Abstract
The tensile and reverse-bending fatigue behaviors of the SAE 6150 steel in the dual-phase (DP), fully martensitic, and tempered states, respectively, have been investigated using mechanical tests, scanning electron microscopy (SEM), energy-dispersive x-ray (EDX) microscopy, and optical microscopy. Residual stresses, inherent microcracks, and retained austenite films in the martensitic steel, quenched from 900 °C, lead to the development of inferior tensile and fatigue strength. Tempering at 700°C relieves the residual stresses associated with martensite, causes the precipitation of microalloy carbides (MACs), and thus results in superior strength, increased fatigue resistance, and moderate ductility. The DP microstructure, consisting of martensite islets in a ferrite matrix, gives rise to a combination of good strength, excellent ductility, and commendable fatigue characteristics. MAC in the tempered steel and martensite islands in the DP variant enhance fatigue performance by causing crack tip deflection and concomitant crack path tortuosity. Strain incompatibility between martensite and ferrite in the DP steel, and cementite films and ferrite in the tempered variant are identified as fatigue crack initiation sites.
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