A Fig. 12 — Schematic representations. A — Disrupted and small precipitations of carbide along the δ/δ grain boundary at 100% Ar shielding gas condition; B — precipitations of carbide along the δ/δ grain boundary increases with the addition of 5% to 10% CO2 in shielding gas mixture; C — grain boundary and intragranular carbide precipitation were observed with the addition of 20% CO2 in gas mixture. A Fig. 13 — Schematic representation of three different types of microstructural combinations observed in the welds due to variation in CO2 content in shielding gas mixture with Creq/Nieq ratios and MT temperatures. A — Stable austenite formation, low martensite content, and low carbide precipitation; B — metastable austenite formation, high martensite content, and moderate carbide precipitation; C — stable austenite formation, low martensite content, and high carbide precipitation. cantly through fusion boundary diffusion during cooling from high temperature (1200° to 1300°C). In the present study, it was observed that the carbon content of the welds increased with the increase in CO2 content — Fig. 5. Therefore, it is likely that at the same heat input more carbon should diffuse through the fusion boundary into the CGHAZ region with the increase in CO2 content. Higher carbon content expands the loop to a certain extent and, hence, increases the - phase formation. The -phase formed at the high temperature is metastable in nature because of low nickel content in CGHAZ, and therefore further transformed (solid-state phase transformation, i.e., a) into lath martensite along the grain boundary during continuous cooling — Fig. 15. The grain size of CGHAZ is also notably affected by different shielding gas compositions — Fig. 10. In general, CGHAZ produced coarse grains; although CGHAZ produced by pure Ar (J1) attributes maximum grain growth and became finer with an increase in CO2 content. Therefore, the grain size of different CGHAZ can be written in decreasing order as follows: J1J2J3J4. The decreasing trend of grain coarsening with the increase in CO2 content can be explained by the formation of -phase and lath martensite along the ferrite grain boundaries. Higher -phase and lath martensite formation, associated with the diffused carbon content, can restrict the -ferrite grain growth at the high peak temperatures experienced during welding (Ref. 29). The formation of lowest -phase and/or lath martensite along the grain boundary is unable to restrict the grain coarsening in the CGHAZ of J1, whereas, the increase in CO2 content increases the carbon diffusion and thus increases the -phase and lath martensite content, which ultimately increases the degree of grain fineness in CGHAZ. Correlation of Mechanical Properties with Microstructure Microhardness The microhardness values are plotted in Fig. 16 and the average microhardness of different welded zones are given in Table 6. The average hardness of weld metal (~350 HV) is higher than that of the HAZ (~316 HV) and BM (~195 HV). Furthermore, as expected, among the four types of welds, J2 and J3 having relatively higher martensite content (Fig. 6C) and finer grain size (Fig. 10), have provided slightly higher hardness. On the other hand, HAZ having coarser ferrite grains (Fig. 14) with lesser amount of lath martensite and separately placed dislocations provide lower hardness — Fig. 16. Interestingly, J1, having pure Ar shielding, had the lowest HAZ hardness, whereas hardness slightly increased with the increase in CO2 content. An increase in CO2 content increases the metastable -phase formation along the grain boundary at high temperature, which further transformed to lathe martensite (through solid-state phase transformation) upon cooling WELDING RESEARCH 110-s WELDING JOURNAL / APRIL 2015, VOL. 94 C C B B
Welding Journal | April 2015
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