S G Iron Charge Calculation

Plan a practical spheroidal graphite iron charge mix with metallic burden split, treatment additions, and a clear visual breakdown. This calculator is ideal for foundry engineers, melting supervisors, and process planners who need a fast estimate before final chemistry correction and shop-floor verification.

Charge Mix Calculator

• Metallic burden percentages must total 100%.
• This tool gives a planning estimate, not a substitute for spectrometer-based correction.
• Actual additions depend on carbon equivalent, sulfur, magnesium recovery, and temperature practice.

Calculated Output

Chart shows the estimated metallic burden and treatment additions used to reach the required poured metal.

Expert Guide to SG Iron Charge Calculation

SG iron, also called ductile iron or nodular iron, is one of the most versatile ferrous foundry materials in modern manufacturing. Its excellent combination of strength, ductility, machinability, and castability makes it the preferred choice for pressure pipes, automotive brackets, hubs, housings, wind components, machine frames, valve bodies, and a wide range of heavy engineering parts. The phrase SG iron charge calculation refers to the practical process of determining how much pig iron, steel scrap, foundry returns, carburizer, ferrosilicon, magnesium treatment alloy, and inoculant must be charged into the furnace to produce a target quantity of acceptable liquid metal.

A good charge calculation does much more than divide raw materials by percentage. In a real foundry, the charge mix has to support the final chemistry, carbon equivalent, sulfur balance, magnesium recovery, pouring temperature, slag control, shrinkage behavior, and the desired matrix structure after solidification and heat treatment. If the charge is too heavy in low-carbon steel scrap, for example, the heat may require excessive recarburization and silicon adjustment. If it contains too much return scrap, the melt may become difficult to control because of accumulated tramp elements or inconsistent residual magnesium. For that reason, experienced foundry engineers use charge calculation as the first planning step, then refine the melt with thermal analysis, spectrometry, and process control.

What Is Included in an SG Iron Charge Calculation?

At the planning stage, most SG iron charge calculations include five major elements:

• Required poured metal: the total molten iron needed for molds, runners, gates, and risers.
• Melt loss: furnace and handling loss due to oxidation, slagging, dross, and transfer.
• Metallic burden split: pig iron, steel scrap, and foundry returns as percentages of the base metallic charge.
• Base chemistry correction additions: carburizer and ferrosilicon.
• Nodularization and inoculation additions: magnesium ferrosilicon alloy and inoculant.

Simple working formula: Metallic Charge Required = Required Poured Metal / (1 – Melt Loss %). Once the metallic charge is known, each metallic constituent is calculated by its selected percentage. Add treatment and inoculation separately according to plant practice.

Why Charge Calculation Matters in Ductile Iron Production

Unlike ordinary gray iron, SG iron depends on successful graphite nodularity. This is only achieved when sulfur is controlled, magnesium treatment is effective, inoculation is timely, and the base iron chemistry supports nodule formation. A weak charge plan can create several downstream problems:

1. Excessive carbide tendency from a poor carbon-silicon balance.
2. Low nodularity due to inadequate magnesium treatment or poor base metal quality.
3. Slag and dross defects caused by high oxidation and poor melting practice.
4. Inconsistent tensile strength and elongation because the matrix shifts from ferritic to pearlitic unintentionally.
5. Higher cost per tonne when expensive virgin materials are used unnecessarily.

That is why foundries usually balance three competing goals: predictable metallurgy, low cost, and stable production. The ideal charge is rarely the cheapest theoretical mix. Instead, it is the blend that consistently delivers target properties at a manageable process window.

Typical Raw Materials Used in SG Iron Melting

Most foundries rely on a blend of pig iron, selected steel scrap, and internal returns. Pig iron provides stable carbon and low residuals, making it valuable when high nodularity and low tramp elements are critical. Steel scrap lowers cost and helps control carbon equivalent, but it usually requires recarburization and silicon correction. Foundry returns improve economics and maintain familiar chemistry, although excessive use can concentrate residual elements and increase variation.

Charge Material	Typical Carbon (%)	Typical Silicon (%)	Typical Sulfur (%)	Practical Use in SG Iron
Pig iron	3.8 to 4.5	0.5 to 1.5	0.01 to 0.04	Improves base quality, supports carbon level, low residuals
Steel scrap	0.05 to 0.25	0.02 to 0.30	0.01 to 0.05	Economical coolant and metallic iron source, needs recarburizer
Foundry returns	3.2 to 3.9	1.8 to 2.8	0.008 to 0.03	Cost-effective, chemistry familiarity, monitor residual buildup
Carburizer	95 to 99 fixed carbon	Very low	Low sulfur preferred	Raises carbon and helps restore carbon equivalent
Ferrosilicon 75	Low	About 72 to 76	Low	Raises silicon, supports graphitization and inoculation practice

Understanding the Main Calculation Logic

Suppose a foundry needs 1,000 kg of poured SG iron and expects a 4% melt loss. The metallic base charge required is:

1,000 / 0.96 = 1,041.67 kg

If the foundry uses a metallic burden of 35% pig iron, 25% steel scrap, and 40% returns, the metallic inputs become:

• Pig iron = 1,041.67 × 0.35 = 364.58 kg
• Steel scrap = 1,041.67 × 0.25 = 260.42 kg
• Returns = 1,041.67 × 0.40 = 416.67 kg

Then additions are applied according to plant practice. If carburizer is 8 kg per tonne of metallic charge, ferrosilicon is 12 kg per tonne, magnesium alloy is 1.2% of poured metal, and inoculant is 0.3% of poured metal, then:

• Carburizer = 1,041.67 × 8 / 1000 = 8.33 kg
• Ferrosilicon = 1,041.67 × 12 / 1000 = 12.50 kg
• MgFeSi treatment alloy = 1,000 × 1.2% = 12.00 kg
• Inoculant = 1,000 × 0.3% = 3.00 kg

The total planned furnace-side material becomes the metallic burden plus these additions. In real production, however, magnesium recovery may vary with sulfur, ladle geometry, treatment temperature, and alloy particle size. Therefore, the final value should always be cross-checked against historical process data.

How Target Grade Affects the Charge Mix

Different SG iron grades require different process emphasis. Ferritic grades such as ASTM A536 60-40-18 and 65-45-12 favor cleaner base iron, lower pearlite-promoting residuals, controlled manganese, and effective inoculation. Stronger pearlitic or high-strength grades such as 80-55-06 or 100-70-03 may tolerate a different silicon and alloying strategy, and sometimes require copper, tin, or other pearlite promoters depending on local specification and section size.

ASTM A536 Grade	Minimum Tensile Strength	Minimum Yield Strength	Minimum Elongation	Common Matrix Tendency
60-40-18	60 ksi	40 ksi	18%	Predominantly ferritic
65-45-12	65 ksi	45 ksi	12%	Ferritic to ferritic-pearlitic
80-55-06	80 ksi	55 ksi	6%	Ferritic-pearlitic
100-70-03	100 ksi	70 ksi	3%	Pearlitic
120-90-02	120 ksi	90 ksi	2%	Highly pearlitic or alloyed

These standard property levels are important because they influence how conservative the charge should be. For grades needing high ductility, foundries often reduce contaminated scrap and maintain stronger control of sulfur, phosphorus, titanium, and residual carbide-forming elements.

Best Practices for Accurate SG Iron Charge Calculation

• Know your real melt loss. Use historical furnace records rather than textbook assumptions. A 2% error in melt loss can significantly alter the metallic burden on larger heats.
• Verify the chemistry of returns. Internal returns are economical, but they are not chemically neutral. Residual magnesium, silicon pickup, and trace elements can accumulate over time.
• Track carbon recovery. Carburizer efficiency depends on furnace type, particle size, bath agitation, and when the carburizer is added.
• Control sulfur before treatment. High sulfur increases magnesium consumption and can destroy nodularity.
• Separate base correction from treatment alloy. Carburizer and ferrosilicon adjust the base iron, while MgFeSi is mainly for nodularization and sulfur neutralization.
• Use thermal analysis and spectrometry. A charge plan is only the starting point. Final correction should be measured, not guessed.

Common Mistakes in Foundry Charge Planning

A frequent mistake is assuming that the same charge mix can be used for every casting family. Thick-section ductile iron, pressure-retaining castings, and high-elongation grades often need cleaner and more controlled charge materials than general-purpose engineering castings. Another mistake is treating all steel scrap as equivalent. The residual element content of scrap can vary substantially depending on the source. Coated, oily, galvanized, or mixed scrap can destabilize the process and should be controlled carefully.

Many shops also underestimate the influence of treatment practice. The same metallic charge can yield different nodularity results depending on ladle cover, reaction chamber design, superheat, and post-inoculation timing. This is why world-class foundries build a melting database that connects charge composition with final chemistry, nodule count, mechanical properties, and defect rates.

How This Calculator Should Be Used

The calculator on this page is designed as a practical first-pass estimator. It lets you define the required poured metal, apply melt loss, set the metallic burden percentages, and add common SG iron process additions. It is especially useful in these situations:

• Preparing a standard furnace charge sheet before a melting campaign.
• Comparing low-pig-iron and high-return scenarios for cost and process control.
• Training junior engineers or supervisors on the structure of an SG iron burden calculation.
• Generating a fast estimate before final chemistry correction with plant instrumentation.

For production use, you should still confirm the result with actual raw material certificates, lab-tested sulfur and carbon levels, and your plant’s measured recovery factors. The more data you feed back from real heats, the more valuable any charge calculator becomes.

Reference Sources for Foundry and Iron Data

For broader technical and industrial context, these authoritative resources are useful:

• U.S. Geological Survey: Iron and Steel Statistics and Information
• U.S. Department of Energy: Steel Industry Resources
• OSHA Foundry Industry Safety Information

Final Takeaway

SG iron charge calculation is fundamentally a controlled balance between quantity, chemistry, treatment practice, and economics. The goal is not merely to charge enough metal to fill the molds. The real objective is to create a clean, consistent base iron that can be nodularized and inoculated reliably while still meeting the customer’s mechanical property targets at a competitive cost. If you approach charge planning with measured melt loss, disciplined raw material selection, and a feedback loop from chemistry and property testing, your foundry will achieve much better consistency in both casting quality and metal yield.