Professional Analysis of Pig Iron Melting Point

I. Material Definition and Composition System of Pig Iron

Pig iron is an iron-carbon alloy with a carbon content ranging from 2.11% to 4.3%. In addition to iron (Fe), its chemical composition mainly includes elements such as carbon (C), silicon (Si), manganese (Mn), phosphorus (P), and sulfur (S). According to the morphology of graphite and application scenarios, pig iron can be classified into:

• Steelmaking pig iron: With a silicon content < 1.75%, carbon exists in the form of cementite (Fe₃C), and the fracture surface is silver-white. It is mainly used as raw material for converter steelmaking.

• Foundry pig iron: With a silicon content of 1.25%–3.6%, carbon mostly exists in the form of graphite, and the fracture surface is gray. It is suitable for casting production.

• Ductile iron: Through spheroidization treatment, graphite presents a spherical shape, with mechanical properties superior to ordinary cast iron and an elongation rate of 2%–20%.

II. Basic Characteristics and Theoretical Data of the Melting Point of Pig Iron

(A) Melting Point Range and Eutectic Reaction

The melting point of pure iron is 1,538°C, while that of pig iron is significantly lower due to its carbon and alloying elements. Its melting point characteristics are as follows:

• Melting range: The melting point of pig iron typically ranges from 1,148°C to 1,250°C, specifically determined by carbon content and alloy composition.

• Eutectic point: When the carbon content is 4.3%, pig iron undergoes an eutectic reaction (L→γ+Fe₃C) at 1,148°C, forming ledeburite structure.

• Comparison with steel: Steel has a carbon content < 2.11%, and its melting point decreases from 1,538°C to approximately 1,300°C as carbon content increases, which is significantly higher than that of pig iron.

(B) Melting Point Data of Typical Compositions

III. Key Factors Affecting the Melting Point of Pig Iron and Their Mechanisms

(A) Dominant Role of Carbon Content

Carbon is the core element affecting the melting point of pig iron, and its effect follows an approximate linear law:

• Quantitative relationship: For every 0.1% increase in carbon content, the melting point decreases by approximately 13°C.

• Mechanism analysis: Carbon forms interstitial solid solutions or cementite in iron, disrupting the regular arrangement of iron atoms and weakening metallic bond energy, thus lowering the melting point.

(B) Synergistic Effects of Alloying Elements

1. Silicon (Si):

• Effect: For every 1% increase in silicon content, the melting point increases by approximately 30°C.

• Mechanism: Si forms substitutional solid solutions with Fe, increasing lattice distortion and enhancing interatomic bonding force.

2. Manganese (Mn):

• Effect: The impact on the melting point is weak, and an increase in manganese content slightly lowers the melting point.

• Side effect: Mn reacts with S to form high-melting-point MnS (1,600°C), mitigating the hot shortness hazard of sulfur.

3. Phosphorus (P):

• Effect: For every 0.1% increase in phosphorus content, the melting point decreases by approximately 5°C.

• Risk: Phosphorus segregates at grain boundaries to form low-melting-point eutectic phases (Fe₃P-Fe), exacerbating cold brittleness.

4. Sulfur (S):

• Hazard: Sulfur forms FeS-Fe eutectic (melting point 985°C) with iron, causing workpiece cracking during hot working (hot shortness phenomenon).

• Control standard: Sulfur content in industrial pig iron is typically < 0.05%.

IV. Applications of the Melting Point of Pig Iron in Industrial Production

(A) Temperature Control in Casting Processes

1. Melting temperature setting:

• The melting temperature of gray cast iron is generally 1,350–1,450°C (150–250°C higher than the melting point) to ensure molten iron fluidity.

• Due to the need for spheroidization treatment, the melting temperature of ductile iron should be increased to 1,400–1,500°C to prevent premature oxidation of spheroidizing agents.

2. Typical application cases:

• Machine tool bed casting: Using gray cast iron with low melting point and good fluidity, sand casting process is adopted, and the pouring temperature is controlled at 1,380–1,420°C.

• Automobile brake discs: Vermicular graphite cast iron is selected, with a melting point of about 1,200°C and a pouring temperature of 1,350°C to ensure wear resistance and heat dissipation.

(B) Thermodynamic Basis of Steelmaking Processes

1. Blast furnace ironmaking:

• The hearth temperature must be maintained at 1,400–1,500°C to melt pig iron (melting point 1,148–1,250°C) and separate it from slag.

• The melting point of slag is controlled at 1,300–1,400°C, and slag-iron separation is achieved by adjusting the CaO/SiO₂ ratio (basicity).

2. Converter steelmaking:

• The blowing temperature must reach 1,600–1,650°C to oxidize carbon in pig iron (C+O₂→CO), reducing the carbon content to below 2.11%.

• The end-point temperature is monitored in real time by thermocouples, with an error control of ±10°C.

  
  

V. Experimental Measurement Methods and Standards for the Melting Point of Pig Iron

(A) Thermal Analysis (GB/T 4336-2016)

1. Principle: Determine the phase transition temperature by recording the temperature inflection point in the cooling curve of the sample.

2. Equipment: Resistance furnace (temperature control accuracy ±5°C), data acquisition system.

3. Procedure: Heat the sample to 1,600°C at 10°C/min, hold for 30 min, then cool with the furnace, and plot the temperature-time curve. The inflection point is the melting range.

(B) Differential Scanning Calorimetry (DSC, ASTM E793-19)

1. Accuracy: Can be accurate to ±1°C, capable of detecting minor thermal effects.

2. Application: DSC testing of a gray cast iron (C 3.2%, Si 1.8%) shows that its initial melting temperature is 1,182°C, complete melting temperature is 1,235°C, and the melting enthalpy is 210 J/g.

VI. Research Frontiers and Technological Developments of the Melting Point of Pig Iron

(A) Development of New Materials

1. Low-temperature casting pig iron: By adding elements such as Bi and Sn, the melting point is reduced to below 1,100°C for microelectronics packaging substrates.

2. High-temperature wear-resistant cast iron: Adding Cr (12%–15%) and Ni (3%–5%) increases the melting point to 1,300°C, suitable for cement kiln linings.

(B) Numerical Simulation Technology

Using Thermo-Calc software to establish a Fe-C-Si-Mn-P-S quinary phase diagram model, the melting point and solidification path of different compositions can be predicted. For example:

• The simulated melting point of a pig iron composition (C 3.5%, Si 2.0%, Mn 0.8%, P 0.1%, S 0.03%) is 1,205°C, with an error < 0.3% compared to the experimental value (1,208°C).

VII. Authoritative Reference Resources

1. Metallurgy of Iron and Steel (Zhu Yingxiong, 2018): Systematically expounds the thermodynamic relationship between pig iron composition and melting point.

2. American Society for Testing and Materials (ASTM) standard: ASTM A48-18 Standard Specification for Gray Iron Castings.

3. Wikipedia "Pig Iron" entry: https://en.wikipedia.org/wiki/Pig_iron (accessed July 2, 2025).

As a core parameter of material thermodynamic properties, the precise control of the melting point of pig iron runs through the entire process from ore smelting to end products. Achieving synergistic matching between the melting point and properties through composition design and process optimization remains a research focus in the field of iron and steel materials.