A summary of the Blasch paper that compares three lining structures with finite element analysis.
Better refractory grades help against thermal shock.
This paper asks what the structure of the lining adds.
Large induction furnace linings crack mainly because of mechanical stress from thermal expansion: the coil side stays cool, the metal side runs at melt temperature, and a lining that acts as one body cannot absorb the difference. In a finite element comparison by Blasch engineers, rammed and mortared brick linings showed similar peak stresses on the hot face, while a segmented, mortar free lining with designed expansion gaps showed a much lower overall stress at operating temperature.
| Item | Detail |
|---|---|
| Title | Effects and Analysis of Thermal Stresses on Large Induction Furnace Refractory Linings for Molten Metal Applications |
| Type | Technical paper, Blasch Precision Ceramics, Albany, New York |
| Year | 2016 |
| Authors | P. Geers, Market Manager Molten Metals; W. Russell, Application Engineer |
A coreless induction furnace heats the charge from the inside through eddy currents, while a water cooled coil keeps the outside of the lining cold. For many aerospace alloys the melt sits around 1,600 °C. The result is a large temperature difference across a few inches of refractory, and that gradient is what holds the metal back. It is also what loads the lining with strain.
The authors point out that the problem grows with size. Furnaces from about 500 kg capacity upward see a clearly higher tendency to fail from these compounding stresses.
The paper models a 3 ton lining as a cylinder fixed at the bottom. As it heats, the lining grows upward, by as much as 7 mm in height for an alumina or MgO based material, and the inner diameter expands more than the outer one. That puts the hot face in compression and the outer surface in tension.
Three constructions were then run through the same finite element model with identical inputs: a monolithic rammed lining, a mortared brick lining and a segmented brick lining. The only variable was the construction. Stress was compared as von Mises stress, which the authors use for comparison rather than as an exact failure value.
Rammed lining: the inner face carries higher stress than the outer face, which weakens grain boundaries and leads to cracks through the lining.
Mortared brick: peak stress on the inner face is similar to the rammed lining, because the bricks act as one body until the mortar, the weakest part, gives way.
Segmented, mortar free lining: each brick has expansion gaps sized for the different growth of inner and outer face. The gaps close as the lining heats, and the overall stress stays much lower than in the other two. On cooling, the segments shrink back to their original size.
Cracks in a one piece lining also feed themselves. When the furnace cools, metal runs into the cracks, and on the next heat the trapped metal expands and opens them further.
A lining that behaves as one piece will crack until it has become many pieces, so the paper builds the pieces in from the start and gives each one room to grow.
Segments handle the horizontal and uneven stresses, but the lining still grows in height. The paper pairs the segmented lining with a spring loaded hold down system: a top clamp connected to springs in the furnace, set so the lining can expand and contract through the full melting cycle.
This combination is the basis of the InterLok segmented crucible lining, which is installed with a backup ram behind the segments. The authors note that the same idea, segments that move plus a system that holds them, could benefit other ceramic assemblies.
If you run large vacuum or air melt furnaces, the practical question is whether your lining fails at the hot face, in the mortar joints or through metal penetration. The answer decides whether a better grade or a different structure is the next step. The InterLok white paper presentation walks through the system, and the technical library lists the related papers on core setters.
Large coreless induction furnaces for molten metal, from about 500 kg capacity, used in air and vacuum melting and in most alloy investment casting.
No. It compares stress fields from a finite element model. Campaign data for InterLok linings are not part of this paper.
Mortared bricks behave as a single body until the mortar cracks, so the stress on the hot face is similar to that of a rammed lining.
With a spring loaded hold down system on top of the lining, which keeps pressure on the segments while letting the lining expand and contract.
Tell us the furnace capacity, the alloys and how your current lining fails. A Blasch engineer sends the paper and looks at your case.
