Advanced quantification of the carbide spacing and correlation with dimple size in a high-strength medium carbon martensitic steel

Abstract Interparticle spacing in materials science is a key parameter controlling ductile fracture. It is generally evaluated as the distance between particle centers of mass. High strength medium carbon martensitic steels contain a high density of carbides, whose size is non-negligible with respect to the distance between them. Current microstructural quantification methods do not take the shape of analyzed particles into account, thus being not necessarily relevant to materials containing a high density of particles that trigger void nucleation and ductile fracture. This work reports on the potency of a simple full field particle quantification method based on a watershed algorithm that takes the shape and spatial distribution of particles into account. Three microstructures were produced by heat treating a hot-rolled 40CrMo4 steel bar. Cementite carbide populations of each microstructure were quantified using carbon extractive replicas, conventional image processing and application of the developed algorithm. A comparison of results from a full field Voronoi tiling algorithm revealed pronounced differences in the shape of distributions. The watershed-based method was shown to allow efficient interpretation of the relationships between carbide distribution and ductile fracture features.