Improvement of sub-critical fatigue crack growth life by nano-structuring of pearlite

Nano-structured pearlitic steels with interlamellar spacing between 60-160 nm have been developed in bulk by austenitizing and subsequent austempering at different temperatures. Their response to subcritical fatigue crack propagation has been studied using a compact-tension specimen while varying interlamellar spacing and maintaining similar prior austenite grain size. Pearlite with the finest lamellae spacing shows the best resistance to fatigue crack propagation sustaining the highest number of cycles and the largest cyclic stress intensity factor (Delta K) before failure. For low Delta K, the crack propagation behaviour was invariant with microstructural features and the crack propagated along softer ferrite phase contained in a single colony of pearlite. At higher cyclic stress intensity factor the finest lamellae pearlite with better strain accommodation ability, smaller CTOD and smaller reversed plastic zone size offered the best resistance to crack growth. Hence, for pearlitic steels, it is possible to enhance the strength as well as fatigue crack growth tolerance by decreasing the inter-lamellar spacing.