Date of Award
Dissertation - Restricted
Doctor of Philosophy (PhD)
Raymond A. Fournelle
Martin A. Seitz
The fatigue behavior of a quenched and tempered Nb-bearing high strength low alloy (HSLA) steel (Fe-0.08C-1.43Mn-0.17Mo-0.034Nb) heat treated to give two different microstructures of the same yield strength has been studied by light and electron microscopy, low and high cycle fatigue tests, x-ray line profile analysis, stress relaxation tests, fatigue crack propagation tests and measurements of the plastic work required to propagate a fatigue crack. One heat treatment (40H5) produced a quenched and tempered martensitic structure without NbC precipitates while the other (55H10) produced a quenched and tempered martensitic structure with fine coherent NbC precipitates.
The 40H5 heat treatment exhibited a large amount of cyclic softening during low cycle fatigue. This was accompanied by dislocation unpinning from carbon atmospheres, a rearrangement of the dislocation substructure into a cell structure and a consequent decrease in internal stress and microstrain. On the other hand the 55Hl0 heat treatment was observed to soften to a much lesser extent than the 40H5 heat treatment. This behavior was accompanied by dislocation unpinning and the development of a rather uniform distribution of dislocation debris rather than by the development of cell substructure. The internal stress and microstrain were observed to initially increase and then decrease slowly. Initial hardening was attributed to the increase in dislocation density and subsequent softening was associated with the degradation of fine coherent NbC precipitates.
In high cycle stress controlled fatigue the 55H10 heat treatment showed a little better fatigue resistance than the 40H5. This was attributed to better resistance to crack initiation for the 55H10. Crack propagation rate in the 40H5 heat treatment was 2 to 5 times slower than in the 55H10 heat treatment. Similar S.E.M. fractography for both heat treatments could not explain apparent crack growth rate difference. However, this difference could be explained by measuring much larger plastic energy required to propagate a fatigue crack accompanying much larger plastic zone size in front of a propagating crack in the 40H5 heat treatment.