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Extensive service at 540uC led to changes in the composition, morphology, size, and distribution of the original carbides that were present in the pre-service steel. This service did not seem to introduce weakening voids along grain boundaries. At low creep stress levels the stress versus Larson-Miller parameter (LMP) curve of the postservice steel indicates better performance than the lower bound of the pre-service steel. The opposite performance was found at high creep stress levels. The service led to only a slight reduction in fracture toughness of the steel. Exposure to post-service accelerated creep tests led to further gradual reduction in fracture toughness. The reductions in fracture toughness were proportional to the creep strain and duration. The formation of spherical carbides during the accelerated creep tests promoted void formation during the fracture process and thus contributed to the observed additional reduction in fracture toughness. MST/6187