Correlations Between Fractographic Features and Plane-Strain Fracture Toughness in an Ultrahigh-Strength Steel

Abstract

Fractographic analyses have been conducted on fracture surfaces produced by plane-strain fracture-toughness tests on notched tension-test specimens of 0.45C-Ni-Cr-Mo steels with different sulfur contents (0.008, 0.025, and 0.049 percent), tempered at 400, 800, and 900 F, to establish a relationship between features of the fracture surfaces and the plane-strain fracture-toughness parameter (KIc). The primary differences in the fracture-surface topography among the specimens with different sulfur levels and different tempering treatments were: (1) an increase in number and a decrease in the spacing of sulfide-nucleated dimples with increasing sulfur content and (2) a decrease in the extent of a “stretched” zone between the fatigue-crack and overload-fracture regions of the fracture surface. For each tempering temperature, the plane-strain fracture toughness decreased with a decrease in average sulfide spacing (that is, with an increase in sulfur content). Although, as expected, the sulfide spacing did not change with tempering temperature, the extent of the stretched zone increased with increasing tempering temperature from 800 to 900 F and, therefore, with increasing fracture toughness. A stretched zone was not observed in the steels tempered at 400 F (those with KIc values about 50 ksi√ in. and below). Comparisons of the variations in the size of the stretched zone with the process-zone size and the crack-opening displacement models for planestrain fracture toughness of these 0.45C-Ni-Cr-Mo steels indicated that the variations in the stretched-zone size were in good agreement for both models. Therefore, this stretched zone appears to be related to the size of the zone of plastic instability at the crack tip just prior to unstable fracture. The sulfide-particle spacing did not show good quantitative agreement with either the process-zone size or the crack-opening displacement. The deleterious effect of sulfide particles on plane-strain fracture toughness appears to be related to a decrease in fracture strain with increasing sulfide content that results in a decrease in the zone of plastic instability at the tip of a crack.