Continuous melting and intermittent melting have a great influence on the service life of the lining. In continuous smelting, the lining is always in a hot state and is little affected by temperature changes. During intermittent operation, every smelting furnace lining will change from low temperature to high temperature to low temperature periodically. The result of this sudden change in cooling and heating will cause cracks in the lining, which will reduce the service life of the lining.  

In addition to magnesia-alumina spinel lining, magnesia-quartz lining has poor resistance to rapid cooling and rapid heating. In the process of heating and cooling of the lining, the linear expansion or shrinkage of the sintered layer of the lining is 0.9%. That is to say, every batch of smelting 1 furnace, the volume of the lining will produce an expansion and contraction. This rate of change below 800 ° C. If the lining is in continuous melting conditions, the temperature of the lining wall will not be lower than 800 ° C, so that the rate of change of the volume of the lining is very small, the time of cracking will be delayed, and the life of the lining will be improved. Therefore, in order to extend the life of the lining, the crucible temperature must be kept above 800℃ when the large-scale power frequency furnace of melting cast iron is shut down.  

During the smelting process, the solution will penetrate into the refractory matrix through the capillary channels in the refractory matrix and erode the lining. Infiltration into the interior of the refractory matrix; CaO, SiO2, FeO in slag; Fe, Si, Ai, Mn, C in liquid steel, and even metal vapor, CO gas and so on. These infiltrating components are deposited in the capillary channels of refractory materials, resulting in discontinuity between the physical and chemical properties of the refractory working surface and the original refractory matrix, and cracks, spalling and structural porosity will occur under the sudden change of operating temperature. Strictly speaking, this damage process is much more serious than the dissolution damage process.

Taking basic lining as an example, the increase of magnesium oxide content and viscosity is conducive to reducing the erosion of lining and improving the slag collection effect.

1, the effect of melting temperature on the service life of the lining  

The high temperature of molten steel in the smelting process will cause the temperature of slag to rise, the erosion of furnace lining will be intensified, and the furnace lining will be damaged along the slag line prematurely. In addition, the high temperature will accelerate the fluidity of the liquid steel and accelerate the crack penetration into the lining, which intensifies the chemical erosion of the liquid metal on the lining. While trying to control the melting temperature, do not appear for a long time high temperature or heat preservation waiting for casting, too high temperature not only easy to burn the alloy, but also increase energy consumption. In short, the higher the melting temperature, the lower the service life of the lining.

Under the action of high temperature, the SiO2 in the lining material will have a reduction reaction with the C in the steel (iron) liquid, and the lining will be rapidly eroded.

High temperature holding is the main cause of lining erosion under normal conditions, and the high temperature holding time should be reduced as much as possible. High temperature iron, low temperature casting, has always been the operating criteria. However, each casting has its melting process requirements, when reaching the appropriate temperature, do not pursue excessive overtemperature, and often observe and measure the temperature. Meet the process requirements, to achieve low temperature melting, rapid temperature rise. The longer the high temperature liquid steel stays in the lining, the greater the erosion of the lining.

When a foundry uses quartz sand material as lining material to produce cast iron, the iron liquid inspection is qualified, and the pouring is normal, but when the casting quality is checked, it is found that the carbon element is low and the silicon element is high, resulting in the casting is unqualified. The reason is actually caused by the blind pursuit of high temperature iron production. After the liquid iron sample is sent, the temperature on the furnace is maintained, resulting in the evaporation of carbon monoxide gas under the action of high temperature, and the furnace lining material is melted under the action of high temperature - a large number of silica elements in the quartz sand penetrate into the liquid iron.                                   

2, the influence of smelting steel on the service life of the lining                             

When melting high carbon and high manganese steel with low melting point and good fluidity, the penetration of molten steel to the lining will be intensified, and the service life of the lining will be reduced.

The furnace lining has a relatively high service life when smelting low carbon high nichcr steel grade with poor fluidity.

Materials with a high melting point require a higher temperature, and a long period of high temperature will also reduce the service life of the lining.

Therefore, high carbon steel and high manganese steel should be smelted in the later stage of the furnace lining when arranging the smelting steel.

In addition, smelting different materials should pay attention to the properties of metal materials, the previous furnace smelting metal with high sulfur content, the next furnace smelting high manganese steel is easy to cause corrosion of the furnace wall. Therefore, high carbon steel and high manganese steel should be smelted in the later stage of the furnace lining when arranging the smelting steel.

3, the impact of slag on the service life of the lining

The pH of the slag should be compatible with the material of the furnace lining. The appropriate slag should be selected according to the material of the furnace lining. Basic slag is suitable for magnesia lining, but it can be eroded by high CaO slag and SiO2 slag, and excessive _CaF2 will also erode basic lining, resulting in premature corrosion of slag line area. Acid slag is suitable for quartz lining, magnesium aluminum lining can only be suitable for weak alkaline or neutral slag. Aluminum oxide lining at high temperature in different pH will show a typical amphoteric, it can adapt to different pH slag, compared to acid lining and alkaline lining is slightly worse.

The metal materials added to the furnace will bring a variety of different oxides, and the slag composition of different materials and different furnace times is not the same. Most of the oxides, carbides, sulfides and various forms of compound compounds present in the slag will chemically react with the furnace lining to generate new compounds with different melting points. Some low melting point oxides generated in the reaction, such as iron olivine (FeOSiO2) and manganese olivine (MnOSiO2), are generally within the range of 1200 ° C. Low melting point slag has excellent fluidity, which may form a flux effect and cause severe chemical erosion to the lining and reduce the service life of the lining.             

The high melting point slag generated in the reaction, such as mullite (_3Al2O3·2SiO2), magnesium olidine (2MgO·SiO2), etc., and some high melting point metal elements can reach a melting point of more than 1800℃, and there is a more complex interpenetration between the high melting point slag and the low melting point slag suspended in the metal liquid. These slag easily adhere to the furnace wall and form a serious accumulation of sticky slag, affecting the power, melting rate and capacity of the electric furnace, until affecting the life of the furnace lining. There are three steps you can take to eliminate this situation:

(1) By strengthening the management of charge into the furnace: greatly reduce the sand, rust, oil and waste containing various impurities and return charge into the furnace is an important measure to prevent the induction furnace lining erosion and slag;

(2) Formulate the appropriate process: according to the material of the furnace lining and the fused alloy, formulate the charging sequence and the appropriate melting process to minimize the chance and time of contact between impurities and the furnace lining is also an effective way to prevent the induction furnace lining from sticking slag;

(3) The use of new auxiliary materials to clean oxides and impurities in liquid metal and slag: such as the use of multi-element composite deoxidizer to replace a single element deoxidizer. The single element deoxidizer deoxidation product has a high melting point, exists as a solid particle in liquid metal, is difficult to discharge, and is easy to adhere to the furnace wall, and rare earth elements, although the deoxidizer has a strong deoxidation capacity, but the density of its deoxidizer products is not easy to exclude from liquid metal, and thin

The earth element is also easy to react with SiO2 and _Al2O3 in the silica and alumina lining materials to erode the furnace wall.  

Multi-element composite deoxidizer such as silicon, aluminum, barium and calcium has strong deoxidation ability, and the deoxidized product is a liquid particle that is easy to float and remove. A variety of slag compounds and a variety of harmful gases in liquid metal, if can not completely remove the slag caused by the furnace lining, but also make the casting form a variety of casting defects, the use of high-quality slag removal agent can remove the scum on the surface of liquid metal, for miscible and suspended in liquid metal impurities and gases can not be removed.

The service life of lining is longer than that of non-vacuum smelting under vacuum, which proves that slag will reduce the service life of lining.

4. The influence of melting operation on the service life of the lining

Whether the metal charge can melt smoothly during the melting period has great influence. The tightness of the charge is directly related to the melting speed of the charge. In order to quickly heat and melt, the charging should be tight, but also to avoid the phenomenon of bridging in the melting process, which will make the lower part of the melted metal liquid overheating, the metal liquid suction and increase the burning loss of alloying elements, prolong the melting time, high temperature and intense eddy current will seriously erode the furnace wall, there is a bridge and bridge treatment damage to the furnace wall. When the metal liquid level is too low, special attention should be paid to the feeding, if the bulk material is added and the electric furnace is input with high power, the bottom will produce serious overheating and aggravated erosion, and the static pressure under the middle and lower parts of the entire furnace lining. Big, hot. High, scour. Serious. After the molten metal reaches the appropriate temperature and quality requirements, it should be powered off, so that it is not only safe and reduces energy consumption, but also protects the lower part of the furnace lining from being damaged by the erosion of the superheated molten metal. The melting time should be shortened to reduce the power consumption of molten steel.

Seven measures can be taken to eliminate the effect of melting operations on the service life of the lining  

(1) Before starting the furnace, the power supply cabinet, water cooling, tilting mechanism and other equipment should be carefully checked again to minimize the shutdown failure caused by equipment failure, especially the shutdown of the full furnace (commonly known as ";furnace"), at this time, the furnace should be repaired before the liquid iron solidifying. Otherwise, due to thermal expansion and cold contraction, uneven heating, the lining will crack, seriously affect the life of the lining, resulting in early furnace leakage accident.

(2) Smelting... When furnace steel, the lining has just been made, and the lining wall has not been fully sintered, the first few furnaces should use cleaner metal materials, and try to avoid materials with complex composition, rust and oil, especially oil-immersed waste iron filings. Because the composition is complex, containing more rust and oil, there is a complex interpenetration and miscibility between the oxides formed by the waste and the return charge at high temperature and high oxygen, which is easy to form viscous slag and erode the lining, which will make the lining loose and melt.

Slow feeding, should be less material batch, with the melting and remove the slag to reduce the slag. Low power smelting should be used in the early smelting stage, and the impurities brought in will not only increase the energy consumption, but also affect the quality of the casting. After melting every day, the molten steel in the furnace is emptied, and the furnace cover is sealed to slow the cooling.

(3) The effect of the "bridging"; phenomenon on the service life of the furnace lining caused by the furnace charge during the melting period. Large. When there is "bridging";, the charge is separated from the liquid steel, and the temperature of the liquid steel will be overheated to a very high temperature, which seriously affects the service life of the furnace lining. When dealing with the phenomenon of "bridging";, excessive vibration and hammering will cause the furnace lining to be damaged.

Pay attention to the mix of the charge and loose the charge in time to avoid the shed material in the furnace and prevent local overheating to avoid the local violent erosion of the furnace lining.

The seriously sticky sand and corroded charge should be cleaned before being put into the furnace. The bulk charge with small block degree and light corrosion should be added in the early stage, and the bulk charge with large impact on the lining should be added slowly after preheating in the middle and late stage to slow down the impact on the lining, especially the lining that is not completely sintered in the early stage.

(4) Due to the variety of scrap steel, (such as: pig iron block, modified pig iron, slag steel, slag iron, internal recycling scrap, purchased scrap steel, etc.) The volume and single weight of the charge are very different, and the scrap site is limited and the production rhythm is fast. It is difficult for workers to achieve a reasonable mix, resulting in large slag fluctuations, the temperature is very unstable, so each factory should be based on the size of the furnace, the actual situation of scrap and scrap stacking site, and at the same time to prevent loading too much or too little resulting in large slag fluctuations, to standardize the loading system: both scrap block block and single weight.

(5) All kinds of slagging agents will have an impact on the service life of the furnace lining.

Under the action of high temperature, such as high-calcium and silica-calcium slagging agent will react with the lining material and seriously erode the lining matrix.  

(6) When the steel is produced, especially the liquid steel can not be finished at one time, the furnace is not returned in time, and the furnace temperature is not in a state of insulation but in a state of high temperature; The upper part of the furnace lining is always under the condition of high temperature air firing and the lower part of the furnace body is under the condition of local electromagnetic stirring. Will seriously and accelerate the erosion of the lining.

(7) When stopping melting, the cooling water flow should be reduced in time and the furnace cover should be covered. The furnace lining is cooled slowly to reduce the thermal shock to the lining and prevent cracks in the cooling process.