Abstract：The macroscopical and microscopic structures on F65 forging billet and hub forging were analyzed by means of ultrasonic testing, microstructure inspection, metallographic microscope, tensile testing machine and scanning electron microscope to discuss the causes of cracking, which provided a feasible method for the research of defect characteristics of highpressure transmission pipe flange in deepsea oil production. The results showed that the cracking was caused by the residual stress of the structure. The cause of the crack was not related to the metallurgical defects and hydrogeninduced cracks of the material. The cracks were mainly propagated along the bainite in the strip structure, which was related to the strip microstructure with alternating distribution of coarse and uneven ferrite and bainite. The superposition of the structural stress of forgings and the residual shear stress of diagonal forging provided the external conditions for the cracks. The internal cause of the cracks was the uneven microstructure.
2021, Vol. 39 
