To prevent the corrosion of the inlet part with aqueous ammonia injection, high chromium corrosion-resistant materials have been applied for the welded joints. The bending fatigue strengths of the flange-pipe welded joint samples were defined by the fatigue experiments, modeling that high fluctuating stresses exist in the inlet and outlet flange-pipes of the muffler caused by the vibration of a moving vehicle. The factors that caused the fatigue to failure such as the welding bead shape and metallographic structure have been identified by the local stress measurement, FEM stress simulation, microscopic observation, and SEM-EDS composition analysis. By comparing the sample A having a smaller flank angle with the sample B having a larger flank angle, the result suggested that the difference of the bending fatigue strengths at 2x105 cycles was 24% when based on the nominal stress, and the difference was 10% when based on measured maximum stress. The shape effect was 14% because the fatigue strength of the sample B was 14% higher than that of the sample A subtracting 10% of the material effect. The maximum stress was non-linearly proportional to the weld flank angle. The crack starting point of the sample A was at the weld interface, a boundary between the weld acicular phase and the pipe ferrite phase. The crack starting point of the sample B was not at the boundary between the weld lathy phase and the pipe ferrite phase, but it was at the weld metal which had a phase like the ferrite and the crack propagated across the pipe metal. After the etching by the strong acid, the sensitization in the grain boundary occurred in the HAZ of the 17Cr flange, but it did not occur in the HAZ of the 30Cr pipe. The high chromium content was effective for the corrosion resistance.