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.

Narrow gap submerged arc welding method accompanied with multilayer and multipass technology was used to manufacture advanced 9Cr and CrMoV dissimilarly welded joint used as a newly developed turbine rotor. The aim of this investigation was to evaluate the high cycle fatigue (HCF) behavior of the welded joint at room temperature. Uniaxial-stress controlled HCF tests at stress ratio R = −1 were performed with specimens chipped from the welded joint of mockup and the S–N curve up to 1.0 × 108 cycle lifetime was obtained. It was found that the fracture location transferred from heat affected zone (HAZ) of CrMoV side to weld metal (WM) with decreasing stress amplitude. The microstructure of the welded joint was characterized and microstructure diversity was found to be responsible for the failure locations both in the CrMoV–HAZ and WM. Fracture morphology of failure samples were also investigated by a scanning electron microscope. It is detected that the stress amplitude required to drive the inclusion to be the crack initiation of the CrMoV–HAZ lies behind the transition. With decreasing stress amplitudes, void in the WM more easily tends to be the initiation of a fatigue crack than inclusion.