CLADDING TECHNOLOGY SHANXI CO., LTDPreheating Implementation Techniques for Weld Overlay Cladding
Overview and Technical Significance
Preheating is one of the most critical process variables in weld overlay cladding operations, particularly when joining dissimilar materials such as carbon steel substrates with nickel-based or stainless steel overlay layers. The primary purpose of preheating is to reduce the cooling rate of the weld metal, thereby minimizing the risk of hydrogen-induced cold cracking (HIC) in the heat-affected zone (HAZ) and improving ductility in high-hardness martensitic regions. In bimetal pressure vessel fabrication, where the base material is often a low-alloy steel such as 16MnR or Q345R and the overlay layer is a 304L or Inconel 625 alloy, the mismatch in thermal conductivity and thermal expansion coefficient between the two materials makes preheating even more essential. According to NB/T 47014 and ASME IX, the preheat temperature must be selected based on the carbon equivalent (CE) of the base material, the thickness of the material, and the diffusible hydrogen content in the weld metal. A general rule of thumb derived from engineering practice is that for carbon equivalent values above 0.40%, a preheat temperature of at least 100–150°C should be applied, while for CE values above 0.50%, preheat temperatures of 200–300°C are typically required.
Comparison of Preheating Methods
The three primary methods for implementing preheat in cladding operations are resistance heating blankets, induction heating, and flame preheating. Each method has distinct advantages and limitations that must be considered in the context of the specific application.
| Parameter | Resistance Heating Blanket | Induction Heating | Flame Preheating |
|---|---|---|---|
| Typical Temperature Range | 50–600°C | 100–1200°C | 200–1000°C |
| Heating Uniformity | High | Medium | Low to Medium |
| Setup Time | 30–60 min | 15–30 min | 10–20 min |
| Cost per Hour | Low | Medium | Low |
| Portability | Medium | Low | High |
| Temperature Control Accuracy | ±5°C | ±10°C | ±20°C |
| Suitability for Large Vessels | Good | Poor | Good |
| Risk of Local Overheating | Low | Medium | High |
Resistance heating blankets offer the most uniform and controllable preheating, making them the preferred method for critical cladding applications such as hydrogenation reactors and high-pressure heat exchangers. The blankets are typically equipped with embedded thermocouples and a PID temperature controller, allowing precise regulation of the surface temperature. However, their application requires flat or gently curved surfaces, which limits their use on complex geometries such as nozzles, manholes, and small-diameter pipe connections.
Induction heating is particularly effective for localized preheating of thick sections and pipe intersections where resistance blankets cannot be easily applied. The electromagnetic induction principle generates heat directly within the material through eddy currents, resulting in rapid heating without the need for direct contact. However, the heating depth is limited to the skin depth, which for carbon steel at typical frequencies is approximately 2–5 mm. This means that induction heating may not effectively raise the core temperature of thick sections, potentially creating a temperature gradient that is detrimental to weld quality.
Flame preheating using oxy-acetylene or oxy-propane torches is the most portable and flexible method, suitable for field conditions and large-scale fabrication shops. However, achieving uniform preheat temperatures across a large area is challenging, and there is a significant risk of local overheating, which can cause grain growth and softening of the base material. For this reason, flame preheating should be used only when other methods are impractical, and the temperature must be verified at multiple locations across the preheat zone.
Heating Zone Requirements and Uniformity Control
The heating zone for preheating must extend at least three times the plate thickness on each side of the weld, and must not be less than 100 mm, as specified in GB/T 150 and ASME VIII Div.1. This requirement ensures that the thermal mass of the surrounding material is sufficient to moderate the cooling rate of the weld metal. For a 50 mm thick pressure vessel shell, the preheat zone must therefore extend at least 150 mm on each side of the weld, which in practice often translates to a heating area of 300–400 mm width.
The common issue of preheat uniformity exceeding acceptable tolerances is a persistent challenge in engineering practice. Temperature variations of more than ±25°C across the preheat zone can lead to differential thermal expansion, residual stresses, and uneven cooling rates, all of which compromise the integrity of the overlay layer. To address this issue, the following measures should be implemented:
- Use of multiple thermocouples distributed across the preheat zone, with a minimum of three measurement points for each 100 mm of weld length.
- Layering of insulation blankets over the heating zone to reduce heat loss and improve temperature uniformity.
- Staged heating, where the temperature is raised gradually in increments of 50°C, allowing thermal equilibrium to be reached at each stage before proceeding to the next.
- Use of a temperature controller with proportional-integral-derivative (PID) regulation to minimize overshoot and oscillation.
Engineering Practice and Defect Analysis
In a recent project involving the cladding of a hydrogenation reactor shell with Inconel 625 overlay on a 16MnR base plate, the preheat temperature was set at 200°C based on the carbon equivalent of 0.48% and the diffusible hydrogen content of the welding consumables. Resistance heating blankets were used, with insulation blankets layered over the heating zone. Temperature measurements were taken at five locations across the preheat zone, and the maximum variation was found to be ±12°C, which was well within the acceptable tolerance. The resulting overlay layer showed no cold cracks, and the hardness profile in the HAZ was within the specified range of 250–300 HV.
Conversely, in a field fabrication project where flame preheating was used for a large storage tank, temperature measurements revealed a variation of ±45°C across the preheat zone. The welds in the cooler regions exhibited microcracks in the HAZ, which were detected during magnetic particle inspection (MT). This case underscores the importance of rigorous temperature monitoring and the limitations of flame preheating for critical applications.
Key Reflections and Recommendations
The selection of preheating method should be based on a systematic evaluation of the material properties, geometry, thickness, and available equipment. Resistance heating blankets should be the default choice for critical cladding operations, while induction heating is suitable for localized preheating of thick sections. Flame preheating should be reserved for non-critical applications or field conditions where other methods are impractical. In all cases, temperature uniformity must be verified at multiple points, and the heating zone must meet the minimum dimensional requirements specified in the applicable standards. A preheating procedure should be documented in the welding procedure specification (WPS) and verified during procedure qualification testing, with periodic requalification to ensure continued compliance.