WELDING RESEARCH Fig. 5 — CGHAZ microstructure of HY-100. A — Optical; B — bright-field TEM. Fig. 6 — CGHAZ microstructure of HSLA-100. A — Optical; B and C — bright-field TEM. tic of lower bainite (Ref. 15). Therefore, the HSLA-100 CGHAZ microstructure is a mixture of lath martensite and bainite, which has hardness in the range of 325 to 340 HV. The CGHAZ microstructure of HSLA-65 is shown in Fig. 7A–C. It can be seen in Fig. 7A that a small amount of ferrite forms along the prior austenite grain boundaries. The morphology of packets of parallel laths free of precipitates can be seen in Fig. 7B, which is the feature of lath martensite. Similar to HSLA-100, directionally aligned intra-lath cementite platelets can be observed in Fig. 7C, which confirms the presence of lower bainite. The difference is that these cementite platelets are coarser than the ones forming in HSLA-100. The formation of a mixture of ferrite, bainite, and martensite results in a HSLA-65 CGHAZ with the lowest hardness (300 to 317 HV) among the three steels. The Implant Test Results A series of composite weldments of specimen plates and steel implants welded with the same parameters was subject to different levels of tensile loading after welding, and the time to failure at each stress level was recorded. These data were used to generate the implant test curves for the three steels, as shown in Fig. 8A–C. The tensile stress is equal to the load divided by the cross-sectional area of the root diameter of the 10-32 thread. Because the lower critical stress (LCS) is an important index to quantify HIC susceptibility, tests run at the highest stress at which the implant specimen does not fail after 24 h tensile loading WELDING JOURNAL 23-s Table 3 — Implant Test Results Steel CGHAZ Max CGHAZ Tensile Lower Critical Nominal Yield NCSRb Embrittlement Hardness Strengtha Stress Strength Indexc (HV) ksi (MPa) ksi (MPa) ksi (MPa) HY-100 440 212 (1462) 72 (496) 100 (689) 0.72 0.34 HSLA-100 340 154 (1062) 83(572) 100 (689) 0.83 0.54 HSLA-65 317 145 (1000) 76 (524) 65 (448) 1.17 0.52 (a) The CGHAZ tensile strength is converted from the CGHAZ max hardness using the ASTM hardess conversion chart. (b) NCSR stands for normalized critical stress ratio, which is the ratio of lower critical stress to nominal yield strength. (c) Embrittlement index is the ratio of lower critical stress to the CGHAZ tensile strength.
Welding Journal | January 2013
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