To prevent corrosion of the inlet part with aqueous ammonia injection, high chromium corrosion-resistant materials have been applied for welded joints of mufflers. Bending fatigue strength of welded joint samples of flange pipes was defined through fatigue experiments, modeling that high fluctuating stresses exist in the inlet and outlet flange pipes of a muffler caused by the vibration of a moving vehicle. Factors that caused fatigue to failure such as welding bead shape and metallographic structure were identified through local stress measurements, FEM stress simulations, microscopic observations, and SEM-EDS composition analyses. By comparing with sample A having a smaller flank angle and sample B having a larger flank angle, the results suggested that the difference of bending fatigue strengths at 200,000 cycles was 24% when based on nominal stress, and the difference was 10% when based on measured maximum stress. The maximum stress was inversely proportional to weld flank angle non-linearly. The crack starting point of sample A was at weld interface, a boundary between weld acicular phase and pipe ferrite phase, but that of the sample B was not at the boundary between weld lathy phase and pipe ferrite phase. After etching with strong mixed acids, the sensitization in grain boundary occurred in the 17Cr flanges, but it did not occur in the 30Cr pipes.