Study on the microstructure and toughness of dissimilarly welded joints of advanced 9Cr/CrMoV
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n Huo and Peng Wang Shanghai Key Laboratory of Materials Laser Processing and Modification, School of Materials Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, People’s Republic of China; and Shanghai Turbine Plant of Shanghai Electric Power Generation Equipment, Shanghai 200240, People’s Republic of China (Received 1 June 2016; accepted 21 September 2016)
Dissimilar joints of advanced 9Cr/CrMoV have been successfully welded by narrow gap submerged arc welding using multi-layer and multi-pass techniques. The objective of our study is to establish the correlation between impact toughness and microstructural characteristics of the welded joints. Impact toughness tests were conducted in a wide range of temperature from 60 °C to 80 °C for different regions in the dissimilar joints. The fracture appearance transition temperature of base metal of 9Cr, CrMoV and weld metal were tested as 23 °C, 9 °C and 2 °C respectively, which all satisfied the service requirement. Optical microscope and scanning electron microscope revealed that weld metal and base metal of CrMoV comprised martensite and bainite while 9Cr was composed of lath martensite. The low toughness in the latter region arose from large grains with excessive carbide precipitates. Nonuniform microstructure in the heat-affected zone of 9Cr side caused different crack propagation paths and subsequently led to large variations of absorbed energy. When crack propagates along carbon-enriched zone in heat affected zone, the absorbed energy was 48 J. With crack deviating far from carbon-enriched zone, the absorbed energy increased to 147 J. Examination on fracture surfaces revealed the typical brittle fracture appearance in 9Cr and inter-granular fracture mode in heat-affected zone of 9Cr side when crack propagated along carbon-enriched zone.
I. INTRODUCTION
In China, coal-fired power plants will continue to play a pivotal role in supplying reliable and low cost electricity in the foreseeable future. To mitigate the environmental impact from pollutants and greenhouse gas, the power industry has to improve the thermal efficiency by implementing ultra-supercritical (USC) combustion technology.1,2 Higher combustion temperature and vapor pressure exert extra stress on all moving parts and the pipes, which necessitates the application of advanced materials with much enhanced mechanical properties.3–5 For instance, advanced 9Cr steel was developed for the turbine rotor,6–9 a critical component in the thermal power steam-turbine systems. However, 9Cr steel is extremely expensive and a large forged rotor of 9Cr steel is difficult to manufacture. A more economical approach is to substitute part of 9Cr steel that is exposed to
Contributing Editor: George M. Pharr a) Address all correspondence to this author. e-mail: [email protected] DOI: 10.1557/jmr.2016.381
intermediate-pressure with cheaper CrMoV steel,10,11 and join the two parts by welding. For large-scale rotors, narrow gap submerged arc welding (NG-SAW) is a proven cost-effective and efficient manufact
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