The failure mechanism and causes of a low-carbon steel fuel pipe were investigated via fractography,metallographic examination and chemical analysis. The steel pipe and bracket are connected by copper brazing. The seg-regation and diffusion of copper atoms occur along the grain boundaries on the surface of fuel pipe due to a poorly controlled brazing temperature,which reduces its plasticity and strength;on the other hand,the poorly controlled postweld cooling rate leads Widmanstatten-ferrite to generate inside the steel pipe. In the platform test,the brazed zone is subjected to periodic loading processes of large cyclic stress. Due to the stress concentration effect at the mechanical damage on the weld surface,a fatigue crack source is generated under the local high stress condition and the initial crack extends towards the fuel pipe,which leads to a cracking failure at the early stage of its service life.