Fatigue behaviour of a nickel alloyed sintered steel

The relationship between microstructure and microscopic damage mechanisms of a powder metallurgy steel submitted to cyclic stresses was studied. Samples were prepared from Distaloy AE(TM) (Fe-4wt.%Ni-1.5wt.%Cu-0.5wt.%Mo) mixed with natural graphite (0.8 wt.%). The material's average density is about 7.40 kg m(-3). Porosity and phases of the initial microstructure were characterised. Fatigue tests were carried out at R = 0.1 both on unnotched and notched specimens. During the tests, the details of damage initiation and crack propagation were surveyed by light or scanning electron microscopy until the material failed. Slow crack growth was studied in detail. Two propagation modes were identified. First, the crack propagates in the I mode, then the crack forks off to preferentially follow the sintered bridges. The crack growth rate law of each mode was determined. The change of the crack propagation path was linked to the formation of secondary microcracks in the sintered necks during cycling. These observations and fractographic analysis show that damage mechanisms strongly depend on the microstructure. In particular, the presence of inclusions, the network of interconnected pores and the austenitic sintered bridges appear to be critical parameters. (C) 1998 Elsevier Science S.A. All rights reserved.