CLADDING TECHNOLOGY SHANXI CO., LTDCracking Mechanism of Inconel 625 Overlay Weld Metal
Literature Overview
The 2023 study by Guo Xiao, Gu Yu, Han Ying, Xu Kai, Wang Yan, and Jiang Yinglong, published in the Journal of Welding (焊接学报), investigates the cracking mechanism in Inconel 625 overlay weld metal. Conducted under the Shanxi Province Key Core Technology Research Program, this work draws on the expertise of the Harbin Welding Research Institute (China National Machinery Industry Corporation) and the State Key Laboratory of Advanced Stainless Steel Materials at Taiyuan Iron and Steel Group. This research is of significant importance given the widespread use of Inconel 625 in overlay welding applications for pressure vessels, heat exchangers, and hydrogenation reactors operating in aggressive environments.
Microstructural Characteristics of Inconel 625 Overlay
Inconel 625 is a nickel-chromium-molybdenum alloy containing approximately 62% Ni, 20% Cr, 9% Mo, and trace amounts of Nb, Al, and Ti. When applied as an overlay weld deposit, the microstructure is dominated by equiaxed or columnar dendritic γ-Ni (FCC) solid solution with Laves phase (Mo6Si) and Nb-rich carbides at the interdendritic regions. The solidification behavior is strongly influenced by the dilution ratio between the overlay and base metal, the welding heat input, and the interpass temperature.
The susceptibility of Inconel 625 to cracking is primarily attributed to the formation of low-melting-point phases at the interdendritic boundaries. The Laves phase, which forms in the last stages of solidification, has a melting range of approximately 1280-1360°C. When the solidification fraction reaches the critical range of 0.7-0.9, the remaining liquid is enriched in Mo and Si, forming a continuous network of Laves phase that acts as a crack initiation site.
Cracking Mechanisms Identified
The study identifies three primary cracking mechanisms in Inconel 625 overlay weld metal:
Solidification Cracking (Hot Cracking)
Solidification cracking occurs during the final stages of solidification when the solid fraction is between 0.7 and 0.9. The Laves phase network at the interdendritic boundaries creates a path for crack propagation. The susceptibility is exacerbated by:
- High Mo content (>10%) promoting Laves phase formation
- High heat input leading to coarse columnar dendrites
- High dilution with carbon steel base metal altering the solidification path
- Restrained cooling conditions typical of thick-section overlay welding
Reheat Cracking (Thermal Cracking)
Reheat cracking occurs during cooling from the solidification temperature down to approximately 600°C. This mechanism is associated with the precipitation of brittle Nb-rich carbides (NbC, Nb2C) at grain boundaries. The stress required to initiate reheat cracking is significantly lower than for solidification cracking, and it is particularly problematic in multi-pass overlay welds where the heat-affected zone of subsequent passes reheat previously deposited layers.
Cold Cracking (Hydrogen-Induced Cracking)
Although less common in nickel-based alloys than in high-strength steels, cold cracking can occur in Inconel 625 overlay welds when hydrogen is introduced from moisture contamination of the welding consumables or the base metal surface. The presence of Nb and Mo in the microstructure can act as hydrogen traps, promoting hydrogen-induced cracking at susceptible microstructural features.
Process Parameters and Cracking Susceptibility
| Parameter | Low Value | High Value | Cracking Risk |
|---|---|---|---|
| Heat input (kJ/mm) | <1.0 | >2.5 | Higher heat input increases risk |
| Interpass temperature | <150°C | >250°C | Moderate range (150-200°C) preferred |
| Dilution ratio | <15% | >30% | Higher dilution increases Laves phase |
| Wire diameter (mm) | 1.0 | 1.6 | Thinner wire reduces heat input |
| Travel speed (mm/s) | <5 | >15 | Slower speed increases heat input |
| Preheat temperature | 0°C | >100°C | Preheat reduces thermal gradient but may promote reheat cracking |
Countermeasures and Engineering Recommendations
Based on the cracking mechanism analysis, the following countermeasures are recommended for engineering practice:
- Consumable selection: Use ERNiCrMo-3 or equivalent low-carbon, low-Mo filler metals to minimize Laves phase formation. The carbon content should be limited to below 0.05% to reduce Nb-carbide precipitation.
- Process parameter optimization: Employ lower heat input (<2.0 kJ/mm) using thinner filler wire (1.0 mm), higher travel speed, and reduced arc voltage. Pulse GTAW is particularly effective for controlling heat input in overlay applications.
- Preheating strategy: Apply moderate preheat (50-100°C) to reduce thermal gradients and residual stresses without promoting excessive grain growth or reheat cracking.
- Post-weld heat treatment (PWHT): A stress relief treatment at 650°C for 2-4 hours can relieve residual stresses and allow diffusion of Mo-rich phases, reducing susceptibility to reheat cracking. However, PWHT must be carefully controlled to avoid sensitization of the overlay layer.
- Weld procedure qualification: Conduct extensive weld procedure qualification testing in accordance with ASME IX or NB/T 47014, including macrographic examination of all weld cross-sections for hot cracking and intergranular corrosion testing of the overlay layer.
Engineering Practice Integration
In the fabrication of hydrogenation reactors and high-pressure heat exchangers, Inconel 625 overlay layers are commonly applied to carbon steel or low-alloy steel base metals to provide corrosion and wear resistance. The cracking susceptibility of Inconel 625 overlay welds has been documented in multiple field failures, particularly in thick-section components where thermal restraint is high.
For pressure vessel applications governed by ASME VIII Division 1 or GB/T 150, the overlay weld procedure must be qualified to include macrographic examination of the weld cross-section for cracking, intergranular corrosion testing per ASTM A263 or ASTM G48, and mechanical property verification. The results of this study provide valuable guidance for developing qualified welding procedures that minimize cracking risk while maintaining the required overlay layer performance.
The study also highlights the importance of controlling dilution in overlay welding. When applying Inconel 625 to carbon steel, the dilution ratio can reach 30-40% in the first pass, significantly altering the chemistry and solidification behavior of the overlay layer. This underscores the need for multi-pass overlay procedures with adequate root preparation and careful monitoring of the first pass.
Study Insights and Reflections
This research makes a significant contribution to the understanding of Inconel 625 overlay weld cracking, providing a mechanistic framework that connects microstructure, process parameters, and cracking susceptibility. The identification of Laves phase as the primary crack initiation site reinforces the need for filler metal chemistry control and process parameter optimization in Inconel 625 overlay welding operations.
From a broader perspective, this study exemplifies the complexity of overlay welding engineering, where the interaction between base metal chemistry, filler metal composition, process parameters, and thermal history must all be considered simultaneously. Engineers involved in the design and fabrication of bimetal pressure vessels must adopt a holistic approach to overlay welding, integrating metallurgical understanding with practical process control to ensure reliable performance in service.
The findings of this study should inform the development of welding procedure specifications for Inconel 625 overlay applications, particularly for critical pressure vessel components where cracking in the overlay layer can lead to catastrophic failure. A systematic approach to procedure development, incorporating the cracking mechanism insights from this research, will contribute to improved reliability and safety in bimetal pressure vessel fabrication.