Analysis of Inner Crack of Super 304H Steel Ultra-Supercritical Boiler Tube in Hot Piercing Process

Super 304H ultra-supercritical boiler tube is made by hot piercing method, and internal crack is a representative defect. The influencing factors of Super 304H hot piercing were analyzed by using scanning electron microscope（SEM）. The results show that Super 304H seamless tube with internal cracks has coarse axial grains and large austenite grains compared to seamless tube without internal cracks. A large number of precipitation NbC, Nb（C,N） in the grain boundaries is one of the main causes of hot piercing cracking. Local Nb（C,N） aggregates along the grain boundary, on the one hand, it reduces the binding force of the grain boundaries and leads to the decrease of material plasticity; on the other hand, it weakens its ability to inhibit austenite subgrain growth.