The preheating zone generates the most severe thermal gradients because it subjects the cold Titanium/Steel clad plate to an immediate, intense heat source before internal conduction can catch up. Upon entry, the plate’s surface is rapidly heated by convection and radiation from high-temperature furnace gases. Because heat takes time to conduct to the center, a massive disparity forms between the rapidly heating surface and the cold core, creating the peak cross-sectional temperature difference during the entire process.
The physical lag between surface heating and internal conduction creates a temporary but critical "shock" phase where temperature gradients are highest. Managing this specific zone is the single most important factor in preventing thermal stress-induced interface delamination.
The Physics of the Temperature Spike
The Mechanism of Surface Heating
When the clad plate enters the preheating zone, it transitions instantly from a cold state to a high-temperature environment.
The furnace gases transfer heat aggressively to the exterior of the plate through convection and radiation.
The Conduction Lag
While the surface temperature spikes, the core temperature does not rise in unison.
Heat requires time to travel through the thickness of the metal via conduction.
This delay means the exterior is effectively "hot" while the interior remains "cold," creating a steep thermal slope across the plate's cross-section.
The Peak Gradient Point
This specific phase represents the maximum divergence in temperature across the plate.
In subsequent zones, such as the heating or soaking zones, the core temperature begins to catch up, causing the differential to narrow.
Therefore, the preheating zone is uniquely critical because it is the only point where the temperature gap widens aggressively rather than narrowing.
Implications for Material Integrity
Thermal Stress Accumulation
The large temperature difference generated in the preheating zone translates directly into physical forces within the plate.
The expanding surface layers pull against the cooler, more rigid core, generating significant thermal stress.
Risk of Delamination
If this initial temperature gradient is uncontrolled, the resulting stress can exceed the bond strength between the Titanium and Steel layers.
This leads to interface delamination, compromising the structural integrity of the clad plate before it even reaches the rolling stage.
The Necessity of Precise Control
To mitigate this risk, precise initial temperature control is required within the heating equipment.
Operators cannot simply maximize heat input in this zone; they must balance the heat transfer rate against the material's ability to conduct that heat inwardly.
Understanding the Trade-offs
Heating Speed vs. Material Safety
There is a natural tension between production speed and material safety in the preheating zone.
Increasing the heating rate improves throughput but exacerbates the cross-sectional temperature difference, raising the risk of delamination.
The Role of the Soaking Zone
It is important to distinguish the preheating zone's volatility from the stability of the soaking zone.
Supplementary data shows the soaking zone is designed to provide minimal fluctuations, allowing the plate to reach internal thermal equilibrium.
While the soaking zone fixes the temperature differences, it cannot undo damage caused by excessive gradients formed earlier in the preheating zone.
Simulation and Optimization
Numerical simulations indicate that adjusting furnace parameters allows for finding an optimal balance.
However, increasing temperatures in later stages (like the soaking zone) primarily raises the core temperature and only slightly affects the gradient, confirming that the preheating zone remains the primary driver of temperature differences.
Making the Right Choice for Your Goal
To achieve high-precision rolling results, you must optimize the heating curve based on your specific quality priorities.
- If your primary focus is Preventing Delamination: Prioritize conservative temperature settings in the preheating zone to minimize the initial cross-sectional gradient, even if it slightly extends total heating time.
- If your primary focus is Rolling Consistency: Ensure the subsequent soaking zone provides sufficient residence time to fully eliminate the gradients generated during preheating, ensuring uniform plasticity.
- If your primary focus is Process Efficiency: Utilize walking-beam furnace models to simulate gas temperatures and residence times, identifying the maximum heating speed that stays within safe stress limits.
Mastering the preheating zone is not just about heating the metal; it is about pacing the energy transfer to preserve the bond between layers.
Summary Table:
| Factor | Preheating Zone Impact | Soaking Zone Impact |
|---|---|---|
| Primary Function | Rapid initial surface heating | Thermal equilibrium (core catch-up) |
| Temp. Gradient | Reaches maximum peak (highest risk) | Minimizes and narrows |
| Material Risk | Interface delamination due to stress | Reduced risk if preheating was controlled |
| Heat Transfer | High convection & radiation | Low fluctuation, steady state |
| Control Goal | Balance heating speed vs. conduction | Uniformity for rolling consistency |
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References
- Zhanrui Wang, Hui Yu. Numerical investigation on heating process of Ti/Steel composite plate in a walking-beam reheating furnace. DOI: 10.2298/tsci231108082w
This article is also based on technical information from Kintek Furnace Knowledge Base .
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