Research on Fracture Behavior under Cyclic Stress of Drill Pipe with Microduplex Structure

The fatigue strength and characteristics of crack initiation and propagation under cyclic stress of the drill pipe with microduplex structure are studied by fatigue testing machine, scanning electron microscopy dynamic tensile device and transmission electron microscopy. And the effect by microstructure on fatigue fracture behavior of the said drill pipe is analyzed and discussed.The results show that the evolution of the micro-fracture mode from intergranular-transgranular fracture to transgranular fracture may appear because of introducing low bainite structure or bainite + martensite interface. As a result, the deflection of cracking direction and secondary cracking are increased, and the resistance of the material against crack propagation is accordingly enhanced. Therefore it is possible for the the drill pipe to obtain fatigue performance better than that in case of tempered sorbite structure, and still keep similar mechanical properties by means of introducing an appropriate proportion of low banite/martensite duplex structure which leads to a longer crack initiation cycle.