Reasons for fracture of matching 42CrMo steel flexible connector for continuous oil pipe

The flexible connector of the continuous oil pipe in a certain oilfield fractured. The reasons for the fracture of flexible connectors were analyzed using macroscopic observation, chemical composition analysis, scanning electron microscopy analysis, metallographic examination and hardness testing. The results show that the fracture property of the flexible connector was fatigue fracture. The design of the transition fillet radius in the flexible connector structure was too small, the cutting parameters for turning were not reasonable, and there were micro hole defects with different shapes in the base material, which caused stress concentration in the material. The unreasonable design of heat treatment process parameters for flexible connectors led to excessive hardness and low plastic deformation capacity, which promoted the propagation of fatigue cracks. Under the action of low cycle fatigue load and alternating load, microcracks initiated at the stress concentration point, and the cracks continued to propagate, ultimately led to the fracture of the flexible connector.