Abstract
The main scope of this study investigated the occurrence of liquation cracking in the heat-affected zone (HAZ) of IN738 superalloy weld, IN738 is widely used in gas turbine blades in land-based power plants. Microstructural examinations showed considerable amounts of γ’ uniformly precipitated in the γ matrix. Electron probe microanalysis (EPMA) maps showed the γ-γ’ colonies were rich in Al and Ti, but lean in other alloy elements. Moreover, the metal carbides (MC), fine borides (M3B2 and M5B3), η-Ni3Ti, σ (Cr-Co) and lamellar Ni7Zr2 intermetallic compounds could be found at the interdendritic boundaries. The fracture morphologies and the corresponding EPMA maps confirmed that the liquation cracking in the HAZ of the IN738 superalloy weld resulted from the presence of complex microconstituents at the interdendritic boundaries.
Highlights
The superior tensile strength, creep and oxidation resistance at elevated temperature make Ni-base superalloys used extensively in industrial gas turbines [1]
The back-scatter electron (BSE) images can be used to distinguish solidification products in a cast superalloy according to their image brightness or phase contrast
Image brightness and phase contrast of the interdendritic microconstituents, the γ-γ’ colonies and γ matrix are revealed by high darkness in the figure
Summary
The superior tensile strength, creep and oxidation resistance at elevated temperature make Ni-base superalloys used extensively in industrial gas turbines [1]. A wide range of Ni-base superalloys, from solid solution-strengthened to highly-alloyed precipitation-hardened materials, have been achieved to satisfy the requirements of high-temperature performance and environmental corrosion resistance. IN738, a cast Ni-base superalloy, is strengthened by extensive precipitation of γ’ (Ni3 (Al,Ti)) and metal carbides (MC) to achieve excellent mechanical properties at elevated temperature [2,3]. After a long term of service at elevated temperature and high stress, the turbine blades eventually suffer from fatigue and creep damages, which degrade the microstructures and/or introduce defects into the components. Rejuvenation treatments are reported to be able to restore the microstructures and mechanical properties of degraded superalloys [4,5,6]. Repair-welding is chosen as the major way to refurbish cracked or damaged blades in gas turbines [1]
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