edly support the close proximity of the toughness values as illustrated in Fig. 18A and D. The cleavage facets depict the reduction in toughness up to a certain extent but do not deteriorate so much because of the certain ductile nature of the rupture, which is incorporated by the microstructural constituents. Conclusions From the present study, the following conclusions can be drawn: 1) Higher CO2 content in shielding gas mixtures leads to an alteration in fluid flow mechanism, resulting in flatter, wider, and laterally deep penetrated bead profile, which ultimately increases dilution. 2) Welds J2 and J3 with 5% and A 10% CO2, respectively, provide a higher amount of martensite along the grain boundary. The dominant path of martensite transformation is g a. However, gea transformation is also realized in the case of pure Ar and 20% CO2 mixtures. Also, the precipitation of M23C6-type carbides increases with higher CO2 content. 3) Three types of microstructural combinations are possible in the weld metal. First, stable austenite is formed together with a lower amount of martensite formation and less carbide precipitation along the grain boundary. Second, metastable austenite together with a higher amount of martensite is formed and there is comparatively higher carbide precipitation. Third, stable austenite is formed together with low martensite and higher WELDING RESEARCH B precipitates. Again, the weld metal has a fine grain structure irrespective of shielding gas mixtures; however, the variation of grain size in the weld metal is probably controlled by the variation in g phase and M23C6 precipitation along the grain boundary. 4) In general, the high-temperature heat-affected zone of all the welded joints is characterized by coarse ferrite grains surrounded by the g-phase and lath martensite. However, higher CO2 content restricted grain coarsening in the CGHAZ up to a certain extent. 5) The weld metal is harder than the HAZ followed by the base metal. Differences among the weld metal hardness values are trivial with the change in shielding gas mixture. However, HAZ hardness slightly increases with the increase in CO2 content. APRIL 2015 / WELDING JOURNAL 113-s Fig. 18 — Fractographs of weld impact specimens: A — J1; B — J2; C — J3; D — J4. C D
Welding Journal | April 2015
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