In the foundry, there is a common saying: “30% Installation, 70% Sintering.” If building the furnace lining is constructing the body, then Sintering is the process that gives the lining its soul.
For acidic linings (primarily silica sand / SiO2), the melting process of the very first heat is the sintering process. If this heating curve takes a wrong turn, the internal crystal structure of the lining will become chaotic, leading to cracking, run-outs (leakage), or a halved service life.
This article reveals, from the perspective of microscopic phase transformation, why heating rates and holding times are “life or death” factors for the furnace lining.
I. Core Mechanism: Quartz’s “Transformation” Journey (Phase Change)
Silica sand lining is not static at high temperatures; it undergoes a series of violent Polymorphic Transformations. Each crystal form possesses a different density and volume. The internal stress generated by these volume changes is the fundamental reason we must strictly control the heating curve.
1. Initial Stage: α-Quartz to β-Quartz
- Temperature Range: Approx. 573℃
- Physical Change: This is the most dangerous moment in the early phase. Low-temperature α-Quartz transforms into high-temperature β-Quartz.
- Critical Consequence: The volume suddenly expands by about 0.82%. While the number seems small, in a densely rammed layer, this instantaneous expansion creates massive thermal stress. If the heating is too fast at this point, micro-cracks—commonly known as “spalling” or peeling—will occur on the lining surface.
2. Critical Stage: β-Quartz to Tridymite
- Temperature Range: 870℃ to 1470℃
- Physical Change: With the help of Boric Acid (H3BO3) or Boron Oxide (B2O3) acting as a mineralizer, quartz transforms into Tridymite.
- Critical Consequence:This is the phase change we desire most. Tridymite expands by about 16% compared to quartz.
- Why is it the “Hero”? This massive volumetric expansion fills the voids between lining particles, making the sintered layer extremely dense and blocking the penetration of molten iron. Simultaneously, Tridymite offers excellent thermal shock stability, which is the foundation of a long lining life.
3. High-Temp Stage: Tridymite to Cristobalite
- Temperature Range: > 1470℃
- Physical Change: In the hottest zone contacting the molten iron, some Tridymite transforms into Cristobalite.
- Critical Consequence: Cristobalite has a high melting point (1713℃), high hardness, and corrosion resistance. It forms the hardest “armor” of the lining. However, if the Cristobalite layer becomes too thick or deep, severe contraction during cooling will cause cracks.
II. Why “Strictly Control Heating Rate”?
The essence of controlling the heating rate is to balance the contradiction between “Venting” and “Phase Change Expansion.”
1. Avoiding “Bursting”: The Escape of Moisture
Lining materials contain physical water (absorbed moisture) and chemical water (water of crystallization from the decomposition of Boric Acid).
- When water turns to steam, its volume expands 1600 times.
- The Risk: If heating is too fast, the internal steam cannot escape through the dense sand layer in time. The accumulating pressure causes local spalling or even micro-“explosions” within the lining.
2. Taming “Expansion”: Forming the Gradient
The ideal lining structure must be a three-layer structure:
- Sintered Layer (Hot Face): Highly dense, composed of Cristobalite + Tridymite.
- Semi-Sintered Layer (Transition Zone): Primarily Tridymite, moderate strength, stops crack propagation.
- Loose Layer (Back-up/Coil Side): Un-sintered silica sand, remains loose to provide insulation and a mechanical buffer.
If heating is too fast: Heat conducts rapidly to the deeper layers, causing the transition and even the loose layers to undergo phase change and sinter prematurely. Without the buffer of the loose layer, the furnace body has no room to maneuver during cooling contraction, inevitably leading to through-cracks that directly threaten the coil.
III. The Importance of Low-Temperature Holding: The Invisible Battlefield
Many operators tend to be impatient during this stage, believing nothing happens in the low-temperature zone. This is a grave mistake! The low-temperature holding period (usually between 200℃ ~ 400℃) is the most critical time for chemical reactions.
| Process | Explanation | Why must we hold temperature? |
| Boric Acid Dehydration | Boric acid (H3BO3) starts decomposing into metaboric acid at 170℃, finally becoming Boron Oxide (B2O3) around 450℃. | This process releases a significant amount of water vapor. If not held to allow time for the steam to vent, this trapped moisture becomes a “bomb” at high temperatures. |
| Network Building | Boron Oxide begins to melt and coat the silica sand particles. | This is like the glue beginning to spread. Only if the “glue” is spread evenly can the subsequent Tridymite transformation occur uniformly at high temperatures. |
Summary: Low-temperature holding is for thorough venting and binder activation. Skipping this step is like building a skyscraper on wet concrete.
IV. Conclusion and Practical Advice
The “First Heat” of a lining is not just about melting scrap steel; it is the in-situ manufacturing of a high-performance ceramic composite material.
- Slow before 600℃: To allow the 573℃ crystal transition to pass smoothly and for Boric Acid to fully dehydrate.
- Stable between 900℃ ~ 1200℃: This is the golden period for Tridymite formation; appropriate holding time increases the density of the sintered layer.
- Decisive at Tapping: The first heat should not be emptied too quickly. It is best to leave some molten iron (a “heel”) to avoid thermal shock caused by rapid cooling.
Remember: Any time saved on the sintering heating curve will eventually be paid back double in the form of “patching” and “run-outs.”
