A troubleshooting guide to silica dry ram refractories.
Optimizing refractory performance is a difficult task. A tool assisting foundrymen is the evaluation of spent refractory samples. By evaluating "post-mortem" refractory samples, foundries can determine the mode of failure, eliminate the causes and improve refractory performance.
This refractory "troubleshooting guide" discusses the most common lining problems with silica dry ram refractories. Typically, a refractory lining is removed prematurely due to superheating, finning, spalling, erosion, buildup or saturation.
Silica dry ram refractories are used in coreless induction melting and holding furnaces for iron. This article contains cause/effect diagrams for all six major refractory melt operation-related problems, as well as photographs of silica dry ram refractories that were exposed to these problem operating conditions.
Many of these conditions are frequent and often unavoidable in many foundries. The simple reduction of many of these conditions, however, can result in longer refractory performance. If furnace operators can identify the specific mode of failure for every refractory campaign, they can identify and eliminate those adverse conditions for maximized lining life.
Superheating of a refractory lining is caused by:
* trapped metal within the refractory cross section;
* superheating of the metal bath.
Bridging results from poor charging. If cold charge accumulates on the surface of the melt, a bridge can form. If the bridge remains solid, the continuing induction power superheats the metal below. Figure 1 shows a refractory exposed to a bridge that was removed from the taper area of a coreless induction furnace. The bridge occurred in the middle sidewall and resulted in the superheating of the melt below. The tapered fused silica hotface shown in the sample often accompanies a bridging condition.
Trapped metal within the refractory cross section also causes superheating. This trapped metal draws power and superheats an isolated area of the refractory cross section. As shown in Fig. 2, a bolt was knocked into the refractory cross section at the middle sidewall during installation. During the initial sinter, this bolt "drew power" and caused an isolated area to superheat. This area eroded rapidly, causing the refractory to be removed after only three days of operation.
The final operating condition that results in superheating is lack of temperature control. Silica dry ram refractories have a melting point below 3100F (1704C). A furnace on high power easily achieves these temperatures in short periods of time. Therefore, it is essential that temperature controls are in place to ensure that this condition doesn't occur.
Metal fins penetrate the refractory cross section and result in ineffective refractory performance. Fins are the result of metal infiltrating cracks. The orientation of the cracks is consistent with the mode of failure. Cracks are typically oriented in three ways:
Vertical cracks are typically the result of thermal cycling, while horizontal cracks are usually due to installation-related problems.
Vertical cracks occur when the refractory experiences thermal shock upon cooling. These cracks propagate in a radial fashion throughout the refractory.
Molten metal can infiltrate these cracks, as shown in Fig. 3. These fins propagate through the hotface and freeze within the refractory cross section, causing premature refractory failure. To cure this problem, foundries must minimizes the thermal shock conditions a silica refractory experiences by optimizing the cooldown and heatup of the refractory lining.
Horizontal, or lamination, cracks occur because there is an inadequate knitting between adjacent layers, resulting in discontinuities that metal can penetrate. This problem is eliminated by proper installation.
A secondary cause for horizontal cracking/finning is refractory lining hang-up, which occurs near the spout of coreless induction furnaces. During cooling of the furnace, the refractory can adhere to the spout and cause horizontal cracking. This failure mode is structurally related to the support refractory system and is corrected by pre-installing the spout. If the spout is pre-installed to the same contour as the grout/cast ring surface, the potential for refractory lining hang-up is reduced.
Random cracking often results from mechanical abuse. As shown in Fig. 4, the hotface has an erratic crack pattern as a result of improper charging. The random fins are evident in the refractory cross section.
Another cause of random cracking and finning is form removal. While it's unusual to remove the form with silica dry ram refractories, cracking and finning can occur if the form is removed and the hotface is disrupted.
Spalling occurs when layers of sintered refractory lining suddenly "break" from the refractory wall. Generally, spalling is caused by improper installation of sintering, yet is also associated with thermal cycling and mechanical stress conditions. Spalling is classified as:
* steam spalling;
* mechanical abuse spalling;
* differential expansion spalling;
* pinch spalling;
* thermal spalling.
Steam spalling results in a moisture-bearing refractory. If this moisture-bearing refractory is heated too rapidly, pressurized steam is generated within the lining. If the steam has no place to exit the refractory, steam pressure builds up and results in a spall.
If the grout or refractory castable ring structure has not been thoroughly cured, excess moisture permeates the refractory and results in a hotface spall. This condition typically occurs during the initial sinter schedule. A spall can also occur from a moisture leak in the furnace cooling system. Any excess moisture can saturate the refractory and result in a steam spall.
Mechanical abuse spalling is a severe case of the random cracking/finning failure mode discussed previously. If the charge material excessively impacts the refractory lining, it can break pieces of refractory from the wall. This is often due to large (or large quantities of) charge materials impacting an isolated section of the refractory wall.
Differential expansion spalling results from severe refractory hotface saturation. Saturation of the hotface alters the expansion profile of the refractory cross section. During thermal cycling of the refractory, the metal-saturated hotface expands at a different rate than the rest of the refractory lining. This results in a separation of the refractory at the saturated/unsaturated interface, as shown in Fig. 5. A lens-shaped void is beginning to form at an interface about 1 in. behind the hotface. This void will increase in size and result in hotface loss.
Pinch spalling occurs when the refractory is in severe compression. Pinch spalling is often found in the floor of coreless induction furnaces. If the floor is not perfectly flat or slightly concave, the expansion of the refractory floor results in a compressive force placed on the refractory, which results in a sheer plane or hotface spalling.
Thermal spalling occurs during a rapid change in temperature. Although these refractories resist thermal changes, rapid increases or decreases in temperature can result in the development of internal stress planes. If these stress planes are sufficient, actual refractory spalling can occur.
Erosion is the reduction in lining thickness due to chemical or mechanical means.
Chemical erosion results in the gradual loss of refractory thickness due to corrosion of the lining. Typically, contaminants from the charge or alloying materials result in chemical attack. Sources in the charge stream are either trace metals that easily oxidize or inadvertent metallic contamination in the scrap.
Contaminants in the alloying system are often trace ingredients such as manganese magnesium, calcium and iron oxide that become corrosive upon oxidation.
Figure 6 shows a refractory lining exposed to chemical attack. This specific erosion condition was the result of calcium carbide carryover from the desulfurization process. A glassy slag coating is observed as an infiltration of this highly corrosive slag throughout the refractory matrix. This corrosive slag slowly dissolves the refractory material. Refractory lining life is improved by reducing the amount of calcium carbide carryover or by decreasing the fluidity of the slag.
Mechanical erosion is the result of the mechanical abuse. If the charging system repeatedly impacts an isolated area of the refractory lining, it gradually abrades that lining away. This condition is typical in foundries using a continuous charge feeding system that impacts a particular area of the refractory.
Mechanical erosion is also attributed to the stirring action occurring in lower-frequency furnaces. The large meniscus and stirring features of lower frequency furnaces result in a gradual abrading of the refractory hotface.
Buildup is characterized by an increase in refractory-wall thickness caused by the precipitation or adherence of metallic oxides to the refractory hotface. The sources of the metallic oxides are often the same as those that create a chemical erosion condition, but the difference is these contaminants are typically inert to refractories.
The source of these metallic oxides is either oxidation of the molten metal or contaminants charged into the furnace.
Figure 7 shows a refractory sample that experienced silica buildup, it was caused by molding sand being inadvertently charged back into the furnace as an attachment to gates and risers. Lining life is increased by eliminating or reducing the amount of sand adhering to the charge material.
Saturation occurs when the hotface refractory is impregnated with metal or metallic oxides. Molten metal saturation is typically the result of a low-installed density refractory or premature molten metal exposure.
A refractory installed to a low density saturates readily with molten metal. This saturation is the result of the refractory absorbing the liquid metal.
The other cause for metallic saturation is premature molten metal exposure. If liquid metal is exposed to the refractory prior to hotface formation, saturation results. Figure 8 illustrates a refractory hotface that was exposed to liquid metal before a proper hotface was formed. Extensive metal saturation occurred and an erratic hotface surface resulted. Lining life is optimized by following a sinter schedule that ensures proper hotface development prior to molten metal exposure.
Metallic oxide saturation can occur if slag chemically attacks a refractory lining. Slag penetration of a refractory cross section can result from a low-density refractory. Again, a low-installed density refractory matrix acts much like a sponge.
Any saturation has an adverse effect on lining the saturation alters the entire chemical, thermal and mechanical behavior of the refractory lining, and results in less than optimal refractory performance.
Figure 9 represents an ideal silica dry ram refractory cross section. This sample was removed from the taper of a coreless induction furnace melting gray iron.
The white cristobalite zone measures more than 1 in., and the quartz backup measures about one third of the refractory cross section - both ideal conditions. This refractory was installed to an optimal density and sintered according to specification, which resulted in a dense, properly developed hotface that resists slag and metal attack.
Also, furnace operations were controlled, so this refractory was not exposed to extensive thermal shock conditions or superheating. Therefore, no cracking, finning or superheating occurred during this campaign. This refractory achieved optimal lining life due to ideal operational conditions.
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|Author:||Doza, Douglas K.|
|Date:||Jun 1, 1995|
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