Fatigue of pure iron and of iron containing a small quantity of carbon after strain ageing

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Abstract
The question of the sharp fatigue limit of iron and steel has been investigated by obtaining fatigue curves for single crystals and polycrystalline material carefully purified from carbon and nitrogen by long annealing at 1480°c in hydrogen (average carbon and nitrogen content near the surface ≤0·0002%; yield point absent). In addition, curves for the same material with controlled amounts of carbon and nitrogen re-introduced, and curves for plain Armco iron specimens with about 0·015% carbon, quenched and strain aged, i.e. with dislocations strongly anchored by interstitials, were determined, for comparison with curves for the same material with carbon in solid solution or precipitated. It was shown that pure iron, like mild steel, shows a sharp fatigue limit in the region of 106 cycles, but at a lower stress level according to the lower strength of the material. Hence this behaviour in fatigue appears to be characteristic of high purity iron, and is likely to be an intrinsic property of the metal. On the other hand, the specimens containing carbon quenched and strain aged, show smooth fatigue curves with a significant slope in the region of about 107 cycles and a fatigue limit reached above this value. It is suggested that anchoring of dislocations due to interstitials is not stable but is progressively broken down in regions of increasing volume in the course of fatigue.