Tundish Working Lining: Castable, Trowelling Mix or Dry Vibratable?
A tundish working lining looks like a single layer of material, but it decides three things at once: whether a sequence can be cast to plan, whether inclusion levels stay inside specification, and whether the installation and dry-out fit into the maintenance window. On the shop floor the argument is usually “castable lasts longer” versus “trowelling mix is faster”. Neither statement is complete. The real dividing line is water input, forming method, and how the heat-up schedule matches the turn-around rhythm.
1. What actually separates the three routes
A castable is mixed with water, vibrated into a monolithic lining, and bonded by cement or a sol system; it needs curing and a staged dry-out. A trowelling mix is a paste with high water content, applied by hand or machine in a 20–40 mm layer; it carries the most water and depends on dry-out more than the others. A dry vibratable mix contains no water at all; the bond develops through particle packing and sintering additives at temperature, with the heat of the steel completing what the burner starts.
Put simply: castables bond through hydration, trowelling mixes use water as a placing medium, and dry mixes use none. More water means higher risk of steam spalling but greater placing flexibility. Less water means better spalling resistance but far stricter requirements on compaction quality and former accuracy. That is why campaign lengths for the same tundish can differ widely without any of the materials being “bad” — each route simply fails in a different place.
2. Water demand and installed thickness
Water addition is the parameter most often ruined by “feel”, and the three materials sit in completely different ranges.
Low-cement and ultra-low-cement castables: 5.0–6.5%, judged by “flows under vibration, does not bleed when vibration stops”. Above 6.5% the dried strength, corrosion resistance and thermal shock resistance all drop, so the upper limit matters more than the lower one.
Trowelling mixes: 18–24%, because they are placed as a paste. Do not judge them against castable water figures, but do not top up water for easier trowelling either; above the upper limit, shrinkage cracks after drying are obvious.
Dry vibratable mixes: zero. Only the specified sintering additive or liquid agent is permitted, never water. A dry mix that has been wetted is effectively scrap, because water destroys the sintering bridges between grains before they can form.
Thickness follows the same three-way split. Trowelling mix goes on in 20–40 mm layers, two passes, with the second finished before the first sets. Castable working linings are usually 80–150 mm, formed and vibrated until the surface turns creamy and bubbles stop; over-vibration segregates the mix. Dry mix runs 60–120 mm, placed in loose layers of no more than 100 mm, compacted to roughly 0.75–0.8 of the loose depth.
3. Dry-out: where trowelling mixes fail
The three materials cannot share one heat-up curve.
Trowelling mix carries the most water and needs 8–12 h of natural drying first, then 4–6 h at 200–300℃ on a low flame, and finally 800–1000℃ on the tundish burner with a 1–2 h hold. Skip the natural drying stage and blistering plus spalling is almost guaranteed, usually showing up as slabs coming away during the second heat.
Castables need curing first: ambient above 5℃, 24 h before stripping, then three stages — room temperature to 300℃ at no more than 15℃/h with a 4 h hold to release free water, 300–600℃ at no more than 20℃/h with a 3 h hold, and 600–1000℃ with at least a 2 h hold.
Dry mix has no free water to release. After compaction it can go straight to the burner, 2–4 h at 200–300℃ to drive off adsorbed moisture, with the rest of the bond formed by the steel.
One rule covers all three: after dry-out, any through-crack wider than 1 mm, or a hollow sound when tapped, means the area must be cut out and replaced. Do not count on the next heat sintering it back together.
4. Campaign length and the residual-thickness limit
Under typical slab and billet casting conditions, working lining life is roughly 4–8 heats for trowelling mix, 8–15 for castable and 10–20 for dry mix. The spread is wide because the real variables are tundish capacity, casting time per heat, steel grade and turn-around — not the material designation.
Do not retire a tundish on heat count alone. Watch residual thickness: below 30–40 mm, or a local washout deeper than half the original thickness, means the tundish comes out. Replacing a tundish early always costs less than one breakout.
5. Four questions that settle the choice
First, is the steel grade sensitive to carbon pickup, such as low-carbon or ultra-low-carbon? If yes, prefer low-cement castable or a low-carbon trowelling mix; for dry mix, verify the carbon content first.
Second, does a single sequence run beyond 8 hours? If yes, choose castable or dry mix. A thin trowelling layer erodes faster over a long sequence.
Third, is the maintenance window shorter than 6 hours? If yes, dry mix wins because it needs no curing; trowelling mix is second; castable needs 24 h of curing and often will not fit.
Fourth, does the tundish change format frequently, with many irregular sections? If yes, trowelling mix adapts best; castable needs formwork and becomes expensive to re-shape.
If two or more answers point to the same route, the decision is made. If they conflict, prioritise window time, then steel grade, then campaign length — missing the window leads directly to an unplanned tundish change.
6. Pre-installation checklist
One: permanent lining clean, free of loose material and standing water — the leading cause of hollow spots. Two: anchors or studs tight, with height not exceeding the working lining thickness. Three: mixer weighing verified, water calculated on dry material weight rather than estimated by volume. Four: trowelling mix batched and used in small lots, discarding anything past its initial set. Five: dry mix checked for moisture and caking, with damp material rejected. Six: vent holes and purging plug surroundings kept open, not sealed by slurry. Seven: burner thermocouples correctly placed, and actual stage temperatures logged rather than set points. Eight: bottom-to-wall junction compacted or trowelled properly, the most common location for hollowing.
Selection is really about aligning three constraints on one sheet: material behaviour, schedule and steel grade. No route is universally better. Hold the three lines — water addition, dry-out curve and residual-thickness limit — and any of them will deliver a stable tundish campaign.
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