p. carlo ratto fused cast...

In the age of ‘virtual’ techno-logy, when processes, includ-ing manufacturing, seem to be predictable to the point where optimization of process variables (and introduction of new ones) can be reliably performed through computerized models, it is rather surprising having to admit that some manufacturing technology is not, in the age of communi-cations and social media, com-pletely under the reassuring con-trol of ‘science & technology’.

While refractories in general are among the oldest materials handled by mankind, and one of the most ubiquitous mate-rials that supported the Metal

Ages of men, it is to be said that in only less than a couple of centuries the production of refractories has switched from empiric practice to a complex and branched field of science.

While sinter refractory materi-als, produced through a range of different forming and firing techniques, have been studied and understood enough to be able to predict, with a reasonable margin of error, the behaviour of a given refractory under opera-tional conditions, there is an area where the large amount of inde-pendent and connected variables and the technical difficulty of collecting (measuring) some of

them has, so far, been a chal-lenge that several have faced but none have completely defeated.

ODD COMBINATIONSWell in the middle of this area,

fused-cast represent a peculiar combination of refractory science and foundry practice, transferred into a field of temperatures where most of the traditional foundry rules are subverted.

The huge amount of techno-logical parameters, and among them some of very difficult quan-ti-fication, have, so far, made a sufficient degree of scientific definition impossible; as a result some capability stays in the area

FUSED CAST REFRACTORIES:SCIENCE OR CRAFT?

P. Carlo Ratto

84 glass machinery plants & accessories 5/2014

ANAYLSIS

This article discusses the more technical approach to fused cast manufacturing methods and how the advent of new modelling techniques is greatly enhancing the process.

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of empiric ‘know-how’ or, if you prefer, well within the domain of craft or art.

To the sceptics, it should be enough to say that, for example, the physical condition of a fused refractory inside the EAF furnace, just before the pouring process, is far from a complete understand-ing... For example it is generally acknow-ledged that in the case of a typical AZS (alumina-zirconia-silica composition) we deal with a liquid ceramic but, is it a homoge-neous mono-phase liquid, in other words a ‘real’ liquid?

And at which temperature are

we operating and therefore refer-ring to when studying the charac-teristics of this peculiar liquid?

The fact that leaving this liquid for a few minutes without stir-ring you produce a well evident sedimentation of a liquid phase with higher zirconia content, hardly agrees with the idea of dealing with a plain, single-phase, homogene-ous liquid.

The generally accepted idea that reactivity and other physical para-meters of the alumina used as a raw material for fusion does indeed influence some crucial qualitative performance of the cast blocks (e.g. the level of crack occurrence lead-ing to block rejection) is incom-

prehensible if we accept the idea that alpha corundum batched-in is completely fused within the liquid ceramic before casting. Complete fusion, in fact, should destroy any ‘memory’ of the raw material status.

Since within the furnace, before casting, we deal with temperatures of around 2,000°C (depending on the chemistry we are going to pro-duce), no traditional measurement means (TCs) can be utilized and only optical pyrometers are capa-ble of handling that range. Due to the way the EAF operates, how-ever, optical measurement cannot be practically performed inside the furnace.

The alternative generally fol-lowed utilizes a double-colour optical pyrometer looking at the pouring stream; the obvious inho-mo-geneity of temperature between liquid strings within the pouring stream is then averaged through statistical evaluation of a number of fast measurements on spots. But the emissivity coefficient of such a liquid (and its variation with the fast change in temperature) is not well known and, therefore, abso-lute temperature measurement is

not reliable; as a consequence, this temperature measurement can only be utilized as a ‘reference param-eter’ by the experts that handle the know-how, saying that ‘when temperature measurement provides a number within a given range, then it is OK for proper casting temperature’.

STICKY REFERENCESOf course, the consequence is

that for each company and each furnace, ‘experts’ are managing reference numbers that are not easily transferrable to another com-

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pany, another furnace, or another expert. In other words, experiences are not automatically applicable as general practices, and this is one of the root causes for difficult diffu-sion of fused-cast refractory manufacturing technology.

Another section of the manu-facturing process, consequential to the pouring stage, also deals with remarkable complexity and a large number of parameters of very difficult measurement: liquid ceramic (citing for example the case of an AZS) poured in a cold mould flasked inside a cooling (or annealing) medium initiates a cool down process in a thermal field and, simultaneously, the crystal-line species precipitate, while single crystal nucleuses grow at a speed determined by complex diffusion processes. The chemistry of the liq-uid phase therefore changes during the precipitation process, leaving a residual amorphous phase having a quite different composition from the initial whole liquid ceramic.

For an AZS fused-cast, the final crystalline phases are alpha corundum (Al2O3) and badde-leyite (ZrO2), partially as inde-

pendent phases and partially as pseudo-eutectic co-precipitation. The residual amorphous phase is a soda-silicatic glass, saturated with alumina and zirconia.

The crystal morphology (acicu-lar or isometric corundum) and crystal size, the glassy-phase dis-tribution among crystals changes in different zones of a refractory block, generally following the iso-thermal surfaces; the amount of co-

precipitated or independent crystalline phases also depends on the cooling down process.

The intensity of this macro inhomogeneity depends on the size of a specific block and the thermal flow through the mould, which, in turn, depends on the cooling/annealing package ther-mo conductivity.

We are not referring to minor changes, in fact, it is known that, for example in a 36 per

cent ZrO2 AZS you can have zones with 32 per cent and others with 42 per cent ZrO2, depending on these segregation processes.

As a result of several studies, it is confirmed that the quality of crystalline phases and the way the glassy phase is distributed are closely related to corrosion resist-ance performance and tendency to generate glass defects, and ultimately, to refractory quality in application.

COMPUTER MODELLING?Once more, the number of

para-meters related to the above mentioned range of results is so large (including mould quality, cooling/annealing medium, cast-ing configuration for specific size/shapes, pouring velocity and liquid temperature, liquid chemistry and crystal nucleation parameters, ...) that a computer model capable of providing reliable results on how to operate to effectively manufacture a specific size/shape is far from being produced.

When a new shape/size of block, significantly far from known prac-

tice, is to be produced, ‘experts’ have nothing to do but estimate proper initial parameters and go through a trial-and-error routine to optimize the results.

It is also to be stressed that track-ing back observed manufacturing malfunctions (e.g. the occurrence of a higher than wished level of cracks) to the primary causes is neither a univoque nor an easy exercise; it is a game where good technologists perform better than not-so-good ‘experts’.

No fused-cast refractory manu-facturer will ever easily admit, but every player, even the ‘best west-ern’ have, from time to time, epi-sodes when production has bursts of defectiveness, as if the manu-facturing process were moment-arily out of control. These alarming and expensive episodes, of course, trigger immediate and worrying corrective actions based on previ-ous experiences. In my long and diversified experience, I must admit that in some cases the problem disappears (or does not) without a clear connection with the corrective actions. Sometimes, as a conse-quence of the long manufactur-ing cycle (cause and effect can be separated by more than a month), the problem has fortunately already disappeared when we detect the issue at the tail end of the process.

This is further evidence of the incomplete scientific definition of the manufacturing process; it explains the unpleasant feeling of having the process not completely under control, and that, therefore, you can potentially have something going wrong without complete knowledge of the occurrence.

This kind of discomfort is some-thing that experts of this technol-ogy sometime experience, though will not be pleased to admit.

CONTROL FREAKS?Is all this enough to say that

we should give up the attempt to put our manufacturing processes under better control?

86 glass machinery plants & accessories 5/2014

ANAYLSIS

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Surely not; it is obvious that the more we know, the better we meas-ure, the more we keep under con-trol with a reproducible practice, then the less we will face episodes of apparently uncontrollable and erratic increases of scrap rates and also the worsening of average qual-ity levels that goes along with bursts of defects.

We must convince ourselves that fortuity has nothing to do with the observed episodes and that, on the contrary, it is always a matter of our lack of knowledge in the area of parameters and their inter-connection.

Case-Based-Reasoning (CBR) has been tentatively applied to build a better organized database of experiences to be utilized when a new situation or a not new undesir-able occurrence appears, in order to improve the problem-solving process and make it less dependant on so-called experts’ know-how.

Every advanced company has gone through various experiences in order to organize technology and know-how in something more scientific and less reliant to ‘crafts-man experience’.

Though the results of such attempts have, (so far and to my knowledge) not been completely satisfactory, it is out of doubt that the reached stage of this continu-ous improvement is one of the most important (or should we say ‘the’ most important) traits that differ-entiate a good and well experienced company from others that manage a much less controlled process.

If a nice company can have an episode of significant devia-tion in the final products quality once a year, other producers can have multiple episodes or can even be systematically in that condi-tion. Some, of whom I have direct experience, do not even have the knowledge of being under such condition!

Now, it is clear that having a higher scrap rate due to bad qual-ity of products in your process is

not only unbearable and unac-ceptable in term of costs, but also largely undesirable in terms of average quality of the ‘acceptable products’ sold.

Establishing an efficient track-ing system, properly connecting a given product to the manufac-turing condition through a multi-week production cycle, the ade-quate collection of manufacturing parameters (from raw materials to finished goods) and the in-depth knowledge of the quality of finished goods are, in fact, the only way to reliably collect ‘cases’ that stay at the base of accumulated manufac-turing know-how.

BUILDING KNOW-HOWBuilding this ‘book of knowl-

edge’ is a continuous improvement process, the main way to reduce costs (the average scrap rate is a major cost factor) and improve quality.

We must now stress that most of this know-how is site-specific, due to sensitivity to certain vari-ables related to raw materials and equipment; as a consequence, transferring (and therefore buy-ing/selling) know-how from one

company to another is an almost impossible operation, unless the whole manufacturing configura-tion is transplanted, a practice that often turns out to be unre-alistic due to the need to procure some of the materials locally.

In other words, know-how can-not be simply transferred from one company to another; on the con-trary what can be done is to apply operational concepts that stay at the root of building the specific ‘book of knowledge’ for each par-ticular situation.

The journey from handicraft to science and technology is a work in progress for everybody making fused-cast refractories, but the way of doing it is different for different players, and there is no guarantee to eventually reach a final target.

And thinking about it, this must be the reason why people like me stay in this business, old and yet ever new, where art and science come together in an intriguing world of stone and fire.

This article was first published in Asian Glass, June/July 2014 and was authorized for republication by the author P. Carlo Ratto.

glass machinery plants & accessories 5/2014 87

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