Compression-shear fracture of nickel-based superalloy during rotary tube piercing

Large-diameter nickel-based superalloy seamless tubes are the key component of pipeline transmission system in nuclear power, thermal power and petroleum industries. The rotary tube piercing (RTP) process is suitable to prepare large-diameter seamless tubes. However, the high temperature dynamic compression-shear fracture, also known as separation layer defect (SLD), is inevitable in the RTP process of nickel-based superalloy. In the study, the SLD was systematically studied by the combination of simulation and experiment. Firstly, the isothermal compressionshear experiments were designed to study the fracture behaviors. The results reveal that the plasticity of Inconel 718 alloy decrease with the increase of temperature and strain rate within the range of temperature 1080-1160 degrees C and strain rate 5-40/s, and the shear angle has significant effects on the fracture behavior. Secondly, based on the isothermal compression-shear experiment results, the SLD was explored by RTP experiments. The results reveal that the compression-shear deformation extent has significant effects on the SLD, and the folding defect is generated before the SLD is observed. The fracture morphology further confirms that the SLD belongs to typical shear fracture, which is generated under specific strain and stress state. Finally, the formation mechanism of SLD is revealed by combining finite element simulation and experiment. It is found that the formation of SLD includes crack initiation and crack propagation. Crack initiation is related with the maximum shear strain and maximum shear stress, and the crack propagation is controlled by radial deformation gradient.