proper practices for optimizing silica refractory campaign life.

by:AIRWOLF     2020-06-14
Following reasonable fire-resistant practices can extend the life of the Coreless Induction refractory lining at a higher cost
Efficient and efficient melting part.
Foundry using Coreless Induction furnace often miss production and cost
Because they have to turn off the operation to replace the fire-resistant lining of the Furnace, there is an opportunity to save both due to problems and regular re-wiring.
In many cases, this activity is still too early, because if the refractory lining is properly taken care of and concerned, it may bring longer production activities to the foundry.
Many practices affect the life of the silicon refractory lining of the Coreless Induction furnace.
These effects include formal design and construction, installation, sintering, cooling and re-heating.
In order to extend the service life of the refractory lining, the foundry staff must understand the impact of these practices on the operation.
This paper summarizes the practice of maximizing movement life and explains the theory behind each practice.
Ensuring that these practices are followed can prevent premature loss of the lining and improve the bottom line impact of the melting operation.
Form design and construction objectives: maintain form rigidity throughout the installation and sintering process.
There are two key functions in form: transmitting vibration (mechanical)
Energy of refractory materials used for compaction (
During formal vibration)
And maintain rigidity during sintering until sufficient strength is formed before the form melts.
Many form design details must be considered in order to achieve these objectives, including: * proper form thickness; * smooth welds;
* Cross support in form;
* Form construction;
* Form placement.
Proper shape thickness: consumables can be built from 0 for furnaces less than 5 tons. 25 in. (6 mm)plate steel.
For larger stoves, the thickness is 3/8. (10 mm)
Must be used to maintain rigidity.
As suggested by the furnace manufacturer, the external dimensions of the form are the same as the finished lining configuration.
It must be 3/8 in if using a removable form program. (10 mm)
Thick with cone, easy to disassemble.
Removable form with thickness less than 3/8.
Easily deformed and outdatedof-
Turns round due to repeated heating, cooling and stress removal.
Smooth Weld: in order to facilitate disassembly and uniform overall refractory lining, the shape weld should be smooth on the ground.
Any form of irregularities will result in irregularities in the refractory lining.
Cross support of form: for movable form, horizontal and vertical support is essential to maintain the form side straight and rigid during vibration and sintering.
The position of the braces should not exceed 24. (610 mm)
In addition to maintaining form rigidity.
Internal support must be found so that it does not interfere with the installation or use of the vibrator.
Due to the need to place the cost, the consumables cannot use cross support;
However, in order to maintain their rounding, they still need internal rigid rings.
Due to the increased size and weight of the shape, the support of the shape becomes more important in larger furnaces.
Anchor in form: removable and consumable form anchor on the furnace body structure to prevent movement during the installation and vibration of refractory materials.
If there is no anchoring, a shape will be forced up during vibration, resulting in a higher vibrationthan-
The desired height of the refractory floor and possible defects in the floor refractory, which may result in subsequent peeling.
Form structure: The form must be concentric in order to provide a uniform lining configuration.
This helps prevent uneven erosion of refractory materials and ensures that the lining is also affected by the magnetic field.
The bottom of the form must also be perpendicular to the center line of the form so that it remains Square after installing the properly leveled bottom refractory.
The detachable form is usually tapered (1-2 [degrees]
From top to bottom)or collapsible.
Due to its shape, the tapered form can be easily disassembled after preheating.
Foldable form is also easy to disassemble.
But attention must be paid to ensuring that it is rigid and that there is no open seam that allows the refractory to enter, which will result in low density surface defects.
Precautions must also be taken when using segmented tables.
As in foldable form, the seams must be tight to prevent the entry of refractory materials.
In addition, all forms must have a \"flat bottom\" with complete welds placed on the floor to prevent the refractory from entering the table and to ensure full contact with the base refractory.
Any gap caused by a concave or convex shape may cause the bottom to peel off shortly after the sintering is completed.
Form placement: in order to achieve a uniform wall thickness, the form must be properly installed in the furnace.
It must be centered at the bottom and top of the furnace and remain stationary on the fire-resistant floor.
Uneven shape causes uneven thickness of the side wall.
In addition, the uneven detachable form will damage the refractory during disassembly.
Installation objective: to optimize the density of refractory materials.
The main goal when installing refractory materials is to optimize the installation density to prevent the saturation of refractory materials caused by molten metal and slag.
Saturation changes the entire chemical, thermal, and mechanical behavior of the refractory lining, resulting in lower fire resistance than optimal.
Two ways to install silicone refractory in a coreless furnace are: vibration tampering and vibration form.
Vibration tampering installation: important factors to consider when using vibration tampering include the design of the Ram foot, the thickness of the refractory layer, \"de-
\"Procedures and tools, stamping technology.
Ram foot design-
When vibrating the floor, a 6. (152 mm)
Feet 1/8 in diameterin. (3 mm)
Diameter hole spacing is 1. (25 mm)
The center is standard.
The hole on the foot needs to be in de-
Drying of fire-resistant materials during vibration [
Diagram omitted in figure 1].
During the installation of the floor, the circular tamping foot design is very useful and helps to tamping along the inner week of the furnace.
When vibrating the side wall, use a curved Ram foot that matches the shape and the radius of the side wall.
This makes it easy and complete to hit around the form and next to the sliding surface.
The curvature of the tamping foot depends on the size and radius of the furnace.
The feet are also designed with 1/8-in. (3 mm)
Diameter hole of De-
Drying of refractory materials.
The recommended width of the side wall Ram foot is 66-
75% thickness of the side wall.
Too wide stamping feet are not easy to install between the shape and the side wall.
Therefore, due to the close cooperation between the shape and the sliding surface, there is a possibility of destroying the sliding surface material during stamping.
The tamper feet of the floor and side walls should be flat and heavy so that maximum pressure can be applied evenly to the refractory layer.
Thickness of refractory layer-
Dry silica refractory shall be installed in a range of not more than 5. (125 mm)
Thick loose layer. A 5-in.
The loose layer increases the density to about 4.
After proper compaction.
The thickness of the layer is greater than 5 in.
The effectiveness of reducing vibration tampering has been demonstrated, resulting in low installation density and possible stacks. \"De-
Programs and tools-
The bifurcation of refractory is to remove
Air and start the compaction process before tamping.
The fork of each loose layer is done by means of a fork tool or shovel, with four passes around the form to fully eliminate
Air of refractory materials.
The length of the forked spike depends on the thickness of the loose refractory layer.
For example, a 5-in. (125 mm)
The refractory layer needs a 5-in.
Peak to ensure the entire refractory layer is removed
It also helps to weave adjacent layers [
Diagram omitted in figure 2].
After vibrating each layer, the fork is also used to scrape and consolidate the surface of the refractory before adding new materials.
When scraping the newly smashed surface, the fork should penetrate 0. 75 in. (19 mm)
Enter the refractory material.
This is done in order to redistribute the coarse particles separated around the impact foot, and also to prevent the layering between layers.
Solid technology
When hitting the floor, start at the center, in the peripheral ram.
To ensure compaction, the entire floor should be hit at least four times.
The side walls should hit at least four channels around the template.
A downward force should also be applied to tampering to ensure that the maximum vibration energy is transferred to the refractory.
Use light pressure on first pass to avoid burying your feet in refractory and increase the pressure each time you follow through as the material becomes more dense.
For example, if the floor is 10 in.
And then two floors 5-in.
Loose Fill and Layer 3-in.
Loose filling should be installed in turn
Ventilation, impact, scratch, then bottom-
Scrape to where the table will be placed.
Before starting the side wall installation, the area between the form and the furnace coil must be scratched.
5-each side wall-in.
There are loose layers of four channels.
Installation by means of formal vibration: regular use of formal vibration on medium and large furnaces.
As with the tampering method, details must be carefully noted in order to reach the maximum refractory density.
The most commonly used vibration systems include: Standard vibration rigs with pneumatic or electric vibrators, vibration crossover systems, and three-legged system.
No matter which system is used, the process of installing the floor, setting up the form, and adding and deleting the form
The drying of refractory materials is the same.
When using the form vibration, the fire-resistant floor is usually installed with vibration tampering.
Another way to compact the refractory at the bottom is to attach the vibrator to a rigid steel plate of about 2. 5 in.
Smaller than the inner diameter of the grouting furnace.
The larger plate damages the grouting material and may damage the coil, while the smaller plate goes deep into the refractory rather than riding horizontally on the bottom surface.
The vibration plate must have 1/8-in. (3 mm)de-
In 3-in. (75 mm)centers.
This ensures that the air is not trapped on the floor.
Once the bottom is installed and leveled, the steel lining form is set.
Then constrain the top of the lining form with a wedge, or by welding steel bars on the top of the form and on the upper structure of the furnace.
During the vibration of the form, do not use the internal weight in the form, such as scrap or starter blocks, as they suppress the vibration energy.
Next, draw the space between the table and the stove.
The dry refractory is then introduced into the space between the table and the furnace at 5-in. loose layers.
When pouring materials to the back of the form, a large funnel can be used to help reduce dust and isolation.
In order to reduce segregation and eliminate segregation, each layer should be forked
Ventilate the material before the next layer is introduced.
Using a fork tool with the same length of the tip teeth as the thickness of the layer helps to mix adjacent layers togetheraired.
Vibration Program
Once the space behind the table is filled, the vibration begins.
As the refractory is precipitated and cured, continue to add more materials to the top of the furnace.
Two simple measurements of the effectiveness of the installation are the total amount of dry silica used and the vibration frequency.
Easy monitoring of vibration frequency with simple Reed
Type tachometer to verify that the form has sufficient vibration energy.
Follow the minimum acceptable frequency advice for the vibration equipment used.
Standard vibration rig-
Material Handling-
Pneumatic vibrators are usually used with square vibrators
Welded or bolted to the steel rig at the lower part of the form in four positions.
To facilitate the transmission of vibration energy from the rig to the material, the rig should be as hard as possible.
To ensure that all areas of the furnace are treated equally, the vibrator should run for at least 10 minutes on each leg of the rig.
Larger furnaces need to relocate the rig in a higher form and then repeat the vibration procedure.
Any loose connection will show excessive noise and movement, accompanied by loss of vibration per minute (VPM).
The minimum acceptable frequency of most normally working vibrators is 3200 VPM.
Vibration Crossover System
With this system, the vibration rig can be repositioned from above, thus eliminating the need for a person to climb into the form to move the vibrator.
Four wooden strips made of hardwood floors (
Like oak or pecans)
, Is placed in the form as a guide to the installation of shoes on the cross.
The location of the cross should be 1-
The third way up from the bottom of the form, then tighten the wooden strip using a hydraulic ram.
The Cross must perform the same pressing on all four wooden strips.
If not, the wooden strips can be adjusted or adjusted to make the pressure of the Cross equal to all four sides of the table.
In large furnaces, the Compaction effect is best when the material vibrates in two positions inside the shape
Third, the distance up from the bottom and-
Third, the distance down from the top.
When the vibration cross is in position, when running the vibrator at low speed, tighten it on the Batten to reduce friction.
Raise the vibrator to full speed and check its tightness multiple times during installation.
The vibrator should be operated 8-
The minimum frequency of the silicone refractory is 4600 VPM for 15 minutes.
Note that this is a fairly high frequency compared to conventional vibrations.
For large furnaces, the vibration time at the top and bottom is extended by 20 minutes. Three-
Leg vibrator system
Another option for the form vibration is the expansion leg vibrator.
Typically, the device uses three legs, although similar devices with four or more legs have been sold. Impact-
The type or rotating vibrator is mounted on each leg and these vibrators rest tightly on the shape through a spring or other mechanism.
The vibrator assembly pulls up during operation so that the vibration is passed to the top and bottom of the form.
Although it has been effectively applied in the foundry for many years, this method has several disadvantages compared with other methods, including: * more moving parts, resulting in higher maintenance requirements;
* Difficulty in controlling the vibrator rig in the cone part of the form;
* Due to the lack of positive attachment to the shape, the system is less rigid, reducing the overall efficiency of vibration transmission to refractory materials;
* Careful crane operation is required to properly control the speed at which the vibrator is removed from the form;
* The frequency cannot be monitored because the system uses multiple independently operated vibrators.
Despite these shortcomings, it is possible to achieve good results if the vibrator is properly used and maintained.
Purpose of sintering: to develop a hot surface for fire resistance.
Proper sintering of silicon refractory materials helps to minimize metal penetration by developing a good thermal surface.
By following the sintering schedule that allows the phase transition of silica particles, a suitable hot surface can be obtained.
The following figure shows the temperature at which the silica particles are transformed into different multi-forms, resulting in a change in the volume of refractory materials.
Crystal transformation (Density in g/[cm. sup. 3])[
Mathematical expression omitted
Appropriate heating rate and holding time are used to adapt to the transformation of refractory materials.
In order to achieve proper thermal surface development, special attention must be paid to the heating speed and retention time.
The holding time is usually 1600-2000F (870-1093C)
2 hours to adapt to mass expansion (12%)
In the process of transformation from beta quartz to three leaf insects.
This volumetric expansion of refractory reduces the opening pores, thus reducing the possibility of penetration of molten metal.
The conservative slope rate allows the transfer of heat to the refractory and helps with phase change.
Because the refractory is usually an insulator, it takes more time for heat transfer.
Final temperature hold applied 100F (38C)
Higher than the maximum tap temperature.
This last reservation allows the transition from tridymite to Square Quartz.
This again results in volume expansion, closing the open air holes and increasing the strength of the refractory.
Few silicone refractory materials remain above 2900F (1595C)
Due to the temperature limit.
The traditional sintering method is the most commonly used and recommended sintering method.
The method of accelerated sintering and liquid sintering is also adopted.
They tend to reduce the performance of the refractory lining.
Conventional Sintering plans use ramp rates and hold times to ensure phase transitions occur during the development of the thermal surface.
Accelerated sintering plans use increased ramp rates and reduced or excluded hold times.
This reduces the development of the thermal surface, and due to the lack of the development of the thermal surface, greatly increases the potential of metal penetration and reduces the service life of refractory materials.
Cooling the actual target: minimize the depth and size of the thermal crack.
This technology minimizes the impact of thermal cracking on the performance of refractory liners.
For those operators who find it necessary to completely close the furnace for maintenance or to close on weekends, it is essential to understand the impact this has on the performance of the refractory.
Obviously, it is better to keep all refractory materials at a constant working temperature to avoid thermal stress and final thermal cracking.
This may not apply to all operations due to scheduling and/or stove size.
When closing is an absolute requirement, it is important to recognize that it is best to cool the refractory lining quickly.
Contrary to what many operators believe, the rapid cooling of the hot surface of the refractory helps to control the development of the crack.
The development of many small cracks is promoted by rapidly cooling the hot surface.
However, these cracks are shallow in depth, so it is easier to close at subsequent re-heating than the larger and smaller thermal cracks that may be formed during slow cooling.
Details of the process include completely emptying the stove after the last heating: the fan or air hose should then be placed opposite the nozzle (
Usually called 6 points)
And open immediately to promote the side wall airflow.
Across the floor, up from one side of the spout. !
T helps to remove any slag or metal adhesion between the refractory and the nozzle.
By drilling the area free of charge, you will make sure that the refractory lining does not attach to the spout and there will be no horizontal cracks.
Figure 3 provides an overview of typical configurations and processes.
When it is not necessary to cool the furnace to the ambient temperature, another option for rapid cooling is to use a gas torch to keep the refractory in a thermal equilibrium state of more than 16 degrees Fahrenheit (815C).
As shown in the figure.
4, Square Quartz, as the main component of the thermal surface, does not shrink sharply until the temperature is lower than 16 degrees Fahrenheit (815C).
If the refractory is kept below 16 degrees F (815C)
The furnace can be recharged and put into operation immediately.
Re-heating process objective: to ensure that the metal does not penetrate the thermal cycle crack.
After the refractory is cooled below 16 degrees F (815C)
, Heat is required for expansion refractory lining and sealing hot cracks.
All cracks must be completely closed before the liquid metal is exposed.
The length of time required to completely close the hot crack depends on the size of the furnace.
The larger the furnace, the greater the mass of the refractory, so the longer it takes to reach the heat balance.
The program includes loading the furnace with clean, tightly packed cold materials.
At least one thermocouple must be used to monitor the temperature.
In furnaces larger than 10 tons, two thermocouple are essential.
Typically, the stove is then heated to 300F/hr (165C/hr)
Until 200F (110C)
Lower than the melting point of the metal charge. (
Due to the reduction in the volume of refractory materials, the smaller furnace can be heated at a faster speed, while the larger furnace requires a slower heating time. )
For the period of time outlined in Table 1, the furnace must remain at this temperature.
This will ensure that all hot cracks are sealed before the liquid metal is exposed.
This procedure will reduce the possibility of short-term refractory movement due to metal processing.
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