Influence of microstructure on near- threshold fatigue-crack propagation in ultra-high strength steel

Fatigue crack propagation behaviour of an ultra-high strength, silicon-modified AISI 4340 alloy steel (300-M) has been investigated in moist air over an extremely wide range of growth rates from 10^‑8 to 10^‑1 mm/cycle. Particular emphasis has been devoted to the influence of microstructure on fatigue-fracture behaviour near the threshold stress intensity, ∆K_o below which crack growth cannot be detected. By varying microstructure through quench and tempering and isothermal transformations, the threshold stress intensity and near-threshold crack-propagation rates are observed to be influenced by mean stress (load ratio), material strength, grain size, and impurity segregation. The threshold ∆K_o for crack propagation is found to be inversely related to the strength of the steel, and a relationship between ∆K_o and cyclic yield stress is observed. It is shown how near-threshold crack -growth resistance can be improved by (i) cyclic softening, (ii) coarsening the prior austenite grain size, and (iii) controlling impurity segregation to grain boundaries. These effects are contrasted with crackpropagation behaviour at higher growth rates. A semiquantitative model is developed to rationalize nearthreshold fatigue crack growth behaviour, based on the environmental influence of hydrogen, evolved from crack tip surface reactions with water vapour in moist air.