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By comparing the microstructure of hot extruded forming tube and burst tube as well as the analysis of crack and fracture of burst tube, the microstructure and crack formation mechanism of GH3625 alloy tube during hot extrusion were studied. The results show that the microstructure of the burst tube and the formed tube are equiaxed crystals, but the cracking of the burst tube causes the stress concentration at the grain boundary to be released. Besides, no deformation twins are formed in the microstructure and there is no uneven grain size in the radial direction of the tube. The root cause of crack formation is that the extrusion ratio is too high, which causes the adiabatic heat of the tube to be severely heated during the hot extrusion process, so that the low melting Laves phase melts and diffuses into the surrounding matrix. Under the action of high tensile stress at the exit of the mold, these cracks continue to propagation and eventually join together, causing the tube to burst. The higher cooling rate of the fracture surface leads to a shorter time for the passage of the microstructure through the austenite region, more recrystallized nucleation core and the grain growth process is hindered, so that a very fine recrystallized grain is formed on the fracture surface.