Fatigue resistance and microstructure of experimental dual phase Fe/2Si/0. 1C steel

The fatigue behavior of an experimental dual phase Fe/2Si/0.1C steel has been examined as a function of constituent morphology with the objective of developing ferritic-martensitic microstructures with optimum strength and fatigue crack propagation resistance. Microstructure containing fine globular or coarse martensite within a coarse-grained ferritic matrix were found to show the highest fatigue threshold stress intensity range ..delta..K/sub o/ values reported to date (to our knowledge) and certainly the highest combination of strength and ..delta..K/sub o/ for steels (..delta..K/sub o/ values above 19 MPa..sqrt..m with yield strengths in excess of 600 MPa). Such unusually high crack growth resistance is attributed primarily to a meandering crack path morphology which promotes slower crack extension rates from crack deflection and roughness-induced crack closure mechanisms.