CLADDING TECHNOLOGY SHANXI CO., LTDEffect of Interpass Cooling Time and Arc Striking Method on Cladding Quality
Literature Overview
This 2018 study published in Hot Working Technology, conducted by researchers at the Xinjiang University School of Mechanical Engineering under the National Natural Science Foundation of China (Grant No. 51365053) and the Xinjiang Uygur Autonomous Region Youth Science and Technology Innovation Talent Training Project (gn2015yx008), investigates two critical process parameters that significantly influence cladding quality: interpass cooling time and arc striking method. These parameters, while seemingly operational details, have profound effects on the metallurgical quality, mechanical properties, and defect susceptibility of weld overlay cladding layers.
Technical Analysis of Interpass Cooling Time
Interpass cooling time is the duration between the completion of one cladding pass and the initiation of the next. This parameter directly controls the cooling rate of the previous pass and the starting temperature of the next pass, thereby influencing the microstructure and mechanical properties of the overlay layer.
Influence Mechanisms
The interpass cooling time affects cladding quality through several interconnected mechanisms:
- Thermal cycle control: Shorter cooling times result in higher starting temperatures for subsequent passes, reducing the thermal gradient and cooling rate. This promotes coarser grain growth and potentially softer microstructures.
- Residual stress management: Inadequate cooling time can lead to excessive heat accumulation, increasing residual stresses and the risk of cracking. Conversely, excessive cooling time can create large thermal gradients that also promote cracking.
- Phase transformation: The cooling rate determines the phase transformations that occur in the overlay material. For example, in martensitic cladding materials, slower cooling rates promote retained austenite formation, which can improve toughness but reduce hardness.
- Hydrogen diffusion: Longer cooling times allow more time for hydrogen to diffuse out of the weld metal, reducing the risk of hydrogen-induced cracking. However, if the interpass temperature is too low, cold cracking can occur.
Recommended Interpass Cooling Times
| Cladding Material | Base Metal | Recommended Cooling Time (min) | Target Interpass Temperature (C) |
|---|---|---|---|
| Inconel 625 | Carbon steel | 5-15 | 200-300 |
| 309L stainless | Carbon steel | 3-10 | 150-250 |
| Hardfacing (Cr-C) | Carbon steel | 2-8 | 100-200 |
| Nickel-based hardfacing | Low-alloy steel | 5-20 | 200-350 |
| 316L stainless | Stainless steel | 2-8 | 100-200 |
The study found that for typical nickel-based and stainless steel cladding applications, an interpass cooling time of 5-10 minutes provides an optimal balance between cooling rate control and production efficiency.
Arc Striking Method Analysis
The arc striking method refers to the technique used to initiate the welding arc at the start of each cladding pass. This seemingly minor operational detail can have significant effects on the quality of the cladding layer, particularly at the start and end points of each pass.
Common Arc Striking Methods
- Direct arc striking on the cladding surface: The arc is struck directly on the existing cladding layer. This method can cause surface gouging and contamination from oxide and scale, leading to inclusions and poor fusion at the arc start point.
- Arc striking on a starting tab: A sacrificial starting tab is welded to the component, and the arc is struck on the tab. The tab is removed after welding, providing a clean arc start. This is the preferred method for high-quality cladding.
- Arc striking on a separate area: The arc is struck on a non-critical area of the component, and the arc is then transferred to the cladding start point. This method is used when starting tabs are not practical.
- Hot-start technique: The arc is struck and held for a brief period before moving into the cladding pattern. This allows the base metal to warm up and reduces the risk of cold cracking.
Quality Implications of Arc Striking Methods
| Arc Striking Method | Surface Quality | Fusion Quality | Defect Rate | Suitability |
|---|---|---|---|---|
| Direct on cladding | Poor - gouging marks | Incomplete fusion common | High (15-25%) | Not recommended |
| Starting tab | Excellent | Good fusion | Low (2-5%) | Preferred for critical applications |
| Separate area | Good | Moderate fusion | Moderate (5-10%) | Acceptable for non-critical areas |
| Hot-start | Good | Good fusion | Low (3-8%) | Good for thick cladding layers |
The study demonstrated that the use of starting tabs reduced the defect rate at arc start points by approximately 70 percent compared to direct arc striking, with the most significant improvement in fusion quality and surface integrity.
Integrated Process Optimization
The interaction between interpass cooling time and arc striking method creates additional complexity that must be managed in practice:
- When using direct arc striking, shorter interpass cooling times are required to maintain the base metal temperature above the minimum threshold for fusion, but this increases the risk of excessive heat input and microstructural coarsening.
- When using starting tabs, longer interpass cooling times are acceptable because the tab provides a controlled arc start regardless of the base metal temperature, allowing better control of the thermal cycle.
- The combination of optimal interpass cooling time (5-10 minutes) with starting tab arc striking produced the highest quality cladding layers, with defect rates below 3 percent and consistent mechanical properties throughout the overlay.
Engineering Practice Recommendations
Based on the findings of this research, the following recommendations are provided for cladding operations:
- Always use starting tabs for critical cladding applications where defect-free overlay layers are required.
- Monitor interpass temperature using infrared thermometers or contact thermocouples, maintaining the temperature within the specified range for the particular cladding material.
- Document interpass times in the welding procedure specification (WPS) and enforce compliance through welding procedure qualifications (WPQ).
- Train welders on proper arc striking techniques, with particular emphasis on the importance of clean arc starts and consistent arc travel patterns.
- Implement in-process inspection at regular intervals to verify that interpass temperatures and arc striking practices are being followed.
The research highlights that even seemingly minor process parameters can have outsized effects on cladding quality. A systematic approach to process optimization, combining fundamental metallurgical understanding with practical operational considerations, is essential for achieving reliable and repeatable cladding performance in production environments.