An overload failure mode, initiating with ductile fracture and transitioning to brittle cleavage fracture, was positively identified in the failed deck sockets. Metallographic observations show that cleavage fracture initiation occurs 0.060-0.08-inch from the flame-cut surface. The transformed microstructure and hardness measurements indicate that the flame-cut surface carbon content substantially exceeds the base level of 0.18-0.20 wt. %, may be higher than 1.0 wt. %. Charpy tests indicate that ABS AH36 steel plate is within the ductile-to-brittle transition region at 32℉. Metallographic examination, three point bend tests and Charpy tests indicate that plastic deformation occurs prior to cleavage fracture. 6. Charpy results and metallographic observations indicate that the cold worked region plays an important role in cleavage fracture initiation. 7. Flame-cutting experiments show that the surface cracks form immediately after flame cutting, in the absence of an applied stress. 8. Flame-cutting experiments and metallographic observations show that carburization occurs upon flame cutting and is dependent upon the fuel/O_2 ratio. A sufficiently high O_2 cutting flame eliminates the hardened layer entirely. In summary, the overload failure mode and the transition from ductile to brittle fracture is facilitated by the combination of (1) the high hardness and carbon-rich transformed layer introduced by the carburizing cutting flame and (2) increased hardness due to the cold working coining process, and (3) high residual stresses produced by the manufacturing process (not discussed in this paper).
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