Mechanisms of fatigue crack initiation in annealed, quenched and tempered 4340 steel

Abstract

The mechanisms of fatigue crack initiation in annealed and quenched and tempered 4340 steel were characterized. Several axial fatigue specimens were tested at a strain range of 1.5 per cent to various fractions of fatigue life. The tested specimens were thoroughly examined using a scanning electron microscope (SEM) and an atomic force microscope (AFM). The latter technique provides a better resolution and is also capable of providing quantitative surface topographical information. In annealed 4340 steel, the initial fatigue damage is shown to accumulate in the form of steps between ferrite and cementite laths in the pearlitic microstructure. Subsequent damage accumulation occurs by formation of slip bands which are formed by joining several adjoining steps. Cracks initiate from the slip bands. In quenched and tempered steels, the fatigue damage accumulates at discontinuities at a more rapid rate than in the remaining regions of the specimen, leading to crack initiation and growth emanating primarily from the discontinuities. The roughness of the specimen surface increases with fatigue damage in both microstructures, as demonstrated from the measurements obtained from AFM studies. Such correlations may be useful in predicting the remaining life of cyclically loaded structural components.