The microstructural characteristics of a simulated heat-affected zone (HAZ) in SA213-T23 (2.25Cr-1.6W steel) used for boiler tubes employed in thermal power plants were investigated using nital, alkaline sodium picrate, and Murakami's etchants. In order to investigate the microstructure formation process of the HAZ in the welding process, simulated HAZ specimens were fabricated at intervals of 100 • C for peak temperatures between 950 and 1350 • C, and the microstructural features and precipitate behavior at various peak temperatures were observed. The alkaline-sodium-picrate-etched microstructures exhibited a black dot or band, which was not observed in the natal-etched microstructure. As the temperature increased from 950 to 1350 • C, the black dot and band became wider and thicker. Experimental analyses using an electron probe micro-analyzer, electron backscatter diffraction, and transmission electron microscopy revealed the appearance of austenite in the black dot region at a peak temperature of 950 • C; its amount increased up to a peak temperature of 1050 • C and thereafter decreased as the peak temperature further increased. The amount of M23C6 decreased with an increase in peak temperature. Based on these results, we investigated the behaviors of austenite and M23C6 as functions of the peak temperature.
Microstructure evolution and the creep fracture characteristics of dissimilar welds between Ni-based superalloy Inconel 740H and High Cr ferritic steel Gr.92 were investigated. The dissimilar welds were fabricated by shielded metal arc (SMA) welding process with Ni-based austenitic Inconel 182 superalloy. Postweld heat treatment at 760 o C for 4 hours was conducted to temper the fresh martensite formed in the heat-affected zone (HAZ) of Gr.92, and to form γ΄ strengthener in Inconel 740H. The weld interface between the Gr.92 and weld metal was characterized by a discrete line with minimal inter-diffusion, whilst the weld interface on the Inconel 740H side showed an unmixed zone having epitaxial columnar growth from the base metal. It was observed that the fresh untempered martensite formed at the Gr.92-sided coarse-grained HAZ next to the weld interface during PWHT. This may be due to higher Ni contents diffused from the Ni-based weld metal. A sharp increase in hardness was detected at the coarse-grained HAZ of Gr.92 while the hardness minimum occurred at the fine-grained and intercritical HAZ (FG/IC HAZ). Fracture after creep at 650 o C/80 MPa and 700 o C/35 MPa occurred at the FG/IC HAZ of Gr.92, commonly known as a type IV fracture. This may be attributed to the strain localization at the FG/IC HAZ due to the smaller grain size as well as the lowest hardness. The elemental migration across the weld interface was not significant. It was also found that a needle-like Nb-rich δ-phase formed at the interdendritic region of the weld metal after creep at 700 o C/35 MPa.
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