Iron Boiling Point and Melting Point: Thermodynamic Data

The boiling point of iron is 2,861°C (5,182°F, 3,134 K). Its melting point is 1,538°C (2,800°F, 1,811 K). Between these two temperatures, iron exists as a liquid metal — a range of 1,323°C separating the solid-liquid transition from the liquid-vapor transition.

These two temperatures bracket the complete liquid range of elemental iron and define the thermodynamic limits that govern every high-temperature process involving iron: steelmaking, casting, arc welding, plasma cutting, and high-vacuum metallurgical processing. The melting point is where iron becomes workable as a liquid; the boiling point is the temperature beyond which iron atomizes into vapor — a phenomenon relevant to electric arc furnace steelmaking, plasma spray coating, and the physics of arc welding at very high current densities.

Iron Boiling Point and Melting Point: Complete Reference Values

All values from the NIST Standard Reference Database and the CRC Handbook of Chemistry and Physics:

Additional thermal properties:

• Latent heat of fusion (melting): 247 kJ/kg (13.81 kJ/mol)
• Latent heat of vaporization (boiling): 6,090 kJ/kg (340 kJ/mol)
• Heat capacity (solid, 25°C): 449 J/(kg·K) (25.1 J/(mol·K))
• Heat capacity (liquid): 820 J/(kg·K) (45.8 J/(mol·K))

Source: NIST Standard Reference Database, WebBook; CRC Handbook of Chemistry and Physics, 104th Edition.

The latent heat ratio — vaporization (6,090 kJ/kg) vs. fusion (247 kJ/kg) — shows that boiling iron requires approximately 24.7 times more energy than melting it. This energy relationship is fundamental to steelmaking thermodynamics: raising a ton of molten steel from 1,538°C to the boiling point and fully vaporizing it would require approximately 25× the energy needed to melt it from a solid. This is why iron vapor is limited to extreme arc and plasma conditions rather than appearing in normal foundry or steelmaking practice.

Melting Point of Iron: 1,538°C in Depth

The melting point of iron — 1,538°C (2,800°F, 1,811 K) — is the temperature at which the body-centered cubic (BCC) delta-iron lattice collapses into the disordered liquid state at standard atmospheric pressure.

Why iron melts at this specific temperature

Iron’s melting point reflects the strength of its metallic bonding. Iron has the electron configuration [Ar] 3d⁶ 4s². The 3d electrons participate in metallic bonding (through hybridized d-electron wave functions), producing stronger cohesive energy than metals with only s-electron bonding. The melting point scales with this cohesive energy: iron at 1,538°C melts much higher than sodium (98°C, weakly bonded s-metal) or aluminum (660°C, primarily s-electron bonding) but lower than tungsten (3,422°C, strong d-electron bonding) or chromium (1,907°C).

Iron’s allotropic transitions below the melting point

Iron passes through three crystal structure transitions between room temperature and the melting point:

The δ-iron phase (BCC, 1,394–1,538°C) is significant in steelmaking because it is the phase that solidifies first from the melt in low-carbon steel. The δ-iron range of approximately 144°C is why the carbon content dramatically affects whether steel solidifies through δ-iron or directly into γ-austenite, which in turn governs the risk of solidification cracking in continuous casting and welding.

Iron’s melting point and steel

Pure iron melts at 1,538°C. Every steel grade — from A36 mild steel to M2 high-speed steel — melts at a lower temperature due to alloying. Carbon has the dominant effect: each 0.1% increase in carbon content lowers the liquidus by approximately 7–9°C. At the iron-carbon eutectic composition (4.3% C), the melting point reaches its minimum of 1,148°C — the lowest temperature at which any iron-carbon mixture can melt.

The practical range for commercial steels: A36 (0.25% C max) melts at approximately 1,510–1,530°C; high-carbon tool steels (0.8–2.0% C) melt at 1,350–1,430°C; cast irons (2.5–4.5% C) melt at 1,148–1,300°C.

Boiling Point of Iron: 2,861°C in Context

The boiling point of iron — 2,861°C (5,182°F) — is the temperature at which the vapor pressure of liquid iron reaches 1 atmosphere and the metal transitions from liquid to vapor. This temperature is far above anything encountered in conventional metalworking:

• Arc welding electrode tip temperature: ~3,000–4,000°C (briefly exceeds Fe boiling point)
• Plasma cutting temperature: 10,000–30,000°C (massively exceeds Fe boiling point; full vaporization at cut front)
• Basic oxygen furnace (BOF): ~1,600–1,700°C (well below boiling; above melting)
• Electric arc furnace (EAF): ~1,600–1,700°C normal, up to 3,000°C near the electrode tip
• Laser cutting focal zone: surface temperature exceeds boiling in some configurations

Why iron boiling matters in practice

Arc welding metal transfer: In gas metal arc welding (GMAW/MIG), the electrode tip reaches temperatures where iron approaches or exceeds its boiling point. This drives the explosive formation and ejection of metal droplets — the mechanism behind spray transfer mode at high current. The iron vapor that forms at the electrode tip ionizes to become part of the arc plasma.

Laser cutting iron and steel: In high-power fiber laser cutting, the cutting mechanism involves both melting and partial vaporization at the focal zone. The laser power density at the beam waist (typically 10⁶–10⁷ W/cm²) is sufficient to raise the surface temperature above 2,861°C in the cutting process — producing a vapor cavity (keyhole) that allows deep penetration. The vapor pressure inside the keyhole helps eject molten material. The iron boiling point is the temperature threshold above which this keyhole mechanism becomes active.

Steel decarburization: In basic oxygen steelmaking, oxygen is blown through the molten steel. CO and CO₂ bubbles form at the iron-oxygen-carbon interface and agitate the melt, but the temperature (~1,600°C) remains well below the iron boiling point. Excessive overheating risks localized vaporization of volatile alloying elements (Zn, Mg, Pb) before iron itself boils.

Thermite reaction: The classic thermite reaction (2Al + Fe₂O₃ → Al₂O₃ + 2Fe) produces liquid iron at approximately 2,500°C — approaching but still below the boiling point. Thermite products are liquid iron droplets, not iron vapor.

Iron Thermodynamic Data: The Complete Phase Diagram Summary

The thermodynamic state of iron depends on temperature and pressure. At standard atmospheric pressure, the phase sequence is:

Solid iron phases → Liquid iron → Iron vapor

The temperature-enthalpy relationship, accounting for all phase transitions:

The total energy from room temperature solid iron to fully vaporized iron vapor: approximately 8,362 kJ/kg.

The vaporization enthalpy (6,090 kJ/kg) dominates — it is 24× the melting enthalpy (247 kJ/kg). The enormous energy cost of vaporization is why metalworking processes are designed to work in the melt temperature range (1,538–2,000°C for steelmaking) rather than the vapor range. Laser cutting and plasma cutting do briefly exceed the boiling point at the cut front, but only in the microscopic focal zone — not throughout the bulk metal.

Iron Boiling Point vs. Other Metals: Context for Processing

The significance of the liquid range for steelmaking and casting:

Iron’s liquid range of 1,323°C (melting at 1,538°C, boiling at 2,861°C) is moderate. Steel is typically cast at 50–100°C above the liquidus — around 1,580–1,650°C for carbon steel — which is only 1,200–1,280°C below the boiling point. There is ample headroom for superheating, degassing, and alloying operations without approaching vaporization.

The zinc boiling point coincidence: Zinc boils at 907°C — below iron’s melting point. This is why welding galvanized steel is hazardous: the zinc coating on the steel surface reaches its boiling point (907°C) and vaporizes before the base steel even melts. Zinc oxide aform immediately, requiring ventilation. Similarly, in laser cutting of galvanized steel, the zinc vaporizes ahead of the iron melt front, producing zinc oxide fume regardless of how quickly or slowly the cut proceeds. This is not an avoidable consequence of parameter selection — it is a direct result of zinc’s thermodynamic properties relative to iron.

Frequently Asked Questions About Iron Boiling Point

Q: What is the boiling point of iron?

2,861°C (5,182°F, 3,134 K) at standard atmospheric pressure. This is the temperature at which liquid iron’s vapor pressure reaches 1 atmosphere and the metal transitions from liquid to gas. Source: NIST WebBook standard reference data for the element Fe.

Q: What is the melting point of iron?

1,538°C (2,800°F, 1,811 K) at standard atmospheric pressure. Pure iron (delta-iron phase, BCC crystal structure) melts at this temperature. All steel grades melt at lower temperatures because dissolved carbon and other alloying elements depress the solidus below pure iron’s value.

Q: How much hotter is the boiling point of iron than its melting point?

The boiling point (2,861°C) is 1,323°C higher than the melting point (1,538°C). This 1,323°C liquid range gives iron ample temperature headroom for steelmaking, casting, and welding operations well above the melting point without approaching vaporization. For comparison, the energy required to vaporize iron (6,090 kJ/kg) is approximately 24.7 times greater than the energy required to melt it (247 kJ/kg).

Q: Does iron actually boil in laser cutting?

At the microscopic focal zone of a high-power fiber laser, yes — the surface temperature briefly exceeds iron’s boiling point (2,861°C). This creates a vapor cavity (keyhole) that assists melt ejection and allows deep penetration cuts. The bulk material never reaches this temperature — only the surface at the beam waist. The laser cutting process operates in a regime where both melting and localized vaporization occur simultaneously in a sub-millimeter zone.

Q: Why does steel have a lower melting point than pure iron?

Because dissolved carbon and other alloying elements disrupt the iron crystal lattice, lowering the energy required to transition from solid to liquid. Carbon has the largest effect: at 0.25% C (A36 steel), the liquidus is approximately 1,520–1,530°C — about 10–18°C below pure iron. At the iron-carbon eutectic composition (4.3% C), the melting point reaches its minimum of 1,148°C — nearly 390°C below pure iron.

Q: What happens to alloying elements when steel approaches its boiling point?

At temperatures approaching steel’s boiling point, volatile alloying and tramp elements vaporize preferentially. Zinc (boiling point 907°C) vaporizes well below the steel melting point. Magnesium (1,091°C boiling), lead (1,750°C boiling), and manganese (2,061°C boiling) all have boiling points below iron’s. In electric arc furnace steelmaking, manganese loss by vaporization near the arc zone is a real consideration. In laser cutting of steel alloys, the selective vaporization of lower-boiling constituents at the kerf face can affect cut-edge composition compared to the bulk.

References

1. National Institute of Standards and Technology (NIST), NIST Standard Reference Database 69: NIST Chemistry WebBook, available at webbook.nist.gov. (Iron thermophysical properties: melting point 1,538°C (1,811 K), boiling point 2,861°C (3,134 K), latent heat of fusion 247 kJ/kg (13.81 kJ/mol), latent heat of vaporization 6,090 kJ/kg (340 kJ/mol); ITS-90 adjacent fixed point status for iron melting point.)
2. Haynes, W.M., ed., CRC Handbook of Chemistry and Physics, 104th Edition. CRC Press, Boca Raton, FL, 2024. (Melting and boiling points for all elements, including Fe, Zn, Cu, Al, Ni, Cr, W; lattice parameter and crystal structure data for α-, γ-, and δ-iron; enthalpy of fusion and vaporization data; critical temperature and pressure for iron.)
3. Chase, M.W., et al., NIST-JANAF Thermochemical Tables, 4th Edition. American Institute of Physics, 1998. (Standard thermochemical data for iron and iron compounds: heat capacities for solid and liquid iron, enthalpy changes through all phase transitions from 298 K to above boiling point; the authoritative source for the complete thermodynamic energy budget table in this article.)
4. ASM Handbook, Volume 1: Properties and Selection — Irons, Steels, and High-Performance Alloys, 10th Edition. ASM International, Materials Park, OH, 1990. (Iron phase transformation temperatures: α-iron (BCC, <912°C), γ-iron (FCC, 912–1,394°C), δ-iron (BCC, 1,394–1,538°C); Curie temperature 770°C; effect of carbon content on iron liquidus depression; steel casting temperature practice relative to pure iron melting point.)

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Dr. Liu Che

Dr. Liu Che holds a Ph.D. in Engineering from Dalian University of Technology and completed postdoctoral research at the Mechanical Engineering Postdoctoral Fellowship of Harbin Institute of Technology. His research focuses on core algorithms for large-scale laser equipment integration, laser cladding remanufacturing, and real-time intelligent monitoring systems for processing. He possesses profound academic expertise in cutting-edge fields such as laser beam spatial shaping and the mechanism analysis of laser processing of special alloys. He has led numerous solutions to technical challenges in cutting and surface modification of high-hardness materials in the heavy machinery industry, demonstrating strong theoretical foundations and outstanding engineering implementation capabilities.

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