Weld Repair of Cracked Defa Cannon Mount Frames

摘要

The weld repair of fatigue cracks found in the mounting frames of the Defa gun pack, fitted to Mirage III aircraft, is described. These cracks, caused by normal gun firing stresses, occurred well short of the frame design life as a result of metallurgical degradation in the form of a decarburized layer on the surface of the cast steel frames. Weld repair was by the Tungsten Inert-Gas process, using a low alloy Ni-Cr-Mo steel consumable. This produced a weld deposit with overmatching strength and toughness, with the aim of conferring a fatigue resistance at least equal to that of the sound parent metal, so as to prevent the recurrence of cracking. This performance was validated by laboratory fatigue-life (S-N) tests and fatigue crack growth measurements in weld metal and in sound casting. A detailed description is given of preweld preparation, the weld repair procedure, and postweld finishing including weld profile blending and surface protection. (Author)