MIA86MA048

After gear retraction drg takeoff, at aprx 150' AGL, the aircrew heard a loud noise & noted the #2 eng had failed. They secured the eng, dumped fuel & landed without further incident. An exam of the #2 eng revealed the combustion chamber outer case, pn 616315, had failed from hi stress, low-cycle fatigue. The failure originated at a 4.6 inch crack at the rear drain boss to case weld. At the last shop inspn, a suspected crack was visually obsvd in this area. The case was prepared for non-destructive testing by grit blasting it with aluminum oxide beads to strip off nickel-cadmium plating. However, the testing did not reveal a crack. Therefore, the case was recoated with nickel-cadmium & returned to svc. After the failure, an exam of the fracture area revealed the presence of nickel & cadmium (0.5 to 0.75 inch, centered on the fracture origin area). There was evidence that grit blasting (prior to non-destructive testing) had mechanically closed the prev crack, thus preventing detection. The operator revised the shop procedure to a shell blasting method.