Carburizer Addition Methods in Induction Furnace Melting

What a Carburizer Does in Iron and Steel Melting

A carburizer - also written carburiser, or recarburizer when the emphasis is on restoring carbon that has been lost - is a carbon-bearing charge addition whose only job is to lift the carbon content of a heat to the value demanded by the final specification. In a cupola the coke bed supplies most of the carbon, but in an induction furnace or an electric arc furnace the carbon budget is decided almost entirely by the metallic charge and by what is deliberately added to it. Steel scrap is normally low in carbon, often far below 0.3% C, while grey iron and ductile iron specifications call for roughly 3.0-3.8% C and many steel grades sit between 0.05% and 1.0% C. The gap has to be closed with a carburizer, and it has to be closed without introducing more sulfur, ash, nitrogen or moisture than the specification can tolerate.

Carbon is also consumed inside the furnace. Oxidation at the bath surface, iron oxide carried in the slag, and dust losses all remove carbon from the heat, so the carbon charged must always exceed the carbon required in the finished iron. The difference is expressed as carbon recovery: recovery = (carbon found in the melt / carbon added) x 100%. Recovery is the most useful single number in the melting shop, because it converts a certificate value into a practical dosing figure. Two foundries using the same grade of artificial graphite can report recoveries that differ by twenty percentage points purely because one of them adds the material under the charge and the other throws it onto an open bath.

Why the Addition Point Changes Everything

Recovery is not a property of the material alone. It is a property of where the material is placed, how hot the melt is when it arrives, and how much oxygen and slag it meets on the way in. Carburizer buried beneath the solid charge before melt-down is shielded from the atmosphere and is surrounded by metal that will dissolve it as soon as a bath forms, so recovery is high. Carburizer thrown onto a bare, open bath after melt-down floats, burns and is captured by slag faster than it can dissolve. Carburizer added to a ladle, either in the ladle bottom or along the tapping stream, has only a few seconds of contact with turbulent metal, so only part of its carbon can go into solution.

A second controlling idea is carbon solubility. Carbon dissolves in liquid iron up to a maximum that rises as the temperature falls, reaching the eutectic point at 4.3% C and 1153 degrees C on the stable iron-graphite system. That is why a partially melted, comparatively cool bath can absorb a larger single dose of carbon without becoming saturated, and why a thin, superheated bath rejects carbon into graphite flotation or into the slag. The practical consequence is that a low-temperature carburizing step is not merely a time saver; it is a way of working closer to the eutectic, where the driving force for dissolution is still favourable and oxidation losses are smaller because the carburizer never sits exposed on an open surface.

Addition route	Typical carbon recovery	Key conditions	Dominant losses
Charged on the furnace bottom and covered with metallic charge	About 90-95%	Dry carburizer, dry and clean charge, no exposure to air	Dusting during charging, fines drawn into the fume
Added to the melt after melt-down and deslagging	About 85-95%	Bath at roughly 1400-1500 degrees C, FeO-rich slag skimmed first, strong stirring	Flotation into slag, oxidation at the bath surface
Low-temperature practice with a solid charge cover	Bulk pickup above 1.0% C in one operation	Partial melt, low bath temperature, carburizer pressed below the surface	Very slow dissolution if the bath cools too far
Ladle addition, ladle bottom or tapping stream	About 40-60%	Fine grade near 100-300 mesh, turbulent stream, stirring after tap	Short contact time, dusting, atmospheric oxidation
Powder injection with a carrier gas	Falls steadily as carbon approaches saturation	Lance held deep in the bath, nitrogen preferred as carrier, steady feed rate	Carrier-gas cooling of the bath, splash, dust in the off-gas

Charging Carburizer with the Metallic Charge

The most efficient way to put carbon into an induction furnace heat is to put it in at the bottom. The usual sequence is a light layer of clean metallic charge on the furnace bottom, then the weighed carburizer, then the balance of the charge on top, with the heaviest and densest scrap loaded directly over the carbon so that the layer is compressed and cannot float. Because the carburizer is never in contact with the atmosphere, oxidation loss is small and recovery commonly reaches the low nineties as a percentage of the carbon charged.

The dose is calculated from the carbon balance, not from habit. Take a 5 tonne heat of scrap analysing 0.15% C, a target of 3.20% C, and an expected recovery of 95%: the carbon that must be added is (3.20 - 0.15) / 0.95 = 3.21% of the heat weight, that is about 160 kg of carbon, which for a 99% fixed-carbon artificial graphite corresponds to roughly 162 kg of material. The same arithmetic carried out with a 90% recovery figure would ask for about 169 kg, so a badly chosen recovery assumption is worth nearly 10 kg of carbon per heat - small in one heat, but not small across a year of production.

Two practical rules keep this route reliable. First, the carburizer must be dry: moisture carried in with the charge dissociates in the bath, adds hydrogen, and produces violent spitting that damages the lining and throws material out of the furnace. Second, the carbon equivalent, usually taken as CE = %C + (%Si + %P) / 3, must be checked against the process window. Charging carbon to the charge is safe in the sense that large carbon pickup is possible at low temperature, but if the silicon level of the charge is also high, the melt can become hypereutectic and graphite flakes or nodules will float out of the bath as kish, giving a carbon analysis that is lower than the carbon balance predicted.

Late Carbon Adjustment After Melt-Down

When the analysis comes back from the laboratory and the carbon is short, the correction must be made carefully. The first step is to remove slag from the bath surface, because a slag rich in iron oxide attacks carbon directly by the reaction FeO + C giving Fe + CO. Adding fresh carburizer under a reactive slag simply feeds the slag. Skim first, then add.

Second, the bath should be at a temperature that allows rapid dissolution without excessive oxidation - for cast iron this is normally in the range of about 1400-1500 degrees C - and it should be stirred. Electromagnetic stirring in a mains-frequency induction furnace, or mechanical stirring with a graphite rod or a purpose-made plunger, keeps the carburizer particles in contact with fresh metal instead of letting them float in a raft on the surface. Third, the carbon should be added in small increments rather than one large dose. A dose of 20-30 kg on a 5 tonne heat, allowed to dissolve under stirring for three to five minutes before the next increment, usually achieves a recovery approaching 90%, whereas the same material tipped in as a single heap may recover barely half of its carbon because the heap insulates and protects only its own top layer.

Verification matters as much as the addition. Each increment should be closed out by a carbon determination on a sample taken from a well-stirred bath, because a sample taken from the surface of a bath that has not mixed will read low and tempt the operator into a second, unnecessary addition that leaves the heat over carbon. Sampling and sample preparation practice for ferroalloys and related furnace additions is standardised; GB/T 4010 for the sampling of ferroalloys for chemical analysis and ISO 3713 for ferroalloys sampling and sample preparation describe the principles that a foundry laboratory follows when it validates incoming material and bath samples.

Low-Temperature Carburizing Practice

The low-temperature method works on a deliberately different principle. Only part of the charge is melted, the bath temperature is deliberately kept low, and the whole carburizer dose for the heat is added at one time. The solid portion of the charge that remains is then pushed down over the carburizer, so that the carbon is trapped between molten iron and cold, unmelted scrap and cannot float to the surface or be exposed to air. Under these conditions a bulk carbon pickup above 1.0% C in a single operation is achievable, which is a large dose to place in one step by any other route.

The metallurgical logic is straightforward. Carbon solubility in liquid iron increases as the temperature falls towards the eutectic, so a cooler bath can accept more carbon before it becomes saturated, and the risk of kish flotation is reduced. The trapped layer of carburizer is also protected from the oxygen in the furnace atmosphere, so the carbon that is not yet in solution is not being destroyed while it waits. The limits of the method are equally clear: if the bath cools too far, the dissolution rate falls away, the carburizer remains as undissolved lumps, and the next step is a long, expensive period of superheating to recover the heat. The low-temperature practice therefore needs a disciplined temperature reading before the addition, a full dose weighed in advance, and immediate covering of the carburizer with solid charge. It suits shops where the charge mix is known and reproducible and where the melting operator can judge the amount of unmelted charge by experience.

Out-of-Furnace Carburizing in the Ladle

Adding carbon after the iron has left the furnace is possible, but the recovery is fundamentally poorer and the reasons are physical rather than metallurgical. Ladle additions rely on the material dissolving while it is in contact with the metal, and that contact time is short. The commonest practice is to place a fine graphite powder, typically in the region of 100-300 mesh, in the bottom of the preheated ladle before tapping, or to introduce it into the tapping stream so that the turbulence of the falling metal carries the particles downwards. After the tap, the ladle is stirred to give the particles a second chance to dissolve. Even with good practice, roughly half the carbon added this way is normally recovered, with the remainder lost as dust, as oxidation at the open surface, and as material that never reaches the metal at all.

Two details separate a workable ladle addition from a wasted one. The particle size has to be fine enough for rapid dissolution but coarse enough not to be blown away; very fine powders are the most reactive and the easiest to lose. The ladle has to be hot, so that the iron neither freezes around the particles nor loses its stirring energy in the first seconds of the tap. Ladle carbon correction is best treated as a safety net for a small shortfall, not as the primary carburizing route for a heat.

Powder Injection with a Carrier Gas

Injection of graphite powder through a lance submerged in the bath is the fastest way of getting carbon into a melt, because it delivers the material directly into the metal rather than onto its surface, and because the fine powder presents an enormous surface area to dissolution. Nitrogen is the preferred carrier gas because it is inert towards carbon at steelmaking temperatures, but compressed air is used in some production conditions; when air is used, the oxygen reacts with carbon to form carbon monoxide, and two useful things follow - the reaction releases heat that offsets part of the cooling effect of the injected gas, and the carbon monoxide produces a locally reducing atmosphere that protects the remaining carbon from further oxidation.

The efficiency of injection, however, is not constant. Carbon has to dissolve, and the driving force for dissolution falls as the melt approaches saturation, so the amount of carbon recovered per kilogram injected drops steadily as the heat gets richer in carbon. A useful mental check is the stoichiometric one: one tonne of iron needs only about 10 kg of carbon to rise by one percentage point if every particle dissolved. Real injection practice needs several times that figure, and the multiple grows as the target carbon rises. Injection is also a cooling operation, so a heat that has been injected heavily may need a short superheat afterwards, and the lance should be held well below the surface to avoid throwing powder out of the bath as splash.

What Decides Carbon Recovery in Practice

The variables that move recovery can be grouped into four families. Material properties include fixed carbon content, ash, volatile matter, sulfur, moisture and particle size distribution. Process conditions include bath temperature, quantity of slag, iron oxide content of that slag, and whether the bath is covered. Method of addition includes the choice of the route described above, the size of each increment, and whether stirring is applied. Finally, discipline factors decide the rest: whether the carburizer is weighed rather than shovelled, whether the material is stored dry, whether samples are taken from a properly mixed bath, and whether the weighing and sampling records are kept.

Particle size deserves a comment of its own, because it cuts both ways. Fine material dissolves quickly and is therefore attractive for ladle and injection work, but it also dusts, fluidises and oxidises more readily, and fines are the first fraction to be lost from a charge. Coarse material survives handling and stays in the furnace, but its dissolution is slow, and in a cold bath coarse lumps can still be found undissolved at tap. A sizing specification with a controlled proportion of fines, verified by sieve analysis, is worth more than an impressive headline carbon figure.

Slag basicity and iron oxide level are usually underestimated. A well-reduced, lime-fluxed slag protects carbon, while a viscous, iron-oxide-rich slag actively consumes it. Shops that melt only light and rusty scrap should expect lower recovery than shops charging clean, heavy scrap, and should budget for a slightly larger carbon addition for the same target analysis.

Specifying and Accepting a Carburizer

An incoming carburizer should be bought against a written specification, not a trade description. The specification needs a minimum fixed carbon, a maximum ash, a maximum volatile matter, a maximum sulfur, a maximum moisture, and a particle size distribution with a stated oversize and fines fraction. For graphitic grades the fixed carbon figure is usually determined on a dried basis, and the ash represents silica, alumina and other mineral residue that will end up in the slag and add to the heat's energy demand. For petroleum coke grades, volatile matter is a direct indication of how completely the coke was calcined, and it is determined by the established coke test ASTM D3175 for volatile matter in the analysis sample of coal and coke. Flake graphite grades are conventionally described by fixed carbon content, and GB/T 3518 covers the specification of flake graphite.

Sampling and particle size verification are part of acceptance just as much as the chemical certificate. GB/T 4010 for sampling ferroalloys for chemical analysis and ISO 3713 for ferroalloys sampling and sample preparation provide the systematic approach of increment sampling and sample division that avoids the classic error of testing a handful of material taken from the top of a bag. Particle size distribution of furnace and ladle additions is assessed against the relevant granularity specification, and for ferroalloy-type additions the granularity determination follows GB/T 13247. Where a carburizer is delivered together with ferrosilicon or other ferroalloys on the same order, the packing, marking and delivery documentation for those ferroalloys follows the general requirements of GB/T 3650, which is the usual reference for ferroalloy packing, storage, transport and certificates.

Sulfur deserves particular attention because its effect depends on the product. In ductile iron the magnesium used for nodularisation is consumed by sulfur, so every 0.01% of sulfur in the charge consumes magnesium and reduces the efficiency of the treatment; a low-sulfur carburizer is therefore essential, and the graphite microstructure of the finished casting is assessed visually according to graphite classification practice such as ISO 945-1 to confirm that nodularity has not been harmed. In grey iron, sulfur is better tolerated but still influences carbide stability and section sensitivity. Ash is a cost in every case: it forms additional slag, carries away iron as slag inclusions, and increases the energy needed per tonne of good casting.

Common Mistakes and How to Avoid Them

The same handful of errors appear in almost every shop that reports poor carbon recovery. Carburizer is added on top of a slag-covered bath instead of after deslagging, and the slag eats it. It is added to a bath so cold that the particles never dissolve, or so hot and thin that they oxidise on entry. It is stored in an open heap next to a wet wall and then charged with several per cent of moisture. It is dosed by volume rather than by mass, so a wetter or coarser batch delivers less carbon than the previous one. It is used in a ductile iron heat without regard to sulfur content, and the magnesium treatment then fails or needs a larger addition. And it is used as the only tool for carbon control, when a balanced charge design with a known mix of returns, pig iron and scrap would have delivered the required carbon with far less addition.

A final, frequently overlooked mistake is to confuse the certificate figure with the recoverable figure. A carburizer with 99% fixed carbon does not deliver 99 kg of dissolved carbon per 100 kg charged. Recovery depends on the route and the conditions, as the table above shows, and dosing calculations should always use the shop's own measured recovery for a given route rather than the number printed on the bag.

Storage, Handling and Safety

Carburizers are hygroscopic to a degree and should be kept in a dry, ventilated store on pallets or in sealed bags, away from standing water and from furnace areas where steam and condensate are present. The oldest stock should be issued first, and opened bags should not be left exposed for long periods in humid weather. Graphite powder in particular is very light and dusty: it migrates through the shop, coats surfaces, and because it is electrically conductive it can cause short circuits in switchgear and motors. Extraction at the point of discharge, enclosed transfer points, and regular cleaning are practical countermeasures.

Respirable dust is a health issue. Artificial graphite and calcined petroleum coke are mainly elemental carbon, but natural graphite carries a mineral fraction, and any crystalline silica in that fraction is a respiratory hazard. Local exhaust ventilation at the charging and bag-opening points, together with suitable respiratory protection, eye protection and gloves, is the normal control set. Bulk deliveries should be discharged into covered bins, and the transfer of powder into ladles should be done from a position that does not expose the operator to the plume of dust that rises when fine carbon meets hot metal.

FAQ

Q: Where is the best place to add a carburizer in an induction furnace?
On the furnace bottom, as a layer between a small pad of metallic charge and the main charge above it. The carbon is then shielded from the air and dissolves as the bath forms, and recovery commonly reaches about 90-95%.

Q: What carbon recovery should be assumed for a ladle addition?
Roughly 40-60% for a fine graphite powder added in the ladle bottom or on the tapping stream, provided the ladle is hot and the iron is stirred after tapping. Treat ladle addition as a small correction, not as the main carburizing route.

Q: Why does carbon recovery fall as the melt gets closer to saturation?
Because dissolution is driven by the difference between the actual carbon content and the saturation value. As the bath approaches saturation the driving force falls, so each additional kilogram of carbon dissolves less completely, and the excess can float out as kish or be taken up by the slag.

Q: How much carbon does it take to raise one tonne of iron by one percentage point?
Stoichiometrically about 10 kg of carbon per tonne. In practice the requirement is several times larger, because only part of the addition dissolves; the multiple depends on the addition route, bath temperature, slag condition and particle size.

Q: Is sulfur in a carburizer a problem?
It is critical in ductile iron, where sulfur consumes the magnesium added for nodularisation and can spoil graphite nodularity. Low-sulfur graphitic carburizers are therefore preferred for ductile iron. In grey iron the tolerance is wider, but sulfur still has to be counted in the charge balance.

Q: How should carburizer moisture be controlled?
Store material dry, issue oldest stock first, and specify a maximum moisture level with incoming material. Water carried into the bath dissociates, adds hydrogen, and causes violent spitting, so wet carburizer is never a candidate for the charge.

Q: Which documents should accompany a delivery of carburizer?
A certificate showing fixed carbon, ash, volatile matter, sulfur, moisture and particle size distribution, backed by a sampling and preparation procedure such as GB/T 4010 or ISO 3713. Particle size claims are verifiable by sieve analysis, and granularity of ferroalloy-type additions is assessed under GB/T 13247.