Sg Iron Charge Calculation

Estimate a practical spheroidal graphite iron charge mix using metallic inputs, target chemistry, and alloy recovery assumptions. This calculator helps foundry teams plan return scrap, pig iron, steel scrap, carburizer, FeSi, nodulizer, and inoculant for a cleaner, more controlled melt.

Charge Mix Calculator

Enter your batch size, target chemistry, and raw material percentages. The calculator estimates the base metallic charge and required alloy additions.

Calculated Output

Expert Guide to SG Iron Charge Calculation

SG iron charge calculation is one of the most important control activities in a modern ductile iron foundry. SG iron, also called spheroidal graphite iron or nodular iron, depends on a controlled combination of carbon, silicon, magnesium treatment, inoculation practice, and low residual contamination. While the melt shop may discuss charge building in simple terms such as “more returns” or “add more pig,” the actual calculation is a chemistry balancing exercise with process losses layered on top. A good charge calculation does not only aim for low raw material cost. It also protects nodularity, tensile properties, elongation, machinability, and casting consistency.

In practical foundry work, the charge mix usually begins with three metallic streams: return scrap, pig iron, and steel scrap. Return scrap contributes economy and process familiarity because it comes from the same casting family. Pig iron helps maintain predictable carbon and low tramp elements. Steel scrap provides dilution of residuals and can make chemistry adjustment easier, but excessive steel requires larger carburizer additions and may increase melting energy or reaction intensity. Once the metallic fraction is established, the metallurgist or melt supervisor calculates the chemistry gap between the base charge and the target composition. That gap is then closed using recarburizer, ferrosilicon, and treatment or inoculation alloys.

What the SG Iron Charge Calculation Actually Solves

At its core, charge calculation answers five questions:

1. How much return scrap, pig iron, and steel scrap should be charged for the target melt size?
2. What carbon and silicon are already present in that metallic burden?
3. How much carbon must be added through carburizer to meet the target carbon level?
4. How much silicon must be added through FeSi, nodulizer, or inoculant to reach target silicon after expected recovery?
5. How much treatment alloy is needed to produce nodular graphite and stable microstructure?

These questions matter because SG iron is less forgiving than gray iron. In gray iron, free graphite flake formation can tolerate a broader process window. In SG iron, you are trying to create spheroidal graphite through magnesium treatment and controlled post inoculation. Sulfur, oxygen, residuals, and chemistry drift all compete against that goal. A weak charge practice can push a foundry into magnesium fade, chill tendency, low elongation, carbides, or dross defects.

Why Carbon and Silicon Matter So Much

Carbon and silicon are the two first numbers most foundry teams review when building an SG iron charge. Carbon promotes graphitization and influences the carbon equivalent. Silicon strongly affects graphitization, ferrite formation, shrinkage tendency, and matrix balance. Too little carbon can increase chill risk and force excessive recarburizer addition late in the process. Too much carbon may raise flotation tendencies or alter feeding behavior. Too little silicon may reduce graphitization support; too much silicon can push hardness downward in ferritic grades or create embrittlement concerns in some high silicon ductile iron systems.

Because carbon and silicon are measured in percent but purchased in kilograms, charge calculation converts chemistry targets into absolute mass. For example, a 1000 kg melt at 3.70% carbon contains 37.0 kg of carbon. If the metallic charge only provides 30.0 kg of carbon, the foundry must add enough recarburizer to supply the missing 7.0 kg, corrected for recovery. If the recarburizer is 98% fixed carbon and the furnace recovery is 85%, each kilogram added does not contribute a full kilogram to the final melt. The same mass balance logic applies to silicon additions through FeSi 75 or silicon-bearing treatment alloys.

A professional charge sheet should never ignore recovery. Alloy additions calculated without recovery assumptions almost always understate actual consumption and create heat-to-heat drift.

Typical Charge Material Roles in SG Iron

• Return scrap: Economical and chemically familiar, but quality depends on segregation control and internal scrap discipline.
• Pig iron: Adds stable chemistry, usually higher carbon, and lower residual elements than mixed steel scrap.
• Steel scrap: Useful for dilution and cost balance, but demands more recarburization and stronger chemistry control.
• Carburizer: Raises carbon to target, often through petroleum coke, synthetic graphite, or similar recarburizing products.
• FeSi 75: Corrects silicon and supports graphitization.
• Nodulizer: Supplies magnesium, calcium, and silicon to transform flake graphite tendency into nodular graphite.
• Inoculant: Improves nucleation, reduces chill tendency, and stabilizes graphite count.

Comparison Table: Typical Base Charge Material Chemistry

Material	Typical Carbon %	Typical Silicon %	General Foundry Use	Charge Calculation Impact
Internal SG return scrap	3.30 to 3.70	2.00 to 2.60	Primary low cost recycled metallic	Reduces fresh alloy demand if chemistry is tightly controlled
Foundry pig iron	3.80 to 4.40	0.80 to 2.00	Stable chemistry and low residual support	Raises base carbon and improves predictability
Low residual steel scrap	0.05 to 0.25	0.01 to 0.08	Dilution of residuals and matrix control	Lowers base carbon and silicon, increasing additive need
FeSi 75	0	Approximately 75	Silicon correction and inoculation support	Primary silicon balancing tool in charge sheets
Recarburizer	95 to 99 fixed carbon	Negligible	Carbon adjustment	Recovery sensitive and timing sensitive

The numeric ranges above are representative values used in many foundry planning models. Actual purchasing specifications vary by producer, furnace practice, and grade requirements. The key lesson is not the exact number alone, but the direction of its effect. Pig iron tends to raise base carbon. Steel lowers base carbon and pushes recarburizer demand upward. High internal return rates may be economical, but they demand excellent segregation and analysis discipline. If returns from mixed grades enter the wrong charge, manganese, copper, chromium, or phosphorus can drift quickly.

How to Build a Reliable SG Iron Charge Sheet

A reliable charge sheet starts with actual data, not assumptions copied from last month’s logbook. The best practice is to use recent spectrometer results for returns and maintain purchase certificates or internal average values for pig and steel streams. Then use a standard mass balance approach:

1. Set the target melt weight in kilograms or pounds.
2. Allocate metallic percentages among return scrap, pig iron, and steel scrap.
3. Multiply each metallic weight by its carbon and silicon percentage.
4. Add all carbon contributions to obtain base carbon mass.
5. Add all silicon contributions to obtain base silicon mass.
6. Calculate target carbon mass and target silicon mass from the desired final chemistry.
7. Subtract the base values from the target values.
8. Correct the gap using recovery-adjusted carburizer and FeSi additions.
9. Add treatment alloy and inoculant according to sulfur level, treatment practice, section size, and fading behavior.

For many shops, the biggest hidden error is overconfidence in return chemistry. A foundry may assume returns always match the target grade. In reality, returns can shift if they include heavy section castings, runner systems from another family, or oxidized surface losses from repeated remelting. This is why some melt supervisors cap return percentages for critical jobs or maintain separate bins for ferritic and pearlitic SG iron returns.

Comparison Table: Typical Mechanical Property Targets for Common Ductile Iron Families

Common Ductile Iron Family	Tensile Strength MPa	Yield Strength MPa	Elongation %	Charge Strategy Implication
Ferritic ductile iron 60-40-18 class equivalent	Approximately 414 minimum	Approximately 276 minimum	Approximately 18 minimum	Favors lower residuals, cleaner charge, stable inoculation, and moderate silicon
Pearlitic ductile iron 80-55-06 class equivalent	Approximately 552 minimum	Approximately 379 minimum	Approximately 6 minimum	Often uses matrix-promoting alloy control and tighter residual management
Higher strength 100-70-03 class equivalent	Approximately 689 minimum	Approximately 483 minimum	Approximately 3 minimum	Requires very disciplined chemistry and microstructure control

These property levels demonstrate why charge calculation is not simply an accounting task. The metallic burden and additive practice shape the chemistry foundation that later influences matrix structure, graphite form, and final mechanical performance. In ferritic grades, excess residuals or poor treatment can easily reduce elongation. In pearlitic or higher strength grades, the wrong charge mix can create inconsistent hardness or machinability. A charge calculation that looks cheap on paper may become expensive if it drives scrap, rework, or customer claims.

Recovery Factors and Why They Change

Recovery is not fixed across all operations. Carburizer recovery may change with furnace type, particle size, addition timing, bath turbulence, temperature, and holding time. FeSi recovery can vary with slag carryover, melt superheat, and addition point. Magnesium treatment efficiency also depends heavily on sulfur content in the base iron. If sulfur is high, more magnesium is consumed in desulfurization before it can shape graphite. This is why treatment alloy percentages must be tied to actual base iron conditions rather than copied from a standard recipe.

Many foundries start with rule-of-thumb recoveries such as 80% to 90% for carburizer and 85% to 95% for silicon additions in induction melting. That is useful for early planning, but high control shops refine these assumptions with historical data. If your average chemistry after correction consistently lands below target, either your recovery factors are too optimistic or your raw material chemistry assumptions are outdated.

Common Errors in SG Iron Charge Calculation

• Using nominal chemistry values instead of recent actual analysis.
• Allowing mixed returns from multiple grades into one charge stream.
• Ignoring residual element buildup from scrap recycling loops.
• Calculating chemistry to tapping but forgetting treatment and inoculation silicon pickup.
• Applying the same nodulizer rate regardless of sulfur level or treatment method.
• Assuming all steel scrap is low residual and compositionally neutral.
• Forgetting that alloy additions change total melt mass slightly.

How This Calculator Should Be Used in Practice

The calculator on this page is designed as a practical planning tool. It estimates a base metallic charge from return scrap, pig iron, and steel scrap percentages. Then it computes the carbon and silicon gap to target chemistry and converts those gaps into carburizer and FeSi additions using recovery assumptions. Finally, it estimates nodulizer and inoculant based on percentage of melt. This mirrors the logic used in many production charge sheets.

Still, real foundry use should always include final adjustment using spectrometer readings, thermal analysis, treatment response, and process-specific observations. For example, if your treatment alloy contributes significant silicon pickup, the target pre-treatment silicon should be lower than the final desired silicon. Likewise, if your foundry experiences notable magnesium fade during holding, your treatment practice may need stronger timing control rather than simply more alloy.

Best Practices for Better Charge Control

1. Maintain separate return streams by grade and casting family.
2. Use low residual, low sulfur charge materials for critical SG iron jobs.
3. Verify pig iron, steel scrap, and return chemistry regularly.
4. Track actual recovery by comparing planned versus measured final chemistry.
5. Control slag and oxidation to improve alloy efficiency.
6. Review treatment and inoculation timing, not only chemistry totals.
7. Keep a rolling database of heat records to refine charge assumptions.

Useful Authoritative References

For broader metallurgy, process control, and industrial safety context, see these authoritative resources:

• National Institute of Standards and Technology (NIST)
• Occupational Safety and Health Administration (OSHA)
• Michigan Technological University Materials Science and Engineering

In summary, SG iron charge calculation is a controlled mass balance problem connected directly to graphite shape, matrix structure, casting soundness, and final properties. The better your input data, segregation discipline, and recovery tracking, the more consistent your ductile iron production becomes. A premium foundry operation treats the charge sheet as a live metallurgical control document, not a rough estimate. That is exactly why a calculator like this is useful: it provides a structured starting point for precise, repeatable decision making.

Disclaimer: This calculator is intended for process planning and educational use. Final foundry decisions should be confirmed by laboratory analysis, thermal analysis, treatment trials, and your internal metallurgical standards.