Microstructure Evolution of a Simulated Coarse-Grained Heat-Affected Zone of T23 Steel During Aging
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TRODUCTION
ASME SA-213 T23 (2.25Cr-1.6W-V-Nb) is a new low-alloy heat-resistant steel developed for manufacturing water wall panels, superheaters and reheaters in ultra-super critical boilers (USCB). T23 steel was modified on the basis of T22 (2.25Cr-1Mo) steel by reducing C, substituting W for Mo and adding V, Nb, N and B.[1] Substituting W for Mo can enhance solution strengthening and suppress the coarsening of M23C6 and MC during long-term creep, while V and Nb can form fine and dispersive (V, Nb) (C, N) during tempering for dispersion strengthening, and B contributes to improving hardenability.[2,3] After an appropriate heat treatment, T23 steel exhibits a stable bainitic microstructure with excellent creep properties. The creep rupture strength at 600 C of T23 steel was about 1.8 times greater than that of T22 steel.[4,5] In addition, reducing C lowers the
YONG LI and XUE WANG are with the School of Power and Mechanical Engineering, Wuhan University, Wuhan 430072, P.R. China. Contact e-mil: [email protected] Manuscript submitted June 5, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
hardness of the heat-affected zone (HAZ) and cold cracking susceptibility of T23 steel. Masuyama et al.[1] reported that T23 welds had a hardness < 350 HV 10 in HAZ without post-weld heat treatment (PWHT). Bendick et al.[6] proposed that PWHT can be eliminated when welding T23 tubes with a thin wall thickness (d < 10 mm) because the hardnesses of the weld metal (WM) and HAZ were < 350 HV 10. However, T23 welded joints without PWHT were found to be prone to cracking during operation in the early stage.[7–11] Premature failure of T23 welded joints caused by cracking resulted in severe steam leakage, which threatened the operational safety of power plants and caused massive economic loss. It has become a difficult problem to solve. T23 was found to be susceptible to stress-relief cracking (SRC).[12,13] Coarse-grained HAZ (CGHAZ) was prone to cracking during PWHT. T23 welded joints without PWHT were also prone to cracking during high-temperature service. Long et al.[10] studied the reason for cracking in T23/12Cr1MoV dissimilar steel welded joints of reheater tubes after service for about 8000 hours and found that cracks initially generated in CGHAZ exhibited typical features of SRC. Ren et al.[11] analyzed the failure of T23/TP347H dissimilar steel welded joints in high temperature reheaters and found
that SRC in CGHAZ of T23 steel caused the failure. Moreover, Mohyla et al.[14] found that secondary hardening and embrittlement occurred in the CGHAZ of T23 steel during service at 550 C. The generation of SRC and embrittlement in T23 welded joints was closely related to the microstructure evolution of CGHAZ during service. Therefore, it is necessary to study the microstructure evolution in CGHAZ of T23 steel during exposure at high temperatures, which contributes to supply information to explain the reasons for property degradation and early failure that take place in CGHAZ.
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EXPERIMENTAL PROCEDURE
The investigated materia
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