Coreless Induction Furnace Linings: Silica Ramming Mass and the Sintered Skin
A dry powder rammed against a steel former, sintered by the first melt, that cracks itself on purpose and heals on the next heat. Most linings are less dramatic.
The lining of a coreless induction furnace is unusual: it goes in as a dry powder, gets rammed against a steel template, and only becomes a ceramic when the first heat melts through it. Once you understand the three zones it forms, the rest of the operating practice makes sense.
How a coreless lining is built
The mass is installed dry against a former, the steel template that shapes the crucible. It is added in lifts and compacted by ramming or vibration, the former is left in place, and the first melt sinters the working face. From then on the lining is replaced on a scheduled interval measured in heats, with patch repairs at the top collar and spout in between.
Why silica, and when not
Silica ramming mass is high-purity quartzite, usually well above 98% SiO2, graded and blended with a small borate sintering additive. For cast iron and many steels it is the default because it is inexpensive, it has low thermal expansion once it has converted to cristobalite (so it tolerates thermal shock and resists spalling), and it forms a tight, low-penetration sintered face that tends to heal itself.
It is not universal. Where silica pickup would contaminate the melt, or where the practice is strongly basic or high in manganese, an alumina or spinel dry mass is used instead, and very basic operations move to magnesia.
The three zones after sintering
- Sintered zone at the hot face: dense and ceramic-bonded. This does the work and contains the melt.
- Transition zone behind it: partly bonded, and it is the cushion that gives the whole lining its thermal-shock tolerance.
- Loose zone against the coil grout: still powdery. It is a compressible buffer and, more importantly, a wear indicator. If it sinters all the way through to the coil, the lining is near the end of its life and the run-out risk is rising.
The best part of a silica induction lining is the part that never does its job. That loose powder at the back is what tells you when to stop.
The cracks you want, and the ones you don't
A silica ramming lining develops a network of fine vertical hairline cracks as it cools, a result of the cristobalite phase change. These close up on reheating and are completely normal. What is not normal is a single wide radial crack, or horizontal cracking, either of which is a reason to inspect before the next melt.
Installing and sintering it
Installation quality decides lining life more than almost anything else. Under-ramming leaves the mass porous and weak and invites metal penetration. Over-ramming or grain segregation causes layering and cracking. The former has to be aligned, and the grain sizing has to be right through every lift.
The first heat is a sintering schedule, not a production heat. Heat slowly through the quartz inversion near 573 °C, hold, then raise to sintering temperature at the rate the supplier specifies. Rushing it cracks the skin before it has formed.
The loose backing layer and the coil leak-detection system are the safeguards against a molten-metal run-out. Never chase extra heats past the wear indicators. That is the one place on the furnace where "a bit more life" is not worth it.
The short version
Silica ramming mass sinters itself, cracks itself on purpose, and heals those cracks on the next heat, as long as it was rammed properly and sintered on a proper schedule. Our silica ramming mass is graded for coreless induction and foundry linings; tell us the furnace size and the melt and we will help with grain sizing and the push schedule.