Study On High Temperature Low Cycle Fatigue Characteristics Of Cr-Mo Steel Pipe

In recent years,with the improvement of industrial technology,the service environment of high temperature and high pressure pipelines in the fields of petroleum,petrochemical and energy power generation has become increasingly harsh.Low cycle fatigue failure caused by thermal stress is one of the most important failure modes of pipelines under high temperature and pressure conditions.Ferritic heat-resistant steel,austenitic heat-resistant steel and nickelbased alloy steel are the most widely used high-temperature pipelines at present.Low-alloy ferritic Cr-Mo steel has become the preferred steel for high-temperature and high-pressure pipelines because of its low price,excellent high temperature mechanical properties,and good creep resistance and oxidation resistance.At present,the high temperature pipe steel has not been completely localized,and there is still a certain gap with the developed countries in the field of high-end products.Some products are dependent on imports.The basic research and localization of high temperature resistant pipe steel need to be strengthened urgently.P91 and80 H steels are representative steels of 9Cr-1Mo series and 1Cr-0.5Mo series,respectively.The high temperature and low cycle fatigue characteristics of the two kinds of Cr-Mo steels are studied by experiments.The fracture mechanism of Cr-Mo steel was analyzed by means of micro-characterization.Combined with the fatigue behavior and fracture mechanism of the two materials,the appropriate damage parameters were selected to predict the fatigue life,and the reliability of the life prediction model was evaluated.The specific research results and main conclusions are as follows:(1)The fatigue life of P91 and 80 H steel is closely related to strain amplitude and temperature.The increase of strain amplitude and temperature will accelerate the fatigue failure of the material.The low cycle fatigue damage of the material is mainly controlled by plastic strain amplitude and cyclic strain ratio.The plastic strain amplitude and cyclic strain ratio of the two materials will increase with the increase of temperature,and the effect will gradually decrease with the increase of strain amplitude.Under the same strain amplitude,the plastic strain amplitude of 80 H steel is more sensitive to temperature.(2)The tensile fracture characteristics of the two kinds of Cr-Mo steels will change with the increase of temperature.From room temperature to 350?,the composite fracture changes from brittle-ductile to ductile.The low cycle fatigue cracks of the two kinds of materials originated on the surface of the sample,and the main fracture was transgranular fracture.With the increase of temperature,the distance between fatigue bands in the propagation zone increases,and the size of the secondary crack also increases,indicating that the high temperature environment accelerates the crack propagation.The increase of the average fatigue strip width of P91 steel at 350? is only about one third of that of 80 H steel,and there is no obvious oxidation trace on the surface of the extended zone of P91 steel,which indicates that temperature has little effect on the low-cycle fatigue behavior of P91 steel.It is believed that the difference of Cr content is the main reason for the difference of oxidative damage between the two materials.(3)The fatigue life of the material decreases exponentially with the increase of strain amplitude.The fatigue life is negatively correlated with temperature under the same strain amplitude.The larger the strain amplitude,the smaller the influence of temperature on fatigue life;By comparing the results of four fatigue life prediction models,it can be found that choosing reasonable fatigue damage parameters is the key to accurately predict the fatigue life.Ignored creep damage,Sehitoglu damage model is more reliable in predicting the low-cycle fatigue life of the two materials at 350?,which indicates that oxidative damage is an important reason for the fatigue life of the two materials at high temperature to be smaller than that at room temperature.