Metallographic and fractographic analyses are performed on A533B low alloy plate steels made by three steelmaking practices. The fatigue crack growth rates of these steels had been previously established in all six possible testing orientations. By using quantitative image analysis to quantify the nonmetallic inclusion structures of these steels, graphical and statistical correlations were established between the two material constants of the fatigue crack growth rate equation and various inclusion parameters. The average area of an inclusion on the metallographic cross section corresponding to the plane of fracture was found to be the best single parameter for establishing a correlation applying to all testing orientations. Scanning electron microscopic investigations of the fracture surfaces of the fatigue crack propagation specimens indicated that the inclusions (Type II manganese sulfides and alumina galaxies) exerted a predominant effect on the fatigue crack propagation behavior. Both studies provide additional evidence that the differences in fatigue crack growth rates between and within the steels of this investigation can be a result of the nonmetallic inclusion structures of the materials.

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