Failure Analysis of Co-Based Superalloy Component and TIG Surfacing Process Optimization

Literature Overview

This paper by Guo Ying and colleagues, published in Hot Working Technology (2022, Vol. 51, No. 13, pp. 151-156), presents a comprehensive study of failure analysis and repair process optimization for a Co-based superalloy component (UMCo50) used in the hot section of pure oxygen burners in coal chemical applications. The research was conducted jointly by Beijing Hanghua Energy Conservation and Environmental Protection Technology Co., Ltd. and Datang Hulunbuir Fertilizer Co., Ltd., addressing a real-world industrial problem of end-face cracking in burner nozzle components.

Background: Co-Based Superalloys in Coal Chemical Applications

Co-based superalloys such as UMCo50 (UNS R30603) are widely used in high-temperature applications due to their exceptional resistance to oxidation, hot corrosion, and thermal fatigue. In coal chemical pure oxygen burner systems, the hot-section components (particularly the outer nozzle) are exposed to extreme conditions:

• Operating temperatures exceeding 1000°C.
• High-velocity oxygen-enriched gas flow.
• Thermal cycling due to burner start-up and shutdown.
• Potential sulfur and vanadium hot corrosion from coal-derived feedstock.

The outer nozzle is a critical component that directs the oxygen flow and must maintain dimensional integrity under these severe conditions. End-face cracking represents a serious failure mode that can lead to burner malfunction, reduced efficiency, and potential safety hazards.

Failure Analysis

Observed Damage Modes

The study identifies several distinct damage modes on the failed outer nozzle surface:

Root Cause Analysis

The failure analysis reveals a multifactorial degradation mechanism:

1. Thermal fatigue: Cyclic thermal loading during burner operation causes repeated expansion and contraction of the end face. The high thermal conductivity of the Co-based alloy combined with the thick section geometry creates significant thermal gradients, leading to fatigue crack initiation at stress concentration sites.
2. Residual stress: The original manufacturing process (likely casting or hot working) introduced residual stresses that, combined with thermal stresses, exceed the fatigue limit of the material.
3. Microstructural degradation: Prolonged exposure to high temperatures causes grain growth, carbide coarsening, and potentially phase transformation, reducing the fatigue resistance of the material.
4. Environmental attack: Sulfur and vanadium species in the coal-derived gas contribute to hot corrosion, which accelerates crack propagation by embrittling the material at crack tips.

TIG Surfacing Repair Process

The repair strategy involves TIG (GTAW) surfacing of UMCo50 alloy onto the same base material to restore the component's surface integrity and dimensional accuracy. The repair process is optimized using a multi-factor orthogonal experimental design with range analysis.

Orthogonal Experimental Design

The study employs a multi-factor orthogonal array to systematically evaluate the influence of key process parameters on surfacing quality:

Optimal Process Parameters

The range analysis of the orthogonal experiments identifies the following optimal parameters:

Process Justification

The selected parameters are justified by the following considerations:

• Welding current of 120 A: Provides sufficient heat input to achieve complete melting of the UMCo50 wire and adequate fusion with the base material, while avoiding excessive heat input that could cause distortion or microstructural degradation.
• Surfacing speed of 10-11 cm/min: Balances deposition rate with bead quality. Too slow a speed would increase heat input per unit length, while too fast a speed would produce insufficient penetration and poor bead profile.
• Interpass temperature of 60±10°C: This low interpass temperature is critical for Co-based superalloys because:
• It limits grain growth in the HAZ and previously deposited layers.
• It reduces residual stress accumulation between passes.
• It prevents the formation of brittle intermetallic phases that can precipitate at elevated temperatures.
• 2-3 surfacing layers: Sufficient to build up the required surface thickness while minimizing total heat input and residual stress. More than 3 layers would increase the risk of cracking and distortion.

Quality Verification and Production Validation

The optimized surfacing process was validated in actual production, demonstrating its practical applicability. The verification included:

Engineering Practice Implications

This study provides valuable guidance for the repair of Co-based superalloy components in coal chemical applications:

• Failure prevention: Understanding the multifactorial failure mechanism (thermal fatigue + residual stress + environmental attack) enables proactive maintenance strategies, including regular inspection of hot-section components and scheduled replacement before failure occurs.
• Repair feasibility: The successful TIG surfacing repair demonstrates that Co-based superalloy components can be economically restored rather than replaced, which is particularly important for large, expensive components such as burner nozzles.
• Process control: The strict interpass temperature control (60±10°C) is a critical process parameter that must be monitored and controlled during production. This may require the use of infrared thermometers or thermocouples for real-time temperature monitoring.
• Material selection: Using the same alloy (UMCo50) for both the substrate and the surfacing wire ensures metallurgical compatibility and avoids the introduction of dissimilar material at the repair interface.

Study Reflections

This paper exemplifies the integration of failure analysis and process optimization in a practical engineering context. The transition from understanding why the component failed to developing a reliable repair process is a critical competency for maintenance engineers. The use of orthogonal experimental design for process optimization is particularly noteworthy — it provides a systematic, statistically sound approach to parameter selection that is more efficient than trial-and-error methods.

The finding that a low interpass temperature of 60±10°C is optimal for Co-based superalloy surfacing is a valuable insight. This is significantly lower than the interpass temperatures commonly used for steel welding (typically 150-250°C) and reflects the unique metallurgical requirements of Co-based superalloys, which are susceptible to grain growth and phase transformation at elevated temperatures. Engineers working with Co-based alloys should adopt this temperature control practice as a standard procedure.

The production validation of the optimized process provides confidence in its practical applicability. For coal chemical facilities operating pure oxygen burner systems, this study offers a proven repair methodology that can extend component life and reduce maintenance costs, contributing to the overall reliability and safety of the process plant.