The drying and sintering of the lining of furnaces for melting ferrous metals is carried out by induction heating. The use of boric anhydride instead of boric acid improves and accelerates the sintering process and increases the reliability of the lining. The template is filled with starting blocks, a large charge (sprues, trimmings) or pig iron. To remove physical moisture, the crucible is first dried at a temperature of 280-250 ° C for 2-4 hours. The crucible is dried by a gas burner or by briefly switching on an unloaded furnace without water supply to the inductor. If the quartzite before calcination is calcined to 300-400 ° C, then exposure at 250 ° C is not necessary (Fig. 1). Then, at the minimum power of the power source, turning the loaded furnace on and off provides a temperature rise at a speed of 150–200 ° C / h to 570 ° C, then at a rate of <100 ° C / h to 900–920 ° C. The temperature is measured by two thermocouples installed opposite each other inside the furnace at the same level equal to half the height of the crucible (near the crucible wall).
During sintering, the temperature of the cooling water on the furnace should be kept at 20-30 ° C above the room temperature to avoid condensation of water on the coil. For this purpose, it is best to use heated water, you can also reduce the flow of cold water.
After reaching a temperature of 920 ° C, the power smoothly increases and continues heating at a rate of 50-100 ° C / h until the charge melts. The duration of the heating of the furnace to complete melting of the metal depends on its capacity and ranges from 6 to 16 hours. Specific values of power during the sintering period are set depending on the type of load and capacity of the furnace. Figure 1 shows the optimal temperature graph of sintering the acidic lining of a furnace with a capacity of 10 tons.
Image1. Sintering schedule for acid lining of an induction crucible furnace with a capacity of 10 tons:
And – heating the mixture to the melting point; B – melting of the charge; B – heating of the liquid metal; G – endurance; dash-dotted line – non-calcined quartzite; solid – calcined quartzite.
For a furnace with a capacity of 30 tons, BBS recommends heating the furnace to 800 ° C at a rate of 100 ° C / h, then until the charge is melted at a rate of 150 ° C / h with a power equal to 14% of the nominal.
For a furnace with a capacity of 60 t, it is recommended to heat the charge to 1000 ° C at a speed of no more than 70 ° C / h, at 1000 ° C make an exposure of 6 h. Next, bring the metal to melting, load the crucible to full and hold at 1550-1600 ° C 8-10 hours
As the melting into the furnace of any capacity, add the charge until the crucible is completely filled. After the charge is completely melted, the slag is downloaded and sintering of the lining is achieved by holding the melt at a temperature 50 ° higher than the temperature of the melted metal at the discharge.
The level of molten metal during sintering must be kept sufficiently high (to the level of the lower edge of the drain toe) to ensure good sintering of the crucible and collar. Exposure at sintering temperature depends on the furnace capacity; for furnaces with a capacity of up to 10 tons, it should be 2 hours. During sintering, attention should be paid to the fact that quartzite expansion in the temperature range from 573 to 700 ° C is one of the most significant and important processes. . In this temperature range, quartzite expands most intensively (by about 1.5%), and then at higher temperatures, the volume of quartzite almost does not change (Img. 2).
Image2. Quartzite lining sintering zones:
M is metal; I – zone of complete sintering; II – low-sided zone; III – buffer zone (unheated); A – beta quartz; B – alpha quartz; B – alpha tridymite; G – alpha cristobalite; D – vitrified quartzite; 1 – sheet asbestos; 2 – asbestos fabric; 3 – refractory coating; 4 – inductor.
When stuffing, the quartz mass is compacted from 1.7 to 2.0-2.1 kg / dm3, and then during sintering it turns into a mechanically strong crucible body, which must be resistant to the action of metal and slag. Therefore, the degree of sintering of the inner layer of the lining should be such that this layer could prevent the penetration of metal through the pores of the lining.
The sintering temperature of pure quartzite is about 1700 ° C, i.e. significantly higher than the operating temperature, by adding boric anhydride or boric acid, the sintering temperature is reduced. The sintering process begins after melting of boric anhydride (450 ° C) and ends at 1500 ° C. Due to the sintering process of dry quartzite mass, 3 lining zones are formed in the crucible wall of the furnace (see Img. 2): Zone I – full sintering adjacent to the molten metal, Zone II – low-strength, Zone III – non-sintered mass or buffer zone.
I. The zone of complete sintering is characterized by intense, glassy sintering of quartzite grains. The width of this zone should be approximately 10 to 20% of the wall thickness, and the outer layer 10-15 mm thick, facing the melt, should have a high density, since it serves as a refractory lining. Due to the formation of borosilicate glass, quartzite grains are sintered in this layer into a monolith, the porosity of which ranges from 9 to 13%. In this case, the strength of the sintered layer in bending ranges from 10 to 18 MPa.
Ii. The low-adhesion (frit) zone is characterized by ceramic hardening of quartzite grains, has significant porosity; In this zone there are separate grains of quartzite interconnected by boron glass. The thickness of this zone should be from 30 to 40% of the crucible wall thickness. The undersized layer is formed in the middle part of the lining as a result of the intensive formation of boron silicate. Individual grains of quartzite are glued together with a glassy phase in which fine grains of quartzite are dissolved. The bending strength of this layer is from 2.2 to 4.5 N / mm2. The porosity of the layer is in the range of 13-16%, its compaction occurs as a result of the formation of the liquid phase and, to a lesser extent, due to the transformation of α-quartz into α-tridymite, which is associated with the expansion of the crystal volume by 15%.
Iii. The buffer zone is characterized by the presence of unbound grains and powdered particles that adhere to each other due to the angular shape of ground quartzite particles. The width of this zone is from 20 to 30% of the thickness of the lining layer. The buffer zone gives the crucible its elasticity and protects it from the formation of through cracks and from the passage of metal to the inductor. The buffer zone at the inductor has almost the same mechanical properties as the compacted mass before sintering. The bulk density of the buffer zone varies from 2.05 to 2.2 g / cm3, and the porosity ranges from 17 to 23% with a quartzite density of 2.65 g / cm3. In the buffer zone, boron anhydride B2O3 is melted at a temperature of about 450 ° C. At temperatures above 573 ° C, β-quartz is converted to α-quartz. This process is reversible and is associated with an increase in the volume of the crystal by 1.3%. At a temperature of approximately 700 ° C, the formation of the vitreous phase of boron silicate begins, the volume of which increases as it approaches the sintering zone and the temperature rises further. In the boundary zone (850 ° C), cementation of individual grains of quartzite by the borosilicate vitreous phase is observed.
In order to save metal consumed in the manufacture of the melted template, sintering of the quartz lining can be performed by rapid heating of the removable template.
After the template has a good fit, the crucible walls are packed with quartz mass, compacting it with a vibrator. Starting blocks are loaded into the newly made crucible pattern, the furnace is turned on and heated to 800-900 ° C to prevent signs of metal melting, then the furnace is turned off, the lid is closed and after 3 hours the water supply to the inductor is stopped. During rapid heating of the template, the surface of the dry padding mass is sintering without melting the template. The thickness of the sintered layer in three hours is 2-3 mm. The particles of ground quartzite used for this purpose should have an angular structure. Sand is not suitable for this purpose. Very important is the use of dry ramming mass with the addition of boric anhydride. Boric acid is not suitable for this purpose. Attention should be paid to the fact that the quartzite moisture does not exceed 0.2%.
When using a removable template for sintering crucible, it is desirable to subject any quartzite to calcination up to 300-400 ° C. After sintering and cooling the lining, the starting blocks are removed. There are 2 ways to use removable templates. The first way is to use the main collapsible template of steel and an auxiliary template of micanite, which wraps and fastens the main template with insulating tape on the main template. For sintering the crucible, the starting block is used (blanks 80-100 mm thick) laid on the furnace bottom, or packages of tin, fine scrap are laid, then cast iron with a temperature of 1300-1350 ° C is poured into the crucible. The casting of the crucible with cast iron must be carried out continuously 30-40 minutes before the level of the furnace’s collar. The second method is to use a conical well-worked out removable template covered with alcohol-based graphite paint to form a crucible. Holes are drilled in the wall of the template to allow air to pass when the template is removed from the molded crucible. The template is heated to 400 ° C with a gas torch, cooled and removed. A metal cross (support) is installed to draw a conical template over the furnace, the template is slightly loosened before drawing out, and a large-capacity crane is not required.
In all cases, the authors point to the necessity of using boric anhydride as a sintering additive, since boric acid contains 44% of water, which, when the crucible is heated, loosens the lining structure and deteriorates the insulation condition of the inductor.
For 25-t furnaces, the template is made of 3 parts with different taper. With good care, the removable template of the 5-ton furnace was taken out 1060 times. Removable templates can be used by all enterprises that currently use ICT and IST series furnaces with a capacity of up to 10 tons.
1. Sassa V.S. Lining induction furnaces. M .: Metallurgy, 1989, 232 p.
This article was taken from this resource.