Study On Microstructure And Mechanical Behavior Of Oil (Natural Gas) Pipeline

As the fast development of national economy, the demand of petroleum and natural gas grows so rapidly in China that the average yearly growth rate is up to12% in recent years. It is estimated that the petroleum consumption will be over 400 million tons in 2020 state, and 60% of them will depend upon imports at that time.Compared with other ways, the pipeline transportation is characteristic of more quantity, more safely, less invest and lower cost as one of the ways to transport petroleum and natural gas for long distance. At present the total length of pipeline has amounted to 2.7×10~6 km in the world. The development of our country delays behind the advanced western countries such as Russia and USA. However the engineering project "transporting the natural gas from the West to the East" attracting worldwide attention has opened the new chapter of pipeline construction in our country.Along with the fast developing of pipeline transportation, the security attracts more and more attention owing to the fact that the geological condition, environmental change as well as the corrosion of the pernicious impurity in the oil and gas can all cause various damages of operating pipeline, reduce the lifetime and even bring about catastrophic accident.This research originated from the engineering project "transporting the natural gas from the West to the East" .The pipeline material X70 steel has been selected as an object of study to systematically study the microstructure and mechanical behavior, and so establish the relation between welding parameters, residual stress and fatigue crack propagate for three kinds of pipeline F 711, F 219 and F159 .The microstructure of matrix, welding and HEZ has been studied by means of metallograph, SEM and quantitative metallograph, the measurement of residual stressconducted on X-ray stress unit, the study of fatigue crack propagate rate for different welding carried out on MTS HF fatigue testing machine. It was shown that:(1) The different welding parameters have considerable effect on the microstructure homogeneity of welding seam and the HEZ as result of block ferrite in certain weldings by different heat inputs and cooling conditions .(2) The fatigue crack propagate rate of welding for F711 and F219 pipeline corresponds to the Paris rule. The block ferrite can cause the "microstructure stress concentration" during the fatigue process, and so accelerate fatigue crack propagate and the finer microstructure is necessary to ensure the fatigue property and pipeline security. Therefore besides the metallurgical during welding process is also very important.(3)Within the welding parameters scope, the welding residual stress does not obviously affect fatigue crack propagate.The above results can be as a reference for designing the reasonable welding parameters, and offer experimental basis to study fatigue propagate properties and security of pipeline, as well as technical support for security assessment of engineering "transporting the natural gas from the West to the East",...