June 4, 2009

Effects of Compacting (Forming) Pressure on Sintering and other Properties of Refractory Bricks

We assume that the reader is already aware with the concept of ‘Sintering’, types of sintering and also the effects of sintering on refractories. In this article we will discuss on the effects of compacting pressure or applied pressure on sintering and various other properties of refractory bricks.

It has been established much before by Budnikov and Blyumen that sintering processes and reactions in the solid-state are interrelated and proceed with on the phase boundaries, as in a heterogeneous system. The basis of sintering, according to their broad definition, is the capacity of the solid phase to recrystallize, which, in turn, is related to the physiochemical nature of the crystal. Pressure is said to be an important factor in accelerating reactions in solid state and in facilitating sintering at relatively low temperatures in a refractory brick.

Precautions must be taken to eliminate any pressure variation during compaction of the refractory shape. The main deleterious effect of pressure variation is the corresponding differences in green bulk density resulting into non-uniform shrinkage after firing and some sort of distortion of warping is inevitable. The frictional force between the die wall and the powder is directly proportional to the radial stress at the wall. During a uniaxial pressing, the applied stress is in the axial direction and is parallel to the die (mould) wall. For a given axial stress the resultant radial stress depends on the fluidity of the powder under compaction. For example both the radial and axial stresses are equal when a liquid is compacted. However, when a non-elastic and incompressible solid is under axial compaction, there should not be any radial stress. Thus, it is desirable to decrease the powder fluidity in order to minimize the radial and frictional stresses or the density and stress gradients in the refractory brick.

There is no doubt that the forming pressure affects the firing behavior of the refractory materials. Theses effects may be due to:

>> Decrease in pore size and better particle contact,

>> Strain energy added due to plastic flow,

>> Strain energy added due to particle interlocking, or

>> Fracture of particles at contact points.

In general increasing pressure enhances the Green Density, decreases Shrinkage, and often increases the Fired Density of refractory bricks. Higher compacting (forming) pressures may cause plastic flow, increased strain energy, or particle fracture, which causes further increase in bulk density in refractory bricks. The effect of these variations on firing properties of a refractory brick depend on the firing time and temperature, and the nature of the refractory aggregates or refractory raw materials used, but in general decreased pore size due to compaction or particle fracture leads to increased density at lower firing temperature in a refractory brick.

What is ‘Sintering’ in Refractory Bricks ? [Read]

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