Why a ferroalloy rather than the pure element
Most alloying elements melt at far higher temperatures than iron. Chromium melts at 1907 °C, molybdenum at 2620 °C and tungsten at 3422 °C, whereas a steel bath generally sits at around 1600 °C.
Alloyed with iron, the melting point falls markedly, the density comes closer to that of the bath and the element dissolves quickly. Ferroalloy production is also far more economical than producing the pure metal: reduction from ore to the metallic state happens in a single step.
Production routes
- Carbothermic reduction
- The ore is reduced with coke or coal in a submerged arc furnace. High-carbon ferrochrome, ferromanganese, ferrosilicon and silicomanganese are produced this way. It is economical, but the product is high in carbon.
- Metallothermic reduction
- Aluminium or silicon is used as the reductant instead of carbon. Low-carbon ferrochrome, ferrovanadium, ferroniobium and ferrotitanium are generally produced this way. The reaction is exothermic and, with no carbon input, yields a low-carbon product.
- Perrin process and decarburisation
- Lowering the carbon of a high-carbon ferroalloy by blowing oxygen or through slag reactions. This is how medium-carbon grades are obtained.
The main ferroalloys
| Ferroalloy | Typical use |
|---|---|
| Ferrochrome (FeCr) | Stainless and alloy steel. HC, MC and LC grades by carbon level. |
| Ferromanganese (FeMn) | Manganese source and deoxidiser. Carbon and structural steels. |
| Silicomanganese (SiMn) | Supplies silicon and manganese together; common in structural steel production. |
| Ferrosilicon (FeSi) | Deoxidation, inoculation in cast iron, electrical steel. |
| Ferrovanadium (FeV) | High-strength low-alloy steels, tool steels. |
| Ferromolybdenum (FeMo) | Creep resistance and hardenability; pipeline and pressure vessel steels. |
| Ferrotungsten (FeW) | Tool and high-speed steels. |
| Ferroniobium (FeNb) | Microalloyed pipeline steels, grain refinement. |
| Ferrotitanium (FeTi) | Tying up carbon and nitrogen, stabilisation in stainless steel. |
| Ferroboron (FeB) | Increasing hardenability, low-alloy steels. |
| Ferrophosphorus (FeP) | Special applications requiring a controlled phosphorus addition. |
| Ferrozirconium (FeZr) | Deoxidation, inclusion modification, grain refinement. |
Why the carbon level is critical
In stainless steel, carbon combines with chromium to form chromium carbides at the grain boundaries. This depletes the surrounding region of chromium and leaves the material open to intergranular corrosion — the phenomenon known as sensitisation.
That is why low-carbon ferrochrome (generally C < 0.1% or 0.2%) is used in stainless steel production. High-carbon ferrochrome is preferred in processes where carbon is not a problem or is removed later, and it is markedly cheaper.
Writing only "ferrochrome" in your specification is not enough. Always state the carbon limit, the chromium range and the upper limit for silicon.
Particle size and packaging
Ferroalloys are usually shipped as lump and crushed to size against the order. The particle size is chosen to suit the addition method:
- For ladle addition, a small size (10–50 mm, for example) gives fast dissolution.
- For furnace charging, a coarser size is preferred.
- For wire feeding and injection, powder or granules are needed.
- The proportion of very fine material should be capped; fines cause loss and dust problems.
Packaging options are big bags, steel drums and bulk shipment. Items sensitive to oxidation need closed packaging.
Writing the specification
- State the range of the main element (Cr 60–65%, for example).
- State the upper limits for carbon, silicon, sulphur and phosphorus.
- Give the particle size range and the acceptable percentage of fines.
- State the packaging and the lot size.
- Add the certificate type and the sampling method to the order text.
Frequently asked questions
What is a ferroalloy?
A ferroalloy is an intermediate product formed by iron and one or more alloying elements. It is used to carry the alloying element into a steel or foundry bath; compared with adding the element in pure form, its melting point is lower, it dissolves faster and it costs markedly less.
Which ferroalloys are used most?
Ferrochrome, ferromanganese, silicomanganese and ferrosilicon are the most widespread by tonnage. They are followed by special items such as ferrovanadium, ferromolybdenum, ferroniobium, ferrotitanium, ferroboron and ferrotungsten.
How is a ferroalloy produced?
There are two main routes. In carbothermic reduction the ore is reduced with coke in a submerged arc furnace; it is economical but the product is high in carbon. In metallothermic reduction aluminium or silicon is used instead of carbon, and low-carbon grades are generally produced this way.
What should a ferroalloy order state?
The range of the main element, the upper limits for carbon and silicon, the sulphur and phosphorus limits, the particle size range, the acceptable proportion of fines, the packaging and the lot size. The certificate of analysis and the sampling method should also be written into the order.
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