CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Interpass Time on Microstructure Evolution in Multi-Pass Overlay Weldments
Literature Overview
This 2017 study published in Thermal Processing Technology by Xu Yan, Jiang Xiangsheng, Zhou Jianping, Xue Ruilei, and Yilihama Abudureimu from Xinjiang University School of Mechanical Engineering and Xinjiang Weiao Technology Co., Ltd., investigates the influence of interpass time on the microstructure of multi-pass overlay weldments. Funded by the Xinjiang Uygur Autonomous Region Science and Technology Talent Training Project (gn2015yx008) and the Xinjiang High-Tech Research Project (201113129), this work addresses a practical but often overlooked aspect of overlay welding process control.
Core Technical Content
Interpass time—the duration between successive welding passes—is a critical process parameter that directly influences the thermal cycle experienced by previously deposited layers. In multi-pass overlay welding, the thermal history of each pass is determined not only by the welding parameters of that specific pass but also by the cooling behavior between passes. The interpass time effectively controls the starting temperature for each subsequent pass, which in turn governs the solidification microstructure, phase transformations, and residual stress state.
Thermal Cycle Analysis
The relationship between interpass time and peak temperature can be described through the following framework:
| Interpass Time | Starting Temperature for Next Pass | Thermal Cycle Characteristic | Microstructural Consequence |
|---|---|---|---|
| Short (< 30 min) | High (250-400 °C) | Slow cooling rate, high peak temperature | Coarse grains, reduced hardness, possible over-tempering |
| Moderate (30-90 min) | Medium (100-250 °C) | Moderate cooling rate | Balanced microstructure with good toughness |
| Long (> 90 min) | Low (< 100 °C) | Fast cooling rate, low peak temperature | Fine grains, high hardness, potential for cracking |
The thermal cycle experienced by each pass can be characterized by several key parameters: peak temperature (Tp), cooling rate at 800 °C (t800), and cooling rate at 500 °C (t500). Short interpass times result in elevated starting temperatures, which reduce the effective cooling rate and shift the microstructure toward coarser, more tempered conditions. Conversely, long interpass times allow the previously deposited material to cool significantly, resulting in faster cooling rates and potentially harder but more brittle microstructures.
Microstructural Evolution Mechanisms
The microstructure of overlay weldments evolves through several mechanisms that are sensitive to interpass time:
- Solidification structure: The primary solidification mode (dendritic, columnar, or equiaxed) depends on the thermal gradient and solidification rate, both of which are influenced by the starting temperature for each pass.
- Phase transformations: In martensitic overlay systems, the starting temperature determines whether the microstructure undergoes direct martensitic transformation or partial austenite retention. High starting temperatures can lead to retained austenite formation, which improves toughness but may compromise dimensional stability.
- Grain growth: Prolonged exposure to elevated temperatures between passes promotes grain growth in the previously deposited layers, particularly in the heat-affected zones of those layers.
- Tempering effects: For martensitic overlays, the heat from subsequent passes acts as a tempering treatment for the previously deposited layers. Short interpass times result in more significant tempering, reducing hardness but improving toughness.
Process Control Recommendations
Based on the principles established in this research, the following interpass time recommendations can be derived for different overlay applications:
| Application | Recommended Interpass Time | Rationale |
|---|---|---|
| Hardfacing for wear resistance | 30-60 minutes | Maintain high hardness while avoiding excessive brittleness |
| Corrosion-resistant overlay | 20-40 minutes | Ensure adequate austenite formation for corrosion resistance |
| High-temperature service | 45-90 minutes | Promote grain growth for improved creep resistance |
| Low-temperature service | 15-30 minutes | Maintain fine-grained structure for low-temperature toughness |
The practical challenge lies in balancing the competing requirements of deposition efficiency (which favors short interpass times) and microstructural quality (which may require longer interpass times). In industrial settings, the interpass time must also account for practical considerations such as crew scheduling, equipment availability, and production throughput.
Engineering Practice Integration
In industrial overlay welding operations, interpass time control is often implemented through a combination of passive cooling (air cooling or controlled atmosphere) and active cooling (water cooling or ice application). The choice of cooling method depends on the specific requirements:
- Passive cooling: Allows natural cooling to the target interpass temperature, typically used when moderate interpass times are acceptable
- Active cooling with water: Rapidly reduces the temperature but may introduce hydrogen into the weld, requiring careful control of moisture content
- Controlled atmosphere cooling: Uses inert gas flow to manage cooling rates without introducing contaminants
The implementation of interpass temperature monitoring through thermocouples or infrared pyrometers is essential for ensuring consistent quality. Modern welding monitoring systems can track the thermal cycle in real-time, providing feedback for process optimization.
Key Questions and Reflections
This research raises an important question about the optimal interpass time for specific overlay applications. The answer is not universal but depends on the base material, overlay material, welding process, and service conditions. For example, in the overlay of austenitic stainless steel onto carbon steel, a shorter interpass time may be preferred to maintain the austenitic character and avoid excessive ferrite formation. In contrast, for martensitic hardfacing overlays, a controlled interpass time may be necessary to achieve the desired balance of hardness and toughness.
The practical implication is that interpass time should be treated as a critical process parameter, subject to the same level of control and documentation as welding current, voltage, and travel speed. Quality assurance procedures for overlay welding should include interpass temperature monitoring and documentation, with clear acceptance criteria based on the specific application requirements.