Microstructure Characterization of SAW and TIG Welded 25Cr2Ni2MoV Rotor Steel Metal

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.