Multi-stroke Fracture Performance Research And Evaluation Model Establishment Of Drill Pipe Materials

Drilling link is an important stage in petroleum drilling engineering,and drill pipe plays a very important role in this segment.The drill pipe is subjected to complex altermating stresses in actual working conditions.This action load is often a repeated impact load of small energy.The mechanical properties of the drill pipe subjected to such load are often referred to as repeated impact properties.The performance indicators for this performance are not clearly indicated in current petroleum industry standards,and this performance is usually more difficult to measure than other mechanical properties.The repeated impact load is different from static fatigue and Charpy impact.It is the load type that most components bear in practical work and the bridge between them.Many properties of materials are easy to measure.If these properties can be used to characterize and predict the repeated impact properties of materials,they can be quantitatively evaluated.In this paper,the repeated impact properties of G105,S135,V150 and 165 steel grade drill pipe materials were tested by using the repeated impact fracture fatigue testing machine and the corresponding testing methods developed independently by our research group.An evaluation and prediction model based on basic physical and chemical properties to evaluating repeated impact properties was established by certain mathematical methods.The variation law of repeated impact properties of each material was summarized and compared.The intermal variation of the crystal of the broken specimen was also analyzed by various microscopic observation methods and some useful conclusions were obtained,and the critical speed of each drill pipe without repeated impact fracture is solved.The repeated impact properties of the samples soaked in sodium chloride solution were also tested,and the relevant rules were obtained.The main work and results of this paper are as follows:(1)The basic physical and chemical properties of the four drill pipe materials were tested:tensile performance test,pendulum impact test,hardness test,metallographic analysis,oseillometric impact test;Their repeated impact performance was tested.The performance differences among the materials are analyzed.The 165 drill pipe with the highest strength has the best repeated impact performance in a certain single impact energy range,but the performance decreases rapidly when the impact energy exceeds a certain value.(2)The correlation between basic physical properties and repeated impact performance is analyzed,and a significant index system is selected.The system is used to establish a repeated impact performance evaluation model based on the grey prediction model GM(1,n).The model has passed the Student's t test and the prediction effect is good.This model provides a very good reference basis for material selection under the actual working conditions of drilling engineering.(3)The law of repeated impact fracture of four drill pipe materials under normal conditions(under atmospheric environment,normal temperature and pressure)was analyzed.It was found by EBSD that the crystals of ihc specimens after repeated impact fracture became finer,the orientation of internal crystals has changed;TEM measurements show that there are a large number of dislocations near the repeated impact fracture surface.Dislocations interact with each other and entanglement occurs.Fine sub-crystals are formed within the grains,and this result is consistent with EBSD observations.Therefore,a series of changes have taken place in the interior of the material due to the repeated impact load.As a result,the repeated impact properties is different from Charpy impact properties.According to the theoretical derivation and experimental data,the critical rotational speeds of each material are calculated respectively.The theoretical values of G105,S135,V150,and 165 are 68r/min,77r/min,85r/min,and 95r/min,respectively.The experimental results are 61r/min,68r/min,77r/min,and 81r/min,The relative errors were not more than 15%.(4)The environment of drill pipe corrosion is mostly drilling fluids,most of which contain NaCI.Therefore,the repeated impact performance of four drill pipe materials after corrosion soaking in 3.5%NaCI solution was studied,and the variation rule and difference of the repeated impact properties of four drill pipe materials under this environment was obtained.Also,the micro-morphology of the cross section was observed,and the influence lows of the repeated impact performance of the drill pipe materiasl were analyzed,and the brittleness tendency of the materials after the repeated impact was increased due to the corrosion.