The three zones in the induction crucible: what the break-out reveals about the lining

Four things decide how long a crucible lining lasts: the mix, the compaction during ramming, the sintering curve during the first firing and the mode of operation. If a crucible manages only 80 heats instead of 300, one of these four was the cause — and you can see which one on the broken-out lining. Afterwards you cannot.

What sintering creates

A rammed mix is the same everywhere once it has been installed. During the first heat-up it forms three layers, because the temperature falls from the inside outwards: on the inside the sintered layer, dense and firm, standing in the metal. Behind it a transition layer, partly sintered but not yet bonded through. And on the outside, at the coil, a layer that stays powdery.

This outer layer is not a defect, it is intended. It allows the lining to be broken out at the changeover without damaging the coil, and it slows a crack coming from the inside. A crucible that has sintered all the way through no longer has this reserve.

What the proportions reveal

• Sintered layer too thick, loose layer almost gone: operated too hot or for too long. The lining can barely be broken out any more, and a crack runs right through to the coil instead of petering out in the loose layer.
• Sintered layer too thin: the curve ran too fast or held too low. Wear then starts not after fifty heats, but after the first.
• The layers are uneven around the circumference: usually charging always takes place at the same spot, or the cooling does not work evenly all round. That is not a material problem but an operating problem.
• The layers are uneven over the height: thinner at the top than at the bottom points to a bath line held too low, or to overheating during the melting phase.
• Holes or loose nests in the sintered layer: the compaction during ramming was not clean there. That is the only one of the four factors that was already fixed at installation.

Twenty minutes during the break-out

No more than that is needed: photograph the fracture face, measure the three layer thicknesses at three heights, note down the number of heats. After three campaigns there is a comparison, and the question “material or operation?” is no longer a matter of opinion. Without these twenty minutes it stays one — and is usually decided in favour of whoever argues more loudly.