Role of butter layer in low-cycle fatigue behavior of modified 9Cr and CrMoV dissimilar rotor welded joint

• Published in: Materials in engineering 2014-07, Vol.59, p.165-175
• Main Authors: Wu, Qingjun, Lu, Fenggui, Cui, Haichao, Liu, Xia, Wang, Peng, Tang, Xinhua
• Format: Article
• Language: English
• Subjects: Amplitudes; Butter; Butter layer; Chromium molybdenum vanadium steels; Chromium steels; Crack initiation; Dissimilar welded joint; Fatigue failure; Fracture location; Fracture mechanics; Heat affected zone; Low cycle fatigue; Low-cycle fatigue behavior; Softening behavior; Welded joints
• ISSN: 0261-3069
• DOI: 10.1016/j.matdes.2014.02.022

•Modified 9Cr–CrMoV dissimilar turbine rotor was successfully welded by NG-SAW.•LCF properties of both welded joints were approximate at smaller strain amplitude.•Tempered martensite with amounts of carbides in HAZ contributed to weakest zones.•Matched BL determined LCF properties of whole joint for dissimilar welded rotor. The present work aims at studying the role of butter layer (BL) in low-cycle fatigue (LCF) behavior of modified 9Cr steel and CrMoV steel dissimilar welded joint. The significant difference of the chemical composition of base metals (BMs) makes it a challenge to achieve sound welded joint. Therefore, buttering was considered to obtain a transition layer between the dissimilar steels. The LCF tests of two kinds of specimens without and with butter layer were performed applying strain-controlled cyclic load with different axial strain amplitudes. The test results indicated that the number of cycles at higher strain amplitudes of welded joint without butter layer was greatly higher than that of the joint with butter layer, while the fatigue lifetime to crack initiation (2Nf) became closer to each other at low and middle strain amplitudes. The failure was in the tempered heat affected zone (HAZ) at the CrMoV side for specimens without BL, while the fracture occurred at the tempered HAZ in the BL for specimens with BL. The microstructure details of BM, BL, HAZ and weld metals (WMs) were revealed by optical microscopy (OM). It was found that the tempered martensite was major microstructure for welded joint and much more carbides were observed in tempered HAZ than other parts due to the repeated tempering. Microhardness test indicated a softest zone existing tempered HAZ of BL and also there was a softer zone in tempered HAZ at the CrMoV side due to repeated tempering during welding and post weld heat treatment (PWHT). And scanning electron microscopy (SEM) was applied to observe the fractography. It was indicated that the fatigue crack initiation occurred from the specimen surface and all specimens were ductile–brittle mixed fractures. It is deemed that the softening behavior in BL caused by twice tempering correspondingly decreased the LCF lifetime at higher strain amplitudes. So suitable welding parameters and heat treatment processes became a key measure to ensure LCF property without losing other properties for welded joint with BL.