Precision thermal control and high-pressure deformation act as the driving forces behind the optimization of Invar 36. To answer your question specifically: the industrial heating furnace uniformly brings the ingot to a critical temperature of 900 °C, creating the necessary plasticity for the rolling mill to execute a large deformation process with a 2:1 rolling ratio, effectively halving the material's thickness.
Core Takeaway The collaboration between the furnace and the mill is not merely about shaping the metal; it is a microstructural engineering process. By combining a specific pre-heat temperature with aggressive reduction rates, the equipment introduces high-density dislocations and residual stresses that are essential for minimizing the alloy's thermal expansion coefficient.
The Synergy of Heat and Pressure
The optimization of Invar 36 during warm rolling relies on a strict operational sequence where neither the furnace nor the mill can function effectively without the other.
Thermal Conditioning by the Furnace
The role of the industrial heating furnace is to prepare the material state. It must achieve a uniform temperature of 900 °C.
This specific temperature is critical because it ensures the alloy is malleable enough to be worked without cracking, yet likely cool enough (in the context of "warm" rolling) to retain specific microstructural features generated during deformation.
Mechanical Deformation by the Rolling Mill
Once heated, the rolling mill applies what is classified as large deformation processing.
The target here is a 2:1 rolling ratio. This significant reduction reduces the material thickness by half in a single pass or sequence. This is not a cosmetic smoothing process; it is a fundamental structural alteration of the ingot.
Microstructural Mechanism: The "Why"
The deep value of this collaboration lies in how it alters the internal physics of the Invar 36 alloy. The combination of 900 °C heat and 2:1 compression triggers three specific changes.
Manipulation of Crystal Orientation
The large deformation induces specific preferred crystal orientations within the metal, known as texture.
Most notably, the process alters the intensity of the (111) crystal plane. This reorientation is a direct result of the heavy mechanical load applied by the rolling mill.
Generation of Dislocations
Contrary to some processing methods that seek to eliminate internal defects, this process aims to generate high-density dislocations.
The rolling mill forces the atomic lattice to slip and distort. Because the material is warm rolled rather than fully hot rolled (which might induce complete recrystallization), these dislocations accumulate within the structure.
Introduction of Residual Stress
The process intentionally generates internal residual stresses.
According to the primary data, the synergy of these factors—dislocations, stress, and crystal orientation—is vital. Together, they act to significantly reduce the initial thermal expansion coefficient, which is the defining performance metric for Invar 36.
Understanding the Trade-offs
While general hot rolling (often used for high-entropy alloys) focuses on eliminating defects and refining grains through dynamic recrystallization, the warm rolling of Invar 36 has a different objective.
Plasticity vs. Stored Energy
Standard hot rolling prioritizes plasticity and defect elimination. However, the Invar 36 process prioritizes the retention of stored energy in the form of stress and dislocations.
If the furnace temperature is too high, or the rolling deformation too light, the material might undergo full recovery. This would "heal" the dislocations and relieve the stresses effectively erasing the properties needed to lower the thermal expansion coefficient.
Making the Right Choice for Your Goal
To replicate these results, you must adhere to specific processing parameters that balance workability with microstructural hardening.
- If your primary focus is Low Thermal Expansion: You must ensure the rolling mill achieves the full 2:1 reduction ratio to generate the necessary high-density dislocations.
- If your primary focus is Process Consistency: You must strictly maintain the furnace at 900 °C; deviations may alter the crystal orientation outcomes, specifically the (111) plane intensity.
The collaboration between furnace and mill is ultimately a method of using mechanical stress to engineer thermal stability.
Summary Table:
| Parameter | Specification | Microstructural Impact |
|---|---|---|
| Furnace Temperature | 900 °C (Warm Rolling) | Ensures uniform plasticity and prepares crystal orientation |
| Rolling Ratio | 2:1 (Large Deformation) | Halves thickness and induces high-density dislocations |
| Primary Goal | Low Thermal Expansion | Retention of stored energy and residual stresses |
| Key Crystal Plane | (111) Plane Intensity | Optimized via mechanical load for thermal stability |
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References
- Mehmet Kul, Markus Merkel. Minimum and Stable Coefficient of Thermal Expansion by Three-Step Heat Treatment of Invar 36. DOI: 10.3390/cryst14121097
This article is also based on technical information from Kintek Furnace Knowledge Base .
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