Why do tundish metering nozzles break during use?

In continuous casting, the sizing nozzle of the ladle is a seemingly unremarkable yet critical component. It directly controls the flow of molten steel into the mold; if it fails, the consequences can range from unstable billet cross-sections and slag entrapment to steel leakage, casting interruptions, and even safety incidents. Many field engineers have encountered this perplexing issue: even though the nozzle was properly installed and operated according to process specifications, why does it crack, fall off, or experience localized spalling during use?In continuous casting operations, the sizing nozzle of the ladle is a seemingly inconspicuous yet critical component. It directly controls the flow of molten steel into the mold; if it fails, the consequences can range from minor issues such as unstable billet cross-sections and slag entrapment to major problems like steel leakage, casting interruptions, or even safety incidents.

The service environment of a metering nozzle is extremely severe: the temperature of molten steel is generally between 1520℃ and 1560℃, while the outer surface of the nozzle is exposed to air and cooling water spray, resulting in a huge temperature difference.When the tundish starts casting, the inner wall of the nozzle is instantly heated by high-temperature molten steel, generating intense thermal expansion stress; meanwhile, the outer wall remains relatively cool. This temperature difference between the inner and outer walls forms tensile stress.If the thermal shock resistance of the nozzle material is insufficient, or preheating is not adequate, microcracks will initiate on the inner wall within minutes and rapidly propagate into through-cracks.

Typical manifestations: Longitudinal cracks on the nozzle, radial cracks extending outward from the inner bore.

On-site countermeasures:

• The nozzle must be strictly preheated according to the heating curve before use. Generally, the preheating temperature should reach above 1000℃ with uniform heating inside and outside.
• Use zirconia or zirconia-mullite nozzles with excellent thermal shock resistance, avoiding low-cost but highly brittle materials.

The inner bore of the metering nozzle is subjected to long-term high-speed flow of high-temperature molten steel. Especially when the casting speed is increased, the shear erosion of the bore wall by molten steel becomes very strong.In addition, residual oxygen, iron oxide, and inclusions in molten steel react chemically with the refractory material, causing chemical corrosion.The combination of erosion and corrosion gradually enlarges the inner bore of the nozzle, causing it to lose its metering function. In severe cases, local wall thickness is reduced to a critical limit, leading to sudden fracture under thermal stress or mechanical impact.

Typical manifestations: Significant ovalization or bell-mouthing of the nozzle bore, spalling of the glazed layer on the inner wall.

On-site countermeasures:

• Control the cleanliness of molten steel to reduce mechanical abrasion on the nozzle caused by high-melting-point inclusions.
• Adopt composite materials with high zirconia content; the higher the zirconia content, the stronger the resistance to erosion and corrosion.
• Replace the nozzle regularly according to the casting duration; do not use it beyond its service life.

• Excessively tight fit between the nozzle and the tundish seat brick during installation; forced hammering causes hidden cracks in the preform. Alternatively, an overly loose fit leads to "steel penetration" through gaps, directly causing the nozzle to burst.
• Excessive impact force when closing the stopper rod, with frequent collisions against the nozzle bowl or inner bore edge, resulting in local chipping.
• Improper handling during nozzle replacement, causing hidden damage from manipulator collision.

Typical manifestations: Damage mostly located on end faces, steps, or mounting contact surfaces, with irregular cracks.

On-site countermeasures:

• Operate strictly in accordance with installation specifications, use special tools, and avoid rough hammering.
• Check the fitting dimensions between the seat brick and metering nozzle to ensure a reasonable clearance (typically 0.3–0.5 mm).
• Provide regular training for operators, especially requiring gentle and slow action when closing the stopper rod.

• Unreasonable particle size distribution of raw materials leads to uneven green body density and hidden delamination after firing.
• Improper control of sintering temperature results in internal micro-pores or grain-boundary cracks.
• Stress concentration points generated during inner bore machining (e.g., steps and sharp corners not rounded).

Such defects are difficult to detect in cold-state inspection. Once exposed to molten steel, thermal stress and molten steel pressure immediately amplify the defects, causing sudden failure.

Typical manifestations: Neat fracture surfaces, traces of pre-existing cracks, or obvious pore clusters.

On-site countermeasures:

• Select qualified suppliers with mature processes, and require inspection reports for each batch including thermal shock resistance, apparent porosity, bulk density, etc.
• Conduct random inspections before warehousing; internal cracks can be screened by dye penetrant testing or ultrasonic testing.

Different steel grades have greatly different "destructive effects" on nozzles. For example, molten steel with high manganese and high aluminum content has high fluidity and easily forms high-melting-point Al₂O₃ inclusions, leading to more severe combined erosion and deposition on the nozzle.At high casting speeds, the flow rate of molten steel increases and the corrosion rate rises exponentially. In addition, ultra-long casting duration (e.g., over 8–10 hours) poses a severe challenge to nozzles of ordinary materials, with accumulated material fatigue eventually leading to failure.

On-site countermeasures:

• Select nozzles of appropriate grades according to steel grade and casting speed. Conventional carbon steel can use ordinary zirconia nozzles; aluminum-killed steel and high-manganese steel are recommended to use high-zirconia or zirconia-toughened nozzles.
• Establish a nozzle life log, record the actual steel throughput and casting duration of each nozzle, and adjust the replacement cycle accordingly.

 

 

If you are troubled by frequent breakage of metering nozzles, you are welcome to send us your operating conditions, including steel grade, casting speed, tundish temperature and casting duration. We can recommend the most suitable material solution for you. A quality nozzle is not necessarily the most expensive, but it must be the best match for your production line.

 

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