What Ferrosilicon Nitride Is
Ferrosilicon nitride is a ferroalloy based mainly on silicon nitride (Si3N4), together with free iron, un-nitrided silicon and a small amount of other components. It is used as a nitrogen bearing additive in steelmaking and in refractory production. Refractory grades are supplied as an off white or brown powder, while steelmaking grades are usually off white granular material, because the additive must dissolve in the melt rather than build a refractory structure.
The material is attractive in the melting shop because it acts in three ways at once: it introduces nitrogen, it deoxidises, and its decomposition products help to remove oxygen from the molten steel. The nitrogen it delivers is the part that governs strength, ageing behaviour and weldability of the finished product.
How Nitrogen Enters the Molten Steel
When granular ferrosilicon nitride is plunged into the melt, the nitride decomposes and releases nitrogen that dissolves in the liquid steel. Silicon and iron pass into the steel or into the slag, and the nitrogen that stays in solution is available to form nitrides during solidification and cooling. Because nitrogen solubility falls sharply as the steel solidifies, nitrogen that is not fixed by a strong nitride former can remain in interstitial solution and move later, which is the root of the ageing problems described below.
Addition practice therefore matters as much as the analysis of the additive. Granular material with a controlled size fraction dissolves faster and gives more repeatable nitrogen recovery than a coarse or heavily segregated product, and a deoxidised melt holds nitrogen more effectively than one that is still boiling with oxygen.
Product Forms and Grain Size
Ferrosilicon nitride is graded mainly by grain diameter, because the size fraction decides how quickly the additive dissolves in the ladle or in the mould. A typical granular specification is shown below; the percentage column states the proportion of the product that must fall within the stated size band.
| Product form | Grain diameter | Passing the stated range |
|---|---|---|
| Fine grain | 3~5 mm | above 90 % |
| Common grain | 3~10 mm | above 90 % |
| Coarse grain | 10~50 mm | above 90 % |
Powder and fine granular forms are preferred where nitrogen must be distributed quickly and evenly, for example in nitrogen bearing rebar and in oriented silicon steel, while coarser granules suit larger heats and deeper immersion.
Beneficial Effects of Nitrogen on Molten Steel
Nitrogen is not simply an impurity. In controlled amounts it is a deliberate alloying addition with several useful effects.
Nitrogen combines with aluminium, vanadium, niobium and titanium to form fine nitrides and carbonitrides that pin grain boundaries and refine the as cast and as rolled structure.
In reinforcing bar such as HRB400, nitrogen assisted precipitation raises yield strength so that the same strength level can be reached with less carbon and microalloy addition.
In oriented silicon steel, nitrogen participates in the inhibitor system that controls grain growth during processing, which supports the magnetic properties of the final sheet.
Nitrogen increases resistance to softening at elevated temperature in some grades of steel, because the nitrides formed are stable at temperatures where carbides would coarsen.
Ferrosilicon nitride also deoxidises the melt, so it contributes to a cleaner steel alongside the nitrogen it supplies.
Harmful Effects of Nitrogen and How They Are Controlled
Above the level that the steel can fix as stable nitrides, nitrogen changes from a useful alloying element into a source of defects.
Ageing: nitrogen left in interstitial solution slowly precipitates as iron nitride. The precipitates distort the metal lattice and generate internal stress, so the steel becomes brittle over time and its impact toughness and plasticity fall. Nitrogen raises susceptibility to this strain ageing; it does not improve resistance to it, and a high phosphorus content makes the effect worse.
Blue brittleness: when steel with a high nitrogen content is heated into the 250 to 450 °C range, its surface turns blue while strength rises and impact toughness drops. This loss of ductility in the blue heat range has to be considered whenever a component is formed or straightened warm.
Weldability: as nitrogen content rises, welding performance deteriorates. Porosity, hardening in the heat affected zone and loss of toughness are the usual consequences, so nitrogen bearing steels need a welding procedure matched to their chemistry.
Correction of the interstitial problem: adding a controlled amount of aluminium to the steel forms stable AlN, which limits the formation and precipitation of Fe4N. This improves ageing performance and, at the same time, the AlN particles restrict the growth of austenite grains.
The practical rule is that nitrogen should be specified, not merely accepted. A target nitrogen range, a matched addition of a nitride former and a controlled aluminium level together convert nitrogen from a defect source into a strengthening tool.
Frequently Asked Questions
Q: What is ferrosilicon nitride used for in steelmaking?
It is used to add nitrogen to molten steel, to deoxidise the melt and to support precipitation strengthening and grain refinement in nitrogen bearing steels such as reinforcing bar and oriented silicon steel.
Q: What does the designation FeSi3N4 mean?
It reflects the main constituent of the alloy, silicon nitride (Si3N4), which supplies the nitrogen. Commercial material also contains free iron, un-nitrided silicon and minor components, so the nitrogen content of the product is quoted separately from the analysis of pure silicon nitride.
Q: What causes ageing in nitrogen bearing steel?
Nitrogen that remains in interstitial solution precipitates slowly as iron nitride after cooling. The precipitates distort the lattice and raise internal stress, so ductility and impact toughness fall with time, especially when phosphorus is also high.
Q: What is blue brittleness?
It is the loss of impact toughness that occurs when steel with elevated nitrogen is heated to roughly 250 to 450 °C, the range in which the surface takes on a blue oxide tint. Strength rises while toughness falls, so warm forming and straightening need care.
Q: How does aluminium help when nitrogen is high?
Aluminium is a strong nitride former. It combines with nitrogen to form stable AlN instead of letting Fe4N precipitate later, which improves ageing resistance and pins the austenite grain boundaries during heating.
Q: How should ferrosilicon nitride be stored?
Keep it dry, in sealed packaging or a covered bunker, separate from other ferroalloys. Moisture pick up changes the effective nitrogen recovery and can cause the granular product to cake, which then feeds unevenly into the melt.