Abstract
Low pressure turbine rotors are manufactured by welding thick sections of 25Cr2Ni2MoV rotor steel using tungsten inert gas (TIG) backing weld, and submerged arc welding (SAW) filling weld. In this study, the microstructure of columnar grain zones and reheated zones in weld metal was characterized meticulously by Optical Microscope (OM), Scanning Electron Microscope (SEM) and Electron Back-Scatter Diffraction (EBSD). The results showed that, compared with SAW weld metal microstructure, TIG weld metal microstructure was relatively fine and homogeneous, due to its lower heat input and faster cooling rate than SAW. The maximum effective grain size in TIG and SAW weld were 7.7 μm and 13.2 μm, respectively. TIG weld metal was composed of lath bainite (LB) and blocky ferrite (BF), while SAW weld metal was composed of acicular ferrite (AF), lath bainite (LB)and ferrite side plate (FSP). Tempered martensite (TM) was detected along columnar grain boundaries in both TIG and SAW weld metals, which was related to the segregation of solute elements during weld solidification. Electron Probe Micro-Analysis (EPMA) results showed that the contents of Ni and Mn at the dendritic boundaries were 50% higher than those at the dendritic core in TIG weld. Similarly, 30% of Ni and Mn segregation at dendritic boundaries was also found in SAW weld. In addition, the microhardness of the two welded joints was tested.
Highlights
NiCrMoV steel was widely used to manufacture low pressure rotors in steam turbines, due to its sufficient strength to support the turbines and sufficient, deep hardenability to ensure the suitable microstructure in the center of a large forging [1,2]
In the the present present study, study, the the multi-pass multi-pass weld weld metal metal was was divided divided into into two two regions, regions, which which were were the the columnar grain zone, without any reheated processes, and the reheated zone, which transformed columnar grain zone, without any reheated processes, and the reheated zone, which transformed from heat affected affected zones from columnar columnar grain grain when when subjected subjected to to the the same same thermal thermal cycles cycles as as heat zones in in weld weld metal metal
The reheated zone can be divided into two sub-zones, which the are coarse-grained zone, referred to astoWM-CG, and and the fine-grained zone,zone, referred to asto the coarse-grained zone, referred as weld metals (WM)-CG, the fine-grained referred as WM-FG
Summary
NiCrMoV steel was widely used to manufacture low pressure rotors in steam turbines, due to its sufficient strength to support the turbines and sufficient, deep hardenability to ensure the suitable microstructure in the center of a large forging [1,2]. It is difficult to manufacture large-scale and high-quality heavy section rotors by hot forging directly. Multi-layer and multi-pass welding technologies are utilized for welding thick plates, attributing to their advantage the ability to normalize the pre-layer and/or pre-pass microstructure, and increase the ductility and improve the welding quality [4,5]. In the manufacturing process of large-scale rotors, narrow gap tungsten inert-gas welding (NG-TIG) was performed for backing weld firstly, followed by multi-layer and multi-pass narrow-gap submerged-arc welding (NG-SAW), due to its higher efficiency and lower cost [4,6,7]. Qi et al [12] compared the microstructures
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