Fig. 10 — Average grain size of the weld metal and HTHAZ. Table 7 — Tensile Test Results of Unnotched Base Metal and Welded Joints Sample Specification YS (MPa) UTS (MPa) Elongation Location of Fracture BM 312.2 ±17.4 483.1 ±21.1 29.8 ±1.37 BM J1 314.3 ±8.2 466.8 ±13.7 24.1 ±0.98 BM J2 308.1 ±9.3 473.8 ±10.4 22.7 ±1.24 BM J3 320.2 ±5.7 471.2 ±7.6 23.1 ±0.99 BM J4 328.5 ±8.8 477.3 ±11.7 22.4 ±1.13 BM and J3 due to formation of more metastable phase along the grain boundary. Conversely, the other two welds (J1 and J4) have a relatively stable structure with less transformation, which is somewhat unable to restrict grain growth. Furthermore, EPMA results, as shown in Fig. 11, reveal the segregation and/or concentration pattern of major alloying elements present in different weld metals. Among these, concentration of carbon is most important because it governs the nature of carbide precipitation in weld metal due to the absorption of carbon through the gas metal reaction. In unstabilized FSS alloys, these precipitates are primarily chromium-rich carbides (M23C6) (Ref. 24). The carbide precipitation in welds J1, J2, and J3 is discontinuous and mainly occurs along the grain boundary (Fig. 11A–C), whereas, in weld metal J4, both interand intragranular type of discontinuous precipitation has been recognized (Fig. 11D). Figure 11B–D reveals the amount of carbide precipitation in the welds increases with the increase in CO2 content from 5% to 20% CO2 in shielding gas mixture. Earlier work reported that these precipitates form due to the supersaturation of carbon in the ferrite phase at elevated temperatures (Ref. 25). The ferrite-ferrite (/) grain boundaries are most preferable site for Cr segregation and carbide formation as shown schematically in Fig. 12 due to the negligible presence of austenite (Ref. 26). Therefore, all the welds have grain boundary carbide precipitates mainly along the / grain boundary regions. However, the formation of - phase and a martensite along the grain boundary (Fig. 9) should reduce the tendency to form carbide precipitation (Ref. 27). These phases can significantly alter the alloy segregation by absorbing free carbon rejected from ferrite during solidification and reduce the Cr segregation along the grain boundary (Ref. 27). Hence, existence of such phases (i.e., phase and a martensite) along the grain boundary is the potential cause of irregular or discontinuous carbide precipitation (Fig. 11B–D). Figure 12A–C schematically illustrates the precipitation behavior observed in different welds. Pure Ar (J1) produces lesser and smaller amounts of precipitates (Fig. 12A) due to the fact that the base metal and the filler metal have low carbon content and there are no other sources of carbon addition through diffusion, absorption, and migration due to inert arc environment. Increasing CO2 content (welds J2 and J3) in the arc atmosphere increases the gas metal reaction (between high-temperature ionic gas and molten pool) and absorbs more carbon into the weld pool (Fig. 5) due to dissociation of CO2 at a high temperature. These absorbed or migrated carbons can easily react with the potent carbide formers (i.e., Cr, Fe, etc.) and produces higher amount of precipitates along the grain boundary — Fig. 12 B. However, when the CO2 content is high enough (i.e., 20% CO2 in weld metal J4) in the arc atmosphere some migrated carbon may able to react with the potent carbide formers situated in the intragranular spaces and develop intragranular precipitates — Fig. 12C. Therefore, from the above observations, it can be postulated that three types of microstructural combinations are possible in the fusion zone as schematically shown in Fig. 13. First, stable austenite has been formed together with a lower amount of martensite formation and less carbide precipitation along the grain boundary — Fig. 13A. Weld J1 with pure Ar shielding typically has this type of microstructure. Second, metastable austenite has been formed together with higher amount of martensite formation and moderate discontinuous carbide precipitation — Fig. 13b. Welds J2 and J3 with 5% and 10% CO2, respectively, show this type of microstructure. Third, stable austenite was formed together with a low martensite formation and higher precipitates — Fig. 13C. Weld metal J4 with 20% CO2 shielding typically has this type of microstructure. Evolution of CGHAZ Microstructure The typical coarse-grained HAZ (CGHAZ) microstructure of different welded joints as shown in Fig. 14 reveals ferrite with some martensite. The CGHAZ adjacent to the weld in- WELDING RESEARCH 108-s WELDING JOURNAL / APRIL 2015, VOL. 94
Welding Journal | April 2015
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