Investigation on the weakest zone in toughness of 9Cr/NiCrMoV dissimilar welded joint and its enhancement
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Xionglin Deng and Fenggui Lub) School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China (Received 2 March 2017; accepted 15 May 2017)
The impact toughness and related microstructure of the dissimilar 9Cr/NiCrMoV welded joint fabricated by narrow-gap submerged arc welding were systematically investigated in the paper. Results indicated that the fracture appearance transition temperature (50% FATT) for weld metal was 11 °C, while low and scattered absorbed energies determined by different crack growth paths for the heat affected zone of 9Cr were gained which could not satisfy the requirement of service. However, a dramatically enhanced impact toughness was obtained by optimizing the post-weld heat treatment (PWHT) process. Microstructure characterization revealed that the microstructure evolution from martensitic laths in the previous PWHT metals to a softer ferrite matrix with the supersaturated carbon precipitating from the matrix led to higher toughness in the optimized PWHT materials. In addition, the observation of the fracture morphology found that the fractography varied from brittle fracture to a fracture mode with both brittle and ductile fracture feature with the change of crack growth paths in 9Cr-HAZ.
I. INTRODUCTION
Turbine rotors which are usually subject to high temperature and complex stress environment are the most critical components of ultra-super-critical (USC) steam turbine in modern power plants.1,2 Advanced 9Cr heat resistant steels alloyed with Ni, Mo, etc. which show high temperature stability, high creep strength, good weldability, and outstanding corrosion resistance have been widely applied as high-pressure (HP) structure parts of turbine rotors working in the case of around 560– 630 °C,3,4 while considering the extremely expensive cost of advanced 9Cr, NiCrMoV2Ni4MoV (NiCrMoV) steels which are relatively cheaper have been utilized as low-pressure (LP) structure parts whose working temperature is around 300 °C. Eventually, advanced 9Cr and NiCrMoV steels were fabricated together to satisfy different working conditions. Taking a series of welding methods into consideration, narrow gap submerged arc welding (NG-SAW) was generally recognized to fabricate the dissimilar welded joint via multi-layer and multipass welding process for its high efficiency without losing quality compared with conventional gas tungsten
Contributing Editor: Jürgen Eckert Address all correspondence to these authors. a) e-mail: [email protected] b) e-mail: [email protected] DOI: 10.1557/jmr.2017.222
arc welding (GTAW) or tungsten inert gas welding (TIG) processes.5,6 Post-weld heat treatment for the welded joints to relieve the residual stress and get the microstructure stable is indispensable when manufacturing heavy section and high-temperature structural components that are used in steam power plants.7 Additionally, the PWHT process should be carried out according to the as-supplied state of the base metals and the applied environment of the
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