The early stage of fatigue crack growth in martensitic steel

Abstract

In low ductility and high strength steels, the early stage fatigue behavior associated with non-metallic inclusions is a highly localized phenomenon near the inclusions. However, the nature of the fatigue crack initiation process is not clear. In this paper, a special emphasis is placed on the possible differences in the mechanism of initiation and the early growth of fatigue cracks between a martensitic steel and ordinary ductile materials.The poor adhesion between matrix and aluminum oxide inclusion leads to the formation of an inclusion pit which serves as a simple stress raiser. The fatigue crack originates at the periphery of this inclusion pit at an angle of 45 degrees to the principal stress direction. Metallurgical and micro-fractographical observations revealed that the initiation and early growth of fatigue cracks from non-metallic inclusion are of the shear rather than the tensile mode. Thus, it is concluded that, though the fatigue process is quite localized in the vicinity of inclusion, the mechanism for an initiation and early growth of fatigue cracks is essentially the same as that for ductile materials. The effects of metallurgical heterogeneities in the martensite, such as the prior austenite grain boundaries, packets, and plates on the initiation and early growth of the cracks, are also discussed from the mechanical-metallurgical viewpoint.

Effects of test temperature on internal fatigue crack generation associated with nonmetallic particles in austenitic steels

“…[1,2] While the mechanism of the crack generation and its growth originating from a nonmetallic inclusion near the specimen surface has been discussed, [3,4] it has not been sufficient to understand the internal crack-generation mechanism associated with preexisting defects, such as inclusions. Table I provides a summary of our results [5][6][7][8][9][10] on the fatigue crack-initiation site and subsurface crack origin at cryogenic temperatures.…”

Investigation of Fatigue Damage of Tempered Martensitic Steel during High Cycle Fatigue and Very High Cycle Fatigue Loading Using In Situ Monitoring by Scanning Electron Microscope and High‐Resolution Thermography

“…These parameters include the microstructure of the material itself, that is, defects, such as nonmetallic inclusions, pores, oxides, and the material surface. [1,2] The link between these microstructural parameters and the fatigue damage process can be obtained, when it is possible to observe the respective interactions in one experiment. [3] Tempering steels obtain their particular mechanical properties by a two-stage heat treatment, consisting of normalizing, quenching, and tempering.…”