Long distance pipeline transmission is considered to be the most economic and safest method to transport natural gas in recent days. The pipeline safety problem is a focus of concern in the academic and industrial circles. Wildly used material X70 pipeline steel is tested in this paper. Based on actual conditions of long distance pipeline, experiments were designed by using orthogonal method. Corrosion fatigue crack growth tests were carried out under different H₂S concentrations (2000 ppm, 1000 ppm, 500 ppm), stress ratios (0.9, 0.8, 0.7), and load frequencies (0.0067 Hz, 0.01 Hz, 0.013 Hz). A phenomenological model was built based on test data and Paris formula, which corrects parameters C and n of the Paris formula at the same time. The boundary element method (BEM) was used to simulate the process of corrosion fatigue crack propagation. Fatigue crack growth rates (FCGRs) under different loading conditions were obtained and all the parameters of the phenomenological model were calculated from test data. Compared with traditional Paris formula, this new model shows coupling interactions of stress ratio, load frequency, and H₂S concentration to both slope and intercept of FCGR curve in log-log space. The simulation results show that the largest difference between actual test data and BEM simulation is less than 10%. According to the analysis of coupling interactions of each factor, the slope of fatigue crack growth rate (FCGR) curve in log-log space is mainly influenced by frequency; the influence of H₂S concentration is nonlinear, and FCGR increases significantly with the increase of stress ratio and decrease of frequency. The BEM is suitable to be used in the engineering field to predict the residual life of pipelines.

In this paper, the fatigue crack growth rates of typical pressure vessel material 4130X under different corrosion conditions are investigated, and the effects of corrosion modes and loading frequency on the fatigue crack growth rate of 4130X are discussed. The results show that under the same loading conditions, the pre-corroded crack propagation rate is increased by 1.26 times compared with the uncorroded specimens. The plastic deformation mechanism of the crack tip in air is dominated by phase transformation but the hydrogen introduced by pre-corrosion causes a small number of dislocations at the crack tip. The crack growth rate obtained by corrosion fatigue is four times that of the uncorroded specimen, and the fracture surface shows a strong corrosion effect. The molecular dynamics simulation shows that the hydrogen atoms accumulated at the crack tip make the plastic deformation mechanism dominated by dislocation in the crack propagation process, and the coupling interaction between low frequency and the corrosion environment aggravates the hydrogen embrittlement of the crack tip. In the air condition, the loading frequency has no obvious effect on the crack growth rate: when the frequency decreases from 100 Hz to 0.01 Hz and other conditions remain unchanged, the fatigue crack growth rate increases by 1.5 times. The parameter n in the Paris expression is mainly influenced by frequency. The molecular dynamics simulation shows that low frequency promotes crack tip propagation.

This article studies the effect of microstructure on the corrosion fatigue crack growth behavior of low and medium carbon steels. 45 steel and 15CrMo were selected and five typical microstructures were obtained by different heat treatment processes. The microstructures of 45 steel and 15CrMo after annealing and normalizing are both ferrite and pearlite, but the proportions of ferrite and pearlite in the four microstructures are different. The quenching and tempering microstructure of 45 steel is tempered fine pearlite. Corrosion fatigue tests were conducted on specimens with different microstructures of the two materials. Expressions of corrosion fatigue crack growth rate and da/dNΔK fitting curves of different microstructures were obtained. Comparison of da/dN∼ΔK curves of 45 steel shows that tempered fine pearlite has the best resistance to corrosion fatigue crack growth, and the content of pearlite has a great impact on the corrosion fatigue crack growth behavior for specimens with the microstructure of ferrite and pearlite. In the low ΔK region, the corrosion factor plays a dominant role in fatigue crack growth, and the corrosion resistance of pearlite is weak, which leads to a higher da/dN in the normalized state. In the high ΔK region, fatigue factor dominants, since the fatigue resistance of pearlite is strong, da/dN of the annealed state is higher. The experimental results of 15CrMo showed the same crack growth rate change regulation as that of 45 steel, which further proves the effect of pearlite on the corrosion fatigue crack propagation behavior.

In order to comprehensively study the effect of hydrogen corrosion on the crack growth rate, 45 steel was used as a typical material in this paper and fatigue crack growth rate (FCGR) of under different corrosion conditions and different stress ratios at room temperature were tested. 9 groups of da/dN-Δढ curves were obtained and Paris expressions were fitted. The results show that under the experimental conditions of air and different concentrations of H₂S corrosion environment, the value of parameter n of Pairs expression is basically stable at 3.5. Under the same stress ratio conditions, the FCGR of metals after H₂S corrosion is greater than that of metals in the air environment, and it increases with the increase of corrosion concentration, Stress ratio affects FCGR greatly, and the effect of corrosion time is limited.