Effect of thermal cycling on creep behaviour of 2·25Cr–1 Mo/type 316 steel dissilllilar Illetal vvelds

Williams, John A.; Parker, J. D.

doi:10.1179/mst.1994.10.10.915

Cited by 8 publications

(3 citation statements)

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“…Numerous investigations have been conducted of both service and laboratory induced failures of DMWs, 5,[11][12][13]15,17,[29][30][31][32][33][34][39][40][41][42][43][44][45][46][47][48][49][50][51] and the failure mechanism (i) the difference in CTE between the ferritic and austenitic alloys causes a stress concentration along the interface (ii) an oxide notch often forms neat the fusion line on the ferritic side of the weld that can concentrate the stress even further (iii) the highly localised changes in composition and microstructure lead to large differences in creep strength near the interfacial region (iv) for DMWs prepared with nickel base filler metals, the type I carbides that form along the interface provide a site for nucleation and growth of creep cavities that eventually lead to premature cracking. For DMWs made with stainless steel filler metals, cracking typically occurs along the prior austenite grain boundaries (PAGBs) in the ferritic HAZ at a location of about one or two grains away from the fusion line.…”

Section: Failure Mechanismmentioning

confidence: 99%

Microstructural evolution and high temperature failure of ferritic to austenitic dissimilar welds

DuPont

2012

International Materials Reviews

131

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Dissimilar metal welds between ferritic and austenitic alloys are used extensively in power plants.Premature failure of such welds can occur below the expected creep life of either base metal. This article reviews microstructural evolution in dissimilar welds and describes factors that contribute to premature failure. The microstructure in the as welded condition consists of a sharp chemical concentration gradient across the fusion line. Martensite forms within this gradient due to high hardenability and rapid cooling rates from welding. Upon aging, carbon diffuses down the chemical potential gradient from the ferritic steel toward the austenitic alloy. This can lead to formation of a soft carbon denuded zone in the ferritic steel, and nucleation and growth of carbides in the austenitic steel that produce high hardness. These differences in microstructure and hardness occur over distances of about 50-100 mm. Failure is attributed to the steep microstructural and mechanical property gradients, the large difference in coefficient of thermal expansion, formation of interfacial carbides that promote creep cavity formation, and preferential oxidation of the ferritic steel. Information is also provided on available creep rupture properties, remaining life estimation techniques, current best practices and research in progress.

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Section: Failure Mechanismmentioning

confidence: 99%

Microstructural evolution and high temperature failure of ferritic to austenitic dissimilar welds

DuPont

2012

International Materials Reviews

131

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“…In recent years, nickel-based filler material has been widely used to replace austenitic filler material, which has greatly improved the service performance of DMWs [ 11 , 13 , 14 ]. For example, the service life of 2.25Cr-1Mo/316 DMW fabricated with nickel-based filler metal was five times that of DMW fabricated with austenitic filler metal, [ 15 ] which was due to the reduction of the CTE difference between the two sides of the WM/ferritic BM interface and weakened carbon migration. Therefore, the Ni/Fe interface between nickel-based WM and ferritic BM is the key factor to improving the performance of this type of DMW used in power plants.…”

Section: Introductionmentioning

confidence: 99%

Microstructure Evolution at Ni/Fe Interface in Dissimilar Metal Weld between Ferritic Steel and Austenitic Stainless Steel

Li,

Nie,

Wang

et al. 2023

Materials

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The formation and evolution of microstructures at the Ni/Fe interface in dissimilar metal weld (DMW) between ferritic steel and austenitic stainless steel were investigated. Layered martensitic structures were noted at the nickel-based weld metal/12Cr2MoWVTiB steel interface after welding and post-weld heat treatment (PWHT). The formation of the interfacial martensite layer during welding was clarified and its evolution during PWHT was discussed by means of scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), electron probe microanalysis (EPMA), focused ion beam (FIB), transmission electron microscopy (TEM), energy dispersive X-ray (EDX), transmission kikuchi diffraction (TKD), phase diagrams, and theoretical analysis. In as-welded DMW, the Ni/Fe interface structures consisted of the BCC quenched martensite layer and the FCC partially mixed zone (PMZ), which was the result of inhomogeneous solid phase transformation due to the chemical composition gradient. During the PWHT process, the BCC interfacial microstructure further evolved to a double-layered structure of tempered martensite and quenched martensite newly formed by local re-austenitization and austenite–martensite transformation. These types of martensitic structures induced inhomogeneous hardness distribution near the Ni/Fe interface, aggravating the mismatch of interfacial mechanical properties, which was a potential factor contributing to the degradation and failure of DMW.

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“…Numerous investigations have been conducted of both service and laboratory-induced failures of DMWs 1,14,15,16,18,20,32,33,34,35,36,37,42,43,44,45,46,47,48,49,50,51,52,53,54 and the failure mechanism is now largely understood. There are generally four factors that contribute to premature failure of DMWs during high temperature service:…”

Section: Failure Mechanism Of Dmwsmentioning

confidence: 99%

Review of Dissimilar Metal Welding for the NGNP Helical-Coil Steam Generator

DuPont¹

2010

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EXECUTIVE SUMMARYThe U.S. Department of Energy (DOE) is currently funding research and development of a new high-temperature gas-cooled reactor (HTGR) that is capable of providing high-temperature process heat for industry. The steam generator of the HTGR will consist of an evaporator-economizer section in the lower portion and a finishing superheater section in the upper portion. Alloy 800H is expected to be used for the superheater section, and 2.25Cr-1Mo steel is expected to be used for the evaporator-economizer section. Dissimilar metal welds (DMW) will be needed to join these two materials. It is well known that failure of DMWs can occur well below the expected creep life of either base metal and well below the design life of the plant. The failure time depends on a wide range of factors related to service conditions, welding parameters, and alloys involved in the DMW. The overall objective of this report is to review factors associated with premature failure of DMWs operating at elevated temperatures and identify methods for extending the life of the 2.25Cr-1Mo steel-to-alloy 800H welds required in the new HTGR. Information is provided on a variety of topics pertinent to DMW failures, including microstructural evolution, failure mechanisms, creep rupture properties, aging behavior, remaining-life estimation techniques, effect of environment on creep rupture properties, best practices, and research in progress to improve DMW performance.The microstructure of DMWs in the as-welded condition consists of a sharp chemical concentration gradient across the fusion line that separates the ferritic and austenitic alloys. Upon cooling from the weld thermal cycle, a band of martensite forms within this concentration gradient due to high hardenability and the relatively rapid cooling rates associated with welding. Upon aging, during post-weld heat treatment (PWHT), and/or during high-temperature service, C diffuses down the chemical potential gradient from the ferritic 2.25Cr-1Mo steel toward the austenitic alloy. This can lead to formation of a soft C denuded zone near the interface on the ferritic steel, and nucleation and growth of carbides on the austenitic side that are associated with very high hardness. These large differences in microstructure and hardness occur over very short distances across the fusion line (~ 50-100 m). A band of carbides also forms along the fusion line in the ferritic side of the joint. The difference in hardness across the fusion line increases with increasing aging time due to nucleation and growth of the interfacial carbides.Premature failure of DMWs is generally attributed to several primary factors, including: the sharp change in microstructure and mechanical properties across the fusion line, the large difference in coefficient of thermal expansion (CTE) between the ferritic and austenitic alloys, formation of interfacial carbides that lead to creep cavity formation, and preferential oxidation of the ferritic steel near the fusion line. In general, the large gradient in mechanical...

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Effect of thermal cycling on creep behaviour of 2·25Cr–1 Mo/type 316 steel dissilllilar Illetal vvelds

Cited by 8 publications

References 1 publication

Microstructural evolution and high temperature failure of ferritic to austenitic dissimilar welds

Microstructural evolution and high temperature failure of ferritic to austenitic dissimilar welds

Microstructure Evolution at Ni/Fe Interface in Dissimilar Metal Weld between Ferritic Steel and Austenitic Stainless Steel

Review of Dissimilar Metal Welding for the NGNP Helical-Coil Steam Generator

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