A Fractographic Analysis of the Relationships Between Fracture Toughness and Surface Topography in Ultrahigh-Strength Steels

Abstract

Fracture surfaces of fatigue precracked, notched tension test specimens of experimental oil quenched and tempered (800 F)0.45C-Ni-Cr-Mo steels of different residual sulfur contents, and of two 18Ni maraging steels (250 and 300 grades) were investigated, using electron fractography, by a task group composed of various members of Subcommittee II on Fractography of ASTM Committee E-24 on Fracture Testing of Metals. Sulfur levels of 0.008, 0.016, 0.025, and 0.049 weight per cent in the 0.45C-Ni-Cr-Mo steels corresponded to fracture toughness values (K1c) of 65.3, 55.6, 51.0, and 42.8 ksi√in., respectively. The 250- and 300-grade maraging steels exhibited K1c values of 68 and 42 ksi√in., respectively. The investigation was carried out to determine whether the differences in fracture toughness could be related to changes in fracture surface topography. The differences in fracture toughness among the quenched-and-tempered 0.45 carbon alloy steels of different residual sulfur contents could be accounted for by differences in the frequency (or spacing) of the sulfide inclusions. These particles seemed to have nucleated microvoids in advance of the crack tip, which coalesced and resulted in dimple rupture. The particle spacing in each steel was equivalent to a process zone size based on the Krafft model. A similar correlation was not found for the two grades of 18Ni maraging steel having different toughness levels, possibly because the mechanism of fracture is more complex than for the 0.45 carbon alloy steels. Another fractographic feature, namely, the extent of a “stretched” zone between the fatigue-cracked and overload regions of the fractures, did appear to show some correlation with the KIc levels for both classes of steels, and may be related to the size of the process or plastic zone at the crack tip. This feature, and its significance in terms of planestrain fracture toughness, is being more extensively investigated by Subcommittee II of E-24.