EAF MgO-C Bricks: Wear Mechanisms and Antioxidant Design

1. Why This White Paper Exists

Magnesia-carbon bricks were first developed for the hot spots of electric arc furnaces, and only later moved to converters in the late 1970s. So the EAF is where this material has the longest history. It is also where purchasing and engineering teams still argue the most about it: Is a higher graphite brick better? Is boron carbide worth the price? Why did the same brand last twice as long in the neighbouring furnace?

We make refractories, so we have a view. We also try to say which statements come from published work and which are our own reading. Where we have no solid number, we say so and we do not invent one.

In this paper you will find:

• The four ways an EAF MgO-C brick wears, in plain language;
• What each antioxidant does, and where it can cause trouble;
• How to tailor a brick to each zone of the furnace;
• How to read a used brick, and what to send a supplier;
• A cost per tonne method, and 18 questions with direct answers.

2. How an EAF Magnesia-Carbon Brick Is Built

Three jobs, three ingredients

A MgO-C brick is a team of materials, and each does one job.

• Magnesia grains resist slag and heat. They make up most of the brick.
• Graphite is not wetted by slag, conducts heat well and expands little. That keeps slag out and helps the brick survive heating and cooling.
• A carbon bond (usually resin) holds the brick together and leaves a carbon network after the first heat-up.

The problem is that graphite burns. Everything else in this paper follows from that.

Ranges quoted in the literature

These are general figures from published reviews, not a specification for any one furnace. Your own numbers should come from your supplier’s data sheet and your own furnace.

Fused or sintered magnesia?

One published reference table compares one fused sea-water magnesia (99.07% MgO, 2.6% apparent porosity) with one natural sintered magnesia (95.46% MgO, 8% apparent porosity). That is a single example, not a rule. But it shows the direction: fused grains tend to be purer and denser, so slag finds fewer paths into them. Impurities such as SiO2 and CaO in the grain matter because they form low-melting phases with iron oxide.

In our view, the grain choice should follow the zone. We come back to this in the zone table.

3. The Four Ways an EAF MgO-C Brick Wears

1. Slag dissolves the magnesia

Slag attacks the magnesia in two ways. Silica and lime enter the boundaries between grains and loosen them. Iron oxide (FeO) diffuses into the magnesia grain itself and forms a magnesia-iron oxide solid solution, which melts at a lower temperature than pure magnesia.

A laboratory study at 1,600 °C with 25% FeO slag found the most severe wear at a basicity of 0.5, and little wear from about 2.0 upward. Furnace practice and slag chemistry therefore matter as much as the brick. A brick that does well with a basic, MgO-saturated slag can suffer badly when slag turns acidic.

2. Iron oxide burns the carbon from inside

When slag gets into the brick, iron oxide meets graphite and the reaction FeO + C → Fe + CO takes the carbon away. The brick turns porous and weak, more slag enters, and the loss speeds up. A published review names this reaction as the cause of structural embrittlement in the brick.

3. Air and CO2 burn the carbon where the slag does not cover

The hot face of an EAF wall is not always under slag. Where the brick sees furnace air or CO2, carbon burns directly. Published work places direct oxidation below roughly 1,400 °C, and indirect oxidation by magnesia or slag above it. This is why lower sidewalls and areas that are often uncovered need oxidation resistance, even where slag attack is mild.

4. The brick attacks itself

At high temperature, magnesia and carbon can react: MgO + C → Mg(gas) + CO(gas). Under standard conditions the reaction starts at about 1,850 °C. In a real brick the gas can escape through pores, so it can run at a much lower temperature. The brick loses material from inside, and the result is a weak, open structure. Published tests also report that cutting very fine magnesia (0.1 mm and finer) reduces this kind of damage.

Thermal cycling and mechanical wear

On top of these four, an EAF brick is heated and cooled again and again, hit by scrap, and washed by moving steel. Heating cycles loosen the structure over time. A published converter study found that repeated heating above 1,500 °C and cooling below 500 °C could reproduce real brick damage in the laboratory. Wear by moving steel falls as the brick’s hot strength rises.

Why they feed each other

The four mechanisms rarely act alone. Carbon loss opens pores. Open pores let slag in. Slag brings more FeO. More FeO burns more carbon. If you remember one thing from this section, let it be this: a brick usually dies from a loop, not from one cause. Break the loop at its weakest link, and the whole brick lasts longer.

4. Where in the Furnace the Brick Fails

In many furnaces the upper wall uses water-cooled panels, so bricks work mostly in the lower shell, the slag line and the hot spots. Layouts differ, so check yours (see our complete EAF refractory lining solutions).

The exact order of wear depends on your furnace design, power input, slag practice and scrap mix. We do not give a general life figure. If someone gives you one without conditions, treat it with care.

5. What Antioxidants Do, and What They Cannot Do

An antioxidant is a material that is more eager to react with oxygen than graphite is, or that blocks the path of oxygen into the brick. It protects the carbon, but it does not make a poor brick good.

Aluminium

Aluminium reacts with carbon monoxide and carbon in the brick. A published laboratory study with an argon atmosphere found a liquid containing aluminium and magnesium at low temperature, then, after about 850 °C, aluminium carbide as thin platelets. Above 1,000 °C the aluminium carbide reacts with magnesia to form magnesium aluminate spinel (MgAl2O4), which sits well beside both magnesia and carbon. The overall effect is to fill pores and slow oxygen.

Watch-out (our practice note): aluminium carbide can react with moisture. Keep bricks dry in storage and during transport.

Silicon

Silicon reacts with carbon to form silicon carbide (Si + C → SiC). The new SiC fills pores and protects graphite. It is often paired with aluminium. Published reviews note that aluminium and silicon are the most used antioxidants because they are effective and low in cost.

Silicon carbide (added as SiC)

SiC added as a powder acts in two ways. It resists oxidation itself, and it fills pores. Published converter practice adds SiC to the throat and cone, where gas-phase oxidation is a problem. The same logic suits furnace walls that are often uncovered by slag.

Boron carbide

Boron carbide reacts with carbon monoxide and forms liquid boron oxide, which combines with magnesia to give a liquid MgO-B2O3 phase. In published reviews this liquid fills pores at medium temperatures and limits air access. That is its strength.

Watch-out (our reading, not a published figure): a liquid phase can soften a brick when the face is very hot. We would check it carefully before using boron carbide in the hottest zones, and we would ask the supplier for hot strength data.

What antioxidants cannot do

• They do not stop slag from dissolving magnesia. For that, you need pure, dense grains and a sound slag practice.
• They do not cure a bad heat-up. A wet or fast-heated brick will crack, whatever is in it.
• More is not always better. An antioxidant that reacts too early or leaves a weak phase can cost you hot strength. The overall design matters more than one additive.

6. Tailoring the Brick to the Zone

There is no single best EAF brick. There is a best brick for each zone of your furnace, built from your slag, your scrap and your practice.

“Mid” and “higher” mean relative levels inside your own range of bricks. We do not give fixed percentages, because the right figure depends on the furnace and the supplier’s recipe.

Three trade-offs worth knowing

1. Higher graphite lowers thermal shock risk but also gives the oxidation more to burn. More carbon needs better protection, not just more of it.
2. Purer magnesia costs more but often pays back in the hot spot and slag line, where slag attack drives the loss. In the sidewall, the saving may not be worth it.
3. Metal antioxidants strengthen the brick but can raise cost and handling needs. Use them where they work, not everywhere.

What a custom design needs from you

The more of this you send, the closer the first design will be:

• furnace type, capacity, power and tap-to-tap time;
• lining drawing and brick thickness by zone;
• scrap mix and any direct-reduced iron or hot metal;
• slag practice (basicity, MgO addition, foaming, FeO range);
• current brick, life in heats and how it fails, by zone;
• photos of used bricks and of the furnace in service.

7. Reading a Used Brick: 4-Step Analysis

A used brick is a free test report. Here is how to read it.

Step 1. Photograph the hot face first

Take photos before you cut or clean the brick. Note the zone, the position and the heat count.

Step 2. Cut through the thickness

Cut from the hot face to the cold face and look at the cut surface.

Step 3. Measure the layers

Record the thickness of each layer from the hot face, in millimetres: slag-covered layer, dense layer, carbon-free layer, unaffected core.

Step 4. Decide

What the tests can add

Start with simple tools

Photos, a ruler and an optical microscope tell you a lot. Bulk density and apparent porosity of an unused brick come from standard tests.

Then add instruments when you need them

A scanning electron microscope with element analysis (SEM/EDS) shows which phase holds which element. X-ray diffraction (XRD) names the crystal phases, for example spinel or silicon carbide.

Do not over-read one brick

One brick can mislead. Compare it with an unused brick from the same batch, and with another used brick from the same zone.

8. Counting the Cost per Tonne of Steel

A brick that costs less but lasts fewer heats can cost more per tonne.

An example with made-up numbers, for illustration only:

In this made-up case the dearer brick wins. Add two more lines from your own records: the cost of one unplanned stop, and any extra repair of that zone. An unplanned stop can cost more than the bricks.

9. Six Checks Before You Order

10. How YK Chanshine Works with EAF Teams

We make magnesia-carbon bricks and other refractories for steel plants at our in-house manufacturing facilities. Our process on an EAF project is simple:

1. You send furnace data, slag practice and photos of used bricks.
2. We read the wear pattern with you, by zone.
3. We propose a brick design for each zone and say what we would change and why.
4. We put the storage, installation and heat-up notes in writing.
5. After the trial we read the used bricks together, and adjust.

11. Frequently Asked Questions (FAQ)

12. Reference Sources

The published facts in this paper come from the following. Our own judgements are marked as such in the text.

• Magnesia-Carbon Refractories, Ispatguru
• Magnesia-Carbon Refractories for Converters, Nippon Steel Technical Report No. 125, 2020
• Reaction Chemistry and Microstructure Development of MgO-C Refractories Containing Metal Antioxidants, Guha and Smith, 2006
• The influence of slag basicity and composition on ConArc magnesia-carbon refractories during the blowing phase, A. Muller, Journal of the Southern African Institute of Mining and Metallurgy, 2011