What a Carburizer Does in the Foundry Charge
A carburizer, also called a recarburizer or carbon raiser, is the carbon-bearing additive charged into an electric induction furnace, an electric arc furnace, or a cupola to lift the carbon level of the molten bath. In iron and steel foundries the additive replaces part of the pig iron in the charge mix with steel scrap, which is normally the cheapest and most available ferrous raw material. Because scrap steel carries only about 0.05-0.30 % carbon, the balance must be supplied deliberately, and the carburizer is that source.
Two goals are served at once. First, the target carbon content is reached within specification, which governs graphite structure, fluidity, shrinkage behaviour and machinability of the casting. Second, the cost per tonne of liquid metal falls, because the charge can be built around a high proportion of scrap instead of premium virgin iron units.
Carburizer Types and Typical Specifications
Commercial grades differ mainly in fixed carbon, sulphur, ash, moisture and crystal form. Graphitized materials dissolve quickly with high recovery, calcined petroleum coke is more economical but slower to dissolve, and natural flake graphite dissolves fastest at the highest cost per tonne of carbon.
| Type | Fixed carbon | Sulphur | Ash | Typical size | Behaviour in the bath |
|---|---|---|---|---|---|
| Graphitized petroleum coke | 98.5-99.5 % | max 0.05 % | max 0.5 % | 0.2-5 mm | Fast dissolution, carbon recovery about 85-95 % |
| Calcined petroleum coke | 97-99 % | max 0.5 % | max 1.0 % | 0.5-8 mm | Moderate dissolution, recovery about 70-85 % |
| Graphite electrode scrap, crushed | 97-99 % | max 0.1 % | max 1.0 % | 1-10 mm | Very fast dissolution, recovery about 85-95 % |
| Natural flake graphite | 90-97 % | max 0.1 % | 2-10 % | 0.1-2 mm | Fastest dissolution, highest unit cost |
Particle size matters as much as chemistry. Very fine particles oxidise at the bath surface before they sink, while very coarse lumps float and dissolve too slowly to be useful in a short heat. A practical rule for induction furnaces is 0.5-3 mm for small and medium units and 2-8 mm for large units, with the coarse fraction charged early and the fine fraction reserved for final trimming.
How Much Carburizer to Add
The addition is calculated from the carbon deficit of the charge, the mass of the bath and the expected recovery of the selected grade.
Carbon deficit = target carbon minus the carbon already present in scrap, pig iron and alloy additions.
Addition mass = bath mass x carbon deficit / carbon recovery.
Recovery falls when the bath is oxidised, when the slag is heavy, when the additive is thrown onto a bare molten surface late in the heat, or when the sizing is too fine.
Using typical numbers, a 10 t induction furnace melting scrap at 0.15 % C and requiring 3.4 % C in the final iron, with a graphitized carburizer at 99 % fixed carbon and 90 % recovery, needs roughly 365 kg of additive. In practice the operator charges 80-90 % of that figure with the scrap and trims with a small final addition after a bath sample has been taken.
Charging and Addition Practice
Four habits separate a repeatable carbon result from an erratic one.
Layering. Place part of the carburizer in the bottom of the furnace, cover it with light scrap, then add the remainder with the heavier charge. Buried carbon dissolves into the growing pool and is shielded from the atmosphere.
Early addition. Add most of the carbon while the bath is still forming. Late additions to a fully molten, oxidised bath lose carbon to the slag and the flame.
Keep carbon out of the slag. Material that reports to the slag layer either burns or leaves with the slag; it never reaches the metal.
Bath temperature. 1450-1520 C gives rapid dissolution of graphitized grades. Above about 1550 C, carbon loss and refractory wear increase without any gain in recovery.
Carbon Recovery and Process Control
Recovery is the ratio of carbon retained in the bath to carbon added. Track it heat by heat: weigh the additive, take a bath sample after a fixed stirring time, and compare the analysed carbon with the calculated value. A falling trend points to a heavier slag, a wet or rusty charge, an oversize particle fraction, or a change in the carbon content of the incoming scrap.
Analytical methods for incoming carburizer should be fixed in the purchase specification. Moisture, ash, volatile matter and sulphur are normally determined by standard coke test procedures such as ASTM D3173, ASTM D3174, ASTM D3175 and ASTM D4239, with total carbon by combustion infrared detection. A moisture limit of 0.5 % is typical, because wet additive causes steam, spatter and a measurable recovery loss.
Grade selection also depends on the iron being produced. Ductile iron is sensitive to sulphur, which consumes nodulizing agents and degrades nodularity, so a low-sulphur graphitized grade is preferred and the sulphur balance of the whole charge is controlled rather than the carburizer alone. Gray iron and carbon steel castings tolerate slightly higher sulphur and can use a more economical calcined grade.
Frequently Asked Questions
Q: What is the difference between a carburizer and petroleum coke?
Petroleum coke is the raw refinery residue. A carburizer is a graded product made from it, usually by calcining or graphitizing, then screening to a controlled particle size and checking for sulphur, ash and moisture against a specification.
Q: How much of the pig iron can be replaced by scrap?
With correct carburizer practice, many foundries run charges of 70-90 % steel scrap plus a small amount of pig iron for nucleation. The limit is set by the residual elements that scrap brings in, such as copper, tin, chromium and lead, not by carbon.
Q: Why does recovery drop at the end of a heat?
Because the bath is hotter and more oxidised, the slag is thicker, and there is less scrap left to bury the additive. Late additions therefore recover less carbon, which is why trimming should be small and followed by stirring and re-sampling.
Q: What fixed carbon content should be specified?
Graphitized grades at 98.5 % fixed carbon and above are the usual choice for ductile iron and for heats where recovery must be predictable. Calcined grades at 97-99 % fixed carbon are adequate for gray iron and general steel castings.
Q: Does particle size affect the cost of carbon?
Yes, indirectly. Fine sizing dissolves faster but is lost to oxidation more easily, so recovery drops and more material is consumed. Selecting the size range that matches furnace size and charge practice usually saves more money than buying the cheapest grade available.
Q: Can a carburizer be used to correct a low-carbon heat in progress?
It can, provided the melt is still above the liquidus and there is enough time to stir and re-sample. Correction at the end of a heat is limited by the small time window, so most operators aim to land inside the specification range with the main buried addition.
