---
title: "Ductile iron — density, melting point and properties"
description: "Ductile iron has a density of 7,100 kg/m³ (7,000–7,200). Melting point, strength, conductivity, what it is used for, and whether it floats, rusts or is magnetic."
url: https://atoneplace.net/material/ductile-iron/
site: "At One Place"
---

# Ductile iron

**Density of ductile iron: 7,100 kg/m³ (7,000–7,200)**

7.10 g/cm³ · 443.2 lb/ft³

The same iron as gray cast iron with a trace of magnesium added to the ladle, which makes the graphite form as spheres instead of flakes. That one change roughly triples the tensile strength and turns a brittle material into one that bends before it breaks, at almost no extra cost. It is why buried water mains stopped bursting when the ground shifted: they were cast iron before 1950 and ductile iron after.

## Properties

- **Density:** 7,100 kg/m³ (7,000–7,200) — 7.10 g/cm³
- **Melting range:** 1150–1200 °C
- **Tensile strength:** 400–900 MPa
- **Young's modulus:** 169 GPa
- **Thermal conductivity:** 33 W/(m·K)
- **Thermal expansion:** 11 µm/(m·K)
- **Floats in water:** No
- **Magnetic:** Yes

## What it is used for

- Water and sewer mains
- Crankshafts and steering knuckles
- Gearbox and axle housings
- Manhole covers and hydrants

## How it compares

*Water is 1,000 kg/m³. Anything with a shorter bar than water floats in it.*

- air: 1.2 kg/m³
- expanded polystyrene foam: 20 kg/m³
- rigid polyurethane foam: 40 kg/m³
- Water — the float line: 1,000 kg/m³
- ductile iron: 7,100 kg/m³
- tungsten: 19,250 kg/m³
- gold: 19,300 kg/m³
- platinum: 21,450 kg/m³

*Water is 1,000 kg/m³. Anything below that floats.*

| Material | Density | Relative to water |
| --- | --- | --- |
| rigid polyurethane foam | 40 kg/m³ | 0.04× |
| expanded polystyrene foam | 20 kg/m³ | 0.02× |
| air | 1.2 kg/m³ | 0.00× |
| ductile iron ← | 7,100 kg/m³ | 7.10× |
| platinum | 21,450 kg/m³ | 21.45× |
| gold | 19,300 kg/m³ | 19.30× |
| tungsten | 19,250 kg/m³ | 19.25× |

## Does it rust?

Yes, and readily. Rust is iron oxide, and it is flaky rather than adherent — it does not seal the surface, so oxygen and water keep reaching fresh metal underneath and the process continues until the section is gone. That non-protective quality is what makes rust different from the oxide layers on other metals.

## Is it magnetic?

Iron, nickel and cobalt are the three common ferromagnetic elements, and anything based mostly on them is strongly attracted to a magnet.

## Common questions

### What is the density of ductile iron?

7,100 kg/m³ (7,000–7,200), or 7.10 g/cm³.

### Does ductile iron float?

No. At 7,100 kg/m³ it is denser than water.

### Is ductile iron magnetic?

Iron, nickel and cobalt are the three common ferromagnetic elements, and anything based mostly on them is strongly attracted to a magnet.

## Related pages

- [Does ductile iron float?](https://atoneplace.net/does-it-float/ductile-iron/)
- [Mild steel](https://atoneplace.net/material/mild-steel/)
- [Stainless steel 304](https://atoneplace.net/material/stainless-304/)
- [Brass](https://atoneplace.net/material/brass/)
- [Gray cast iron](https://atoneplace.net/material/cast-iron/)
- [High-carbon steel](https://atoneplace.net/material/carbon-steel/)
- [Stainless steel 316](https://atoneplace.net/material/stainless-316/)
- [Aluminum 6061](https://atoneplace.net/material/aluminum-6061/)
- [Bronze](https://atoneplace.net/material/bronze/)

## Sources

- Typical range compiled from manufacturer specifications — At One Place (/about/)