Why Carbon Control Matters in Casting
Carbon content controls the strength, hardness, wear resistance and machining behaviour of cast iron and cast steel. When the charge contains a large proportion of steel scrap, or when the scrap mix varies from heat to heat, the natural carbon level of the melt often falls below the target analysis of the casting. A carburizing agent, also called a recarburizer, is added to restore carbon to specification without adding unwanted impurities.
Getting the addition right is a question of timing and placement rather than simply of quantity. Carbon is lost to the slag, to oxidation at the bath surface and to dust carry-over, so an addition that is made at the wrong moment or in the wrong form can show a recovery far below expectation. The three methods described below cover the usual furnace types and charge conditions in foundry practice.
Types of Carburizing Agent and Typical Specifications
| Type | Fixed carbon | Sulfur, max | Ash, max | Notes |
|---|---|---|---|---|
| Synthetic graphite recarburizer | 98% – 99% | 0.05% | 0.5% | fastest dissolution, highest recovery, best for strict sulfur limits |
| Graphite petroleum coke | 97% – 99% | 0.1% | 1.0% | good balance of cost and dissolution speed |
| Calcined petroleum coke | 95% – 98% | 0.3% – 0.5% | 1.0% – 2.0% | economical, slower to dissolve, higher sulfur |
| Natural graphite based | 90% – 95% | 0.1% | 3.0% – 5.0% | used where ash and nitrogen are not critical |
Particle size is usually supplied between 0.2 mm and 5 mm. Fine powder dissolves quickly but is easily carried into the slag or lost as dust, while coarse granules sink into the melt and dissolve more slowly. Moisture should be kept low, because wet material causes splashing, hydrogen pick-up and poor recovery.
Method One: Dispersed Addition in Large Electric Furnaces
For furnaces above 5 t working with a single and stable raw material, the dispersed addition method gives the best results. The carburizing agent is charged together with the metal charge in each batch of material, placed in the middle and lower parts of the furnace so that the carbon is protected by scrap from the start of melting.
Calculate the carburizer quantity from the target carbon content and the proportion of the charge in each batch.
Build the charge as a layer of metal scrap followed by a layer of carburizing agent, repeated through the batch.
Place the layered charge in the middle and lower zone of the furnace, not against the upper walls.
Do not break up the carburizing agent during melting; keeping it in coherent lumps prevents it from being wrapped in waste slag and carried away.
With this practice the carbon recovery normally reaches 90% to 95%, because the carbon is introduced into the melt while the metal is still being formed and is never exposed on an open bath surface.
Method Two: Centralized Addition in Induction Furnaces
For a 3 t medium frequency induction furnace charged with a single and stable raw material, the centralized addition method is recommended. The required quantity of carburizing agent is added in one action onto the surface of the molten pool, either as soon as the first melt-down is complete or while a small amount of molten iron remains in the furnace, and the metal charge is then added immediately.
Weigh the full quantity of carburizer before starting the addition so that the dose is not measured in steps.
Add it onto the molten pool in a single pass, spreading it over the surface rather than in a heap.
Press the material into the molten iron promptly so that the particles are wetted by the melt and cannot float on the surface.
Follow with the metal charge without delay, so that the bath surface is covered and oxidation losses stay low.
Under these conditions the absorption rate is above 90%. Delay between adding the carburizer and covering the bath is the most common reason for a lower figure.
Method Three: Fine Tuning After Melt-Down in Small Furnaces
Small medium frequency furnaces charged with high-carbon materials such as pig iron do not need a large carbon addition, and a fine-tuning approach is more appropriate. After the steel or molten iron has melted completely, the carbon level is checked and the small correction is added to the surface of the bath.
Melt down fully and take a carbon reading before deciding the correction.
Add the carburizer to the bath surface in a thin, even layer.
Use the natural vortex of the induction furnace, or manual stirring where the vortex is weak, so that the material dissolves and is absorbed.
Re-check carbon after a short holding period before tapping, and record the actual recovery for the next heat.
With this procedure the carbon absorption rate is around 93%. Because the correction is small, weighing accuracy and sampling discipline determine whether the final analysis lands inside the specification.
Beyond the three methods, a small number of recurring faults account for most of the carbon that is lost in practice:
Low recovery: usually caused by fine or damp material, a thin slag layer that lets carbon oxidise, or an addition made onto an exposed bath. Use dry, correctly sized material and cover the bath quickly.
Sulfur pick-up: a high-sulfur petroleum coke raises sulfur in the final analysis. Select a graphite grade with a declared sulfur limit when the casting specification is tight.
Carbon not dissolving: coarse particles added to a cold or sluggish bath can remain on the surface. Improve stirring or reduce particle size.
Slag wrapped carburizer: breaking up lumps during melting exposes them to the slag. Keep the material coherent and place it in the lower part of the charge.
Inconsistent analysis between heats: variable scrap composition, not the carburizer, is the usual cause. Track charge composition alongside recovery data.
Frequently Asked Questions
Q: Which addition method gives the highest carbon recovery?
The dispersed method used in furnaces above 5 t with a stable charge normally gives the highest recovery, in the range of 90% to 95%, because the carburizer is protected inside the charge throughout melting. The centralized method is close behind at above 90% when the material is pressed into the bath promptly.
Q: Can I simply add more carburizer to compensate for losses?
That is a poor substitute for good practice. Over-addition wastes material, increases sulfur and ash input and can push carbon above specification, which is difficult to correct without diluting the heat. Improve the addition technique first and use actual recovery data in the charge calculation.
Q: Why does my carburizer end up in the slag?
The usual causes are breaking the material up during melting, adding it onto an exposed bath surface or adding very fine powder that floats. Keeping the material in coherent pieces and covering the bath quickly prevents most of the loss.
Q: Does particle size affect the choice of method?
Yes. Coarse granules suit the dispersed and centralized methods because they sink and dissolve steadily. Fine powder is better for the fine-tuning method, where a small correction is added to an already molten bath with good stirring.
Q: How should the carburizer be stored?
Store it dry, under cover and away from moisture and oil contamination. Wet material causes splashing and hydrogen pick-up and its recovery falls sharply, so opened bags should be used promptly or resealed.
Q: Do the same rules apply to both cast iron and cast steel?
The principles are the same, but the operating window differs. Cast iron usually needs larger carbon corrections and tolerates coarser material, while cast steel heats call for tighter sulfur limits and more careful weighing of smaller additions.
