B Fig. 11 — EPMA results of welds, A — J1; B — J2; C — J3; D — J4, show distribution of major alloying elements. terface represents base metal heated above the A3 temperature (i.e., 910°C) during the weld thermal cycle, and is characterized by -ferrite grains surrounded by grain boundary martensite. Figure 14 also depicts that the amount of martensite present in the microstructures is inadequate to control the grain coarsening in CGHAZ. Therefore, despite the partial solidstate phase transformation on cooling, the CGHAZ is characterized by a coarse grain size. TEM micrographs of CGHAZ as shown in Fig. 15 reveal that the CGHAZ contains dispersed dislocation lines. The martensite laths are also present in CGHAZ, though the amount is less compared to the weld metal — Fig. 9. Again, the martensite laths that form in the CGHAZ are in a premature stage. A number of dislocation lines distributed from boundary to the interior of the ferrite grain in the CGHAZ are is also observed. This indicates that distributed dislocations Table 8 — Tensile Test Results of Notched Base Metal and Welded Joints Sample YS (MPa) Notch Tensile NSR Elongation Location Specification Strength (MPa) of Fracture BM 458.8 ±5.77 660.3 ±12.63 1.37 ±0.03 10.8 ±1.26 BM J1 584.8 ±7.32 865.2 ±6.73 1.853 ±0.03 23.5 ±0.82 BM J2 588.3 ±8.43 871.3 ±10.67 1.838 ±0.01 22.4 ±0.87 BM J3 587.5 ±11.23 869.7 ±9.88 1.845 ±0.01 22.9 ±1.03 BM J4 593.4 ±9.11 874.3 ±11.27 1.831 ±0.02 22.2 ±0.91 BM do not favor the formation of martensite laths. Rather, dislocation pileup acts as a precursor to the martensite formation. These dislocation lines are probably the result of local deformation caused by the residual stress (Ref. 21) during the weld thermal cycle. CGHAZ of J1 shows very little lath martensite formation along the grain boundary as illustrated in Fig. 15A. Interestingly, the lath martensite content increases with the increase in CO2 content — Fig. 15B–D. Therefore, the lath martensite present in different CGHAZ can be written in increasing WELDING RESEARCH order as follows: J1J2J3J4. The concentration of lath martensite present in CGHAZ can be explained by the interstitial diffusion mechanism of carbon into CGHAZ from weld metal because an increase in CO2 content increases the area of base metal fusion (Table 4) due to change in weld pool shape and thermal cycle that might act as a driving force to carbon diffusion in CGHAZ. Meyer and duToit (Ref. 28) reported that the higher carbon content in the weld metal might increase the interstitial content of CGHAZ signifi- APRIL 2015 / WELDING JOURNAL 109-s A C D
Welding Journal | April 2015
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