For the motorcycle muffler exhaust pipe rupture fracture morphology and metallographic analysis. Assessment of heat affected zone average grain size of level 3,parent metal grain size grade for level 6. Crack initiation in the region of the welding arc,instantaneous overheat of the region may result in coarse grains,brittleness increases; With inclusions in weld not clean up the local strength is reduced,in the process of serving the stress concentration caused by crack initiation. Crack from the inside to the outside slowly expanding gradually,reached a tipping point instantly exhaust pipe before and after the break.