agation direction. As the crack propagates, region II with different features is shown on the fracture surface. Both quasicleavage (QC) and microvoid coalescence (MVC) can be observed in region II, which are shown in Fig. 9C and D. With further propagation, overload failure will take place. Note that the boundaries separating the three regions are not distinct, and the division of the fracture surface is based on the fracture morphology. Figure 10A–D shows the fracture morphology of an HY-100 implant specimen at a stress level of 80.3 ksi (554 MPa) that failed after 12 min of loading. Similar to the sample shown in Fig. 9, three distinct regions can also be seen on the fracture surface. The fracture mode at crack initiation in region I is essentially intergranular. Again, it is shown that the CGHAZ is the most susceptible to HIC among the HAZ regions. Relative to the previous sample, a small difference in fracture morphology exists in that the area of IG failure increases with decreasing the tensile loading. Both QC and MVC can be observed in region II, as shown in Fig. 10C and D. The fracture morphology of an HSLA- 100 implant specimen at a stress level of 102.4 ksi (706 MPa) that failed after 1.5 min is shown in Fig. 11A–D. Similar to HY-100, the fracture surface can also be divided into three regions as shown in Fig. 11A. Region I with predominant IG fracture can only be observed in a small area close to the thread root, as shown in Fig. 11B. In addition to the clear faceted IG shown on the fracture surface, the prior austenite grain boundary can also be observed on the thread surface, and it is continuous across the boundary separating the fracture surface and thread surface. It shows that cracking initiates in the CGHAZ when a critical amount of hydrogen diffuses to the stress concentration area. The prior austenite grain boundary becomes the weak link under the influence of both hydrogen and stress so that the relative grain boundary sliding occurs in the CGHAZ. That is why the prior austenite grain boundary can be observed on the thread surface. In region II, both QC and MVC fracture modes can be observed as shown in Fig. 11C and D. By decreasing the tensile stress in HSLA-100 to 85.8 ksi (592 MPa), the implant specimen failed after 60 min of loading. The fracture morphology of this sample is shown in Fig. 12A–D. It is shown in Fig. 12B that IG fracture can be observed in a small area of region I. Both QC and MVC can be observed in region II, as shown in Fig. 12C and D. The fracture morphology of the HSLA-65 implant specimen is shown in Fig. 13A–D. As shown in Fig. 13B, there is some faceted IG fracture with a smaller grain size in region I near the thread root even though it is not so clear as compared to the fracture surface of the HY-100 specimen. This is probably because of the mixture of ferrite, bainite, and martensite in the CGHAZ. In region II adjacent to region I, both QC and MVC can be observed, as shown in Fig. 13C and D. The occurrence of IG, QC, and MVC fracture modes on the fracture surface can be explained using Beachem’s model (Refs. 19, 20), as shown in Fig. 14. As- JANUARY 2013, VOL. 92 26-s WELDING RESEARCH Fig. 12 — Fracture morphology of HSLA-100 implant specimen at a stress level of 85.8 ksi (592 MPa) that failed after 60 min of loading. A — General fracture appearance (white arrow indicates the direction of crack growth); B — region I (IG); C — region II (QC); D — region II (MVC). Fig. 13 — Fracture morphology of HSLA-65 implant specimen at a stress level of 77.5 ksi (534 MPa) that failed after 23 min of loading. A — General fracture appearance (white arrow indicates the direction of crack growth); B — region I; C — region II (QC); D — region II (MVC).
Welding Journal | January 2013
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