Fused-Cast AZS for Glass Furnace Sidewalls: Choosing 33, 36 or 41

Fused-Cast AZS for Glass Furnace Sidewalls: Choosing 33, 36 or 41

Fused-cast AZS is the workhorse material for glass furnace sidewalls, yet the most common purchasing mistake is to treat the designations as a grade ladder: 41 at the top, 33 at the bottom, so critical areas get 41 by default. In reality, higher ZrO2 improves resistance to glass corrosion but reduces thermal shock resistance, and raises the risk of zirconia precipitation causing glass defects. Choosing the wrong grade usually does not ruin the brick first — it ruins the glass first, as stones.

1. The designations describe ZrO2, not quality

The core difference is ZrO2 content: roughly 32–34% for AZS-33, 35–37% for AZS-36 and 40–42% for AZS-41, with the balance made up of corundum and a glassy phase formed with Al2O3 and SiO2. Bulk density steps up accordingly — about 3.70, 3.80 and 3.90 g/cm³ — and the glass-phase exudation temperature moves in roughly 1150℃, 1180℃ and 1200℃ bands, subject to the supplier’s technical agreement.

The exudation temperature deserves more attention than the ZrO2 figure, because it defines the range where the brick starts to “sweat”. Exudate migrates into joints and backing, accumulates, and lifts the brick, which shows up as local bulging of the sidewall. Ask for the measured exudation temperature of the actual batch, not just the nominal grade.

2. Casting method decides where the cavity sits

Fused-cast bricks are cast, so a shrinkage cavity is unavoidable. The only question is where it ends up.

Normal casting concentrates the cavity at the pouring end. Yield is high and price is lower, but during installation the cavity face must be turned away from the glass contact side, or upward when used in the bottom. Turn it the wrong way and the cavity becomes a channel for glass to penetrate.

Tilt casting moves the cavity to one end and raises the density of the working body, which suits large sidewall areas under steady corrosion.

Void-free casting typically specifies apparent porosity within 1.0%. It is the most expensive and belongs only where conditions are genuinely severe: the throat, the doghouse corners, around electrode blocks. Treating those areas like ordinary sidewall is a waste in both directions — either extra cost, or an early leak.

3. Joints and expansion: dry-laid sidewalls, no joint wider than 1 mm

AZS sidewalls are normally dry-laid with a thin zircon slurry, joints held within 1 mm and checked layer by layer with a feeler gauge. Do not substitute a fireclay mortar: its shrinkage and low-melting constituents are attacked early in the campaign and leave a void behind.

Expansion has to be designed in. Linear expansion of fused AZS from ambient to 1000℃ is roughly 0.7–0.8%, so sidewalls need a 2–3 mm expansion joint every 1.5–2 m, and the bottom needs a joint around its perimeter, filled with ceramic fibre or left open as designed — never grouted solid.

One detail that gets missed: level bricks with a wooden mallet, never a steel hammer on the face. Fused brick carries residual internal stress, and a concentrated steel impact leaves microcracks that are invisible at installation and become the starting point for corrosion once the furnace is hot.

4. Heat-up: slow down between 600 and 900℃

The AZS heat-up curve cannot be copied from silica or fireclay. A common approach: 10–15℃/h from ambient to 600℃; below 5℃/h between 600 and 900℃; back to about 10℃/h above 900℃; with holding platforms at 600℃, 900℃ and 1200℃, each held 8–24 h depending on furnace type and brick thickness.

Why the 600–900℃ band? It is where the change in glass-phase viscosity overlaps with the zircon phase transition, so the brick expands unevenly. Passing through quickly opens joints and spalls faces, and neither can be repaired once the furnace is in operation. The field rule is simple: if the actual rate exceeded the setpoint in that band, do not try to make up time in the next band — that only pushes the stress into a hotter zone.

Log joint widths during heat-up as well. Joints closing slightly within design range is normal; a joint that widens means the brick in that area has cracked or shifted, and work must stop for inspection.

5. Matching grade to location

Doghouse and throat: AZS-41, void-free casting. This is where corrosion is most severe and where the risk of a glass leak is highest.

Upper and mid sidewall of the melting zone: mainly AZS-36, normal or tilt cast. Near the metal line the brick also sees batch carry-over, so 36 balances corrosion resistance against thermal shock.

Lower sidewall and bottom: mainly AZS-33, with the bottom laid in 33 or 36 and the cavity face upward.

Refining and working end: AZS-33. Temperature is somewhat lower and sensitivity to glass defects is higher, so raising ZrO2 blindly is counterproductive.

6. Incoming inspection and laying checklist

Five checks on arrival: dimensional tolerance, typically around ±1 mm for fused cast, since oversize affects dry joints; face flatness plus chipped edges and corners; clear marking of the cavity position, with unmarked bricks barred from sidewall use; freedom from through-cracks, which are not acceptable in fused cast; and porosity and density certificates matching the agreement for the same batch.

Four checks during laying: level and plumb re-checked every 3–5 courses; joint width spot-checked with a feeler gauge, replacing any brick that exceeds 1 mm rather than filling with slurry; every expansion joint located against the drawing, not squeezed shut by the brickwork; and no debris or slurry allowed into the joints.

Residual sidewall thickness drives the cold repair decision. Design thickness is commonly in the 300–400 mm range; when hot measurements fall to 80–100 mm, or local washouts and seepage appear, plan the cold repair — do not wait for a glass leak to shut down the furnace.