Retention of Delta Ferrite in the Heat-Affected Zone of Grade 91 Steel Dissimilar Metal Welds
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CREEP strength-enhanced ferritic (CSEF) steels have been used extensively for pressure vessel components in fossil-fired and nuclear power plants. Among their desirable properties are adequate creep strength, ductility, corrosion resistance, cost, and weldability, which enable power generation with affordable infrastructure.[1,2] Efforts to maximize efficiency of the plants have resulted in raising operating temperatures, which has demanded use of creep-resistant stainless steels for the hottest regions of the plant including the primary fossil fired generators. However, Grade 91 steel is still used in the lower-temperature heat recovery steam generators of combined cycle power plants. In order to join the high- and low-temperature sections of these systems, dissimilar metal welds (DMWs) are necessary. Early attempts to weld dissimilar steels using stainless steel filler metals resulted in extensive carbon migration near the fusion boundary that created brittle phases and large carbides, which weakened the welds and made them susceptible to cracking near the fusion boundary.[3,4] To help solve this problem, DMWs involving
MICHAEL W. KUPER and BOIAN T. ALEXANDROV are with the The Ohio State University, 1248 Arthur E Adams Dr., Columbus, OH, 43221. Contact e-mail: [email protected] Manuscript submitted October 30, 2018.
METALLURGICAL AND MATERIALS TRANSACTIONS A
CSEF steels are now welded with Ni-based filler metals to reduce the driving force for carbon migration between the dissimilar steels used, which in turn, reduces the formation of hard and soft zones that weakened the creep strength of the welds.[5,6] However, the high concentration of carbide formers in the Ni-based alloys still creates a driving force for carbon migration toward the Ni-based filler metal, particularly during the mandatory postweld heat treatment (PWHT).[7–12] Premature failures near the fusion boundary in DMWs of 2.25-Cr 1-Mo steels have been attributed to carbon migration during PWHT from the CGHAZ into the partially mixed zone.[4,13–15] In the case of Grade 91 steel DMWs with Ni-based filler metals specifically, the higher chromium content of the steel reduces the driving force for carbon migration, resulting in acceptable welds for industrial use with reduced carbon migration during the PHWT. However, premature failures near the phase boundary in such welds have been reported, although the exact failure mechanism is still unknown.[16] The challenge in identifying the root cause of failure is that the mechanism appears to be stochastic, with dependencies on fabrication history, joint design, operating conditions, and metallurgical factors including composition and grain size.[17] A phenomenon that may be related to the unexpected failures, retention of d ferrite in the HAZ, was recently studied in Grade 91 steel DMWs.[18] d ferrite in the HAZ of such welds was identified under the as-welded, heat-treated, and ex-service conditions.
Figure 1 shows the remnants of d ferrite in a DMW that was removed from service because of premature failure at appr
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