Fig. 7 — CGHAZ microstructure of HSLA-65. A — Optical; B and C — bright-field TEM. Fig. 8 — The implant test result curves of the following: A — HY-100; B — HSLA-100; C — HSLA-65. were repeated twice to verify the LCS. The delayed nature of HIC can be seen from Fig. 8A–C, and, as expected, there is a longer incubation time before implant specimen fracture with lower applied stress levels. It is also found that at comparable stress levels, the incubation time of HY-100 is shorter than that of HSLA-100. The implant test results for the three steels are provided in Table 3. The LCS is determined to be 72 ksi (496 MPa), 83 ksi (572 MPa), and 76 ksi (524 MPa) for HY- 100, HSLA-100, and HSLA-65, respectively. Because the three steels have different strength levels, a normalization procedure for critical stress is required. Also, in order to avoid the effect of variation in notch tensile stress on quantitatively rating HIC susceptibility, instead of using the embrittlement index used by Sawhill (Ref. 16), a normalized critical stress ratio (NCSR) is used in the present investigation. This is simply the ratio of LCS to nominal yield strength of the test steels (Ref. 17). Therefore, an exact measure of HIC on percent degradation from nominal yield strength could be determined. As the nominal yield strength is 100 ksi (689 MPa), 100 ksi (689 MPa), and 65 ksi (448 MPa) for HY-100, HSLA- 100, and HSLA-65, respectively. The NCSR is determined accordingly to be 0.72, 0.83, and 1.17 for HY-100, HSLA- 100, and HSLA-65, respectively. Beside the NCSR, a new embrittlement index, which is the ratio of the lower critical stress to CGHAZ tensile strength, is proposed in the present investigation to compare the HIC susceptibility of the three steels. Due to martensite formation in the CGHAZ, the maximum CGHAZ hardnesses is 440, 340, and 317 HV for HY-100, HSLA-100, and HSLA-65, respectively. And they are much higher than the three steels’ re- JANUARY 2013, VOL. 92 24-s WELDING RESEARCH Fig. 9 — Fracture morphology of HY-100 implant specimen at a stress level of 91.3 ksi (629 MPa) that failed after 3 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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