be adopted in the high-cycle area considering the strengthen effect of cryogenic cooling on the material. |
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