How to Prevent Formation of Iron Spots (Crater) in Refractory Bricks as a measure of Quality Control

21-Jan-2023 (Updated)

What makes a refractory product sell? The factors determining the sales success are many and varied. They include market conditions, the nature of the product, and the image of the manufacturing organization in the market as well as of the product created by advertising, the socio-cultural background of customers, credit facilities, Customer Delight and so on.

But one major factor that appears certainly in all conditions (excluding monopoly or extreme scarcity) is product quality as perceived by the customer. The Production Department must accept its fundamental responsibility for the manufactured quality of its refractory products. The workforce with effective training and adequate equipment must be capable of -
1. consistently producing to specification and 

2. recognizing and reacting when an operation goes out of control.

Adequate systems must be established to ensure that corrective action is taken. The quality control for Refractory Bricks refers to the following aspects:

=> Manufacturing defects.

=> Dimensional tolerances.
=> Physical & Chemical material properties.
=> Quantities, Marking, Labelling and Packing.

While Sampling for visual and dimensional inspection, iron spot (crater) is always an important criteria of inspection. It is not an uncommon site or if one had a chance of visiting brick yards of a few Refractory manufacturing units then he must have noticed that in many such plants refractory bricks worth crores (millions) or thousands of tons are lying rejected because of developed iron spots (grey fusion spots) in them. 

Before going into the steps as how to control the formation of these iron spots or iron craters in Refractory Bricks let us discuss a few more things in detail.

Iron Craters or similar Black Spots are actually melt phenomena, caused by mainly iron oxide or lime or some other low temperature fusion and melting elements mostly, alkalies. An Iron spot on the surface of a refractory brick can be tested with a hammer (I assume Refractory persons would be acquainted with this tool :-) ) to determine the possible presence of crater which can be dangerous for the brick and so, for the furnace after lining. Some common reasons/sources of these iron spots and preventive actions required to be taken regularly to stop formation of such black spots and maintain the quality of refractory bricks are outlined below:

Refractory Raw Materials
Preventive Measure: Checking the Raw Material & removing free iron from it before grinding / using. Sorting of Slag / Iron patch especially in calcined fire clay and other calcined raw materials at the kiln yard itself.
Responsibility: Mill house, Calcination Kiln, Checking & RM yard.

Own Rejection (Refractory Grog)
Preventive Measure: Iron containing (contaminated) grog to be kept in strict isolation.
Responsibility: Checking & RM yard.

Outside Grog
Preventive Measure: Visual inspection in the Truck itself by breaking up the lumps into pieces before taking into the raw material yard. Briquette test in the Laboratory.
Responsibility: Checking & RM yard. Laboratory (Quality Assurance).

Maintenance Waste
Preventive Measure: Proper cleaning of waste & scraps after Maintenance of any machine in the shop-floor e.g. iron chips, welding tips etc. before handover to the Production Deptt (there should be a proper Handover – Takeover system). Cleaning before & after maintenance of each machine (Mill House, Mixer, Press etc).
Responsibility: Concerned person of the Maintenance Deptt. Shift in charge of the concerned area (Mill House, Press, and Production).

Free iron in Mill House Silo Material
Actions Required: Due to Beater => regular cleaning of Magnetic drum, Plate & Roller Magnets. Due to Rusted plates => Silo cleaning once in a fortnight.
Responsibility: Mill House. Maintenance. Production departments.

Free iron from Press Hoppers / Mill Hoppers
Actions Required: Due to Rusted plates => Hopper cleaning once in a fortnight.
Responsibility: Production.

These are the certain necessary steps which, if taken properly, can arrest the formation of grey craters and iron spots which become visible on the surface of refractory bricks only after the firing is over that means everything is over!

Sampling of the Refractory Specimen for Inspection

21-Jan-2023

The sample selection or what is known as Sampling, is an important function of refractory inspection. The inspection procedure starts by selecting, generally a minimum number of units, called a ‘Sample’, from a lot. A ‘Lot’ is meant for the quantity of refractory materials from which sample specimens are drawn for inspection. The test results of these samples make the basis for acceptance or otherwise of the refractory lot. Since Refractories are characteristically anisotropic in nature so instead of drawing only one sample, a predefined number of sample specimens are collected from the lot while sampling. Although there are provisions for ‘Hundred percent inspection’ and ‘Sampling inspection’ but, hundred percent inspections is not preferred generally since most of the tests are destructive type besides, consideration of factors like time consumption and the costs involved.

Methods of Sampling

Sampling inspection of both refractory bricks and monolithic refractories can be done by two methods - (1) Inspection by Attributes, (2) Inspection by Variables. 

The first method is performed visually by gauging or counting the number of defects and so, is non-destructive and cost effective. The second method is performed by all tests including the destructive ones on Refractories.

Sample size or the number of samples to be drawn from the Refractory lots to be inspected depends on the type of Standard Method being followed for testing. Sampling and their testing are done strictly according to the specified procedures already mentioned in that Method, the results of which should be binding for both the manufacturer and the customer.

What is Standard Method for Testing of Refractories? (To be posted soon)

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 for reheating Furnaces (to be published soon).

 

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