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Metal Casting Technologies : September 2005
his technical service note is directed to refractories and temperature and the effects placed upon the refractory material when the service conditions are not fully understood or known. TIME AND TEMPERATURE Refractoriness is a property of time and temperature rather than temperature alone. After a refractory has been put into service it is still subject to the effect of temperature plus time. Temperature and time reinforce each other and must be considered together in choosing the proper refractory for any particular furnace. FURNACE ATMOSPHERE AND TEMPERATURE The furnace atmosphere in contact with the refractory lining may vary from highly oxidizing to highly reducing and the difference in their effect upon the fusion temperature of refractories is considerable. Reducing conditions in the furnace will lower the softening temperature and hot load bearing strength of most refractories but for some this decrease is much more than others. One type of reducing condition is an atmosphere with a high percentage of carbon monoxide which can disintegrate some conventional refractories most rapidly at temperatures in the range 480 -- 535 deg.C range. Chlorine, fluorine and alkalies in vapour form, to which conventional refractories are subjected in some industrial processes will reduce the life of many refractories at temperatures far below their normal temperature range. Hydrogen sulphide and sulphur dioxide are harder on conventional refractories than normal products of combustion. In addition, to causing a reducing atmosphere under certain conditions hydrogen will remove silica from refractory. Even a vacuum where there is almost no atmosphere will result in a reduced service life of many refractories. CHEMICAL ATTACK AND TEMPERATURE Chemical attack and/or slag attack are one of the most important and most variable of the factors affecting the ability of refractories to withstand high temperatures. Conditions may vary from a perfectly clean, oxidizing gas flame to actual contact with highly corrosive solids, liquids or vapours. Above a certain minimum temperature which is governed by the chemical composition of both the destructive material and the refractory there is a chemical reaction between the two which results in a reaction product having a fusion point lower than that of the refractory. The attack increases in severity with increasing temperature. The porosity, permeability, strength and chemical and mineral composition of the refractory all have their effect in varying degrees upon its ability to withstand the chemical attack. SPALLING AND TEMPERATURE Spalling is a most important factor causing the destruction of refractory linings. It may be due to mechanical forces resulting from expansion of the refractory caused by improper furnace lining design. This will occur in the low and intermediate temperature ranges. Another type of spalling taking affect here maybe structural spalling that is promoted by a change in structure of the hot face of the refractory. This change may be caused by vitrification from high temperature or by chemical alteration by impurities that have penetrated into the refractory. LOAD AND TEMPERATURE How much temperature will it stand? A refractory may have a fusion point over 1650 deg.C and would withstand temperatures of 1480 deg.C where there is little load. Problems with deformation under load usually occur when the refractory is subjected to heat on more than one face or in high temperature furnace walls or arches that are heavily insulated. The amount of load, the temperature and the time are relative to the performance equation. ABRASION AND TEMPERATURE In many types of service conditions in which refractories are used there is a great variety of different kinds of abrasion. The destructive action of abrasion on a furnace lining may be the result of scraping, scouring, gouging, rubbing, impingement of relatively fine particles or the impact of heavy objects. These abrasive actions may occur at fairly low temperatures or at higher temperatures where the hot face of the refractory is partially softened by heat. Because of the great variety of conditions to be found in the field of abrasion, it is not surprising that a refractory with high resistance to one type of abrasion may not be equally good where another kind of abrasion takes place. REFRACTORIES AND TEMPERATURE What temperature will the refractory stand? Where there is a manufacturer's recommended maximum service temperature. This is normally based on a 24hour laboratory test where the product is heated on one face with a clean oxidizing atmosphere using a furnace fired with natural gas. The product is subject to no destructive service conditions except temperature alone. It is checked for shrinkage, growth, slumping, cracking and vitrification. A maximum service temperature is then assigned. So with temperature measurement being a key control item, all other destructive factors must be given equal consideration for refractory selection. John Pulbrook, Refractech Pty Ltd Refractories and temperature 62 www.metals.rala.com.au