Comprehensive Analysis of Heat Input and Filler Metal Effects on Microstructure and Mechanical Properties in Gas Tungsten Arc Welding of Cobalt-Based FSX-414 Superalloy

Articles in Press

Document Type : Original Article

Authors

1 School of Metallurgy and Materials Engineering, Iran University of Science and Technology (IUST), Tehran, Iran

2 Mavadkaran Engineering Company, MAPNA Group

Abstract

Introduction and Objective: The cobalt-based superalloy FSX-414, owing to its excellent resistance to high-temperature oxidation and hot corrosion, is widely used in the fabrication of first-row stationary vanes in Frame-9 industrial gas turbines. During prolonged service, these critical components are subjected to severe conditions such as thermal fatigue, oxidation, and creep, which can significantly reduce their service life and compromise the reliability of the turbine. The present study focuses on optimizing welding parameters to enhance the durability, mechanical strength, and performance stability of FSX-414 components at elevated temperatures.
Materials and Methods: Repair welding was carried out using the Gas Tungsten Arc Welding (GTAW) process with welding currents of 50, 60, and 70 A applied on FSX-414 substrates. Microstructural evaluation revealed that 60 A provided the most favorable weld quality and was therefore selected for further experiments. At this optimized current, four different filler metals—FSX-414, HS25, HS188, and Mar-M918—were employed. Tensile tests were performed following ASTM E8 at room temperature and ASTM E21 at 650°C to evaluate mechanical behavior. Microstructural characterization and fracture analysis were conducted using optical microscopy and scanning electron microscopy.
Results: For most welded specimens, failure occurred in the base metal rather than in the weld zone, confirming the adequacy of the welding procedure. However, in the specimen welded with HS188 filler metal, fracture initiated within the weld metal. Detailed analysis showed local segregation of chromium and tungsten in interdendritic regions and the transformation of primary MC carbides into M23C6 carbides, which promoted the formation of brittle phases and facilitated crack initiation.
Conclusion: The filler metal Mar-M918 provided the highest tensile strength and toughness at both test temperatures. FSX-414 demonstrated inferior performance, while HS25 and HS188 yielded intermediate results.

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Journal of Advanced Materials in Engineering

Articles in Press, Accepted Manuscript
Available Online from 01 October 2025

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