PSI - Issue 2_A

Y. Nakai et al. / Procedia Structural Integrity 2 (2016) 3117–3124 Nakai, Shiozawa, Kikuchi, Obama, Saito, Makino, Neishi/ Structural Integrity Procedia 00 (2016) 000–000

3120

4

(a) N = 1.00×10 7 cycles

(b) N = 1.68×10 7 cycles

(c) N = 1.295×10 7 cycles

Figure 4: 3D-images of crack and inclusion (0.020S mass%, vertical inclusion).

although part of the vertical crack remained at the bottom of the flaking as can be seen in Figure 3 (b).

3.3. Specimen with sulfur concentration of 0.049 mass%

(a) Material with vertical inclusions Figure 5 shows a surface image of a crack initiation site of specimen with vertical inclusions ( N = 6.00×10 6 cycles, shortly after crack initiation at the surface). Small cracks, those are almost perpendicular to the ball-rolling direction, can be observed. After the initiation of the crack at the surface, observation by SRCL was conducted. Flaking was found to occur at N =7.67×10 6 cycles at the site where the surface crack was observed as shown in Figure 6.

Figure 5: SEM micrograph

of surface crack.

3D images of inclusions and cracks observed by SRCL at N = 6.00 × 10 6 cycles, 6.80 × 10 6 cycles, 7.10 × 10 6 cycles, 7.50 × 10 6 cycles, and 7.67 × 10 6 cycles are shown in Figure 7, where (A) is a top view, (B) is a side view of the specimen, and (C) is the view from the rolling direction. In these figures, red, black, blue, green, and purple indicate crack at N = 6.00×10 6 cycles, 6.80×10 6 cycles, 7.10×10 6 cycles, 7.50×10 6 cycles, and 7.67×10 6 cycles, respectively. It can be seen from these figures that the inclusions, indicated in orange are longer than those in Figure 6. As shown in Figure 7, a vertical crack formed from a cylindrical inclusion with a length of about 60 μ m that reaches the surface, and the crack face is perpendicular to the rolling direction. At N = 6.80×10 6 cycles, the vertical crack propagated to the deepest point of the starter inclusion without propagation along the surface. At N = 7.10×10 6 cycles, a horizontal crack, which was parallel to the rolling

Rolling direction

Rolling direction

Crack

50 m µ

200 m µ

(a) Flaking (b) Enlarged view of (a) Figure 6: SEM micrograph of surface at flaking ( N f = 7.67×10 6 cycles).

contact surface, formed at a depth of 35 μm from the surface. From N = 7.10×10 6 to 7.50×10 6 cycles, the vertical crack propagated in the width direction under the surface, and the horizontal crack propagated in both the width and rolling directions. Another horizontal crack was also formed from the same inclusion. Its formation site was shallower than that of