sume when the implant specimen is subject to loading after welding, the combination of stress intensity factor and hydrogen concentration at the crack tip corresponds to point a in Fig. 14. The hydrogen concentration is not sufficient to initiate a crack, so cracking will not occur immediately. During the incubation period, atomic hydrogen continuously diffuses to the triaxially stressed region, and after some time, it will reach the critical level indicated by point b in Fig. 14. A crack will then be initiated in the CGHAZ and grow intergranularly. As the crack propagates, the stress intensity factor increases while the hydrogen level decreases to point c, promoting a QC fracture mode. As the crack continues to grow, and if the combination of stress intensity factor and hydrogen concentration reaches point d, the fracture mode will change to MVC. If the stress intensity factor continues to increase to the critical value KC, ultimate failure will take place. Microstructure and the fracture behavior taken together can explain the difference in HIC susceptibility of the three steels. As shown from fractography, cracking will always initiate in the CGHAZ, and the intergranular fracture occurs first. For the same welding conditions, the HY-100 CGHAZ microstructure is high-hardness (420 to 440 HV) martensite, while a mixture of bainite and martensite with lower hardness (325 to 340 HV) forms in the HSLA-100 CGHAZ. For HSLA-65, the CGHAZ has the lowest hardness (300 to 317 HV) among the three steels as a result of the presence of ferrite, bainite, and martensite. It has been shown previously (Ref. 10) that the prior austenite grain size is the largest in HY-100, and the smallest in HSLA-65, with HSLA-100 intermediate. Based on the fracture surface observations from the implant tests, HY-100 has the coarsest IG fracture and the largest area of IG fracture region among the three steels, while both of these features are the smallest for HSLA-65. As a result of different grain size, microstructure and associated hardness of the CGHAZ at the same welding condition, the HIC susceptibility of the three steels is different, which is indicated by the value of NCSR and embrittlement index of the three steels. Therefore, it can be concluded that HY-100 is the most susceptible to HIC among the three steels, while HSLA-65 is the least. Conclusions The results of the present investigation can be summarized as follows: 1. In the present welding condition, the hardness of the CGHAZ is in the range of 420–440, 325–340, and 300–317 HV for HY-100, HSLA-100, and HSLA-65, respectively. 2. Lath martensite with a thin film of retained austenite is observed in the CGHAZ of HY-100. For HSLA-100, lath martensite and bainite form in the CGHAZ. While for HSLA-65, a mixture of ferrite, bainite, and martensite forms in the CGHAZ. 3. When the average diffusible hydrogen content is 8.1 mL/100 g, the lower critical stress (LCS) is 72 ksi (496 MPa), 83 ksi (572 MPa), and 76 ksi (524 MPa) for HY- 100, HSLA-100, and HSLA-65, respectively. The normalized critical stress ratio (NCSR) is determined accordingly to be 0.72, 0.83, and 1.17 for HY-100, HSLA- 100, and HSLA-65, respectively. 4. A new embrittlement index is proposed, that is the ratio of the LCS and tensile strength of the CGHAZ, which is approximated by the hardness. Using this approach, the embrittlement index is determined to be 0.34, 0.54, and 0.52 for HY-100, HSLA-100, and HSLA-65, respectively. 5. Based on morphology and fracture mode, the fracture surface of the three steels can be divided into three regions. In region I, the crack will initiate in the CGHAZ and grow intergranularly. Both quasi-cleavage (QC) and microvoid coalescence (MVC) can be observed in region II. Final failure occurs under overload conditions. 6. As the crack initiates and propagates, IG, QC, and MVC fracture mode will occur in sequence. The observation of the three fracture modes on the fracture surface can be explained using Beachem’s model. 7. Among the three steels, HY-100 has the coarsest IG fracture and the largest area of IG fracture, while both of these are the smallest for HSLA-65. 8. Based on the implant test results, HY-100 is the most susceptible to HIC because of the formation of a high-hardness martensitic microstructure and large prior austenite grain size in the CGHAZ. HSLA-100 is less susceptible as a result of formation of bainite and martensite with lower hardness and smaller grain size. HSLA-65 shows resistance to HIC, resulting from the mixture of ferrite, bainite, and martensite with the lowest hardness and smallest grain size in the CGHAZ. Acknowledgments The authors gratefully acknowledge the financial support of the Office of Naval Research, Award No. N000140811000. Grant Officers: Dr. Julie Christodoulou and Dr. William Mullins. The authors would also like to thank Johnnie DeLoach, Matthew Sinfield, and Jeffrey Farren with the Naval Surface Warfare Center Carderock Division, West Bethesda, Md., for providing the steels used in this study and valuable discussions regarding the weldability of these steels. Dejian Liu and Geoffrey Taber are acknowledged for their constructive ideas and help on building the implant testing system. In addition, Badri Narayanan, John Procario, and Garr Eberle with The Lin- WELDING JOURNAL 27-s WELDING RESEARCH Fig. 14 — Combined effect of stress intensity factor and hydrogen concentration at crack tip on the fracture mode.
Welding Journal | January 2013
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