Study On The Effect Of Aging Treatment On Low Cycle Fatigue Behavior Of High Strength Coiled Tubing CT110

CT110 has attracted extensive attention in the oil and gas industry due to its high strength,representing the new advanced coiled tubing.In engineering applications,the unique way of hoisting or running operation and the high-temperature environment are unavoidable during service life.Therefore,it is critical to understand the fatigue behavior of coiled tubing.Based on the study of low cycle fatigue(LCF)under traditional room temperature(RT)conditions,this paper considers the effect of secondary high temperature(204℃)on the LCF behavior of CT110.At the same time,the paper studied the LCF behavior of CT110 by combining the mechanical property of the fatigue analysis method with the microscopic deformation mechanism analysis method.The mechanical properties of fatigue of CT110 at unaging treatment(room temperature,RT)and aging treatment(AT)with 204℃for 12h showed stable cyclic softening behavior.However,the yield strength,proportion of elastic strain amplitude,and elastic modulus of CT110 treated with AT are enhanced.In addition,the cyclic elastic modulus is also affected by the strain amplitude and presents a downward trend with the increase of the strain amplitude.Nevertheless,the energy dissipation and accumulation method can reflect the fatigue damage process and have a linear relationship between cumulative strain energy density and fatigue life in a logarithmic coordinate system,which is more reliable for evaluating fatigue life.Among the three classical strain-life prediction models,the Manson-Coffin model has the best prediction ability for the LCF life of CT110 at RT and AT.The prediction results are distributed in the 2.0 times dispersion zone,followed by the Smith-Watson-Topper and the Landgraf models.The microscopic deformation mechanism of CT110 treated at RT and AT showed similar fatigue fracture characteristics.With the increase of strain amplitude,the fracture morphology shows that the spacing of fatigue striations increases and the final rupture area decreases.The dislocation density of CT110 in both undeformed and fatigue deformation under AT was lower than RT.With the increase of strain amplitude,the cell wall structure of dislocation tends to be tight.The results of TEM and grain size statistics by EBSD confirm that the ferrite grain number and the diffraction spots of SAED increase in the same selected area with the addition of plastic deformation amplitude and plastic deformation time,and the grain near the fracture surface presents thin strips.In addition,the grain size of the CT110 fracture decreased slightly after AT.Owing to the differences in temperature and strain amplitude,CT110 produces the preferred direction of textures and different multiples of uniform density during fatigue.The main direction is parallel(110),consistent with the XRD analysis results.In the LCF,the ferrite grain orientation coordinated toward the most easily slip direction and balanced development,and the LCF of CT110 shows cycle softening.After AT,the shear stressτ_e increase of CT110makes the activity of the sliding system difficult,and the Schmid factorΩof ferritic grain is lower than RT.Therefore,the strength of CT110 increases,and the plasticity decreases in macroscopic mechanical properties.