Fatigue behavior of high-strength steel S135 under coupling multi-factor in complex environments

摘要

High-strength S135 drill pipe is an important component in the oil and gas exploration and development of deep and ultradeep wells under complicated environments. It is of great significance to the study on the corrosion fatigue fracture of high-strength steel S135 in complex environments. In this paper, two samples of smooth and unilateral notch were used to conduct fatigue in air, pre-corrosion fatigue and corrosion fatigue at different temperatures, respectively, to study the corrosion fatigue behavior of high-strength steel S135 under the complex environments of the notch, corrosion medium, temperature and alternating stress. The experimental results indicated that the influences of coupling multiple factors (notch, temperature, corrosion medium and alternating stress) on the corrosion fatigue of S135 steel were greater than that of single factor, and the more factors indicated the greater influence on fatigue. The influence of three kinds of coupling factors, such as the notch, corrosion medium and temperature, was the most significant (80.8%). In single factor, the impact of notch on fatigue was the largest (49.9%), followed by corrosion (41.6%) and temperature (34.4%). Obviously, the notch was the dominant factor of fatigue fracture, and the temperature had a significant influence on corrosion fatigue. It was found that the influence of coupling-factor on fatigue was less than of the superposition of single factor, so the relationship among multiple factors was not superposed but competitive and synergistic relationship. This can be attributed to the fact that the crack was always initiated from the corrosion pits at the notch root under coupling multi-factor in complex environments and the initiation was the combined action of stress concentration, anodic dissolution and temperature. The stress concentration at the notch was the main factor of crack initiation, and the electrochemical activity of at the notch root was the highest under the action of cyclic stress and this place gave priority to corrosive pitting, made the increase of local stress concentration and promoted the initiation of crack. And the temperature provided the energy to the corrosion electrochemical reaction and accelerated the evolution of the corrosion pits, so crack initiation was accelerated. Besides, article established the testing method of rotating bending fatigue in complex environments, revealed the relationship of fatigue fracture as well as the mechanisms of crack initiation and propagation of high-strength S135 steel under coupling multiple factors in complex environment.