A Study On The Low Cycle Fatigue And Fatigue-Creep Interaction Behavior Of 2.25Cr1MoV Steel At Elevated Temperature

Hydrogenation plants are the key equipments in the petrochemical industry which are operated in the long-term high temperature high pressure and hydrogen environment. With the development of petrochemical industry in China, the scale of hydrogenation plants is become larger and the service environment is more harsher, which puts forward higher requirements on the safety of equipment. The failure of these hydrotreating pressure vessels will bring great losses to personal safety and national property. As the typical failure mode for the structures operated at high temperature and high pressure environment, fatigue, creep and their interactions are critical to the safe operation of equipments. It is always a puzzle for the scholars worldwild. 2.25Cr1MoV steel is a vanadium modified steel based on 2.25Cr1Mo which has long been used for the counstruction of hydrotreating pressure vessels. With the addition of vanadium, 2.25Cr1MoV steel has a better strength and hydrogen attack resistance at elevated temperature than traditional 2.25Cr1Mo steel. In this study, a series of uniaxial strain-controlled tensile, fatigue and creep-fatigue tests were conducted to study the low cycle fatigue(LCF) and creep-fatigue interaction(CFI) behavior of 2.25Cr1MoV steel at elevated temperature. In addition, the effect of temperature and mean strain on the low cycle fatigue behavior of 2.25Cr1MoV steel are investigated. The mechanism of creep-fatigue interaction and life prediction method are also disscussed.Major experimental results are as follows:(1) 2.25Cr1MoV steel presents better tensile strength and plasticity at room temperature than those at 455 ?. The material exhibits viscoplastic behavior and higher strain rate responds to the same plastic strain with higher level of stress response at 455 ?.(2) The material features cyclic softening remarkably and masing behavior at 455 ?. Both Manson-Coffin equation and plastic strain energy method show a good ability in the low cycle fatigue life prediction.(3) With the increase of temperature from 355 ? to 555 ?, the steel shows a accelerated cyclic softening. The steel in general shows a increase in plastic strain energy but a reduction in fatigue life with increase in temperature. But the material experiences a dynamic strain ageing(DSA) process at 455 ? which is believed to induce a strengthening to resist the cyclic deformation. As a result, the cyclic stress amplitude and plastic strain energy changes little when temperature increases from 355 ? to 455 ?.(4) The applied mean strain resultes in initial mean stress but has no influence on the cyclic softening behavior at 455 ?. The cyclic stress amplitude, the stable stress-strain hysteresis loop and the fatigue life are not appreciably affected by the mean strain as compared to fully-reversed strain tests.(5) The material exhibits obvious creep-fatigue interactions at 455 ? and 555 ?. The imposed hold time at peak or valley strain is detrimental to fatigue life. Tensile holds are more damaging than analogous compressive holds but considerably less harmful than the combined tensile-compressive hold waveform. Based on the understanding of creep-fatigue interaction behavior and major influential factors, a modified plastic strain energy method is proposed for the life prediction in creep-fatigue interaction conditions and predicts fatigue life well.