THE CORROSION FATIGUE CRACK PROPAGATION OF 316LN AUSTENITIC STAINLESS STEEL

The effect of oxidation behavior on the corrosion fatigue crack initiation and propagation of 316LN austenitic stainless steel (SS) in 320 degrees C water and air environments was investigated by corrosion fatigue cracking test system, X-ray photoelectron spectroscopy (XPS), scanning electron microscopy (SEM) and nanoindentation. The experimental results indicated that the cracks were found to be initiated at first 20% of fatigue life of the specimens tested in 320 degrees C water, while only a few cracks were found until fracture when tested in 320 degrees C air. The crack propagation rates of the specimens tested in 320 degrees C water were obviously faster than those tested in 320 degrees C air especially at lower strain amplitude of +/- 0.5%. This can be attributed to the oxidation behavior of the 316LN SS specimens tested in 320 degrees C water was more serious than those tested in 320 degrees C air. The compositions were more complex of the oxide films formed in 320 degrees C water than those tested in 320 degrees C air, which lead to crack initiation easier in the former. Moreover, the 320 degrees C water made the mechanical properties of the oxide films worse, resulting in a faster fatigue crack propagation rate at crack tip.