Microstructural Evolution of Dissimilar Metal Welds Involving Grade 91
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DISSIMILAR metal welds (DMWs) in the power generation industry between Grade 91 (P91) and austenitic stainless steels are most commonly welded together using Ni-base filler materials.[1,2] These welds connect ferritic steels, used in the low-temperature regions of a power plant, to stainless steels, which are used in the higher temperature and more corrosive regions. A key characteristic of DMWs is the tendency for premature failure, which can lead to significant economic impact due to lost profits and repair costs.[3] The general failure mechanism of DMWs involving low alloy, Cr-Mo ferritic steels (e.g., Grade 22) has recently been reviewed in detail.[4] To summarize, the
SEAN ORZOLEK and JOHN DUPONT are with the Lehigh University, 5 East Packer Ave, Bethlehem PA, 18015. Contact e-mail: [email protected] JOHN SIEFERT is with the Electric Power Research Institute, 1300 West WT Harris Boulevard Charlotte, NC 28262. Manuscript submitted January 23, 2019. Article published online March 12, 2020 2222—VOLUME 51A, MAY 2020
sharp change in microstructure across the fusion line of DMWs in the as-welded condition forms due to a steep chemical concentration gradient across the partially mixed zone (PMZ), resulting in the formation of as-quenched martensite. Prolonged high temperature exposure during post-weld heat treatment (PWHT) or service results in carbon diffusion down the chemical potential of carbon (CPC) gradient from the ferritic base metal towards the austenitic fusion zone. In DMW failures involving Grade 22 steels, carbon diffusion leads to the formation of a row of carbides (Type I carbides) adjacent to a carbon-depleted soft zone on the ferritic side and a carbon-enriched hard zone on the austenitic side.[5] These large differences in microstructure and strength occur over very short distances across the fusion line (~ 50 to 100 lm). Concomitantly, locally high stresses can develop along the fusion line in the DMW from the differences thermal expansion coefficients (CTE) between the ferritic and austenitic steels. As a result of these factors, strain is concentrated in the weak, carbon-depleted zone near the fusion line, thus generating creep voids around the Type I carbides. Eventually the extent of damage is such that continued in-service
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growth leads to the formation of micro- and later macro-cracking. This observed mode of failure may be assisted by local contributors to an enhanced stressstate, temperature cycling or formation of a surface-connected oxide notch. While Grade 22 DMW failures have been extensively studied, few studies have focused on failures involving Grade 91 DMWs. Work conducted to date on failures involving Grade 91 have demonstrated that failure also occurs along the fusion line where creep voids ultimately lead to failure.[6,7] Similar to DMWs involving lower Cr-Mo steels, failure is attributed to various microstructural features including the presence of a carbide-free ferrite band and Type I carbides along the fusion line. The carbides initia
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