Baseline mechanical property data, constant amplitude fatigue crack growth rate data, and single-peak overload test data are presented for HP-9Ni-4Co-30C steel heat treated to three strength levels. These data are then used to evaluate a new model proposed for defining the instantaneous crack growth rate following an overload. The constant amplitude crack growth rates are affected by the strength level of the material with the higher strength exhibiting the faster cracking rates. The magnitude of retardation following an overload cycle is also shown to be influenced by the strength of the material. The lower strength steel displayed significantly more retardation for the same load levels. A general yield zone model is used to predict retarded growth rates. These predictions are shown to correlate quite well with the test data. The model successfully accounts for the different amounts of retardation associated with the different strength levels of the material.

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