spective base metal hardnesses, which are 283, 284, and 201 HV for HY-100, HSLA-100, and HSLA-65, respectively. That means the weld CGHAZ has a higher tensile strength than the base metal for the three steels. If the tensile strength of the CGHAZ can be determined, then the influence of diffusible hydrogen on the CGHAZ degradation can be expressed by the ratio of lower critical stress to CGHAZ tensile strength. However, it is difficult to measure the CGHAZ tensile strength directly from the implant test, that is because if no hydrogen is introduced into the weld, the failure will occur in the lower strength base metal rather than in the higherstrength CGHAZ. As a result, the maximum CGHAZ hardness is converted into the CGHAZ tensile strength according to the ASTM hardness conversion chart, which are determined to be 212 ksi (1462 MPa), 154 ksi (1062 MPa), and 145 ksi (1000 MPa) for HY-100, HSLA-100, and HSLA-65, respectively. The embrittlement index is determined accordingly to be 0.34, 0.54, and 0.52 for HY-100, HSLA-100, and HSLA-65, respectively. The higher the embrittlement index, the lower the HIC susceptibility, which means the degradation of CGHAZ tensile strength due to diffusible hydrogen is not serious. Note that the CGHAZ tensile strength is not experimentally determined but only an approximation; however, it can still be used as an index to evaluate the steels’ HIC susceptibility. Based on the above implant test results, both the NCSR and embrittlement index show that HY-100 undergoes the most serious degradation due to the effect of diffusible hydrogen among the three steels, while HSLA-100 and HSLA- 65 are less susceptible to HIC compared with HY-100. For HSLA-100 and HSLA- 65, their embrittlement index is almost the same, that is because of their relatively lower carbon and alloy addition (lower hardenability as shown in Table 3, Ref. 10) as well as their finer grain size compared with HY-100. However, it should be noted that NCSR of HSLA-65 (1.17) is higher than that of HSLA-100 (0.83), which means the CGHAZ degradation from base metal yield strength due to the effect of diffusible hydrogen for HSLA-100 is more severe than HSLA-65. Thereby, it indicates HSLA-65 has better resistance to HIC than HSLA-100. Fracture Behavior Figure 9A–D shows the fracture morphology of the HY-100 implant specimen at a stress level of 91.3 ksi (629 MPa) that failed after 3 min of loading. It shows that the fracture surface can be divided into three regions, which are region I, region II, and final failure region. Region I is in close vicinity to the thread root, where the highest stress concentration exists. A coarse intergranular (IG) fracture mode is dominant in region I, where the grain size is in the range of 70–90 μm as shown in Fig. 9B. It can be concluded that cracking initiates in the location where CGHAZ and thread root coincides, or somewhere closely behind the thread root (Ref. 18), as a result of stress concentration as well as the presence of coarse-grained lath martensite in the CGHAZ. The white arrow in Fig. 9A indicates the crack prop- WELDING JOURNAL 25-s WELDING RESEARCH Fig. 10 — Fracture morphology of HY-100 implant specimen at a stress level of 80.3 ksi (554 MPa) that failed after 12 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. 11 — Fracture morphology of HSLA-100 implant specimen at a stress level of 102.4 ksi (706 MPa) that failed after 1.5 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).
Welding Journal | January 2013
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