In this study, the production of dual phase microstructure in the core of surface-carburised AISI 8620 cementation steel and the effect of martensite particle size at constant martensite volume fraction on tensile fracture have been investigated, The results showed that specimens with dual phase microstructure in the core exhibited slightly lower tensile and yield strength but superior ductility without sacrificing surface hardness than a specimen with nearly fully martensitic microstructure in the core produced by conventional heat treatment involving quenching from similar to900 degreesC. Tensile properties increased with decreasing martensite particle size. The effects of martensite particle size on tensile fracture has also been observed. In the core of the specimens with dual phase having fine and coarse microstructures, microvoids formed at martensite particles, inclusions and martensite-ferrite interfaces in the necked region. Martensite particle size had an influence in determining martensite cracking. In the specimen with coarse martensite particles, interconnected martensite distributed along ferrite grain boundaries cracked easily and the fracture mode was predominantly cleavage type. Martensite cracking was less frequent and the microvoids were smaller and microvoid density were higher in the specimen with fine martensite particles and the form of fracture was dimple depression type. In this specimen, voids initiated mostly by decohesion at the interface, and by some examples of fracture of martensite. Microvoid coalescence was the dominant form of fracture in both fine and coarse microstructures.. The specimen with nearly fully martensitic microstructure in the core produced by conventional heat treatment showed completely cleavage type fracture. (C) 2002 Elsevier Science Ltd. All rights reserved.