CLADDING TECHNOLOGY SHANXI CO., LTDMicrocrack Analysis of Ni-690 Alloy GTAW Overlay Layer
Literature Overview
This paper by Sun Guohui and Wang Xiaohui from Harbin Electric Group (Qinhuangdao) Heavy Equipment Co., Ltd. (2014, published in Electric Welding Machine) presents a detailed metallurgical analysis of microcracks found in the gas tungsten arc welding (GTAW) overlay layer of Alloy 690 (Ni-690, approximately 58% Ni, 28% Fe, 9% Cr, 5% Mo) applied to steam turbine high-pressure rotors. The study is significant for power generation equipment manufacturing, where Alloy 690 overlay is used to protect rotor surfaces against high-temperature corrosion and erosion. The authors combine optical microscopy, scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and fractography to identify the cracking mechanism and propose preventive measures.
Core Technical Points
Alloy 690 is a nickel-iron-chromium-molybdenum alloy widely used for overlay welding on high-pressure turbine rotors to protect against high-temperature oxidation and erosion. However, the overlay layer is susceptible to cracking due to the combination of high thermal expansion mismatch with the base material, high cooling rates, and the inherent susceptibility of nickel-based alloys to solidification cracking.
The study identifies two distinct types of microcracks in the Alloy 690 overlay layer:
| Crack Type | Location | Morphology | Orientation | Mechanism |
|---|---|---|---|---|
| Type A (solidification cracking) | Center of weld bead | Long, straight, transverse | Perpendicular to weld direction | Dendritic segregation, low melting point interdendritic liquid |
| Type B (thermal cracking) | HAZ / overlay interface | Short, branched, longitudinal | Parallel to weld direction | Thermal stress, low-temperature embrittlement |
The microstructural analysis reveals the following key findings:
- The solidification structure of the overlay layer consists of columnar dendrites growing from the base material interface, with interdendritic segregation of sulfur, phosphorus, and carbon.
- The interdendritic regions contain low-melting-point phases rich in sulfur and phosphorus, which are the primary sites for solidification cracking initiation.
- The cracking susceptibility is influenced by the cooling rate, which is higher for single-pass overlay compared to multi-pass overlay.
- The presence of delta ferrite in the overlay layer (if any) can increase cracking susceptibility by providing preferential crack initiation sites.
The table below summarizes the typical process parameters for Alloy 690 GTAW overlay and their influence on cracking:
| Parameter | Low Value | High Value | Effect on Cracking |
|---|---|---|---|
| Heat input | 0.5 kJ/mm | 1.5 kJ/mm | Low heat input increases cracking risk |
| Cooling rate | 5°C/s | 50°C/s | High cooling rate increases cracking risk |
| Wire feed rate | 4 g/min | 8 g/min | Low feed rate increases cracking risk |
| Arc length | 1 mm | 3 mm | Long arc length increases cracking risk |
| Interpass temperature | 50°C | 200°C | Low interpass temperature increases cracking risk |
| Preheat temperature | 100°C | 300°C | Low preheat temperature increases cracking risk |
Process and Standards Analysis
The study references ASME Section IX for welding procedure qualification and the specific requirements for nickel-based alloy overlay welding. The key qualification requirements include:
- Weld procedure qualification testing with a minimum of three tensile specimens from the overlay layer.
- Microstructural examination of the overlay layer and HAZ at multiple locations.
- Fractography of any cracks found during qualification to determine the cracking mechanism.
- Adjustment of process parameters to eliminate or minimize cracking before production welding.
The study also discusses the role of material chemistry in cracking susceptibility. Alloy 690 with lower sulfur and phosphorus content (< 0.01% S, < 0.01% P) exhibits reduced solidification cracking susceptibility. The authors recommend specifying low-sulfur, low-phosphorus Alloy 690 wire for overlay welding applications.
Engineering Practice Integration
Based on the findings of this study, the following process improvements are recommended for Alloy 690 GTAW overlay:
- Preheat and interpass temperature: Preheat the base material to 250-300°C and maintain interpass temperature at 150-250°C to reduce cooling rates and thermal stresses. This is particularly important for thick-section components where the thermal mass of the base material can create steep thermal gradients.
- Multi-pass welding: Use a multi-pass approach with a total of 2-3 passes to achieve the required overlay thickness. The first pass should use a lower heat input to establish a sound bond with the base material, and subsequent passes should use progressively higher heat inputs to ensure adequate fusion and reduce dilution.
- Wire selection: Use low-sulfur, low-phosphorus Alloy 690 wire (S < 0.01%, P < 0.01%) to minimize solidification cracking susceptibility. The wire should also have a consistent composition and good feeding characteristics.
- Shielding gas: Use pure argon or argon with 5-10% hydrogen as the shielding gas. Hydrogen addition increases the arc energy and penetration, which can help reduce porosity and improve weld quality.
- Post-weld inspection: Perform magnetic particle testing (MT) or penetrant testing (PT) of the overlay surface after each pass to detect and repair any cracks before subsequent passes are deposited. This approach prevents the propagation of cracks through multiple passes.
Key Questions and Reflections
The study raises the fundamental question of whether the cracking susceptibility of Alloy 690 overlay can be completely eliminated or only mitigated. The authors conclude that while complete elimination is difficult, the cracking risk can be reduced to an acceptable level through careful process control and material selection. The key is to understand the cracking mechanism and to design the process to avoid the conditions that promote cracking.
Another important reflection is the role of base material preparation in overlay quality. The base material surface must be clean, free of contamination, and properly machined to ensure good fusion and minimize dilution. Surface roughness, oxidation, and contamination can all contribute to overlay defects and cracking.
Study Insights and Implications
This paper provides a comprehensive metallurgical analysis of Alloy 690 GTAW overlay cracking that is directly applicable to power generation equipment manufacturing. The combination of microstructural analysis, fractography, and process parameter optimization provides a clear understanding of the cracking mechanism and practical guidance for process improvement. For engineers working on turbine rotor overlay welding, this study serves as an essential reference for troubleshooting cracking issues and for developing robust welding procedures that minimize cracking risk. The study reinforces the importance of integrating metallurgical understanding with process engineering to achieve reliable weld overlay quality in demanding applications.