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The microstructure effect is critical in the near-threshold region in terms of fatigue crack propagation. Despite numerous studies on the crack growth phenomenon in the literature, there is still no comprehensive understanding of the mechanism behind it. The fatigue crack growth mechanism occurs in the plastic zone region, which is quite small in size; the order is regarded as microstructural units, particularly at low stress intensities. Microstructural differences caused by heat treatment methods are frequently attributed to changes in monotonic and yield strength, resulting in differences in plastic zone size. The driving force required for crack growth under alternating loading is proportional to the plastic zone size ahead of the crack tip. When the microstructure is modified using isothermal transformations, the stress intensity near the threshold and corresponding crack propagation rates were found to be affected by stress ratio, material yield strength, particle size distribution, and impurity segregation. The crack growth threshold Δ K   0  is discovered to be inversely related to steel strength, and a relationship between Δ K   0  and cyclic yield stress is established. In the scope of this paper, annealed and tempered conditions were investigated to assess near threshold behavior for AISI 4340 steel. Effect of microstructure will be detailed around low stress intensities via performed crack growth tests.

Heat treatment effects on near threshold region for AISI 4340 steels