Microstructural aspects of fatigue crack propagation in a pressure vessel steel 20MnMoNi55

Most thick section steels contain microstructural variations (gradients). Such gradients were simulated in a nuclear pressure vessel grade ferritic steel of type 20MnMoNi55 and the fatigue crack propagation behaviour was studied for high cyclic growth rates (da/dN > 10-8m/cycle). Three heat treated conditions were used; as-quenched, stress relieved and tempered. In one batch, the crack was initiated in a harder microstructural constituent and propagated into progressively softer regions; in the other batch, the situation was the opposite. The results of these batches in the tempered conditions were then compared with results from specimens taken from an industrially heat-treated thick plate. In the simulated specimens, a number of interesting features were observed with respect to crack path, formation of secondary cracks, crack growth rates and fracture appearance. The macroscopic crack propagation was not however particularly sensitive to various parameters such as changes in the carbide distribution, residual stresses or non-metallic inclusions, especially in the tempered condition. This is attributed to the deformation characteristic of the major microstructural constituent, ferrite. Comparison of the results shows that the microstructure and the fatigue crack propagation behaviour of the specimens taken from industrially heat-treated sections can be simulated reasonably well