CLADDING TECHNOLOGY SHANXI CO., LTDOverlay Welding on Large Gear Tooth Surfaces Application of Welding CCT Diagrams
Literature Overview
This study note examines the application of welding continuous cooling transformation (CCT) diagrams in the overlay welding of large gear tooth surfaces, published in 1999 in the journal Welding. The authors, Kong Wei and Kong Fanrong from Xinjiang Shihezi Thermal Power Plant, conducted a practical investigation into the microstructure control of overlay welds on large gear teeth using welding CCT diagrams as a design tool.
This research is particularly relevant to engineers working in power generation, heavy machinery, and industrial drive systems, where large gears with overlay-welded tooth surfaces are used to enhance wear resistance and extend service life. The study addresses a critical practical challenge: ensuring that the overlay weld microstructure is controlled to avoid brittle phases and cracking while maintaining adequate hardness and wear resistance.
Core Technical Analysis
The welding CCT diagram is a critical tool for predicting the microstructure of weld metals and heat-affected zones (HAZ) under continuous cooling conditions. Unlike equilibrium phase diagrams, welding CCT diagrams account for the non-equilibrium cooling conditions that occur during welding, providing more accurate predictions of microstructure and mechanical properties.
The fundamental concept of the welding CCT diagram involves plotting the transformation start and finish times for different phases (ferrite, pearlite, bainite, martensite) as a function of cooling time at different temperatures. The cooling rate during welding determines which phases form in the weld metal and HAZ, and this directly influences the mechanical properties and crack susceptibility of the overlay layer.
For large gear tooth overlay welding, the critical microstructural considerations are:
- Avoidance of martensite formation: Martensite is hard and brittle, and its formation in the overlay layer or HAZ leads to cracking and reduced toughness. The cooling rate must be controlled to avoid the martensite start (Ms) temperature range.
- Promotion of ferrite-pearlite or bainite microstructure: These microstructures provide a good balance of hardness, toughness, and wear resistance for gear tooth applications.
- Control of grain size: Fine grain microstructures improve toughness and wear resistance, while coarse grain microstructures reduce toughness and increase cracking susceptibility.
Key Process Parameters and Countermeasures
The study provides detailed recommendations for process optimization using the welding CCT diagram as a guide. The following table summarizes the recommended process parameters:
| Process Parameter | Critical Value | Effect on Microstructure |
|---|---|---|
| Heat input | 10-18 kJ/mm | Controls cooling rate and microstructure |
| Interpass temperature | 150-250°C | Prevents martensite formation in previous passes |
| Preheat temperature | 150-250°C | Reduces cooling rate in HAZ |
| Weld wire composition | Low carbon (C < 0.2%) | Reduces Ms temperature and martensite susceptibility |
| Weld wire alloying | Mo, Nb, Ti additions | Refines grain size and promotes bainite formation |
| Post-weld heat treatment | 550-650°C for 2 hours | Tempering to relieve stress and improve toughness |
| Cooling rate | < 10°C/s | Avoids martensite formation |
The welding CCT diagram is used to determine the critical cooling rate for avoiding martensite formation. For typical low-alloy steel gear tooth overlay welds, the critical cooling rate is approximately 10-15°C/s. If the cooling rate exceeds this value, martensite formation becomes likely, and the overlay layer becomes susceptible to cracking.
The study also discusses the influence of weld geometry on the cooling rate. For gear tooth overlay welding, the weld geometry is complex due to the curved tooth profile and the varying thickness of the overlay layer. The cooling rate varies across the tooth profile, with higher cooling rates at the tooth tip and lower cooling rates at the tooth root. This requires careful control of the welding sequence and heat input to ensure uniform microstructure across the entire tooth surface.
Engineering Practice Integration
In the context of large gear manufacturing and maintenance, the overlay welding process is typically performed using gas metal arc welding (GMAW) or flux-cored arc welding (FCAW) for high deposition rates and good process control. The overlay layer is designed to provide wear resistance (hardness of 30-40 HRC) while maintaining sufficient toughness to resist gear tooth breakage under impact loading.
The NDT protocol for gear tooth overlay welding includes:
- Visual inspection (VT): 100 percent coverage for surface defects, undercut, and geometric irregularities.
- Magnetic particle testing (MT): 100 percent coverage for surface and near-surface cracks, particularly at the overlay-to-base metal transition zone.
- Ultrasonic testing (UT): Spot check at 20 percent coverage for subsurface cracks and lack of fusion, using a contact probe with frequency of 2.5 MHz.
- Hardness testing: Hardness profile across the overlay-to-base metal interface to verify the absence of brittle martensitic phases.
- Metallographic examination: Cross-sectional examination of the overlay-to-base metal interface to verify the microstructure and the absence of brittle phases.
The study also discusses the importance of overlay layer design for gear tooth applications. The overlay layer should be designed to provide adequate wear resistance while maintaining sufficient toughness to resist gear tooth breakage under impact loading. A multi-layer overlay design with a transition layer (low carbon austenitic) and a wear-resistant surface layer (bainitic or martensitic tempered) is recommended.
Study Insights and Reflections
This literature provides a practical application of the welding CCT diagram as a design tool for overlay welding of large gear tooth surfaces. The key insight is that the welding CCT diagram can be used to predict the microstructure and mechanical properties of the overlay weld under actual welding conditions, allowing for rational process design and optimization.
One particularly valuable aspect of this work is the integration of the welding CCT diagram with practical welding process parameters. The study demonstrates how the welding CCT diagram can be used to determine the critical cooling rate for avoiding martensite formation, and how this critical cooling rate can be achieved through control of heat input, preheat temperature, and interpass temperature.
The practical implication for engineers is that the welding CCT diagram should be used as a standard design tool for overlay welding applications involving high-strength steels or steels with high carbon equivalent. The welding CCT diagram provides a quantitative basis for process design and optimization, reducing the reliance on trial-and-error approach.
The long-term value of this study lies in its demonstration that the welding CCT diagram can be successfully applied to practical overlay welding problems, providing a rational basis for process design and optimization. This approach can be extended to other overlay welding applications involving high-strength steels, including pressure vessels, pipelines, and structural components.