Carbon equivalent (CE) is an important process control parameter in gray iron and ductile iron production.

It helps foundries evaluate how the chemical composition of molten iron may affect solidification behavior, graphite formation, chill tendency, and casting performance.

But how is carbon equivalent actually measured?

In practice, foundries generally use two approaches:

1. Calculate CE from chemical composition
2. Estimate CE through thermal analysis

These methods provide different types of information and are often used together.

This guide explains how carbon equivalent is measured, how the common CE formula works, and how foundries can choose the appropriate testing method.

What Is Carbon Equivalent?

Carbon equivalent is a calculated value used to represent the combined effect of carbon and selected alloying elements on the solidification behavior of cast iron.

A commonly used formula is:

CE = C + (Si + P) / 3

Where:

• C = carbon content
• Si = silicon content
• P = phosphorus content

For example, if a cast iron melt contains:

• C = 3.40%
• Si = 2.10%
• P = 0.06%

Then:

CE = 3.40 + (2.10 + 0.06) / 3

CE ≈ 4.12%

However, this is a commonly used simplified relationship rather than a universal formula for every cast iron grade and process.

Different foundries and analytical systems may use different calculation models.

Therefore, the CE value should always be interpreted according to the material specification and process being controlled.

Method 1: Calculate CE From Chemical Composition

The most straightforward approach is to determine the chemical composition of the molten iron and then calculate CE.

A typical process is:

Molten Iron
     ↓
Take Sample
     ↓
Chemical Composition Analysis
     ↓
Obtain C, Si, P and Other Elements
     ↓
Apply Appropriate CE Formula
     ↓
Calculate Carbon Equivalent

The chemical composition can be determined using methods such as optical emission spectroscopy or other suitable analytical techniques.

Once the carbon, silicon, phosphorus, and other required elements are known, the appropriate CE formula can be applied.

Why Chemical Analysis Is Useful

Chemical analysis provides direct information about elemental composition.

This makes it useful for:

• Furnace control
• Alloy adjustment
• Material verification
• Grade control
• Laboratory testing
• Quality documentation

For example, if carbon content is too low, a foundry may need to make a carbon adjustment.

If silicon is outside the target range, the melt may require further treatment.

Chemical analysis therefore provides an important foundation for CE control.

Method 2: Thermal Analysis

The second common approach is thermal analysis.

Instead of determining CE only from elemental composition, thermal analysis evaluates how a molten iron sample cools and solidifies.

A small molten metal sample is tested under controlled conditions.

The temperature is recorded as the sample cools, producing a cooling curve.

Characteristic thermal events in the curve are then analyzed by the thermal analysis instrument.

Depending on the instrument, cup design, calibration, and calculation model, the system can provide an estimated CE and other metallurgical parameters.

The basic concept is:

Molten Iron
     ↓
Thermal Analysis Test
     ↓
Cooling Curve
     ↓
Characteristic Temperatures
     ↓
Calculation Model
     ↓
Estimated CE

The important distinction is that thermal analysis evaluates solidification behavior, not simply elemental composition.

Chemical Analysis vs. Thermal Analysis

The two methods answer slightly different questions.

Chemical Analysis	Thermal Analysis
What elements are present?	How does the melt solidify?
Measures composition	Evaluates thermal behavior
Can determine C, Si, P and other elements	Can estimate CE and solidification parameters
Useful for chemistry adjustment	Useful for rapid process monitoring
Often requires sample preparation	Designed for fast foundry-floor testing

Neither method should automatically be considered a replacement for the other.

For many foundries, the most useful approach is to combine them.

How Thermal Analysis Estimates CE

This is an important point.

A thermal analysis system does not simply “read” the CE value from the molten iron.

Instead, it measures temperature as the sample cools and then analyzes characteristic features of the cooling curve.

The relationship between composition and solidification temperature allows the system to estimate metallurgical parameters.

Depending on the system, the analysis may consider parameters such as:

• Liquidus temperature
• Eutectic temperature
• Undercooling
• Recalescence
• Cooling curve characteristics

The software then applies its calculation model to estimate CE and potentially other values.

This is why the thermal analysis cup, thermocouple, instrument, calibration, and calculation model all matter.

What Is the Role of the Thermal Analysis Cup?

The thermal analysis cup provides the controlled environment in which the molten metal sample cools.

It is not simply a container.

Its:

• Geometry
• Material
• Wall thickness
• Thermocouple position
• Internal configuration

can affect the cooling behavior of the sample.

For this reason, thermal analysis cups need to be compatible with the instrument and testing method being used.

A different cup design can produce a different cooling response.

This is one reason foundries should use cups designed for their specific thermal analysis system.

For a detailed explanation of cup construction and operation, see:

How Does a Thermal Analysis Cup Work in Cast Iron Analysis?

What Information Is Needed to Calculate CE?

If CE is calculated from chemical composition, the required elements depend on the selected formula.

For the commonly used:

CE = C + (Si + P) / 3

you need:

• Carbon percentage
• Silicon percentage
• Phosphorus percentage

For example:

Element	Content
Carbon	3.50%
Silicon	2.20%
Phosphorus	0.05%

The calculation becomes:

CE = 3.50 + (2.20 + 0.05) / 3

CE ≈ 4.25%

This value can then be compared with the target range established for the particular casting application.

Why CE Measurement Should Be Done During Production

A CE value measured once does not necessarily represent the condition of the melt throughout the entire production process.

Molten iron can change because of:

• Charge material variation
• Carbon additions
• Silicon additions
• Alloy additions
• Treatment
• Inoculation
• Holding time
• Temperature changes
• Return scrap variation

Therefore, many foundries monitor the melt at appropriate points in the production process rather than relying on a single measurement.

The exact sampling frequency depends on the foundry’s process and quality requirements.

When Should a Foundry Measure CE?

There is no universal testing schedule for every foundry.

However, CE measurement may be particularly useful:

After Melting

To evaluate the initial melt condition.

After Chemical Adjustment

To verify that carbon or silicon additions produced the intended result.

After Treatment

To monitor changes associated with the treatment process.

Before Pouring

To confirm that the melt is within the required process range.

When Casting Quality Changes

Unexpected changes in hardness, microstructure, fluidity, or defect rates may justify additional melt analysis.

The objective is not simply to collect more numbers.

The objective is to obtain information early enough to make useful production decisions.

How Accurate Is Thermal Analysis for CE?

Thermal analysis can provide very useful process information, but its accuracy depends on the complete measurement system.

Important factors include:

• Cup quality
• Thermocouple accuracy
• Instrument calibration
• Sample representativeness
• Pouring procedure
• Sample temperature
• Cooling conditions
• Calculation model

Consistency is particularly important.

If the sampling method changes from test to test, the cooling curves may also change.

Therefore, foundries should establish a standardized sampling procedure.

Common Causes of Inconsistent CE Results

If thermal analysis results vary unexpectedly, the problem may not necessarily be the molten iron itself.

Check the following factors.

1. Inconsistent Sampling

Different sampling locations or techniques can produce different samples.

2. Slag Contamination

Excessive slag or foreign material in the sample can affect the test.

3. Different Cup Specifications

Using cups with different designs or specifications may affect cooling behavior.

4. Thermocouple Problems

Poor thermocouple positioning, damaged wires, or unstable connections can affect temperature measurement.

5. Instrument Calibration

The analyzer should be maintained and calibrated according to the equipment manufacturer’s requirements.

6. Different Pouring Procedures

Differences in filling the cup, pouring speed, or sample handling can introduce variation.

For this reason, standardized sampling is just as important as the analytical equipment itself.

Can a Spectrometer Measure CE Directly?

A spectrometer primarily measures elemental composition.

It does not normally provide CE as a direct physical measurement.

Instead, the foundry obtains the required elemental concentrations and then calculates CE using the appropriate formula.

For example:

Spectrometer
     ↓
C + Si + P + Other Elements
     ↓
CE Formula
     ↓
Calculated CE

This provides a chemistry-based CE value.

Thermal analysis approaches CE from a different direction:

Thermal Analysis
     ↓
Cooling Curve
     ↓
Characteristic Temperatures
     ↓
Calculation Model
     ↓
Estimated CE

Comparing these two results can be useful when establishing and maintaining process control.

Which Method Should a Foundry Use?

The answer depends on the purpose of the measurement.

Use Chemical Analysis When You Need:

• Detailed elemental composition
• Alloy adjustment
• Grade verification
• Laboratory analysis
• Documentation of chemical composition

Use Thermal Analysis When You Need:

• Rapid process feedback
• Solidification information
• Quick CE estimation
• Production-floor monitoring
• Comparison between heats

Use Both When You Need:

• Chemical composition control
• Solidification behavior monitoring
• More complete melt characterization

For many production foundries, these methods complement rather than replace each other.

CE Measurement Is Only One Part of Melt Control

It is important not to treat CE as the only parameter determining casting quality.

A suitable CE value does not automatically guarantee a defect-free casting.

Other important variables include:

• Carbon and silicon levels
• Magnesium in ductile iron
• Inoculation
• Melt temperature
• Cooling rate
• Mold design
• Gating system
• Feeding system
• Casting geometry

For example, two castings made from molten iron with the same CE may behave differently if their section thicknesses are significantly different.

Therefore, CE should be used as part of a broader process control system.

Practical CE Measurement Workflow

A practical foundry workflow can be summarized as:

1. Melt Charge Materials
          ↓
2. Take Representative Sample
          ↓
3. Chemical Analysis
          ↓
4. Calculate / Verify CE
          ↓
5. Thermal Analysis
          ↓
6. Compare Melt Behavior
          ↓
7. Adjust Composition if Necessary
          ↓
8. Treatment / Inoculation
          ↓
9. Final Process Check
          ↓
10. Pouring

The exact sequence varies according to the foundry’s production process.

The key principle is to combine composition data with actual solidification behavior.

Frequently Asked Questions

How do you measure carbon equivalent in cast iron?

Carbon equivalent can be calculated from chemical composition using an appropriate formula, or estimated through thermal analysis by analyzing the cooling curve of a molten iron sample.

What is the formula for carbon equivalent?

A commonly used formula is:

CE = C + (Si + P) / 3

The appropriate formula may vary depending on the cast iron grade and calculation method.

Can thermal analysis measure carbon equivalent?

Thermal analysis systems can estimate CE from the cooling behavior of molten iron. The exact calculation depends on the instrument, thermal analysis cup, calibration, and software model.

Is a spectrometer better than thermal analysis for CE?

They serve different purposes. A spectrometer provides elemental composition, while thermal analysis evaluates solidification behavior and can provide rapid CE-related process information.

How often should CE be measured?

The required frequency depends on the foundry’s process stability, production volume, material variation, and quality requirements. Measurements may be performed at important process stages such as after melting, after adjustment, or before pouring.

Does CE alone determine casting quality?

No. CE is an important process indicator, but casting quality also depends on composition, inoculation, treatment, temperature, cooling rate, gating, feeding, mold design, and casting geometry.

Conclusion

Carbon equivalent can be measured in two main ways:

Chemical analysis → Calculate CE from elemental composition

and

Thermal analysis → Estimate CE from molten iron solidification behavior

Each method provides different information.

Chemical analysis is valuable when the foundry needs detailed elemental composition. Thermal analysis is particularly useful when rapid information about melt condition and solidification behavior is required during production.

For reliable CE control, the most important factors are not only the analytical instrument but also:

• Representative sampling
• Consistent testing procedures
• Suitable thermal analysis cups
• Accurate temperature measurement
• Proper instrument calibration
• Appropriate CE calculation methods

For foundries using thermal analysis, SF-Foundry supplies thermal analysis cups, thermocouple-related products, and molten metal sampling solutions designed for foundry process control.

If you are selecting a thermal analysis cup or sampling solution, provide your cast iron grade, thermal analysis instrument, cup dimensions, and testing requirements so the appropriate specification can be evaluated.