September 1980

Document Type


Degree Name



Dept. of Materials Science


Oregon Graduate Center


The effect of microstructure and internal hydrogen on fatigue crack propagation of quenched and tempered AISI 4340 steel has been investigated. The microstructural variables viz., prior austenite grain size, substructure of martensite, retained austenite and type of carbides were altered by choosing various austenitizing temperatures and tempering temperatures. Optical metallography and transmission electron microscopy were used to characterize these microstructural variables. Significant amounts of retained austenite films were observed at the lath and prior austenite grain boundaries, particularly in the coarse grained structures. The retained austenite was mechanically unstable ahead of a crack tip, and as well as during tensile deformation under plane stress and plane strain conditions. The microstructural variations that were induced by various heat treatments used in this study did not significantly affect the threshold stress intensity for fatigue crack growth. Crack growth rates near threshold stress intensity and at intermediate stress intensity ranges were nearly independent of microstructure. However, the specimens with higher fracture toughness exhibited better resistance to crack growth at high stress intensity ranges than the specimens with lower fracture toughness. However, the presence of internal hydrogen enhanced the crack growth rates by an order of magnitude over that of the specimens without hydrogen. The magnitude of enhancement in crack growth rates was dependent on the applied stress intensity range and on the prior austenite grain size. The maximum crack growth enhancement was observed at an intermediate stress intensity range, ΔK = 8 to 15 Ksi√in, (ΔK = 8.8 to 16.5 Mpa√m) for all prior austenite grain sizes. The crack growth rate at low stress intensity ranges (ΔK < 10 Ksi√in) was very sensitive to the prior austenite grain size. Specimens with coarse grained structures exhibited lower crack growth rates than those with fine grain structures. The crack growth mode at low applied stress intensities in the coarse grained structures was predominantly transgranu1ar fracture while in fine grained structures it was mostly intergranular. Crack growth rates in specimens tempered at 280°C for all grain sizes were slightly higher at intermediate applied stress intensity ranges than in specimens tempered at 180°C. These results are discussed in terms of the fracture mode and the diffusion of hydrogen to the prior austenite grain boundaries.





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