Distributed Thermal Profiling in Submerged Entry Nozzles

Key facts

The Submerged Entry Nozzle (SEN) transfers molten steel from tundish to mold at ~1550–1600°C, and historically has only been checked with exterior handheld thermocouples.
A flexible single-crystal sapphire tube (1.0 mm OD, 0.6 mm ID) can bend up to 15°, letting the sensor pass through curved SEN insertion channels.
A 27-inch, 3-FBG sapphire assembly embedded in an SEN wall tracked preheat temperatures up to 1421°C in real time.
The sensor instantly detected a burner shutdown at t = 0.2 hours — a response speed heavy industrial thermocouples can't match.

The Hostile Frontier: Submerged Entry Nozzles in Continuous Casting

In continuous steel casting, the Submerged Entry Nozzle (SEN) is a critical refractory component that transfers molten steel from the tundish into the mold — the same tundish we cover in the multi-depth tundish monitoring case study. Operating under extreme conditions (~1550°C to 1600°C), the SEN regulates liquid metal flow, prevents re-oxidation, and shields molten steel from atmospheric contamination.

Figure 1: SEN Position on the Continuous Casting Deck

Photograph of a continuous casting deck showing the tundish car, tundish, slide gate, Submerged Entry Nozzle (SEN) glowing with molten steel, preheat torches, and preheat stand

The SEN sits directly below the tundish slide gate, glowing red-hot as it channels molten steel toward the mold. An embedded sapphire FBG sensor monitors the wall's thermal profile from within.

Why SEN Monitoring Is Crucial

Clogging & Alumina Build-Up: Incomplete calcium treatment leaves solid alumina (Al₂O₃) inclusions that stick to nozzle walls, restricting steel flow.
Refractory Erosion & Wear: Corrosive slag lines cause localized wall thinning, raising the risk of catastrophic nozzle failure.
Thermal Shock & Cracking: Rapid temperature shifts during preheating or initial pouring can fracture the refractory material.

Historically, steelmakers relied on manual, single-point handheld thermocouples to estimate temperatures from the exterior wall. These measurements lack spatial resolution and fail to provide real-time thermal profiles inside the nozzle wall.

Protective Sheathing: The Flexible Single-Crystal Sapphire Tube

Directly embedding a bare sapphire optical fiber inside an SEN wall is impossible. Molten metal vapors, corrosive fluxes, and thermal degradation quickly break down waveguide boundaries.

To solve this, advanced sensor packaging utilizes a slightly flexible single-crystal sapphire tube sheath (e.g., outer diameter 1.0 mm, inner diameter 0.6 mm) placed inside an outer alumina protective sheath, with the 125 µm sapphire sensing fiber and its cascaded FBGs running through the center.

Key Engineering Attributes

Bend Capability: Thin single-crystal sapphire tubing can withstand bending angles up to 15°, allowing the flexible sensor to pass through curved insertion channels engineered into the SEN.
Impurity Barrier: Prevents volatile zirconia or graphite species from migrating into the sensing fiber at 1600°C.
Mechanical Isolation: Decouples the delicate optical fiber from mechanical expansion stresses of the surrounding refractory ceramic.

Industrial Field Trial: Real-Time Preheat Thermal Profiling

During industrial testing at an operating steel mill, a 27-inch sapphire sensing assembly equipped with three cascaded FBGs was embedded directly into an SEN refractory wall during the burner preheating sequence.

Figure 2: Preheat Thermal Sequence Profile

Real-time, spatially resolved preheat thermal profile across the SEN wall. The three sapphire FBGs (positioned at different depths) each register a distinct temperature as the burner ramps the nozzle toward operating temperature.

Trial Milestones & Thermal Insights

Continuous Tracking up to 1421°C: Real-time spatial tracking captured the preheating temperature ramp across multiple positions along the SEN wall.
Thermal Anomaly Detection: When the burner nozzle was temporarily shut down for maintenance at t = 0.2 hours, the sensor instantly detected the drop in thermal energy, confirming rapid thermal response compared to heavy industrial thermocouples.
Distributed Thermal Mapping: Provided discrete gradient readings (e.g., FBG₁ = 1371°C, FBG₂ = 905°C, FBG₃ = 503°C), giving operators clear visibility into internal heat distribution along the refractory wall.

Conclusion

The SEN trial demonstrates a core advantage of this sensing approach: a cascaded sapphire FBG array turns a single hostile, inaccessible refractory component into a fully mapped, real-time thermal system — something no handheld thermocouple can offer.

Frequently Asked Questions

Why can't conventional thermocouples deliver sufficient data in Submerged Entry Nozzles?

Thermocouples provide only single-point temperature readings, are vulnerable to thermal shock, and cannot be easily embedded along the internal refractory wall to map distributed spatial thermal gradients.

How does single-crystal sapphire tubing protect the sensing fiber in steelmaking environments?

Sapphire tubing acts as a chemical and mechanical shield that prevents volatile refractory impurities from contaminating the sensing fiber while providing flexible installation (bending up to 15°) within curved refractory slots.

References

F. Mumtaz et al., “Distributed Sapphire Fiber Bragg Grating-Based Thermal Profiling of Submerged Entry Nozzles,” IEEE Transactions on Instrumentation and Measurement, vol. 74, article no. 9530112, 2025. DOI: 10.1109/TIM.2025.3593567
D. R. Alla et al., “Cascaded Sapphire Fiber Bragg Gratings Inscribed by Femtosecond Laser for Molten Steel Studies,” IEEE Transactions on Instrumentation and Measurement, vol. 73, art. no. 7000608, 2024. DOI: 10.1109/TIM.2023.3335530