AZS Brick Preheating: A Step-by-Step Heat-Up Guide for Longer Furnace Life
The first 72 hours of heat-up decide years of AZS lining life—master the ramp schedule.
The heat-up of a newly lined glass furnace is the single most critical phase for fused cast AZS bricks. Even the highest-quality AZS-36 or AZS-41 blocks can fail early if the preheating schedule is rushed or uneven. This guide explains the thermal behavior of fused cast AZS, the risks of poor preheating, and a proven step-by-step ramp profile used by glass producers worldwide.
Why preheating AZS bricks is non-negotiable
Fused cast AZS contains approximately 20–25% glassy phase, which acts as a cushion between corundum and baddeleyite crystals. During manufacturing, rapid cooling freezes internal stresses into the block. If the furnace is heated too fast, the glass phase cannot relax these stresses, and microcracks form. Once the furnace is charged with molten glass, these cracks become highways for glass penetration, leading to premature spalling or upward drilling.
Moreover, baddeleyite (ZrO₂) undergoes a phase transformation around 1000–1100°C, causing a sudden volume change. This temperature band must be crossed slowly to avoid cracking the block structure.
The 4-stage ramp schedule
Below is a proven heating curve for AZS-36/41 lined container or float glass furnaces. The total duration is approximately 6–7 days from ambient to operational temperature.
| Stage | Temperature Range | Ramp Rate | Hold Time | Purpose |
| 1 | Ambient → 150°C | 5–8°C/h | 12–24 h at 150°C | Remove residual moisture; equalize temperature through the lining |
| 2 | 150°C → 600°C | 3–5°C/h | 12–24 h at 600°C | Critical stress-relief window; burn out organic binders; expansion joints begin to close |
| 3 | 600°C → 1100°C | 5–8°C/h | 24 h at 1100°C | Baddeleyite transformation band; monitor joint movement closely |
| 4 | 1100°C → Operating Temp | 10–15°C/h | - | Stable ramp to 1520–1580°C after transformation is complete |
Essential heat-up practices
Temporary insulation: Cover AZS sidewalls with fiber blankets during Stages 1–3 to maintain a uniform thermal gradient through the block thickness. Uneven heating causes internal stress worse than a slow ramp.
Temporary crown burners: Use dedicated, controllable burners to supply heat from above, simulating the normal heat flow direction.
Expansion monitoring: Install dial gauges or LVDTs on key block rows. Check every 4 hours. If expansion exceeds 0.3% without corresponding temperature rise, pause the ramp and investigate possible binding.
Joint protection: Ensure vitreous fiber cord in expansion joints is not directly impinged by flames.
What to inspect at 1100°C
At 1100°C, a brief internal visual inspection is possible (with full protective equipment). Technicians should verify:
Expansion joints are still open and even.
No surface spalling flakes on AZS blocks.
Crown skewbacks show no abnormal pinching or displacement.
Any irregular joint closure at this stage signals uneven heating and requires immediate adjustment before proceeding.
Cooling down is equally important
If a mid-campaign repair requires draining the furnace, control the cooling just as strictly. Reduce temperature at no more than 10–15°C/h down to 800°C to protect the residual AZS lining that must return to service.
How Chaoyu supports your heat-up
Every AZS block shipment from Chaoyu includes the recommended heat-up curve for that specific grade. For complex furnace projects, our technical team can participate in the heat-up planning phase, reviewing ramp profiles and expansion monitoring points based on the actual lining design. With our 6-stage QC ensuring low residual stress in the blocks themselves, a well-executed preheating becomes the final link in a 12+ year campaign life.
