Guides/Comparisons
Iron vs steel
iron-vs-steelCast iron and Steel at a glance
This page compares Cast iron and Steel using only values that carry a source, a test condition and an evidence level. Ranges run across temper, heat treatment and product form, measured at room temperature where the source says so.
Steel is iron with a controlled, low carbon content. Cast iron holds more carbon, which sits as graphite flakes or nodules. This page compares the cast irons and the carbon and alloy steels in this database.
- Yield strength: the ranges overlap (Cast iron 165 to 621 MPa; Steel 170 to 1,330 MPa).
- Tensile strength: the ranges overlap (Cast iron 138 to 827 MPa; Steel 310 to 1,600 MPa).
- Density: Steel is higher (Cast iron 7.2 g/cm³; Steel 7.67 to 8.03 g/cm³), a difference of under 10% at the midpoint.
- Young's modulus: Steel is higher (Cast iron 103 to 169 GPa; Steel 190 to 210 GPa), about 1.5 times the midpoint of Cast iron.
- Thermal conductivity: the ranges overlap (Cast iron 50 W/(m·K); Steel 25 to 93 W/(m·K)).
- Yield strength for each unit of density (calculated from each grade's midpoint values): Cast iron 27.3 MPa per g/cm³; Steel 32.6 to 115 MPa per g/cm³.
"Higher" means the whole range is higher, not just one value. Where a value is a specified minimum, the table says "at least".
Side by side
| Property | Cast iron | Steel |
|---|---|---|
| Yield strength | 165 to 621 MPa | 170 to 1,330 MPa |
| Tensile strength | 138 to 827 MPa | 310 to 1,600 MPa |
| Density | 7.2 g/cm³ | 7.67 to 8.03 g/cm³ |
| Young's modulus | 103 to 169 GPa | 190 to 210 GPa |
| Thermal conductivity | 50 W/(m·K) | 25 to 93 W/(m·K) |
| Melting point | no data | 1,432 to 1,526 °C |
| Thermal expansion | no data | 3.44 to 14.6 ppm/K |
| Yield strength per densitycalculated | 27.3 MPa per g/cm³ | 32.6 to 115 MPa per g/cm³ |
| Grades included | Cast iron | Steel |
|---|---|---|
| Materials | Gray cast iron, Ductile iron | A36 structural carbon steel, Carbon steel 1018, Carbon steel 1045, Carbon steel 1215, Alloy steel AISI 4130, Alloy steel AISI 4140, Carbon steel 1010, Carbon steel 1070, Alloy steel AISI 4340 |
When to use which
| Material | Best for | Weakest at |
|---|---|---|
| Gray cast iron | Machine bases, frames, pump housings and engine blocks, where damping, machinability and compressive load matter more than tensile strength | Tensile strength and shock resistance: the graphite flakes act as notches, so the metal breaks in a brittle way |
| Ductile iron | Castings that need the ASTM A536 strengths of 60,000 to 120,000 psi, such as high-strength gears and rollers and pressure-containing valve and pump bodies (Goldens; Daycounter) | Elongation falls as the grade rises, from 18 percent at 60-40-18 to 2 percent at 120-90-02 (Goldens). Properties also depend on the heat treatment that each grade requires (ASTM A536 summary) |
| A36 structural carbon steel | Welded and bolted structural frames, girders and shapes that will be painted or galvanized | Rusts outdoors and corrodes first when it touches copper or tin in a wet electrolyte; minimum strength falls above 650 °F (343 °C) |
| Carbon steel 1018 | Machined shafts, pins and gears, and cost-sensitive structural parts that are coated against rust | Rust in moist air and salt unless it is coated, and strength that depends on whether the bar was cold drawn or hot rolled |
| Carbon steel 1045 | Forged or machined axles, shafts, gears and machine parts that need more strength than low-carbon steel, and that are coated against rust | Rust in moist air and salt unless it is coated, and strength that depends on whether the bar was cold drawn, hot rolled, normalised or annealed |
| Carbon steel 1215 | Screw-machine parts such as bushings, inserts, pins, studs and couplings, where machining speed and surface finish matter more than weldability (Alro; Titanium Industries) | Welding and fatigue. Alro says it is difficult to weld, and Titanium Industries says it fails in parts under severe fatigue stress |
| Alloy steel AISI 4130 | Aircraft and motorsport tube, sheet and bar, and oil and gas parts, where weldability and strength after heat treatment both matter | Hydrogen gas and salt solutions: in one laboratory study the threshold stress intensity fell from 88 to 32 MPa√m as hydrogen pressure rose from 21 to 69 MPa, and salt solution cut corrosion-fatigue life |
| Alloy steel AISI 4140 | Machine parts and structures in medium and large sections that need high strength and toughness after quenching and tempering | Hydrogen gas, which cut notched strength by about 60 percent in one room-temperature study, and dilute sulfuric acid, which corroded it at 176.87 mpy |
| Carbon steel 1010 | Formed, drawn and welded parts, and light structural parts, where cost and ductility matter | Rust in moist air and salt unless it is coated, and strength that depends on the processing condition |
| Carbon steel 1070 | Springs, shafts and hand tools that are heat treated and kept dry or coated (tensile strength 640 to 700 MPa in the supplier range) | Corrosion, which is poor without a coating, brittleness once fully hardened, and poor weldability |
| Alloy steel AISI 4340 | Large sections that need high tensile strength and toughness after quenching and tempering, such as aircraft landing gear, gears and shafts (voestalpine, Saky Steel). | Toughness at the highest strength levels: the minimum Charpy impact energy falls from 42 J at condition U to 9 J at condition Z (voestalpine). No corrosion data were read for this grade. |
Where each is used
| Material | Use |
|---|---|
| Gray cast iron | Brake discs and rotors |
| Gray cast iron | Machine tool bases and mountings |
| Gray cast iron | Internal combustion engine cylinder blocks |
| Gray cast iron | Pump housings and valve bodies |
| Ductile iron | Gears and rollers (100-70-03 and 120-90-02 grades) |
| A36 structural carbon steel | General construction and structural framing |
| A36 structural carbon steel | Girders, structural shapes and angle iron |
| A36 structural carbon steel | Bolted and welded structural joints |
| Carbon steel 1018 | Shafts, pins and gears |
| Carbon steel 1018 | Low-stress structural components |