What are the Reheating Furnaces ? | Refractory Industry | Iron and Steel | Industry Guru

09-Oct-2022

Reheating Furnaces are used for heating the intermediate products of steel like- ingots, blooms or billets at temperatures around 1000 - 12000C before rolling to give them different shapes of angles, channels, bars, slabs, rods & wires etc. The furnace is a chamber having inside refractory lining to conserve heat and generally includes a mechanism for transporting products continuously from one end to the other end. Heat for the furnace is provided by natural gas or fuel oil.

Reheat Furnace which is considered an accessory to the rolling mill is a critical factor in the quality of end-product. There are various types of reheating furnaces which can be differentiated on the bases of - (1) the method of heating, (2) method of charging the reheating furnace, (3) the movement of steel stock in the reheating furnace, and (4) the heat recovery methods.

Based on the method of heating, a reheating furnace can be Combustion Heating type or Electrical Heating type. The combustion heating type furnace can use solid, liquid, or gaseous fuel. Based on the method of charging, the reheating furnace can be classified as Batch type or Continuous type. In batch type reheating furnaces the charged material remains in a fixed position on the hearth until heated to rolling / forging temperature while in continuous type reheating furnaces the charged material moves in the reheating furnace and is heated to rolling temperature as it progresses in the furnace. Based on the movement of steel stock inside the furnace, continuous type reheating furnaces can be further classified as Pusher Furnace, Rotary Hearth Furnace, Walking Beam Furnace, Walking Hearth Furnace, and Roller Hearth Furnace.

Based on heat recovery method a reheating furnace can be either Regenerative type or Recuperative type. Regenerating type reheating furnace uses regenerative burners while Recuperative type reheating furnace uses recuperators for heat recovery from the exhaust gases.

(If you want to know in details about any of the above mentioned types of reheating furnaces then please let us know in the comment section below as we will bring separate post for the same).

In view of saving energy, requirement of Refractories with low thermal conductivity are required particularly for Door, Heaters, Flue-stack and further, for Hearths these criteria are corrosion, abrasion & spalling resistances with good strength. For these purposes, dense low-iron, high-alumina or chrome-magnesia bricks are best suited for Hearth. The side-wall and roof are lined with high-grog alumino-silicate bricks. Precast / Prefired (PCPF) blocks can be used for Burner-block, Seating Well Block, Well Block as well as for Furnace Bottom, made of low-iron high-alumina dense refractories having excellent spalling & abrasion resistances. The advantage of using PCPF blocks is that they can save time for both installation & for drying/preheating inducing more availability of the furnace. In the event of choosing Precast / Pre-fired (PCPF) blocks one has to take care of the design of these blocks which should be such that one person can handle them.

For more details refer to our post: Area wise Refractory specifications and lining procedure for reheating Furnaces

 

Refractory Installation Procedure and Heating Schedule to be followed after starting an Induction Furnace | Furnace Operation

Installation of refractories in any furnace is a tricky process and after the lining is done the most important thing is the heating schedule. That means how the furnace after repairing or with new refractory lining should be started, what should be the rate of heating (rising temperature) and holding time at any particular temperature. You cannot start the furnace by raising its temperature to peak at one go, as otherwise the refractories will be damaged or even the refractory lining may fall apart. The furnace starting heating schedule depends on various aspects including, thermal conductivity of the refractories used.

Here is a step-by-step guide for installation of Refractories (Ramming Masses, etc.) in Induction Furnace also the heating schedule that should be followed after starting the furnace:

 Read: 

• Refractories with their Specifications for Lining for different types of Induction Furnaces
• Refractory Lining Installation of Pipes and Chutes in a Furnace

1. Scrub all loose materials and clean the furnace.

2. Before using heat the refractory material (Ramming mass) at about 100^OC to make it free from moisture. Spread the material at the furnace bottom to about 50 - 60 mm thick layer at each time. Then ram the layer uniformly using a suitable rammer. The material has to be rammed layer by layer to get maximum compaction. Before ramming, little bit poking with a rod help to drive away the air-pockets trapped within the loose refractory material spread.

3. Place the steel former on the rammed bottom. Then fix the steel iron block at the center of the steel former to get uniform thickness throughout the furnace wall.

4. For ramming the upper portion of the side-wall just above the induction coil, mix the dry refractory material with 1-1.5% Sodium Silicate solution and 3-4% water.

5. Best results can be achieved by following the heating schedule for the furnace as mentioned hereunder -

Furnace Heating Schedule | Furnace Operation

Furnace Temperature	Rate of Heating with Holding Time
Ambient temp to 100OC	@ 30OC / hr.
Hold at 100OC.	4-6 hr depending upon the lining thickness.
100O - 800OC.	@ 50OC / hr.
Hold at 800OC.	2 - 3 hr.
800O - 1400OC.	@ 100OC / hr.
Hold at 1400OC.	4 - 6 hr.
1400OC to furnace operating temperature.	@ 100OC / hr.

Related Article

• Induction Furnaces | Channel Type and Coreless Induction Furnaces - Design, Advantages and Disadvantages 

The Functions of Silica Fume (Microsilica) and other Ultrafine or Microfine Additives in Refractory Castables

The unshaped refractories, commonly known as Monolithic refractories are manufactured by suitably blending graded refractory aggregates, binders, fillers & (/or) special additives used for modification of ultimate properties. The refractory aggregates chosen for the formulation of refractory castables have a major contribution in determining their ultimate product quality. Along with this and many other input parameters, especially the high temperature properties of all type of Castables (Aluminous / Basic) depend a lot on the additive’s level and type used. All ingredients, binders, and additives of different chemical compositions and grading are chosen, blended to provide the proper characteristics for various applications of monolithics. 

Effect of Water Addition vs. Ultrafine Additives (Silica Fume) in Castable Refractories

For castable refractories the mobility of various particles is essential for proper placement of the castable. One method to achieve mobility is by adding more water, but this increases porosity & thus affects the performance both at normal & elevated temp. Therefore, addition of water should be minimized. The desired fluidity can be achieved by maintaining the coarser particles separated from one another by suspension of fines and ultrafine additives combined with the state of flocculation within the suspension. The role of the grains of these ‘Microfines or Ultrafine Additives’ can be compared with those of the balls in a ball-bearing.

What are the commonly used Ultrafine Additives in Refractory Castables?

Read: Benefits of using Steel Fibers and Organic Fibers in  Refractory Castables and other Monolithic refractories

The commonly used microfines or ultrafine additives are Silica Fume also known as Microsilica or Fumed Silica, microfine Alumina (Al₂O₃), ultrafine green Chrome Oxide (Cr₂O₃) etc. Silica fume is a by-product of producing silicon metal or ferrosilicon alloys. Silica fume consists primarily of amorphous (non-crystalline) silicon dioxide (SiO₂). Because of its super fine particles, large surface area, and the high SiO₂ content, silica fume is a very reactive.

The addition of Microsilica or Silica Fume has other advantages also, since these micro fine silica particles easily react with alumina present in the material to form Mullite which, in turn, helps in enhancing refractory properties of the product.

The addition of certain percentage of superfine green Chrome Oxide (Cr₂O₃) in Alumina castables increases the slag corrosion resistance & HMOR of the product significantly because of the formation of Alumina-Chrome (Corundum) solid-solution.

There are several manufacturers of fumed silica. Some well-known global brands of fumed silica are of Elkem Materials, AEROSIL of Evonik Industries, Norchem Concrete Products. 

 

Read: Advantages of using Gel Bond and Colloidal Silica in Monolithic Refractories