Fig. 3 — Typical implant test specimen sectioned near the implant axis. HY-100. 5% nital etch. on the as-polished samples using a 1-kg load, in accordance with ASTM E 384-10. Results and Discussion Weld Macrostructure and Hardness Figure 3 shows a transverse section of a typical test weld taken along the axis of the implant specimen. The fusion boundary separating the weld metal and implant specimen is clearly discernable. Due to the excessive grain growth and possible formation of susceptible (high hardness) microstructure, HIC will most likely occur in the CGHAZ region, which is just adjacent to the fusion boundary. Note that the HAZ in the implant specimen is much wider than that in the adjacent plate due to the difference in heat flow and temperature gradient. Vickers hardness measurements were taken along the axis of the implant specimens of the three steels, as shown in Fig. 4A–C. The hardness variation from weld metal to HAZ and the base metal is apparent with the hardness of the weld metal is in the range of 360 to 380 HV. The hardness of CGHAZ of HY-100 is higher than that of the weld metal, which is in the range of 420 to 440 HV. While for HSLA-100 and HSLA-65, their CGHAZ hardness is lower than the weld metal, in the range of 325 to 340 HV and 300 to 317 HV, respectively. The location of the CGHAZ, as shown in Fig. 4A–C, is determined by metallographic observation. It should be noted that the red dotted line is only the approximate boundary between the CGHAZ and adjacent finegrained HAZ (FGHAZ). Microstructure Characterization of CGHAZ of the Three Steels Figure 5 shows the optical and TEM bright-field microstructure of the CGHAZ from the HY-100 implant specimen. Martensite forms in the CGHAZ of this steel, and the packet of the martensite laths (Ref. 12) can be seen in the higher magnification TEM microstructure, as shown in Fig. 5B. The dark thin film between martensite laths is probably retained austenite (Refs. 13, 14). The formation of lath martensite is consistent with a CGHAZ in HY-100 with hardness in the range of 420 to 440 HV. The CGHAZ microstructure of HSLA-100 is shown in Fig. 6A–C. Martensite and bainite with a needle-like morphology form in the CGHAZ as shown in Fig. 6A. Similar to HY-100, because the carbon content of HSLA-100 is relatively low (0.051 wt-%), the martensite formed is of the lath type, whose morphology is clearly seen in Fig. 6B. It is shown in Fig. 6C that parallel laths with small intra-lath platelet-like precipitates form in the microstructure. The precipitates are most likely cementite, and they are directionally oriented. This morphology is characteris- JANUARY 2013, VOL. 92 22-s WELDING RESEARCH Fig. 4 — Vickers hardness measurements taken along the axis of the implant specimen. A — HY-100; B — HSLA-100; C — HSLA-65. Table 2 — Specimen Plate/Implant Specimen Dimensions Specimen Plate Material A36 Steel Plate thickness in. (mm) 0.5 (12.7) Plate width in. (mm) 2 (50.8) Plate length in. (mm) 4 (101.6) Length of the test bead in. (mm) 3.5 (88.9) Hole diameter in. (mm) 0.201 (5.1) Implant Specimen Material HY-100, HSLA-100, and HSLA-65 Total length of implant specimen in. (mm) 1 (25.4) Type of thread 10-32 UNF Pitch in. (mm) 1/32 (0.79) Major diameter in. (mm) 0.1900 (4.83) Minor diameter in. (mm) 0.1517 (3.85) Thread length in. (mm) 0.5 (12.7) Thread angle 60 deg Thread root radius in. (mm) 0.004 (0.1)
Welding Journal | January 2013
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