Microstructural Characteristics and m23c6 Precipitate Behavior of the Course-Grained Heat-Affected Zone of T23 Steel without Post-Weld Heat Treatment

Abstract

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.

Archives of Metallurgy and Materials

“…Therefore, fine secondary nbc carbides were precipitated within the grains, whereas m 23 c 6 carbides were formed mostly at the grain boundaries (Fig. 1(c)-(d)) [10,11].…”

Effect of Austenite-to-Ferrite Phase Transformation at Grain Boundaries on PWHT Cracking Susceptibility in CGHAZ of T23 Steel

“…The M 23 C 6 intergranular precipitates inside the white band are concentrated in C, W, and Cr. It is considered that the depleted zone is formed around the M 23 C 6 precipitate because the M 23 C 6 carbide with high C, W, and Cr components precipitates at the grain boundary [24][25][26][27] when PWHT is carried out at 700 • C. According to a study by Lee et al, the black band in alkaline sodium picrate-etched CGHAZ is the segregated region of C, W, and Cr [19,20]. The black band at 700 • C is also a region where C, W, and Cr components are concentrated.…”

“…However, the research of Belde et al recently found that there is a microsegregation in the matrix where the alloying elements are not uniform, which can affect the mechanical properties and cause cracking in the course of the heat treatment [17,18]. According to Lee et al, the M 23 C 6 precipitate of the base metal in 2.25Cr-1Mo-Ti steel partially dissolves in the thermal cycle of CGHAZ, and the microsegregation of C and Cr appears as a black stripe [19,20]. It is considered that such a microsegregation can affect the mechanical properties of PWHT due to phase transformation other than the matrix.…”

Effect of Austenite-to-Ferrite Phase Transformation at Grain Boundaries on PWHT Cracking Susceptibility in CGHAZ of T23 Steel

“…It is considered that the depleted zone is formed around the M23C6 precipitate because the M23C6 carbide with high C, W and Cr components precipitates at the grain boundary [24][25][26][27] when PWHT is carried out at 700℃. According to a study by Lee et al, the black band in alkaline sodium picrate etched CGHAZ is the segregated region of C, W, and Cr [19,20]. The black band at 700℃ is also a region where C, W, and Cr components are concentrated.…”

“…However, research of M. Belde et al recently found that there is a microsegregation in the matrix where the alloying elements are not uniform, which can affect the mechanical properties and cause cracking in the course of the heat treatment [17,18]. According to Lee et al, the M23C6 precipitate of the base metal in 2.25Cr-1Mo-Ti steel partially dissolves in the thermal cycle of CGHAZ, and the microsegregation of C and Cr appears as a black stripe [19,20]. It is considered that such a microsegregation can affect the mechanical properties of PWHT due to phase transformation other than the matrix.…”