Research On Stress Corrosion Fatigue Of High Strength Steel

With the increasing of the need for oil and gas resource, the situation that oil and gas exploration is facing is becoming more complex, and the drilling workload of the deep and ultra-deep wells is increasing year by year, which results in that drilling is more and more difficult, the drilling tool's working condition is more and more worse. The accidents that drilling tools fractured happens occasionally. Among them, there are a lot of accidents caused by corrosion fatigue, which have a strong impact on drilling safety and economic benefit.The S135,V150 and 165 three kinds of materials were taken as examples in this paper. First, the conventional mechanical property tests were carried out, and the results showed that their mechanical properties met API standard. Then corrosion fatigue tests of drilling pipe material specimen in the organic salt drilling fluid were carried out by rotating bending fatigue machine. The fatigue strengths in the different survival rates and confidence intervals were got by the tests carried out in low stress level using up-down method. In the high stress level, group test method was used. The test data was processed by the three parameter weibull distribution model. And the S-N curves with different survival rates were got, and the characteristics of corrosion fatigue could be seen from them.The effect of loading frequency on corrosion fatigue was analyzed in this paper. For example, the 165 drilling pipe materials, the corrosion fatigue tests in different loading frequency (20Hz,30Hz, and 50Hz) were carried out. The test data was processed by the three parameter weibull distribution model, and fatigue lives in the different stress levels at the survival rate of 0.99. Based on this, the S-N curves was drawn. By comparison, it can be seen from them, the loading frequency had strong impact on 165 drilling pipe materials'corrosion fatigue performance.This paper analyzed the whole process of fatigue crack propagation. And combined with corrosion effect, the factors effect on the corrosion fatigue crack propagation was analyzed. By SEM, analyzing specimen fracture, and EDS, the corrosion of fatigue testing specimen was oxygen corrosion. And the oxygen corrosion mechanism was studied. Combined with the current situation of drilling engineering, besides oxygen corrosion, the other corrosion and mechanism were studied.