Fatigue crack growth mechanism in aged 9Cr–1Mo steel: threshold and Paris regimes

Abstract Room temperature fatigue crack growth (FCG) testing was conducted after thermal aging for 500 and 5000 h at 823 K and for 500 h at 923 K, on 9%Cr–1%Mo steel normalized at 1323 K and tempered at 1053 K. Though crack growth behavior in Paris regime is not much affected by aging, stress intensity factor range for threshold (Δ K th ) is found to increase monotonically. Acoustic emission signal analysis indicates identical crack propagation mechanisms in both threshold and Paris regimes. But SEM fractography reveals micro-voiding in the Paris regime only. Also, theoretical analysis of plastic zone size and inter-particle spacing ( λ m ) predicts microcleavage in the threshold regime. This indicates FCG occurs as a two-step mechanism, but each regime has a different dominant step viz. initial micro-void nucleation and growth at precipitates, followed by eventual micro-cleavage of inter-particle ligaments. Aging induces solid solution dilution, precipitate coarsening and increases λ m . In threshold regime, due to a tougher matrix, and a smaller cyclic crack increment (δ a N ) than λ m , an increase in Δ K th values is observed. Conversely, Paris regime δ a N is greater than λ m , making it rather insensitive to aging.