Discussion On Issues Of Corrosion Fatigue Of X70 Steel

The project of"West-East Gas Transmission"is a national construction and plays an important part in developing national economy. As transporting use, X70 pipeline steel play an important crucial part. In order to research on the stress corrosion performance and the corrosion fatigue characteristic of X70 pipeline steel, the researcher did in-depth quantitative research for X70, which based on the trials.The researcher simulated true environment:1.In the different sulfured hydrogen corrosive environment C(500ppm, 1000ppm, 2000ppm)2.Very low loading frequency (0.0067Hz, 0.01Hz, 0.013Hz)3. True R in the coursing pipe (0.5, 0.7, 0.8, 0.9)Facing problem of the exact measurement of crack growth in the foul environment for long time, the researcher modified experimental device of corrosion fatigue, and adopted a longer pole outside the environment case to measure the displacement of the crack opening in an innovative way. Both hardware and software updates during the experiment effectively excluded the interference of external factors , The method by using the longer guide bar to increase external support of the installation can effectively reduce the measuring tip by 90% of the additional moment ,and solve the problems of the stability, accuracy and convenience of data processing for the long monitoring.By using the mathematics software, compiling data processing procedure and making using of Paris formula for some crack growth rate formulas, the researcher studied deeply the threshold of crack growth, the crack growth curve and got some engineering conclusions with certain value:1.The crack growth coefficient in Paris C has relation with the index m, and the researcher gave their definite math expression C = 5. 01×10^-4(0.2042)^m.2.ΔK_thCF has close relation with R, the concentration C, the frequency f, and R is main impact factor. Orthogonal experimental researches show that R, C, f withΔK_thCF distribute one curved face; with the increase of stress ratio R in the different concentration and frequency,ΔK_thCF all dropped significantly.ΔK_thCF is inversely proportional to the concentration C, and proportional to the very low frequency f.3.The stress corrosion associate platform is characteristic of corrosion fatigue crack. growth curve, and the associate platform move rightward if R is smaller.