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

AbstractFatigue 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 0 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 0 for crack propagation is found to be inversely related to the strength of the steel, and a relationship between ∆K 0 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) ...