Effects of microstructure and strain ageing on fatigue-crack initiation in steel

Abstract

Evidence of the effects of grain size, precipitation hardening, and dynamic strain ageing on the long-life fatigue endurance of smooth mild-steel testpieces is reviewed. The fatigue limit behaviour of annealed mild steel involves a grain-size transition. When dynamic ageing is suppressed, fine-grained conditions still retain a well-defined fatigue limit but coarse-grained samples do not. In strongly locked fine-grained conditions the limit stress corresponds closely with the cyclic stress required to spread plasticity across grain-boundary barriers, but its grain-size dependence is much less than that of discontinuous yielding in monotonic straining. Tests using prestrained and coarse-grained conditions show that dynamic strain ageing can be a sufficient cause of the fatigue limit, but the usefulness of room-temperature ageing as a general fatigue-strengthening mechanism is limited. Atmosphere-locked substructures developed at low cyclic stresses are unstable at higher stress amplitudes. Elevated-temperature ageing which causes carbide precipitation can be more effective, but quench-aged mild steels with very fine dispersions of intermediate precipitates have relatively poor long-life fatigue endurance, owing to the effects of progressive precipitate dissolution during cyclic straining. In contrast, stable dispersions which maintain diffuse slip are remarkably efficient strengtheners. Such observations of the behaviour of mild steel have provided clues to the relative importance of strengthening mechanisms which contribute to the fatigue resistance of tempered martensitic steels. It is likely that the contribution from the matrix substructure is limited by instability in much the same way as the strengthening effects of a strain-aged substructure are limited. Dynamic ageing will promote a fatigue limit, but strengthening by stable dispersion hardening appears to be the dominant source of high fatigue resistance.