CLADDING TECHNOLOGY SHANXI CO., LTDFailure Analysis of Cobalt-Based High Temperature Alloy and Optimization of TIG Overlay Welding Process
Literature Overview
This 2022 study by Guo Ying, Yan Chao, Gu Ming, and Liu Jinyong from Beijing Hanghua Energy Conservation Environmental Protection Technology Co., Ltd. and Datang Hulunbuir Fertilizer Co., Ltd. addresses a critical practical problem encountered in the operation of sulfuric acid plant waste heat recovery systems. The paper investigates the failure mechanisms of cobalt-based high-temperature alloy overlay layers deposited on critical components exposed to aggressive hot sulfuric acid environments, and proposes a systematic optimization of the gas tungsten arc welding (GTAW/TIG) overlay process to extend service life. The work bridges materials science fundamentals with field-level engineering practice, making it highly relevant to engineers working in sulfuric acid production, chemical equipment maintenance, and high-temperature corrosion-resistant overlay applications.
Core Technical Content and Failure Mechanism Analysis
The cobalt-based high-temperature alloys used in sulfuric acid plant applications typically contain 28–35 wt% Cr, 3–5 wt% W, and 0.5–1.0 wt% Mo, providing exceptional resistance to hot concentrated sulfuric acid at temperatures ranging from 300°C to 600°C. The failure modes identified in this study include spalling of the overlay layer, interfacial cracking, and localized pitting corrosion penetrating through the overlay into the substrate.
| Failure Mode | Root Cause | Typical Location | Severity |
|---|---|---|---|
| Overlay spalling | Thermal cycling fatigue at interface | Weld toe and interpass regions | Critical |
| Interfacial cracking | Residual stress concentration | Root pass and last pass | Major |
| Pitting penetration | Incomplete overlay coverage | Edge regions near base metal | Moderate |
| Hot corrosion attack | Sulfur/oxygen ingress at high temperature | Surface exposed to process gas | Major |
The study reveals that the primary cause of overlay failure is the mismatch in thermal expansion coefficients between the cobalt-based alloy overlay (approximately 13.5 × 10⁻⁶/K) and the carbon steel or low-alloy steel substrate (approximately 12.0 × 10⁻⁶/K). During thermal cycling between ambient and operating temperatures, this mismatch generates cyclic interfacial stresses that eventually exceed the bond strength of the overlay-substrate interface.
TIG Overlay Process Optimization
The original TIG overlay process exhibited several deficiencies: excessive heat input per pass, inadequate interpass temperature control, and improper wire feed positioning. The optimized process parameters are summarized below:
| Parameter | Original Process | Optimized Process | Rationale |
|---|---|---|---|
| Welding current (A) | 180–220 | 120–150 | Reduce dilution to base metal |
| Travel speed (mm/min) | 40–60 | 80–120 | Lower heat input per unit length |
| Interpass temperature (°C) | <300 | <150 | Control thermal cycling amplitude |
| Wire diameter (mm) | 2.4 | 1.6 | Finer deposition for better control |
| Shielding gas (Ar%) | 99.5 | 99.9 | Minimize oxidation of Co-Cr-W alloy |
| Number of passes | 4–5 | 6–8 | Thinner individual layers reduce residual stress |
| Preheating (°C) | 200–250 | 100–150 | Reduce thermal gradient at interface |
A key insight from this study is the adoption of a multi-pass, low-heat-input strategy. By increasing the number of passes while reducing the heat input per pass, the maximum temperature gradient at the overlay-substrate interface is significantly reduced. This approach also allows for better grain refinement in each individual pass, improving the mechanical properties of the overlay layer.
Engineering Practice Integration
For engineers managing sulfuric acid plant equipment, this study provides actionable guidance on overlay maintenance strategies. The recommended approach involves:
- Pre-inspection protocol: Conduct ultrasonic thickness measurement and magnetic particle testing of existing overlays before any repair welding to identify subsurface cracks and spalling.
- Surface preparation: Mechanical grinding to remove all defective overlay material, followed by acetone cleaning to eliminate contaminants that could cause porosity.
- Welding sequence: Apply a transition layer using a low-dilution filler matching the base metal composition, followed by the cobalt-based alloy overlay layers with a stringer bead technique.
- Post-weld treatment: Solution heat treatment at 1050–1100°C followed by air cooling to homogenize the microstructure and relieve residual stresses.
The study also emphasizes the importance of monitoring sulfuric acid concentration and temperature profiles at the overlay surface during operation. Deviations beyond the design envelope (typically 70–98 wt% H₂SO₄ at 300–600°C) accelerate hot corrosion mechanisms that no overlay process alone can mitigate.
Key Reflections and Study Insights
This literature demonstrates that overlay welding is not merely a surface application technique but requires holistic consideration of thermal management, metallurgical compatibility, and operational environment. The optimized TIG process achieves a dilution rate below 10%, which is critical for maintaining the corrosion resistance of the cobalt-based alloy. Engineers should note that the 6–8 pass strategy, while increasing welding time by approximately 40%, extends overlay service life by a factor of 2.5 to 3.0 compared to the original process. This trade-off between productivity and durability is a fundamental consideration in maintenance planning for high-value chemical processing equipment.